recurving distributor

Have a 351c with the stock Ford distributor and Petronix points replacement. It is a vacuum advance, I have also switched to a T5 and have highway gears so I am running the car at just shy of 2000 revs when I do 70 on the freeway.

I have the problme when setting the timing that when it is set to get the best MPG the engine does NOT like starting when hot I ave melted a cable trying. When it is set so this does not happen the MPG suffers by about 6!

Been told the best way to deal with this is to recurve the distributor. I have been searching online and found are what looks like 2 different ways of doing this- 1 is to flip a plate that is in the ditributor so that the gap on the plaate is smaller- although if I am on the small gap now this would not work. The other is to replace small springs that can be picked up at O'Reillys for about $6. So neother seems real hard to do or expensive. Just wondeirng whihc one works best or does one do both.

I have also been led to believe by doing this will make the speed the engine gets best torque lower and this in turn will make MPG go up slightly.

I had never heard about anyhing like this until it was mentioned on another thread here.

The answer is yes to both changes. The exact amount of initial advance, mechanical advance, and vacuum advance is dependent on the specific combination of parts you have and how you use the car. It sounds like your concern is highway mileage cruising at light load at 2000 RPM. Under those conditions the engine will tolerate at lot of advance. However, too much advance will cause detonation that can damage pistons, rings, and rod bearings. What initial and total advance are you using that yields the best mileage? Chuck

Flip the breaker plate, or reluctor arms ( whichever one you have) and you should see a marked improvement.

I also strongly recommend an adjustable vacuum advance. With this you can set the static timing retarded significantly which will ease your starting and hot-starting woes. The vacuum advance will advance your timing instantly as the engine starts. You can adjust it to advance the timing as much as you want at idle, giving you great off-the-line performance, and the the recurved distributor centrifugal advance will take over as RPM increase, keeping the improved performance there.

Pretty simple really, and not nearly as intimidating as it appears once you get into it.

When you say it doesn't like starting, are you saying it rev's really slow?

there are a couple ways to recurve the distributor, However i will caution flipping the mechanical advance plate can cause problems with total advance.

the vacuum advance is an additional timing adjustment with regards to vaccum that the engine produces. Typically a distributor recurve starts with the mechanical aspect of the distributor.

you have 4 categories in regards to timing

1) initial Timing

2) total timing (mechanical)

3) Vacuum advance

4) mechanical advance

First you say you have Hot start problems.

This indicates too much initial Advance. you are most likely compensating for a performance loss mid throttle. this is #1

What you want to do set initial so that you get max vacuum at a set idle rpms for max efficiency, this is a careful balance because you can get into a hot no start situation, thus you need to balance initial timing with what an engine can tolerate. Hot Start, dieseling, ping, you need to back off timing until it stops or you will damage the pistons.

In regards to MPG this is the duty of the vaccum advance #3.

#2 total timing is very important and many people ignore it. this is your upper limit of mechanical tolerance and is usually 32-38 Degrees BTCD

exceeding this can blow your engine at high rpms under load.

total advance can be exceeded with the vacuum advance because it is dependant on engine load, low load and you might see 54 degrees of advance at 3000rpms, thats ok because you tune for detonation at that point with the vaccum advance.

So you have a defined rule set a lower end, an upper end, and the tune takes place in between those 2 points.

Curving the distributor comes in between these 2 fixed points.

now setting the upper limit is the job of the mechanical advance plate,

On the plate you will see a number followed by an L

example 8L

this means the mechanical provides 16 degrees of mechanical advance.

in a realistic environment you will see 16L

so 16 X 2 = 32

remember that upper limit 32-38 so that gives you room for 0-6 degrees of initial mechanical advance.

the reason you may need to play with the mechanical advance plate is say you have an after market cam that requires 10 Degrees initial

well your 16L plate is a problem you have too much so you need a smaller angle plate like a 12L which is 24 degrees + 10 and you are at 34 degrees perfect.

so you have adjusting the curve on the distributor end points.

the springs control the rate of advance and so do the weights on the mechanical.

so the true magic of the curve is messing around with the springs to get the right amount of advance depending on engine rpms

Ford gave you one heavy spring and one light spring on the mechanical the heavy spring sat loose and the light spring tight.

the idea was you got a quick advance under 2000 rpms and then the heavy spring took over and released timing much slower to the upper limit.

so kits like mr gasket gave you 2 light springs that allowed full advance at 2000-2500 rpms for max performance.

