351C Timing: Initial, Curve and Total

[HeavyCoupe]

After breaking the cam in on the Stunt Cleveland I started dialing the timing in. A quick search carried me to diyford.com which said initial at 6-12 btdc. Nah, not my Cleveland. It didn't like that at all. A little more looking found me 16 btdc which worked much better and even one guy saying 18 if it will start hot. I understand initial timing, but watching the advance pull all in by 2000 rpm and going over 40* made me realize something wasn't right. Based on my distributor (Pertronix Flame-Thrower 2) without advance stops I'm adding 24* on top of initial, which all adds up on why I'm over 40* total at 2000 rpm with assumed copper springs. I have not pulled the rotor to confirm because I have never curved a distributor before and I didn't know where to start. I'm running 93 octane.

I cruised several forums for my answers and I have come up with a rough plan but I wanted to run it by y'all and see what you thought. Every time some asks for specifics, they get the response that they need to see engine specifics. I can provide every number needed but my paper work is with the car so not tonight. I've read to keep total timing at or below 36*, but it's possible to push as high as 40 as long as it doesn't ping. I've also read the Cleveland like timing in by 3500, but youtube said SBFs like timing in quick, possibly at 2000 rpm like it is now. My plan is set initial at 18* if I don't have hot start issues, I'm at 16* now and aim for 32*-34* as a stating point until the engine gets through the first few hundred miles, then we can start to find the upper limits.

I think understand how it all works. Am I missing something? Does anyone have any suggestions for improvement? I'll attach the most popular timing curves for my distributor, but I have ordered the kit so I will be able to make whatever curve I decide on which is my biggest unknown, I think. It should be here for the weekend which is when I plan to do all of this.

Any input is appreciated.

 

[Paul M]

You should be right in the ballpark with your total "all in" at 36 deg. 36-38 38 being too adv

 

[Stanglover]

Mine is set for 20 degrees crank, 14 deg initial plus about 4-6 deg on the vacuum advance. This is with a stock Autolite distributor re-worked and curved with 1 heavy Ford spring and 1 Mr.Gasket 925D.
More later if you need it. No time right now. Lots has been posted in SEARCH.

 

[basstrix]

Mine is set for 20 degrees crank, 14 deg initial plus about 4-6 deg on the vacuum advance. This is with a stock Autolite distributor re-worked and curved with 1 heavy Ford spring and 1 Mr.Gasket 925D.
More alter if you need it. No time right now. Lots has been posted in SEARCH.

Which vacuum port are you using for vacuum advance? How much centrifugal advance? Curious what your total is and at what RPM.

 

[Stanglover]

Which vacuum port are you using for vacuum advance? How much centrifugal advance? Curious what your total is and at what RPM.

Vacuum is from ported on the Holey 670. I have checked that it has zero vacuum at idle and starts immediately on throttle movement. I think I adjusted the diaphragm to the minimum, so about 4-6 degrees. It's been awhile since I last set or even checked the timing. The total mechanical is 34 degrees and on my engine, that is where it runs best. As said, the dist. is a stock Autolite but with a limit slot re-worked to equal a 10L (.410") which equals 20 degree crank timing.
A cheat would be to use a 13L slot with a piece of NYLON tubing over the post. This is something that can be found on distributors from factory. This in effect cuts the travel to close to that of a 10L. Mechanical timing is all in at around 3K rpm. Spring; I found that using the original heavy spring set just loose and a Mr.Gasket 925D spring set with slight tension gave the best curve/result.
Actually, timing is a job I need to recheck as just lately, the motor has had a slight run-on at (hot) shut down. That may just be because I had to use some midgrade fuel with ethanol and the motor does not like that shit at all. I'm hoping a couple of fill ups with Shell non ethanol will cure that issue. I have never had a problem with that since I did the distributor 5 or more years ago.
Here are the pics again.

 

[Stanglover]

@basstrix, did that give you the info you wanted?

 

[HeavyCoupe]

Thanks for the replies. I’m glad to know I’m on the right track. It did really like 20*, but I figured it would struggle with hot starts. I don’t know what my vacuum advance is, I don’t recall it in the paperwork, but I will look again. I’m leaning towards having the timing all in at 3000. It seems like a good middle ground, and with stanglover having success with it it should be a good place to start.

