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Single vs dual diaphragm distributor 72 Q Code


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Guys apologies for the basic question.....

 

I want to order the correct vacuum lines from Marti http://www.martiauto.com/reproducedparts.cfm

 

There are two sets shown for my car a 72 Q Code. How can I tell if my car has the single or dual diaphragm distributor please? I do not have Ram Air (in fact I don;t believe Ram Air was offered as an option on the 72 Q code.

 

Thanks

Nigel B

 

1972 Mach 1 Cobra Jet 4 speed

 

 

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I've got a dual on my car. I found the coolant actuated vacuum tree which switches the distributor between advance and retard to be worthless. Bypassed the whole thing and just run a vac advance to the distributor. Plugged the rest.

.

FE177109-A1-C8-4-A6-C-9224-AACCFD66-C3-C9.png

 

Rick Bombard

1971 Grabber Green Mach 1 351C 4V

2013 Race Red California Special Convertible

1973 Medium Copper Metallic Convertible 302 4V     SOLD

1953 F-100 SOLD

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The coolant actuated vacuum tree to the distributor is designed to aid in cold engine starts. The Vacuum came from the manifold port, since vacuum from the carburetor was very low during higher rpm's.The timing was advanced on the distributor to accommodate the higher RPM's to get the engine warmed up. Once the coolant, carburetor, and engine reached optimal operating temperatures, it switched back to vacuum port on the carburetor, releasing the vacuum on the diaphragm, allowing the vacuum advance to operate on Carburetor vacuum.

 

Agree on doing away with the dual vacuum.

 

mustang7173

Thanks,

mustang7173 🇺🇸

"Courage is being scared to death but saddling up anyway" -- John Wayne

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I do not know about the 72 but my 73 came with a dual it is 351, 4-V, 4 speed. One of the first things I changed on my car was to put Ford's conversion to a centrifugal advance dual point distributor on. The kit came with a new guts for the dist. that included several sets of weights and springs to tune the advance. It also required that you change the center cam because the position of the points had to be raised in order to get the mechanical guts in the dist..

I always pulled the dist. out and set the points using a dial indicator on the bench. I did not have any way of tuning the distributor back then but seat of the pants. I played with the springs and weights and looked at the timing at different RPM levels. My brain cells with that info have long since died off since I did that in the spring of 73.

The vacuum advance diaphragm is still on the distributor but the arm going to the plate in the dist. is cut off so it does nothing.

I will have to see if the box with the kit is stuck on a shelf in the garage to get a part number. Might still be some on a Ford shelf somewhere. Anyone have a parts book to look it up?

David

When a man is in the woods and talks and no women are there is he still wrong??:P

David

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Thanks everyone. I checked my car last night that is in the shop and yes it has the dual diaphragm as several of you suspected.

 

The guy working on the car (Jeff Schrembi at Oval Repairs in Livonia MI) knows his stuff re older Mustangs, and was surprised that my car still had the points step up inside the distributor and wondered why I hadn't changed over to Pertronix. Apparently you don't have to do any non reversible work so can always revert back, just install the new innards in the distributor.

 

I had a quick read on here and can see several people have changed over, some things to watch out for etc. But what I have works today, and I know will need maintenance in the future. I think I need to do some more reading to form a better opinion. As usual I'm finding the help I get on this forum amazing. Thanks all.

Nigel B

 

1972 Mach 1 Cobra Jet 4 speed

 

 

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The coolant actuated vacuum tree to the distributor is designed to aid in cold engine starts. The Vacuum came from the manifold port, since vacuum from the carburetor was very low during higher rpm's.The timing was advanced on the distributor to accommodate the higher RPM's to get the engine warmed up. Once the coolant, carburetor, and engine reached optimal operating temperatures, it switched back to vacuum port on the carburetor, releasing the vacuum on the diaphragm, allowing the vacuum advance to operate on Carburetor vacuum.

 

Agree on doing away with the dual vacuum.