its always a trade. performance is the inverse of fuel economy this is why people have street timing settings and race timing settings.

so re-curving a distributor is going to aim usually for performance not MPG.

you want MPG you need heavy springs to hold back the engine

you want power and get up and go then light springs and full timing in by 2500-3000 rpms

this is how a distributor is curved.

the vacuum advance is the "patch" that sits on top of the mechanical like an override.

the advance works off engine vaccum, low vacuum high load or acceleration, high vacuum low load/ cruise.

cruise is where you will gain most of the MPG its why a car is rated like 12 city and 20 highway.

in the city you stop and go on highway you sit at cruise for 20 miles at a time.

vacuum advance can be curved like mechanical there are vacuum delays that can make the advance come in slow or fast.

but really once you set the mechanical you work the advance, and what you do is Turn it up all the way to start.

and listen for ping.

each time the engine pings you turn it down 1-2 turns, until ping stops.

that will be optimal for MPG and what an engine can tolerate.

nothing is like the books say, optimal Vacuum means an engine is going to have lean problems usually depending on weather, so you always have a fudge factor towards less efficient to compensate.

all this requires constant testing, and if you change your octane level of fuel you get to start all over again.

you need the engine to run on the hottest and coldest day you will be driving so there is no perfect tune, just perfect for what your engine can tolerate.

all adjustments need to be made when the engine is warmed up.. but you will need to readjust for a lean or rich misfire when the engine is cold.

you are trying to hit a floating target inside a bowl of water basically so nothing is written in stone.

first thing Bring initial timing back to where you do not have a hot start issue.

rev the motor slowly to full advance and see where the top limit is... if the total timing is like 30 then you want to add more and flipping the plate can be a good idea to look into.

be aware, say your currently at 12L and the otherside of the plate reads 16L and you need more top end timing, when you flip it the distributor will be 180 degrees out, so you will need to pull the distributor turn it 180 and reinstall then go through the motions again. if 16L is a little too much and you see 40 degrees on the strobe and balancer scale then you can use a piece of rubber hose on the mechanical advance to limit total throw.

you could also shave the 12L to open it up more.

really if your concerned about mpg you can also change the carburetor settings.

you can lean out jets until the car surges, or use a lower profile accerator cam, change down shooter size, change down power valve size or even use a dual opening power valve that or a staged power valve, but it will depend on what your engine can tolerate.

All great advice, with only one small disagreement: I feel you should dial in your total dynamic maximum advance first to be fully in by whatever RPM you decide on. Then you can work beckwards from there to adjust your other timing parameters to be where you want without affecting the total dynamic timing.

I will say this: there are many enthusiasts out there that spend big bucks on carbs, manifolds, headers, etc...all looking for better performance. Hundreds, ir thousands of dollars for relatively modest performance gains in some respects. But it seems so many of them then simply ignore the one inexpensive and easy-to-adjust item that really will give them about the most noticeable performance improvement overall: ignition timing.

Its kind of a lost art these days, along with carb tuning. It is well worth it to anybody who owns a classic car to study these two items and know how to apply the principals to your car...huge dividends for very little expense.

One advertised feature of the pertronix 111 is the ability of the computer to compensate and retard the timing while cranking...

That is a GREAT feature.

you want MPG you need heavy springs to hold back the engine

you want power and get up and go then light springs and full timing in by 2500-3000 rpms

Click to expand...

One question that I have is how does having overdrive affect this? If the motor was happy with the factory timing without overdrive, is there a rule of thumb for adjusting the advance for highway cruising around 2000rpm? Where would you want the vacuum advance to be fully in, before or after this 2000rpm mark?

well the old way was aiming for a highest vacuum reading at a desired rpm or speed.

highest vacuum means most efficiency. its the 55mph trick people used to do they would sacrifice a total tune in favor of a narrow RPM range to hypermile this was during the first opec embargo.

first you would want a Vacuum gauge on the intake manifold and see what the car is doing at the cruising rpm and speed you want. now you also figure the higher the rpm the more fuel you are using since the mechanical fuel pump is in line with engine rpms. an over drive in general will save mpg due to lower rpms.

so say with the overdrive your mph range is in the 2000s without it with the old 3 speed you might be 2500-3500 depending on rear end gear. so with the overdrive unless there is some drag from something you should be saving mpg