I did search for this but I didn’t see any threads discussing it all in one place, but I can’t say I went to the very last page of results. I also went to several other forums but it’s hard to know which forums and posters to trust. At least with here I can have a conversation versus a post from 7 years ago on a forum that has 16 active members now.

Thanks again.

 

[Bentworker]

I’d probably roll with the silver springs and limit your mechanical advance to about 20 degrees.

I am about 14 initial, 33 total all mechanical no vacuum.

 

[Sheriff41]

I'm running a stock dual-point dizzy with single vacuum advance, a 10L slotted rotor for centrifugal, stock springs. I have it set for 16° initial, 34° total mechanical all in about 2,800-3,000 RPMs. I need to check my vacuum advance adjustment one day. I can't recall if I've ever set it.

 

[Stanglover]

There's one thing to remember. Back in the early 70's, governments were starting to get obsessed with pollution regulations. That is why our cars for example, were coming out of the factory with retarded ignitions. 6 degrees initial was about as good as it got from what I've learned. That means the crank degrees would need to be 30 to arrive at 36 mechanical deg. This is why most Motorcraft or Atuolite distributors on 351C's will be found built using a 15L limit slot. From my knowledge ( which is minimal) only the BOSS 351 had an initial timing of 16 deg. That car of course had a way better distributor, so I would have to assume, never having seen one, that it used a limit slot of 10L (20 deg.)
This said, there is no way that just trying to set the initial timing to 14-16 degs. without altering the crank degs. will result in good timing. Do the math!
I know many have used modern distributors that have all the bells and whistles built in, but most just don't look the part and some are just plain ugly, therefore IF one wants an original looking distributor, rebuilding and recurving is, imo, the only way to go.

 

[Stanglover]

I'm running a stock dual-point dizzy with single vacuum advance, a 10L slotted rotor for centrifugal, stock springs. I have it set for 16° initial, 34° total mechanical all in about 2,800-3,000 RPMs. I need to check my vacuum advance adjustment one day. I can't recall if I've ever set it.

I've often pondered the question, do we even NEED vacuum advance? I mean, we set the timing without vacuum. What advantage does it give?
I (try to) learn something new every day!

 

[Sheriff41]

I've often pondered the question, do we even NEED vacuum advance? I mean, we set the timing without vacuum. What advantage does it give?
I (try to) learn something new every day!

Not really. It just provides a little added advance under initial acceleration and low rpm/light throttle conditions.

 

[giantpune]

I've often pondered the question, do we even NEED vacuum advance? I mean, we set the timing without vacuum. What advantage does it give?
I (try to) learn something new every day!

If you want to test it, while your car is idling, connect the vacuum advance up to manifold vacuum. Your RPMs will shoot up 500, even though you aren't giving the engine any more throttle. I dont recommend driving around like that for your daily driver, but it does help illustrate the concept.

The way I understand it, under certain conditions, the air and fuel mixture in the cylinder is less dense than in other conditions. When the mixture is less dense, the engine needs more advanced timing to burn that charge. If all you care about is your car idling at the line, and then hammering the pedal to the floor, then you'll probably never miss a vacuum advance. If you drive around on the streets, and worry about drivability or the cost of gas, then it doesnt hurt to have one.

 

[mrgmhale]

Re: the tendency of an engine to "run on" (aka "dieseling") after the ignition is turn off (mentioned in one of the posts above), this plagued a lot of engines in the earlier years of increased emission controls back in the 70s and later. There were two basic ways manufacturers tried to fight it, from decreasing idle speeds to using Throttle Position Solenoids (TPS). But, for cars without a TPS, and where idle speeds were still running at 800 or so in Neutral/Park, where the engine will diesel at shut off, the most effective way I found to prevent dieseling was to turn off the ignition when the engine is still in gear (automatic transmissions), or let the clutch drag ever so slightly in gear (and brakes on) when turning the engine off. That little bit of load on the engine at shutdown is enough to help an engine to stop quickly without dieseling.