 

mustang7173

 

I read somewhere that this was also a California requirement to retard the timing when the car would be in slow traffic and running on the hot side to cool the engine down and thereby cut emissions. Not sure where I got that info. Anyone else read this?

Stanglover

Geoff.

 I learn something new every day!

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yup switching to pertonixs takes minutes and you can keep the points and condenser in the trunk to swap back if you have a ignition failure.

 

the dual advance as mentioned above helped for cold starts on big blocks in 72, the idea was retard the timing when cold to make the exhaust temps rise faster, the oem aircleaner base used the snorkel that went through the exhaust shield on the drivers side, thus the intake air would heat faster and bring the engine up to temperature faster. once the engine warmed up the heat sensor on the block switched to normal advance operation using ported vacuum. then the heat sensor on the air cleaner moved the snorkel from warm up to normal intake operation which brought in colder air from the top of the engine instead of by the exhaust.

the operation is simiar to the Distributor Vaccum Control Valve. except that only comes on during emergency cool down when the engine is overheating.

 

all this stuff does not effect performance when operating correctly. you can argue it is some emissions control in reducing bad exhaust gases when the engine is cold.

however all it really does is improve street driving performance and makes the engine less bitchy when warming up.

 

 

In 73 EGR was introduced and that cause timing issues so the Dual vacuum advance was needed to prevent a retard timing issue caused by some throttle positions with the EGR system there was some type of feedback that also messed up the shift points of the automatic transmissions thus in 73 ford installed dual vacuum modulators on transmissions to solve this feedback issue. EGR (exhaust gas recycle) combined with the new more retarded engine timing and other stuff was emmissions and caused tons of engine running issues as it was basically the first emissions controls put on engines. the EGR clogged everything up with carbon from the exhaust.

 

you can change a dual system to a single very easy just replace the vaccum can on the distributor and rehose everything for single advance. you will not notice a change other then hey the engine runs crappier when it is cold, but when it warms up it will be fine.

 

for 73 removing the dual system is a bit more involved because you want to ditch the EGR system and that usually means you need to change the intake manifold to a 71 or 72 version without EGR or go aftermarket and install a aftermarket edelbrock or holley carb.

 

something harder to deal with is converting from duel points to single points in the distributor, so you should make sure you have a single points distributor.

duel points was pretty cool for back in the day before electronic ignitions it reduced the points bouncing issue which limited top end RPMS, so that was why the higher end engines got dual points, and usually dual vacuum advance. setting up dual points is difficult because you have to set the gap between 2 points plates and sort of balance them to work right. many mechanics hated them and so the first time the car came in for a tune up they would either try to convert to single points or pull the distributor and replace it with a single version.

 

today finding good points is REALLY hard it is all made in china junk and the Dwell shifts on them insane every 100 miles. the condensers are garbage today also. you have to basically try and buy NOS vintage stock from ebay and it rarely comes up. i still have a vintage 1970s Accel High rpm Points conversion kit that will sit in a box forever unless an EMP knocks out my electronics ignition system.


Ported timing is what you are thinking about.

 

there was the change from full manifold vacuum to ported.

 

the only difference between the 2 is at Idle RPMS vacuum for ported is 0HG

that difference kept the engine at initial timing when sitting in traffic.

that kept the exhaust temps high and ran the engines hotter sitting in traffic to burn off extra emissions from the tail pipe. this reduced smog majorly with cars sitting in heavy traffic stop and go.

 

ford spec'd 4-6 degrees of initial timing on later cars to increase exhaust temps more. they also re-balanced the distributor mechanical advance stopper plates, so you have to watch out for total timing if you start screwing around increasing initial timing to 10-18 degrees for more power. in 71 your plate would of read L12 in 72 it was like L14

 

it was because they speced like 8-10 degrees in 70-71 for initial so 10+L12 = 10+2X12= 10+24 = 34 total timing, then for 72 you had like 6-8 initial 6+ L14= 6+28 = 34

this is why people talk about the mechanical advance plates needing to be swapped out when increasing initial timing.

so people want like a L6 or L8 plate from a 1969 or 1970 car, because that gives you like a 18 initial timing fudge factor without grenading the engine for more performance.