Now at steady cruise the only thing affecting fuel consumption is the Cruise Jets, so if you jet down 1-2 sizes and keep going as long as the engine does not surge then you lean out cruise and save mpg. your vacuum should go up also, because the engine is leaning out.

each time you touch the pedal the accelerator pump is going to shoot off, that needs to compensate for the overlap between the cruise circuit loosing vaccum and signal and the power valve kicking on, fast pedal movement is just going to turn the accelerator pump on and off. You can reduce shooter size up to lean fire miss and you have to come back up slightly. a secondary adjustment is the Cam that drives the accelerator pump there are different profiles to customize when fuel is delivered, again all depending on your engine, not what you want.

the power valve again can be swapped to come on later rather then early or staged to open at up to 3 different rates. stock the power valve is on or off usually at 6.5 HG vacuum.

The vaccum advance is dependant on engine load, so really you are talking about how fully in the mechanical advance is at a given RPM range. now Again... more timing means more power, if your seriously maximizing MPG then you want the mechanical to kick in as late as possible and pretty much never come fully on. Obviously the car will drive like a sluggish pig in that configuration.

so really you want a stock spring tension that Ford put on the car OEM because that gives the best of both worlds as originally configured. It also means you will want to be light on the throttle since that will burn up gas.

now this is theory because most cams are not going to tolerate this in the motor. your going to get carb backfire and a massive lean condition. and then you get into Fuel injection replacing the carb....

If you Truly want a 71-73 to get 35mpg, then you need to ditch the V8 for a inline 6.

also you need to ditch a secondary carb 4 barrel for a 2 barrel.

additionally a smaller carb will create a higher vacuum and reduced fuel consumption, again there is a lower limit to what you can tolerate. a 750DP is going to use gallons of fuel compared to a stock 550CFM carb.

Heads make a big diffference V4 verse V2 or Milled V2s where the ext and intake valves have been changed to the same size. 50/50 creates the highest mpg.

Headers are a bad thing, they lose heat from the combustion chamber and act like a timing advance of about 2-4 degrees.

another trick.. smaller exhaust higher back pressure will gain you mpg also, you also get a double dip from the carb fuel signal. so everyone puts a 2.25 exhaust for performance, go to a 2.125 or a 1 7/8s pipe and it will increase mpg at the cost of performance.

higher back pressure causes a bounce back wave in the engine. when the exhaust stroke vents it hits a restriction a backwards Air wave goes back into the engine up to the carb and Re-aerates fuel siting on the intake manifold floor that fell out of mix on the original fuel signal (vacuum) into the engine. that re-aerated fuel then goes back down into the combustion chamber for the power stroke, thus you magically got wasted fuel to be burned by the engine which means you can jet down, or reduce accelerator or power valve additional fuel because you already have more and thus boost MPG. this is called a Double dip,,, you double dipped wasted fuel and thus need less fuel from the carb at every rpm. This is what causes a MPG drop in a carburetor when you have an exhaust leak, suddenly the double dip effect stops and you get lean misfire so you compensate by jetting up or increasing pump shot.

When you have an exhaust leak in a modern Fuel injection the O2 sensor gets an incorrect reading and it sees a lean condition because air is getting into a sealed enviroment and thus adjusts more fuel in the injectors to compensate.

the engine is an air pump basically, you force more air in and out of it and it makes more power

you restrict the in and out and your air tank reserve(mpg) lasts longer.

you can't say at 2000rpms your timing must be 16 degrees, every engine is different and each day the environment changes, so you have to take the car out on the road get to a desired RPM and take a vacuum reading and a seat of your pants reading, if you have a oxygen sensor then you want the Air fuel information(you can retro fit a oxygen sensor computer on a carb car, i had one for a while on my 72), from there you say i want to save more fuel or make more power that that rpm, at cruise that is the main jets. if you want to go faster from a standing stop, then you need more fuel and more timing.

but the fun doesn't even stop with ignition changes and carb changes.

you still have final drive gear rations taller the ratio the lower the rpms at cruise the more MPG you save.

a 2.75:1 isn't going to do burn outs like a 3.25:1 but you will get 25mpg because of it.

transmission shift points and how fast a shift occurs effects mpg. you have heard of shift kits, well they trade comfort in gear changes for MPG, it has a performance aspect as well, faster shifts without lag increases mpg. also reduces clutch pack wear inside the trans.

remember the transmission vacuum modulator?

well that adjusts shift points based on engine load and it overrides the mechanical Hydraulic shift points, set it to maximize MPG so you get the engine to shift at low rpms through the gears. your acceleraion will suffer but your mpg will increase.

setting your kick down arm effects mpg how heavy will the pedal be for a shift down for passing.

so i mean you can go nuts .

this is why you have a seat of your pants measurement, you can feel the changes you made and if the car feels really slow then you need to go back a little and trade off mpg for fun factor.

