[Cliff R]

Build looks good aside from dished pistons and 72cc heads. That's a decent amount of "squish" area. SBC's absolutely LOVE a flat top or even domed pistons and "tight" combustion chamber. They also love higher compression and can easily run higher compression ratios on pump gas without issues.

Keep in mind that the very best of the best of the factory SBC offerings (the 327/350hp and 350/350hp) used a domed piston, 64cc heads, a very well designed combustion chamber, good port flow, and tight squish with a .020" steel shim head gasket.

With than much dish (10cc), plus 72cc heads that's 82cc. Not a death sentence but you don't have enough "squeeze" in a "383" build for a well designed HR cam with 230/236 @ .050" duration on a 110LSA. That's the root problem and I'd either increase the compression ratio by a full point or put 10 degrees less cam in it, and I'd move it out on a 112LSA instead of 110 to "tame" things down at idle so it works well with "normal" timing and fuel curves........

[82ltdqs]

My mistake! 65 cc heads. The idea with my builder was a small chamber and a small dish to make an egg shaped chamber. Whatever the reason he said something about a more efficient burn.
He claimed 10.5 compression.

Looks like nothing a Procharger can’t fix

[Cliff R]

65 would be better for sure but still not enough compression for that cam, which is made obvious by my previous post about how an engine should respond to initial timing. Any time you find yourself having to run initial timing WAY off the scale at idle speed you missed the mark one way, the other or both with static compression and duration/LSA/overlap.

Even with that said it's seldom a "show stopper", unless of course you are the dummy who puts a real RAIV cam in a 1977 400 with 6X-8 heads, 8 valve relief pistons .050" in the holes at TDC with a .060" thick head gasket!......Yes that really happens, and a lot more often that you'd think despite the last time I looked it was 2023 and there is a LOT of information on that deal at your fingertips on these Forums......FWIW.....

[Lemans64]

Just something I have run into on my car, similar built sbc 400. big cam, AFR 195 heads, Air Gap Intake. Had a similar
stumble on light throttle acceleration, tried a bunch of stuff, was intermittent, more noticeable when raining. Had same 750HP holley DP
with 4 corner air fuel mixture screws, I found that if I used the front to control idle smoothness, around 1 turn out, and left the rear at
1/2 turn out the stumble would go away, have not noticed it since doing this. Must be right on the edge of being lean. I run a 4.5 PV.
Pretty sure I have 74's in primary side.

[82ltdqs]

Originally Posted by Lemans64

Just something I have run into on my car, similar built sbc 400. big cam, AFR 195 heads, Air Gap Intake. Had a similar
stumble on light throttle acceleration, tried a bunch of stuff, was intermittent, more noticeable when raining. Had same 750HP holley DP
with 4 corner air fuel mixture screws, I found that if I used the front to control idle smoothness, around 1 turn out, and left the rear at
1/2 turn out the stumble would go away, have not noticed it since doing this. Must be right on the edge of being lean. I run a 4.5 PV.
Pretty sure I have 74's in primary side.

Great info. I’m going to try this. I been stuck at work with no time to tune and test.

[Lemans64]

My combo was broke in on dyno and had a few runs on dyno with afr on it. so jet numbers should be good.

[Cliff R]

"So if I’m locked out at 36, with a high stall, it seems the same. Plus this distributor can add vacuum advance whenever I want. And can be reprogrammed to a traditional timing table in 10 seconds."

Don't loose sight of why an engine needs a timing curve in the distributor.

At idle speed the events happen less times per second. Cylinder pressure is high as cylinder filling happens easily with slow movements of the pistons and valves. For a moment forget the fact that well meaning engine builders started throwing cams into these engines with butt-loads of overlap and a lot of off seat timing (duration).

In 1969 for example, the EXCELLENT 300hp 350CID SBC engine was just fine with the initial timing down around 0 to 4 degrees or so. Even down that low it would make close to 20" of vacuum at 650-700 rpm's. It did not want, like, need, or even respond well to adding any more timing at idle speed let alone running 18-20 initial timing plus another 12-15 from the VA at idle speed vial manifold vacuum. Matter of fact if you try that on one of those engines it will probably drive the piston back down the way it's coming up or knock the nose right off the starter trying to crank it up fully warmed up and heat soaked!

Anyhow, as engine RPM's increase the events happen many more times per second. As a matter of physics we must advance ignition timing just to get it in soon enough that the actual point of ignition near TDC happens at the most ideal time for complete combustion and engine efficiency.

The engineers knew this and set factory distributors up with the most ideal timing curve for what they were being used on. They also added timing via the vacuum advance because it is a load sensing device and allowed the use of much leaner mixtures at very light engine load to further enhance the efficiency of these engines for optimum power, less fuel consumption and less emissions. It's all based on a "recipe" for each one of these engines.