 

 

the change required the Distributor Vaccum Control Valve which acted as an emergency cooling mode for engines over heating. this valve changed the distributor to run full manifold vacuum instead of ported which advanced timing and thereby increased engine rpms at idle which made the fan turn faster which cooled the engine back down.

 

performance people hated ported because yes in a way it retarded timing.

however for street drivability it was an improvement.

 

with full manifold vaccum the engine was running advanced timing more often so from a standing start the full mainfold car had advanced timing over the ported mainfold car so the full mainfold car would do a burn out when you stomped on it and the ported car just churped the tires.

 

later when ford moved to open chamber heads and lower compression ported helped with ping/dentonation under certain conditions. because it held back timing as the engine rpm increased otherwise you needed more fuel to compensate. when they started to look at MPG numbers holding back timing made more public sense then increasing more fuel delivery on an engine getting 4mpg already.

 

 

 

 

 

I read somewhere that this was also a California requirement to retard the timing when the car would be in slow traffic and running on the hot side to cool the engine down and thereby cut emissions. Not sure where I got that info. Anyone else read this?

Stanglover

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yup switching to pertonixs takes minutes and you can keep the points and condenser in the trunk to swap back if you have a ignition failure.

 

the dual advance as mentioned above helped for cold starts on big blocks in 72, the idea was retard the timing when cold to make the exhaust temps rise faster, the oem aircleaner base used the snorkel that went through the exhaust shield on the drivers side, thus the intake air would heat faster and bring the engine up to temperature faster. once the engine warmed up the heat sensor on the block switched to normal advance operation using ported vacuum. then the heat sensor on the air cleaner moved the snorkel from warm up to normal intake operation which brought in colder air from the top of the engine instead of by the exhaust.

the operation is simiar to the Distributor Vaccum Control Valve. except that only comes on during emergency cool down when the engine is overheating.

 

all this stuff does not effect performance when operating correctly. you can argue it is some emissions control in reducing bad exhaust gases when the engine is cold.

however all it really does is improve street driving performance and makes the engine less bitchy when warming up.

 

 

In 73 EGR was introduced and that cause timing issues so the Dual vacuum advance was needed to prevent a retard timing issue caused by some throttle positions with the EGR system there was some type of feedback that also messed up the shift points of the automatic transmissions thus in 73 ford installed dual vacuum modulators on transmissions to solve this feedback issue. EGR (exhaust gas recycle) combined with the new more retarded engine timing and other stuff was emmissions and caused tons of engine running issues as it was basically the first emissions controls put on engines. the EGR clogged everything up with carbon from the exhaust.

 

you can change a dual system to a single very easy just replace the vaccum can on the distributor and rehose everything for single advance. you will not notice a change other then hey the engine runs crappier when it is cold, but when it warms up it will be fine.

 

for 73 removing the dual system is a bit more involved because you want to ditch the EGR system and that usually means you need to change the intake manifold to a 71 or 72 version without EGR or go aftermarket and install a aftermarket edelbrock or holley carb.

 

something harder to deal with is converting from duel points to single points in the distributor, so you should make sure you have a single points distributor.

duel points was pretty cool for back in the day before electronic ignitions it reduced the points bouncing issue which limited top end RPMS, so that was why the higher end engines got dual points, and usually dual vacuum advance. setting up dual points is difficult because you have to set the gap between 2 points plates and sort of balance them to work right. many mechanics hated them and so the first time the car came in for a tune up they would either try to convert to single points or pull the distributor and replace it with a single version.

 

today finding good points is REALLY hard it is all made in china junk and the Dwell shifts on them insane every 100 miles. the condensers are garbage today also. you have to basically try and buy NOS vintage stock from ebay and it rarely comes up. i still have a vintage 1970s Accel High rpm Points conversion kit that will sit in a box forever unless an EMP knocks out my electronics ignition system.


Ported timing is what you are thinking about.