Hmm...I'm not sure there is enough information in your post. Could you elaborate, please?

Joking aside, I already know some of what you've mentioned, mostly in a general way, but you said some things I'd never thought about.

What I'm wondering is if Ford chose certain timing curves to fit cruising speed at particular rpm and if so, how should the timing curve fit with a new cruising speed/rpm combination?

This also plays into the torque curve of the motor. If the motor is low on torque at 2k rpm but fine for cruising at 2500 rpm with the stock non-overdrive trans, then installing overdrive drops the engine rpm to point below the power curve and you get a lag if you try to accelerate (unless you downshift). I'm trying to work thru this in my head and it all started years ago with my RX7. Overdrive was great on the highway at 70mph, but if you were doing the urban speedlimit (usually between 55-65mph), it was right on the edge of the two gears. 4th was too high in the rpm and mileage suffered, 5th was too low in the rpm (lugging is REALLY bad for the rotary) and any need to increase speed meant downshifting. But 70 was great. Good mileage and plenty of power, if you don't mind speeding.

I guess what I'm getting at is I'm trying to avoid an rpm/highway speed situation where it's good for neither. So I'm wondering about a drivability thing more than MPG or power.

well what was your rear end ratio. sounds like your rear ratio was not right.

The trans your kind of stuck with the internal ratios, but you have adjustment with the final drive ratio.

it comes down to rollout, how far the tire carries you per revolution.

i know one guy had an issue like this and he bought tires with a slightly taller diameter to force higher speed and slightly lower rpms

then he just swaped one tooth on the speedometer gear to keep it reading correctly.

As for how the timing curve fits well that is the problem because there are basically infinate part combinations that play off each other, plus even 2 cars with the same equipment will be different due to tolerance.

its going to come down to you and how you feel the car is performing.

you might feel the car is sluggish at 2750rpms somebody else might not, so you have to basically sit in the car and say well this isn't right, what needs to change. but you also have to remember a Carb always gives the incorrect fuel/air mixture at all throttle positions. there will be some rpms that will be sluggish, because if you tuned to just get that one area dialed in then elsewhere you will have another issue.

this is like why nascars have to be pushed out of the pits since they are tuned to just run a narrow high rpm range.

ford chose the original curve as the best of all worlds scenario, real world issues like. Altitude, temperature, humidity, bad gas, good gas, octane changes, and then factor in running equipment and all the different gear ratios and transmission and engine combinations. there were limitations. Like if you had A/C they wouldn't let you install a 2.75 ratio on a V4 because it would draw the engine down so far it would run bad. you had to ditch A/C

for the aftermarket they Do make highly tunable distributors, you can sit there with an allen key and without opening the distributor you can make all adjustments externally and see what happens. there are also programable MSD distributors that you can program.

it can be an overwhelming crap shoot also, so many combinations and you may never hit the mark you want because you don't have enough experience. also once you do come close and hit a mark on a nice dry 75 degree sunny day the next day has high humidity and its 85 and suddenly the car is lean misfiring again.

But on your RX7 it seems like your rear end ratio was too short. you were like in between the sweet spot a higher ratio would of dropped the RPMs in 4th and when you did push it into overdrive the rpms would of been higher for a steady cruise. alot of people have modern cars like this and get pissed because the crash box has the wrong gear ratios, its too sluggish on acceleration and then falls out of gear on the top end.

like a person goes OH i want the close ratio trans option its like a 5000$ option so it must be great and its HORRIBLE for street driving and they don't know what happened because the box is optimized for acceleration and you just want to sit at comfortable rpms at cruise and have a wide range of ratio options for like a hilly area.

With the kind of granularity of spark timing you seek, You will really only satisfy this urge by going programmable.

Out of the box is somewhat expensive, but I looked seriously into the jaycar kit (chip magazine).

A lot of users on various forums vouch for them...

They cater for revs, engine load, even knock sensing.

I was almost ready to go for it, but got lazy and was wooed by the features of latest pertronix 111 billet + matching flamethrower coil.