Now along comes the well meaning owner wanting more power from the very stout 300hp 350 engine so they make changes to it. So they venture into the aftermarket and choose a cam with considerably more duration than the factory cam, with a lot more overlap and often later intake closing even with the ICL considerably advanced. Now th engine no longer has good cylinder filling at idle and low RPM's. If they didn't raise the compression to help compensate for the losses advancing the timing becomes mandatory and you will also need to increase idle fuel delivery at the same time because it takes more timing and more fuel to make an engine happy when you lower dynamic compression. For this reason we find ourselves having to advance the initial timing, usually WAY off the scale to get the engine to want to make good vacuum and idle well and even then a lot of these engine builds barely make enough vacuum for power brakes (apprx 10" ). They also don't want to idle smooth, and engine RPM's often drop off considerably when a light load is placed on the engine like putting the trans in gear and loading it with the torque converter......sound familar to anyone out there?

So we, as tuners have to come along and make changes to compensate for parts chosen or a basic engine "recipe" used that is actually making the engine considerably LESS efficient at idle and in the "normal" driving range that an engine that is stock or pretty close to it.

I'm not saying or meaning any of this in a bad way as I do it for a living and have gotten pretty good at it over the years. There are still a few "basic" things to look at here. Any time you find your engine requiring a LOT of timing at idle speed it's really unhappy with the cam vs static compression deal. Even in a pretty stout "high performance" street or street/strip engine build set up to run currently available pump gas w/o issues you should still be able to make adequate vacuum at 700-800 rpm's without any more than 12-14 or so degrees of timing at idle speed. Yes, if you find yourself having to run 20, 25, 30 or even more timing at idle you are basically what I call moving into a "crutch" fix area from too much overlap (bleeding off too much cylinder pressure) or simply not enough static compression for the cam chosen, or a little of both.

When all that happens "custom" tuning is required and whoever is doing the work has to move in and do some testing to find out what the engine wants in terms of timing at every RPM plus fuel required at the same time. So timing/fuel curves are what we are trying to figure out here. You can't change anything else in the equation aside from maybe the octane of the fuel or temperature of the engine to help in any way and either one of those will have very minor effects on the end result and hardly noticed more times than not.

I will say this in closing before I write a book here. When I am asked to tune troubled engines, and I've done countless numbers of them. Typically I'm asked to help after EVERYONE in the equation has already had a shot at it. This will include the owner, all his friends, beer drinking buddies, all the resident "guru's In his area, shops in his area that still do this sort of thing and even the wife's boyfriend will have had a shot at it! At least 95 times out of 100 I fix "carburetor issues" by going into the distributor. They ALWAYS install some "goofy" POS advance curve kit, eliminate vacuum advance or not using it correctly, and the timing is WAY off the scale at idle speed with some of the mechanical timing curve in below 900-1000rpms.

So what I do here is yank all that chit out of the distributor and put a rock solid curve in it based on the engine parameters. If it's going to need more initial timing I shorted it up. Then I'll modify or replace the VA unit to shorten up how much is added because I already know the engine isn't going to like a buttload of timing at light engine load or adding a lot to the mechanical curve I've set up.

Lastly I go into the carburetor and make more fuel available at idle speed and to the transfer slots and main system as well. Once all done I'll close the hood and send the customer on a test drive. I can here them stopping after they are a good distance from the shop and "whacking" it pretty hard, often doing "John Force" burnouts and long-hard pulls on it thru the gears. I just stand outside the shop and chuckle to myself. They come back all grinning from ear to ear right up to the moment I EMPTY their wallet!

So good results with these things involve sort of going back to basics plus enhancing what was already there vs trying to some in with whiz-bang parts sold in a little curve kit plus hand full or larger jets, some pump cams, and optional PV's for your Holley or Holley clone carburetor........FWIW.....

[82ltdqs]

Just an update incase this helps anyone in the future. Used a 12-28 helicoil kit to drill and tap my almost stripped primary nozzle threads. Cleaned it all out and gave the check valve a good tap. Kept the .031 tube squirters in place.

Problem was about 80% gone.

Switched from the orange to the green cam, and the stumble is gone.

Thank you for your help.

[82ltdqs]

Originally Posted by Cliff R

"So if I’m locked out at 36, with a high stall, it seems the same. Plus this distributor can add vacuum advance whenever I want. And can be reprogrammed to a traditional timing table in 10 seconds."

Don't loose sight of why an engine needs a timing curve in the distributor.

At idle speed the events happen less times per second. Cylinder pressure is high as cylinder filling happens easily with slow movements of the pistons and valves. For a moment forget the fact that well meaning engine builders started throwing cams into these engines with butt-loads of overlap and a lot of off seat timing (duration).

In 1969 for example, the EXCELLENT 300hp 350CID SBC engine was just fine with the initial timing down around 0 to 4 degrees or so. Even down that low it would make close to 20" of vacuum at 650-700 rpm's. It did not want, like, need, or even respond well to adding any more timing at idle speed let alone running 18-20 initial timing plus another 12-15 from the VA at idle speed vial manifold vacuum. Matter of fact if you try that on one of those engines it will probably drive the piston back down the way it's coming up or knock the nose right off the starter trying to crank it up fully warmed up and heat soaked!