 

there was the change from full manifold vacuum to ported.

 

the only difference between the 2 is at Idle RPMS vacuum for ported is 0HG

that difference kept the engine at initial timing when sitting in traffic.

that kept the exhaust temps high and ran the engines hotter sitting in traffic to burn off extra emissions from the tail pipe. this reduced smog majorly with cars sitting in heavy traffic stop and go.

 

ford spec'd 4-6 degrees of initial timing on later cars to increase exhaust temps more. they also re-balanced the distributor mechanical advance stopper plates, so you have to watch out for total timing if you start screwing around increasing initial timing to 10-18 degrees for more power. in 71 your plate would of read L12 in 72 it was like L14

 

it was because they speced like 8-10 degrees in 70-71 for initial so 10+L12 = 10+2X12= 10+24 = 34 total timing, then for 72 you had like 6-8 initial 6+ L14= 6+28 = 34

this is why people talk about the mechanical advance plates needing to be swapped out when increasing initial timing.

so people want like a L6 or L8 plate from a 1969 or 1970 car, because that gives you like a 18 initial timing fudge factor without grenading the engine for more performance.

 

 

the change required the Distributor Vaccum Control Valve which acted as an emergency cooling mode for engines over heating. this valve changed the distributor to run full manifold vacuum instead of ported which advanced timing and thereby increased engine rpms at idle which made the fan turn faster which cooled the engine back down.

 

performance people hated ported because yes in a way it retarded timing.

however for street drivability it was an improvement.

 

with full manifold vaccum the engine was running advanced timing more often so from a standing start the full mainfold car had advanced timing over the ported mainfold car so the full mainfold car would do a burn out when you stomped on it and the ported car just churped the tires.

 

later when ford moved to open chamber heads and lower compression ported helped with ping/dentonation under certain conditions. because it held back timing as the engine rpm increased otherwise you needed more fuel to compensate. when they started to look at MPG numbers holding back timing made more public sense then increasing more fuel delivery on an engine getting 4mpg already.

 

 

 

 

 

I read somewhere that this was also a California requirement to retard the timing when the car would be in slow traffic and running on the hot side to cool the engine down and thereby cut emissions. Not sure where I got that info. Anyone else read this?

Stanglover

 

I just found a reference to the dual vacuum distributor in my original 1971 Car Shop Manual, Volume 2, engine, page 23-15-01 where it describes the Distributor Vacuum Control valve. I'm not going to quote it all, but in part it reads " During periods of prolonged idle, should the engine temp rise above normal, the valve close the normal source vacuum port and connects the distributor port to the alternate source vacuum port causing an immediate increase in engine speed, which will continue until the engine temperature returns to normal".

Normal source would be the connection at the carburetor and the distributor port (Advance?).

Interesting comments throughout and very helpful, I too have a dual vac. distributor and I also run Pertronix. My issues are with a badly curved reman. distributor that in hind sight should never have been put in the car the way it is. It is likely going to cost me at least a re-hone and a set of pistons. Maybe a replacement block!!

Geoff.

 I learn something new every day!

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@ Stanglover for the DVCV normal source is ported off the carb, and the secondary is full manifold vac from the intake manifold.

 

it really was a genius way to get the best of both worlds and overlap a problem area with engine overheating.

it was on all A/C equipped cars since the extra load of the AC could send an engine heat over the top on a 90 degree day sitting in traffic not moving.

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My 72 351 H-code automatic also has a dual vaccum. It is original to the engine. I can testify to that based upon how firm it was siezed in the block. Thankfully the machinist was able to break it loose with little damage.

BKDunha

72 Mach 1 H-Code (Concourse driven restoration)

67 S-Code Factory GT with 4-Spd

68 Mercury Cyclone (Pro-Street project)

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@ Stanglover for the DVCV normal source is ported off the carb, and the secondary is full manifold vac from the intake manifold.

 

it really was a genius way to get the best of both worlds and overlap a problem area with engine overheating.

it was on all A/C equipped cars since the extra load of the AC could send an engine heat over the top on a 90 degree day sitting in traffic not moving.