I also bought an independent adjustable knock sensor and a good quality wide A/F ration gauge (to be installed soon) so I'm relatively happy....

When my rich uncle dies, I'd love to go with one of those all in one efi carbies, etc....

well what was your rear end ratio. sounds like your rear ratio was not right.

The trans your kind of stuck with the internal ratios, but you have adjustment with the final drive ratio.

it comes down to rollout, how far the tire carries you per revolution.

i know one guy had an issue like this and he bought tires with a slightly taller diameter to force higher speed and slightly lower rpms

then he just swaped one tooth on the speedometer gear to keep it reading correctly.

As for how the timing curve fits well that is the problem because there are basically infinate part combinations that play off each other, plus even 2 cars with the same equipment will be different due to tolerance.

its going to come down to you and how you feel the car is performing.

you might feel the car is sluggish at 2750rpms somebody else might not, so you have to basically sit in the car and say well this isn't right, what needs to change. but you also have to remember a Carb always gives the incorrect fuel/air mixture at all throttle positions. there will be some rpms that will be sluggish, because if you tuned to just get that one area dialed in then elsewhere you will have another issue.

this is like why nascars have to be pushed out of the pits since they are tuned to just run a narrow high rpm range.

ford chose the original curve as the best of all worlds scenario, real world issues like. Altitude, temperature, humidity, bad gas, good gas, octane changes, and then factor in running equipment and all the different gear ratios and transmission and engine combinations. there were limitations. Like if you had A/C they wouldn't let you install a 2.75 ratio on a V4 because it would draw the engine down so far it would run bad. you had to ditch A/C

for the aftermarket they Do make highly tunable distributors, you can sit there with an allen key and without opening the distributor you can make all adjustments externally and see what happens. there are also programable MSD distributors that you can program.

it can be an overwhelming crap shoot also, so many combinations and you may never hit the mark you want because you don't have enough experience. also once you do come close and hit a mark on a nice dry 75 degree sunny day the next day has high humidity and its 85 and suddenly the car is lean misfiring again.

But on your RX7 it seems like your rear end ratio was too short. you were like in between the sweet spot a higher ratio would of dropped the RPMs in 4th and when you did push it into overdrive the rpms would of been higher for a steady cruise. alot of people have modern cars like this and get pissed because the crash box has the wrong gear ratios, its too sluggish on acceleration and then falls out of gear on the top end.

like a person goes OH i want the close ratio trans option its like a 5000$ option so it must be great and its HORRIBLE for street driving and they don't know what happened because the box is optimized for acceleration and you just want to sit at comfortable rpms at cruise and have a wide range of ratio options for like a hilly area.

Click to expand...

I think the problem with the RX7 was the gearing was stock (3.91, I think) and I suspect Mazda set it up for 70mph cruising, but on a flat level road, the mpg at 55 in OD was pretty good. It gave them a nice mpg rating, but who really drives 55? Plus, the power band didn't really start to take off until 2800rpm (8000 rpm redline), rev it and it's great, rev it and you get to visit more gas stations, too.

Aside from Alan's original questions, I have the factory manual and it shows the advance at specific rpm points (including 1000 and 2000) in cam degrees of advance (which means 2000 and 4000 crank degrees). That is what made me wonder if Ford had a 'reason' and Alan's overdrive drops the 70m[h rpm right into that 2000 crank rpm range. Sometimes there is such a thing as over thinking, then again, engineers can be tricky buggers.

One nice thing about the duraspark units was the fact most had adjustable vacuum advance units from the factory. A small allen wrench thru the vacuum port could be used to fine tune vacuum advance. I do this with my RV. Running in the mountains I could loosen it up about three turns then back again when I came down. Going from 8 to 9 MPG doesn't sound like a lot, but in a cross country trip it saved me a few $$$. Great thread guys!!!

I never read anything like that along my way but then I wasn't looking for that information either I concern myself with the wonders of the c6 and manual verse vacuum secondaries,

That is actually another area to look at for tuning manual trans then you want a manual secondary. Automatic and you want vacuum secondaries and then you can tune from there figuring out when the secondaries should open and dump more fuel and air into the engine. Thing to remember is you cannot convert back and forth from manual to vacuum there are differences with linkage and sometimes internal differences with the carb.

Every car is so different with ratios having it drop just at 2000 or 4000 might be a fluke in regard to a modern rear ratio being between 3.3 and 3.6 ish I guess if you had anything from a 3:1 to 3.75:1 you would be in the ball park and it would appear that everything is happening at specific Rpms.