Funny thing, the electric choke wire going to my electric choke cap had gotten knocked off the other day on our 73 Mach 1. Thus, even though the engine was fully warmed up the choke remained slightly closed, and the engine was puking out a lot of black smoke and was idling at a higher RPM than normal because the high speed idle cam had not been able to drop away on account of the choke being kept slightly closed. The RPM in Drive was about 1,200, and in neutral or part it was a bit higher yet. So, I knew if I simply turned off the ignition in Park I would get lots of dieseling action as the idle was so high on a fully warmed up engine. So, I left it in Drive when I turn the engine off. Whew, it worked, thus sparing me embarrassment of having a run-on situation with a really nice looking and sounding 73 Mach 1 at a Mustang car show where I was parking in my space. heh heh. Once parked I opened the hood, took off the air cleaner, and found, just as I suspected, the electric choke wire had become dislodged , which caused the choke to try to partly close.

On the drive home later on it was clear the excess rich A/F issue was definitely corrected, as the engine was back to its normal idling, the spark plugs were no longer fuel fouled, and there was no more black smoke coming pouring out of the tailpipes. Of course, knowing that spark plugs were fuel fouled from before, and there was likely a sooty mess in the combustion chambers with the prior excess fuel condition meant "somebody" had to blow the carbon out of the engine... Yeah, it had to be done... So, some wild and crazy guy decided to use a long stretch of hardly used country road to open up the throttle to get the black, sooty deposits that likely remained in the combustion chambers (and perhaps even inside the mufflers and inside of the exhaust piping) blasted out of the engine and exhaust. Sure, enough, there was a significant amount of dirty old black stuff that poured out of the engine as the tach and speedo began to race ever higher. That wild and crazy driver kept at it long enough for the black exhaust to go away, and luckily it had cleaned up pretty quickly, as he got close to a double digit MPH level, but was able to back off before hitting a really excessive speed. I wonder who that guy could have been... No doubt some guy who just likes to make sure the inside of his engine remains as cleaned out as possible. Tough work, but someone has to do it.

https://www.youtube.com/shorts/6EnSa7E53mM
https://www.youtube.com/shorts/6EnSa7E53mM

 

[Stanglover]

Re: the tendency of an engine to "run on" (aka "dieseling") after the ignition is turn off (mentioned in one of the posts above), this plagued a lot of engines in the earlier years of increased emission controls back in the 70s and later. There were two basic ways manufacturers tried to fight it, from decreasing idle speeds to using Throttle Position Solenoids (TPS). But, for cars without a TPS, and where idle speeds were still running at 800 or so in Neutral/Park, where the engine will diesel at shut off, the most effective way I found to prevent dieseling was to turn off the ignition when the engine is still in gear (automatic transmissions), or let the clutch drag ever so slightly in gear (and brakes on) when turning the engine off. That little bit of load on the engine at shutdown is enough to help an engine to stop quickly without dieseling.

Funny thing, the electric choke wire going to my electric choke cap had gotten knocked off the other day on our 73 Mach 1. Thus, even though the engine was fully warmed up the choke remained slightly closed, and the engine was puking out a lot of black smoke and was idling at a higher RPM than normal because the high speed idle cam had not been able to drop away on account of the choke being kept slightly closed. The RPM in Drive was about 1,200, and in neutral or part it was a bit higher yet. So, I knew if I simply turned off the ignition in Park I would get lots of dieseling action as the idle was so high on a fully warmed up engine. So, I left it in Drive when I turn the engine off. Whew, it worked, thus sparing me embarrassment of having a run-on situation with a really nice looking and sounding 73 Mach 1 at a Mustang car show where I was parking in my space. heh heh. Once parked I opened the hood, took off the air cleaner, and found, just as I suspected, the electric choke wire had become dislodged , which caused the choke to try to partly close.