Thanks 72Hcode, your comments are most helpful regarding the distributor setting. I think you might have just proved my point to my engine builder re my reman. dist. I have not yet stripped it down, but I think it has the wrong baseplate for the early 71. Very interesting stuff and now I know what to look for.

Thanks,

Stanglover (Geoff)

Geoff.

 I learn something new every day!

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@ Stanglover for the DVCV normal source is ported off the carb, and the secondary is full manifold vac from the intake manifold.

 

it really was a genius way to get the best of both worlds and overlap a problem area with engine overheating.

it was on all A/C equipped cars since the extra load of the AC could send an engine heat over the top on a 90 degree day sitting in traffic not moving.

 

At the risk of displaying my ignorance, what does 'L' stand for with respect to the distributor base plate. Also, it is quite obvious that you are very knowledgeable in these matters. A listing I found on Mustang Tek, shows a distributor # as D0OF-12127-V for the 70-71 351C 4V. I have the specs from my manual, but I'm not sure what I'm reading. My question is, what would be the correct baseplate for this dist. I have a rebuilt in at this time, and until it gets pulled and stripped, I have no idea what it actually is. This MAY be the reason why I am having timing issues and the cause of my problems. To clarify, my engine is a D0AE-L, date 1A7, 351C 4V, 4 speed, no A/C, 3.25:1 open dif.

Your advise would be very much appreciated.

Stanglover.

Geoff.

 I learn something new every day!

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.

exactly what is your timing problem?

 

any ford distributor can be set to whatever spec you want.

 

if you cam and/or compression is no longer stock, the factory setting no longer applies.

 

as i mentioned earlier, you can probably get more power by setting your timing curve for your particular engine . . failure to do this and just assuming the factory setting is best is doing a great disservice to yourself.

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.

exactly what is your timing problem?

 

any ford distributor can be set to whatever spec you want.

 

if you cam and/or compression is no longer stock, the factory setting no longer applies.

 

as i mentioned earlier, you can probably get more power by setting your timing curve for your particular engine . . failure to do this and just assuming the factory setting is best is doing a great disservice to yourself.

 

Hi Barnett468, In a nutshell, the engine was rebuilt in late 2012. The builder is a reputable company and overall, they built a very smooth running motor. They used KB177 flat top pistons, which upped the compression to approx. 10.8 and a Melling MTS-2 cam, which is slightly better than stock, nothing radical at all. The rest is factory stock. The problem is the distributor, which was a recent reman. Cardone, was not curved or checked as far as I know. It was fitted with the PertrixIII and timed to 8-10 degrees. The engine was assembled and broken in at the shop on their stand. I've discussed this at length in previous threads. On a dyno, it was seen that the timing was drifting way up to 39+ deg. and not stable. I'm no expert on this, but the tech told me it was a bad distributor from the get-go. Therein lies my problem. The engine is going back to the builder to be stripped and checked. I suspect chipped pistons and scored bores and who knows what else. Needless to say, it will not go back on the road until I know that everything is correct. I am not sure what type of piston I'll use, they all have different pluses and minuses, but it will have a bit less compression at no more than 10:1. Hyper pistons are fine as long as timing is stable and set conservatively. What would your thoughts on pistons be?

Geoff.

 I learn something new every day!

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the total timing could be adjusted on any distributor.

 

one method is making your own plate, you weld up the gap in the mechanical plate then you cut out the gap just as you need it.

it requires measurement and testing and usually lots of adjustment.

 

another method is bending the end points on the mechanical advance. to shorten the gap..

 

my method,, i stuck a piece of rubber hose on the stoppers to make the gap smaller.

 

the total timing being high isn't necessarily the nail in the coffin for what happened in your case.

there could of been multiple factors. remember even with mechanical advance locked to 34 degrees total you can still have the vacuum advance kick in and drive the advance over 54 degrees this is normal under low load. you most likely just had too much timing coming in too quick and were running hot and lean then that would of nuked the pistons.