Almost anything is possible if you give it enough time and effort. I've gotten 25mpg from a 750dp on a 351 v4 and no overdrive. It was freaky cruising at 70mph for an hour and the fuel gauge basically not moving.. The second I was in stop and go traffic you could watch the fuel gauge move fast to empty.

Really for max mpg you have to go fuel injection, and the works in regards to ecu and sensoring.

well what was your rear end ratio. sounds like your rear ratio was not right.

The trans your kind of stuck with the internal ratios, but you have adjustment with the final drive ratio.

it comes down to rollout, how far the tire carries you per revolution.

i know one guy had an issue like this and he bought tires with a slightly taller diameter to force higher speed and slightly lower rpms

then he just swaped one tooth on the speedometer gear to keep it reading correctly.

As for how the timing curve fits well that is the problem because there are basically infinate part combinations that play off each other, plus even 2 cars with the same equipment will be different due to tolerance.

its going to come down to you and how you feel the car is performing.

you might feel the car is sluggish at 2750rpms somebody else might not, so you have to basically sit in the car and say well this isn't right, what needs to change. but you also have to remember a Carb always gives the incorrect fuel/air mixture at all throttle positions. there will be some rpms that will be sluggish, because if you tuned to just get that one area dialed in then elsewhere you will have another issue.

this is like why nascars have to be pushed out of the pits since they are tuned to just run a narrow high rpm range.

ford chose the original curve as the best of all worlds scenario, real world issues like. Altitude, temperature, humidity, bad gas, good gas, octane changes, and then factor in running equipment and all the different gear ratios and transmission and engine combinations. there were limitations. Like if you had A/C they wouldn't let you install a 2.75 ratio on a V4 because it would draw the engine down so far it would run bad. you had to ditch A/C

for the aftermarket they Do make highly tunable distributors, you can sit there with an allen key and without opening the distributor you can make all adjustments externally and see what happens. there are also programable MSD distributors that you can program.

it can be an overwhelming crap shoot also, so many combinations and you may never hit the mark you want because you don't have enough experience. also once you do come close and hit a mark on a nice dry 75 degree sunny day the next day has high humidity and its 85 and suddenly the car is lean misfiring again.

But on your RX7 it seems like your rear end ratio was too short. you were like in between the sweet spot a higher ratio would of dropped the RPMs in 4th and when you did push it into overdrive the rpms would of been higher for a steady cruise. alot of people have modern cars like this and get pissed because the crash box has the wrong gear ratios, its too sluggish on acceleration and then falls out of gear on the top end.

like a person goes OH i want the close ratio trans option its like a 5000$ option so it must be great and its HORRIBLE for street driving and they don't know what happened because the box is optimized for acceleration and you just want to sit at comfortable rpms at cruise and have a wide range of ratio options for like a hilly area.

Click to expand...

I think the problem with the RX7 was the gearing was stock (3.91, I think) and I suspect Mazda set it up for 70mph cruising, but on a flat level road, the mpg at 55 in OD was pretty good. It gave them a nice mpg rating, but who really drives 55? Plus, the power band didn't really start to take off until 2800rpm (8000 rpm redline), rev it and it's great, rev it and you get to visit more gas stations, too.

Aside from Alan's original questions, I have the factory manual and it shows the advance at specific rpm points (including 1000 and 2000) in cam degrees of advance (which means 2000 and 4000 crank degrees). That is what made me wonder if Ford had a 'reason' and Alan's overdrive drops the 70m[h rpm right into that 2000 crank rpm range. Sometimes there is such a thing as over thinking, then again, engineers can be tricky buggers.

Click to expand...

adm22,

How many miles are on your 351C engine? Could it be a worn or stretched timing chain and gear set? The 351 C's in that time period, from the factory, had plastic covered timing chains, to reduce noise, plus the timing was also retarded several degrees to meet Government emissions regulations. Any engine that has a good condition, tight timing chain gear set, that is set to the factory specs, should start up on the first or second spin. Also, if memory servers me right, the vacuum advance is really usefully in starting the motor, especially when the engine is cold and off the line acceleration. I believe the vacuum advance advances the timing to help with the higher RPM's used to warm up the engine and to provide a better idle.