On the drive home later on it was clear the excess rich A/F issue was definitely corrected, as the engine was back to its normal idling, the spark plugs were no longer fuel fouled, and there was no more black smoke coming pouring out of the tailpipes. Of course, knowing that spark plugs were fuel fouled from before, and there was likely a sooty mess in the combustion chambers with the prior excess fuel condition meant "somebody" had to blow the carbon out of the engine... Yeah, it had to be done... So, some wild and crazy guy decided to use a long stretch of hardly used country road to open up the throttle to get the black, sooty deposits that likely remained in the combustion chambers (and perhaps even inside the mufflers and inside of the exhaust piping) blasted out of the engine and exhaust. Sure, enough, there was a significant amount of dirty old black stuff that poured out of the engine as the tach and speedo began to race ever higher. That wild and crazy driver kept at it long enough for the black exhaust to go away, and luckily it had cleaned up pretty quickly, as he got close to a double digit MPH level, but was able to back off before hitting a really excessive speed. I wonder who that guy could have been... No doubt some guy who just likes to make sure the inside of his engine remains as cleaned out as possible. Tough work, but someone has to do it.

https://www.youtube.com/shorts/6EnSa7E53mM

That's a good story!
In my case, I have checked the choke and even dropped it one notch to bring it off sooner. Idle is around 800 (manual trans) no tps or anything else on this car.
I'm 99% sure it's just crappy gas as this never happened before I put in some 89 mid-grade with f'n ethanol. It's getting very hard to find non-ethanol around here anymore as gas companies put profit over quality gas.
I do need to fill up again, so I guess a hundred bucks on premium non-ethanol Shell hopefully will fix the run-on situation. We'll see I guess and I'll report back later.

 

[Stanglover]

@basstrix, did that give you the info you wanted?

Maybe also should have mentioned that I use a Pertronix II with matching coil.

 

[351c1971]

Sure sounds like more fun then dripping water into the carb to steam clean the combustion chambers.

 

[basstrix]

Maybe also should have mentioned that I use a Pertronix II with matching coil.

My interest was with the centrifugal portion and type of vacuum. Thanks for the details.

I'm running a duraspark distributor, red grommet module & coil with full 12v to coil. Advance plate is 10L with some springs I dug out of my parts bin that bring it all in by ~2500. I've only played with vacuum advance and only connected to manifold vacuum. I also have a Mallory setup. I like how compact the Mallory is. I've checked both on an oscilloscope and the DS has a little bit longer spark duration so I'm running it for now. Mr Hale's input has me thinking I need to move it somewhere with less heat...currently on the apron between rad support and shock tower. I recently added a rev limiter. The pertronix systems have some nice features, but I already have 2 DS distributors and the mallory.

The plot data was recorded with a base of 17...I've since backed that off to 14.

 

[Stanglover]

My interest was with the centrifugal portion and type of vacuum. Thanks for the details.

I'm running a duraspark distributor, red grommet module & coil with full 12v to coil. Advance plate is 10L with some springs I dug out of my parts bin that bring it all in by ~2500. I've only played with vacuum advance and only connected to manifold vacuum. I also have a Mallory setup. I like how compact the Mallory is. I've checked both on an oscilloscope and the DS has a little bit longer spark duration so I'm running it for now. Mr Hale's input has me thinking I need to move it somewhere with less heat...currently on the apron between rad support and shock tower. I recently added a rev limiter. The pertronix systems have some nice features, but I already have 2 DS distributors and the mallory.

The plot data was recorded with a base of 17...I've since backed that off to 14.View attachment 80046

That's a nice touch with the graph.
For springs, adjusting the tension seemed to be very important and took some time to get where it needed to be.
Personally, for correct looks alone, I go with a DuraSpark system if I had need to do it again. The Pertronix II is pretty good and since I ditched the POS P III (commented on many times), I've had no problems. I did however, have a brand new PII that sat on a shelf for 3 years, that was DOA and out of warranty.
The timing is something I do need to recheck due to that minor run-on It's been having lately. It is possible something has shifted and needs adjustment.
As for the Mallory, a friend is running one of those on his 67 with a modded 302. At first he had one that was mechanical only, but switched it to one with vacuum advance. I guess vaauum helps.

 

[HeavyCoupe]

This said, there is no way that just trying to set the initial timing to 14-16 degs. without altering the crank degs. will result in good timing. Do the math!

It didn’t take long for things to get over my head. I don’t understand what you mean here. Or is this something more related to the stock distributors?