 

as for the L on the plate.

it stands for Limit pin.

 

L15 would be 15 degrees but it really means 30 because the distributor spins half speed of the Crank.

so you multiply by 2 to get the total mechanical advance.

L10 is 20

etc...

 

most distributors have have 2 positions on the mechanical advance L15 and L10 are common.

if you happen to be set on the larger number say L15 then you could turn the distributor 180 and use the L10 position.

 

so you need to pull the distributor out of the engine, move the mechanical plate to the smaller number say L10 and now the distributor will be 180 degrees out. so you need to spin the rotor 180 and reinstall back in the engine with the #1 at 1 o'clock position.

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the total timing could be adjusted on any distributor.

 

one method is making your own plate, you weld up the gap in the mechanical plate then you cut out the gap just as you need it.

it requires measurement and testing and usually lots of adjustment.

 

another method is bending the end points on the mechanical advance. to shorten the gap..

 

my method,, i stuck a piece of rubber hose on the stoppers to make the gap smaller.

 

the total timing being high isn't necessarily the nail in the coffin for what happened in your case.

there could of been multiple factors. remember even with mechanical advance locked to 34 degrees total you can still have the vacuum advance kick in and drive the advance over 54 degrees this is normal under low load. you most likely just had too much timing coming in too quick and were running hot and lean then that would of nuked the pistons.

 

as for the L on the plate.

it stands for Limit pin.

 

L15 would be 15 degrees but it really means 30 because the distributor spins half speed of the Crank.

so you multiply by 2 to get the total mechanical advance.

L10 is 20

etc...

 

most distributors have have 2 positions on the mechanical advance L15 and L10 are common.

if you happen to be set on the larger number say L15 then you could turn the distributor 180 and use the L10 position.

 

so you need to pull the distributor out of the engine, move the mechanical plate to the smaller number say L10 and now the distributor will be 180 degrees out. so you need to spin the rotor 180 and reinstall back in the engine with the #1 at 1 o'clock position.

 

I'm learning more every day. I'm sure many others are also benefiting form these posts.

As I said, the engine is going back to the shop and will not be going back in the car until I KNOW it's set up correctly. Your obvious knowledge on this issue is a great help and I thank you so much.

I've asked many people if they know what that 'L' stands for and no one was ever able to tell me...... even those who are supposed to know!! Amazing. Now I know.

As for fuel mixture, when the car was on the dyno, I had O2 bungs inserted in the pipes so they could install the sensors. Fuel mixture was seen to be right on and plug read would support that. I can't get anymore technical than that.

I now have the info I need to get where I need to be with this, so thank you one and all for your invaluable help.

Geoff.

Geoff.

 I learn something new every day!

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.

exactly what is your timing problem?

 

any ford distributor can be set to whatever spec you want.

 

if you cam and/or compression is no longer stock, the factory setting no longer applies.

 

as i mentioned earlier, you can probably get more power by setting your timing curve for your particular engine . . failure to do this and just assuming the factory setting is best is doing a great disservice to yourself.

 

Hi Barnett468, In a nutshell, the engine was rebuilt in late 2012. The builder is a reputable company and overall, they built a very smooth running motor. They used KB177 flat top pistons, which upped the compression to approx. 10.8 and a Melling MTS-2 cam, which is slightly better than stock, nothing radical at all. The rest is factory stock. The problem is the distributor, which was a recent reman. Cardone, was not curved or checked as far as I know. It was fitted with the PertrixIII and timed to 8-10 degrees. The engine was assembled and broken in at the shop on their stand. I've discussed this at length in previous threads. On a dyno, it was seen that the timing was drifting way up to 39+ deg. and not stable. I'm no expert on this, but the tech told me it was a bad distributor from the get-go. Therein lies my problem. The engine is going back to the builder to be stripped and checked. I suspect chipped pistons and scored bores and who knows what else. Needless to say, it will not go back on the road until I know that everything is correct. I am not sure what type of piston I'll use, they all have different pluses and minuses, but it will have a bit less compression at no more than 10:1. Hyper pistons are fine as long as timing is stable and set conservatively. What would your thoughts on pistons be?