Anyhow, as engine RPM's increase the events happen many more times per second. As a matter of physics we must advance ignition timing just to get it in soon enough that the actual point of ignition near TDC happens at the most ideal time for complete combustion and engine efficiency.

The engineers knew this and set factory distributors up with the most ideal timing curve for what they were being used on. They also added timing via the vacuum advance because it is a load sensing device and allowed the use of much leaner mixtures at very light engine load to further enhance the efficiency of these engines for optimum power, less fuel consumption and less emissions. It's all based on a "recipe" for each one of these engines.

Now along comes the well meaning owner wanting more power from the very stout 300hp 350 engine so they make changes to it. So they venture into the aftermarket and choose a cam with considerably more duration than the factory cam, with a lot more overlap and often later intake closing even with the ICL considerably advanced. Now th engine no longer has good cylinder filling at idle and low RPM's. If they didn't raise the compression to help compensate for the losses advancing the timing becomes mandatory and you will also need to increase idle fuel delivery at the same time because it takes more timing and more fuel to make an engine happy when you lower dynamic compression. For this reason we find ourselves having to advance the initial timing, usually WAY off the scale to get the engine to want to make good vacuum and idle well and even then a lot of these engine builds barely make enough vacuum for power brakes (apprx 10" ). They also don't want to idle smooth, and engine RPM's often drop off considerably when a light load is placed on the engine like putting the trans in gear and loading it with the torque converter......sound familar to anyone out there?

So we, as tuners have to come along and make changes to compensate for parts chosen or a basic engine "recipe" used that is actually making the engine considerably LESS efficient at idle and in the "normal" driving range that an engine that is stock or pretty close to it.

I'm not saying or meaning any of this in a bad way as I do it for a living and have gotten pretty good at it over the years. There are still a few "basic" things to look at here. Any time you find your engine requiring a LOT of timing at idle speed it's really unhappy with the cam vs static compression deal. Even in a pretty stout "high performance" street or street/strip engine build set up to run currently available pump gas w/o issues you should still be able to make adequate vacuum at 700-800 rpm's without any more than 12-14 or so degrees of timing at idle speed. Yes, if you find yourself having to run 20, 25, 30 or even more timing at idle you are basically what I call moving into a "crutch" fix area from too much overlap (bleeding off too much cylinder pressure) or simply not enough static compression for the cam chosen, or a little of both.

When all that happens "custom" tuning is required and whoever is doing the work has to move in and do some testing to find out what the engine wants in terms of timing at every RPM plus fuel required at the same time. So timing/fuel curves are what we are trying to figure out here. You can't change anything else in the equation aside from maybe the octane of the fuel or temperature of the engine to help in any way and either one of those will have very minor effects on the end result and hardly noticed more times than not.

I will say this in closing before I write a book here. When I am asked to tune troubled engines, and I've done countless numbers of them. Typically I'm asked to help after EVERYONE in the equation has already had a shot at it. This will include the owner, all his friends, beer drinking buddies, all the resident "guru's In his area, shops in his area that still do this sort of thing and even the wife's boyfriend will have had a shot at it! At least 95 times out of 100 I fix "carburetor issues" by going into the distributor. They ALWAYS install some "goofy" POS advance curve kit, eliminate vacuum advance or not using it correctly, and the timing is WAY off the scale at idle speed with some of the mechanical timing curve in below 900-1000rpms.

So what I do here is yank all that chit out of the distributor and put a rock solid curve in it based on the engine parameters. If it's going to need more initial timing I shorted it up. Then I'll modify or replace the VA unit to shorten up how much is added because I already know the engine isn't going to like a buttload of timing at light engine load or adding a lot to the mechanical curve I've set up.

Lastly I go into the carburetor and make more fuel available at idle speed and to the transfer slots and main system as well. Once all done I'll close the hood and send the customer on a test drive. I can here them stopping after they are a good distance from the shop and "whacking" it pretty hard, often doing "John Force" burnouts and long-hard pulls on it thru the gears. I just stand outside the shop and chuckle to myself. They come back all grinning from ear to ear right up to the moment I EMPTY their wallet!

So good results with these things involve sort of going back to basics plus enhancing what was already there vs trying to some in with whiz-bang parts sold in a little curve kit plus hand full or larger jets, some pump cams, and optional PV's for your Holley or Holley clone carburetor........FWIW.....

Thanks Cliff. Great info. I sat down with my timing table and reprogrammed the distributor. This program will not allow any vacuum advance at idle, so I have to manually add it. Even though the MAP sensor in the distributor requires a full manifold signal, it simulates ported.

It wants about 36 degrees at idle at 900 RPMs, or things get rough. Simulated mechanical advance starts at 1000 RPMs at 22 degrees and ends at 2500 RPMs for 36 degrees. Vacuum advance adds another 14 degrees. It does run smoother. The 14 degrees ob vacuum plus the 22 degree initial equals the 36 idle

[82ltdqs]

This is my timing table…

[steve25]

Glad it's all resolved for you!