All, correct me if I state something here incorrect, When you accelerate from the line, the engine is at operating temperature, your distributor has very little vacuum on the advance diaphragm, as the rpm's climb and your speed increase, the vacuum from your carburetor will use vacuum on the advance diaphragm. So, at Wide Open throttle, there should be minimal or no vacuum on the distributor advance diaphragm.

As you can see in the Ford 351C vacuum advance setup, the vacuum to the distributor is ported to the intake manifold for cold starts, and once the engine is warmed up, it switches to the carburetor vacuum.



What is the condition of your distributor? The do wear out, especially in the shaft area.

Lots of great information in the above threads. The vacuum gauge should always be in your tool kit.

mustang7173

This thread has a lot of great info in it...everybody can benefit from this stuff. For the novice I would strongly recommend "baby steps". Don't dive in with a lot of money and no real cohesive plan. Tackle one small area at a time and judge for yourself how much improvement ( if any) you notice. Start with whatever distributor you have, read up on the internet about that one, and optimize it to the best of your ability. Then evaluate the whole thing before the next step. You will learn the most and spend the least this way.

Learning is fun!

absolutely, baby steps.

a small change can have a big effect on your engine, good or bad.

@mustang7173, yup.

here is where ported verse full manifold vacuum comes in for idle situation.

our cars are on ported vacuum. What that means is at idle there is 0HG vacuum going to the vacuum advance it is completely turned off and only the mechanical is working if the rpm is high.

at WOT you have this peak when you reach max rpms, and vac starts to fall off closing the vacuum advance and again going to mechanical only.

the mechanical is all centrifugal force from the rpm of the engine. which is where the springs come in for tuning the distributor. you tailor how far open the mechanical moves depending on spring tension at a given RPM. this is that fall off people talk about when they reach top speed the car can make. just as they reach peak rpms the vac drops off and the advance cuts out and total timing drops which drops max rpms off the top of the engine and you max out on top rpms.

now there is a lot more going on at the same time secondaries open, power valve open, accelerator pump shot. it is all mixing together to keep the engine going.

then there are 2 types of hesitation. one is lean the other is rich. either way isn't good but rich is not dangerous, lean can grenade a motor.

on Fast acceleration Vacuum drops to zero almost instantly so for a split second the advance comes on but then shuts off and mechanical takes over.

on steady acceleration the vaccum advance comes on and vac rises.

now this website has some good tutorials on vacuum reading

http://www.secondchancegarage.com/public/186.cfm

now full manifold vacuum differs only at idle. it allows 15-20HG of vacuum at idle which turns on the vaccum advance, which raises rpms kind of artificially which makes more power(more timing in early). the purpose of the change from full to ported was emissions. you want retarded timing at idle to knock down the NOx and CO levels of pollution to do that you need high exhaust manifold temperature which occurs with retarded timing. thus ported vacuum was born. now our cars originally came from the factory with both ported and full manifold vacuum options. the Distributor vacuum control valve determined which type your engine was given depending on internal temperature. if you sat in traffic and the engine overheated the DVCV turned on and at idle you went from ported to full vacuum, that raised engine rpms at idle, which reduced heat from the exhaust and at the same time made the cooling fan rpm increase along with idle rpms, thus cooling the engine back down to turn off the DVCV and return to ported emissions standards. it was like a last resort before "THAR SHE BLOWS" from the radiator cap. of course the secondary cooling was the heater core , throw the heat on full blast when it is 100 degrees out.

from a performance aspect you want full manifold vacuum at idle, more timing earlier equals more power. however the distributor needs to be re-curved with less mechanical advance usually and you will need to reset the idle mixture and adjust the vacuum advance. everything kind of ripples down the line and it again really depends on your motor.

there is also fixed timing locking the mechanical advance and not using the vacuum advance this is in a super charger or turbo charger situation, or in a drag situation sometimes.

all this talk or theory is meaning less because it all comes back to the engine itself. you can do things by the book and it does not work because of the engines parts combinations. heads, cam, pistons, stroke, compression, are all over the map so you have to listen and feel what your engine is doing, it really is more art then science because you always end up with a combination far below optimal to cater to all running situations.

on paper my engine should work perfectly with a 550cfm carb stock, in that configuration with 97 octane gas, i was lean backfiring through the carb with enough force that it blew my air filter off and nuked my brake booster. only solution keep adding gas until it stoped, and back off when your eyes are turning red from gas fumes, and listen for detonation at different load and rpm levels and tune for lean spots.