 

hyper pistons are fine but are very sensitive to detonation . . mahle pistons are xlnt.

 

it sounds like your dyno guy was just a dyno guy and not an engine tuner otherwise he would likely not have let the timing get to 39.

 

you have some distributor info now but i would do this.

 

1. you are totally screwing yourself if you use a dual vacuum advance chamber.

 

2. plug the retard side of the chamber hose with a bb.

 

3. set initial timing to 10.

 

4. set the mechanical advance so the limit arm is in a slot that has a maximum of 10 degrees . . if yours says 12, put a thin rubber hose over the limit arm as suggested.

 

5. check the timing at 22oo rpm . . if it is more than around 24 degrees, i would bend the arm on the heavier spring out a little.

 

6. check maximum advance . . it should be no more than around 32 at around 2800 to 3000 rpm.

 

this will be perfectly safe for 10.0 compression if your engine is running at 190 degrees or less.

 

and install a spark plug that is 1 step colder than stock.

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.

exactly what is your timing problem?

 

any ford distributor can be set to whatever spec you want.

 

if you cam and/or compression is no longer stock, the factory setting no longer applies.

 

as i mentioned earlier, you can probably get more power by setting your timing curve for your particular engine . . failure to do this and just assuming the factory setting is best is doing a great disservice to yourself.

 

Hi Barnett468, In a nutshell, the engine was rebuilt in late 2012. The builder is a reputable company and overall, they built a very smooth running motor. They used KB177 flat top pistons, which upped the compression to approx. 10.8 and a Melling MTS-2 cam, which is slightly better than stock, nothing radical at all. The rest is factory stock. The problem is the distributor, which was a recent reman. Cardone, was not curved or checked as far as I know. It was fitted with the PertrixIII and timed to 8-10 degrees. The engine was assembled and broken in at the shop on their stand. I've discussed this at length in previous threads. On a dyno, it was seen that the timing was drifting way up to 39+ deg. and not stable. I'm no expert on this, but the tech told me it was a bad distributor from the get-go. Therein lies my problem. The engine is going back to the builder to be stripped and checked. I suspect chipped pistons and scored bores and who knows what else. Needless to say, it will not go back on the road until I know that everything is correct. I am not sure what type of piston I'll use, they all have different pluses and minuses, but it will have a bit less compression at no more than 10:1. Hyper pistons are fine as long as timing is stable and set conservatively. What would your thoughts on pistons be?

 

hyper pistons are fine but are very sensitive to detonation . . mahle pistons are xlnt.

 

it sounds like your dyno guy was just a dyno guy and not an engine tuner otherwise he would likely not have let the timing get to 39.

 

you have some distributor info now but i would do this.

 

1. you are totally screwing yourself if you use a dual vacuum advance chamber.

 

2. plug the retard side of the chamber hose with a bb.

 

3. set initial timing to 10.

 

4. set the mechanical advance so the limit arm is in a slot that has a maximum of 10 degrees . . if yours says 12, put a thin rubber hose over the limit arm as suggested.

 

5. check the timing at 22oo rpm . . if it is more than around 24 degrees, i would bend the arm on the heavier spring out a little.

 

6. check maximum advance . . it should be no more than around 32 at around 2800 to 3000 rpm.

 

this will be perfectly safe for 10.0 compression if your engine is running at 190 degrees or less.

 

and install a spark plug that is 1 step colder than stock.

 

I know we have got way off track to the original thread posted by nbracken, my apologies to you, but in all there is very valuable info contained here that many of us can and will likely benefit from. Kudo's to those who have offered there time and advice. I for one will be building a much better engine as a result. Can't wait till spring to get it back on the road when I will definitely be posting the remedies and results. Barnett468, your advice will be real help in setting it all up to be a great street driver. Thank you.

Geoff

aka Stanglover

Geoff.

 I learn something new every day!

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