Stubborn Engine Problem

[Bigdav160]

IDK, symptoms sound like a lean misfire due to a lean carb/accelerator pump

Most aftermarket carbs out of the box are pretty dang lean. That's where I would start

[mattfranklin]

One thought on the hesitation and/or misfire... Are you comparing your truck to new vehicles which have nearly zero misfire, or to old cars that almost always have a little of it? When I first started driving most of our cars had A LOT of hesitation, especially on those cold sub-zero Minnesota mornings. So I'm pretty tolerant of a bit of it.

Having said that, Holley has a kit to switch in more aggressive accelerator pump cams. I've done that before. It worked great. If I remember correctly, the yellow one is the biggest and most aggressive. https://www.summitracing.com/parts/h...BoCKwAQAvD_BwE

[twinturboc10]

Is the problem just at constant, light throttle? I believe you mentioned it wasn’t an issue when mashing the throttle. If you shift down a gear while running the same speed where the problem shows up, does it go away?

[toolboxchev]

Originally Posted by drew8mc

Sorry, I'm not following. Are you having the same issues, or are you saying it's just me?

No, what I am saying is I have been having some type of stubborn problem myself. I have just learned to live with it, unfortunately.

Shoot me a pm sometime.

[cadillac_al]

It is a pretty hot engine. I have never had to tune an engine with a rowdy cam like this. That must be the problem here. That performer RPM isn't an air gap is it? Since the carbs aren't making a difference I think I would plunk down $50 and try a Skip White Distributor for the heck of it. Good luck.

[drew8mc]

Originally Posted by Bigdav160

IDK, symptoms sound like a lean misfire due to a lean carb/accelerator pump

Most aftermarket carbs out of the box are pretty dang lean. That's where I would start

When I first hooked up the carb, the plugs told me it was running rich. I jetted down 2 sizes and the plugs looked much better. However, the misfire was not any different between jet sizes. I thought since it was only just off idle that I get that misfire, it may be the intermediate air bleeds were too small. I went up a few sizes there and no real difference.

The sound happens not just when i move the throttle (i.e. when the accelerator pump activates), but also when I hold the throttle for cruising just off idle.

Is it possible it was too rich into the primaries (hence the jetting down helping), but too lean just off idle?

I know one answer to all this would be to get an AFR gauge. But I'm just worried I'll be throwing yet more money at something that may not even be part of the problem.

[drew8mc]

Originally Posted by twinturboc10

Is the problem just at constant, light throttle? I believe you mentioned it wasn’t an issue when mashing the throttle. If you shift down a gear while running the same speed where the problem shows up, does it go away?

Correct. It goes away once I'm into the primaries. It is constant at light throttle, although it doesn't have a noticeable pattern, consistent with RPM for instance. It seems random. Down shifting does make it go away as the throttle opens further.

[drew8mc]

Originally Posted by cadillac_al

It is a pretty hot engine. I have never had to tune an engine with a rowdy cam like this. That must be the problem here. That performer RPM isn't an air gap is it? Since the carbs aren't making a difference I think I would plunk down $50 and try a Skip White Distributor for the heck of it. Good luck.

I put a new intake manifold from Trick Flow which is an air gap type. It did not change this misfire issue. I did try the old HEI cap that was in the old 327. No difference. I did not change the bottom half of the distributor, but I'm not sure how that could produce this problem.

[twinturboc10]

Originally Posted by drew8mc

When I first hooked up the carb, the plugs told me it was running rich. I jetted down 2 sizes and the plugs looked much better. However, the misfire was not any different between jet sizes. I thought since it was only just off idle that I get that misfire, it may be the intermediate air bleeds were too small. I went up a few sizes there and no real difference.

The sound happens not just when i move the throttle (i.e. when the accelerator pump activates), but also when I hold the throttle for cruising just off idle.

Is it possible it was too rich into the primaries (hence the jetting down helping), but too lean just off idle?

I know one answer to all this would be to get an AFR gauge. But I'm just worried I'll be throwing yet more money at something that may not even be part of the problem.

An easy check for a too-lean off idle condition would be going smaller on the idle air bleed or to larger idle feed restrictors to richen the mix before you get into the main circuit.

[mattfranklin]

Originally Posted by drew8mc

I know one answer to all this would be to get an AFR gauge. But I'm just worried I'll be throwing yet more money at something that may not even be part of the problem.

There was a really nice Roadkill episode on tuning the Holley. It was called "Tater Truck" or something like that. In it they systematically worked their way through all of the carb adjustments, jets, power valves, and so on. They used an AFR gauge to check their work. It was pretty cool.

Here's a low resolution version:
https://www.youtube.com/watch?v=CJOx76DC0eM

You can find better versions for free, but the best way is to subscribe ($5/month) to MotorTrendOnDemand.
https://www.motortrendondemand.com/d...ck/0_jerfe1z5/

[drew8mc]

Originally Posted by twinturboc10

An easy check for a too-lean off idle condition would be going smaller on the idle air bleed or to larger idle feed restrictors to richen the mix before you get into the main circuit.

Hmmm. I never tried going the other way with the air bleeds. I will try it and see what happens. Thanks.

[drew8mc]

Originally Posted by mattfranklin

There was a really nice Roadkill episode on tuning the Holley. It was called "Tater Truck" or something like that. In it they systematically worked their way through all of the carb adjustments, jets, power valves, and so on. They used an AFR gauge to check their work. It was pretty cool.

Here's a low resolution version:
https://www.youtube.com/watch?v=CJOx76DC0eM

You can find better versions for free, but the best way is to subscribe ($5/month) to MotorTrendOnDemand.
https://www.motortrendondemand.com/d...ck/0_jerfe1z5/

I was pissed when they left You Tube. I don't want yet another subscription. It should come with your subscription to any of those magazines. I will watch it and see what they do. I may have to break down and get an AFR gauge.

[DieselSJ]

Originally Posted by mattfranklin

There was a really nice Roadkill episode on tuning the Holley. It was called "Tater Truck" or something like that. In it they systematically worked their way through all of the carb adjustments, jets, power valves, and so on. They used an AFR gauge to check their work. It was pretty cool.

Here's a low resolution version:
https://www.youtube.com/watch?v=CJOx76DC0eM

You can find better versions for free, but the best way is to subscribe ($5/month) to MotorTrendOnDemand.
https://www.motortrendondemand.com/d...ck/0_jerfe1z5/

That was a very good episode. I point people to that when they argue that it takes too much time to properly tune EFI. Yeah, well, if you really want to tune your carb then you are easily going to spend just as much time.

[mattfranklin]

Originally Posted by drew8mc

I was pissed when they left You Tube. I don't want yet another subscription. It should come with your subscription to any of those magazines. I will watch it and see what they do. I may have to break down and get an AFR gauge.

I totally understand. But I was so hooked on it I subscribed so could get early episodes. I've been paying my $5 since way before they left YouTube. Addicted. ;-)

[drew8mc]

So I tried two things yesterday.

1. I tried going richer on the idle circuit by going to a smaller air bleed on the idle circuit. No change. Not in the slightest.

2. The quick fuel has a set screw for adjusting when the vacuum secondary comes in instead of changing springs. I set this all the way in for the slowest in coming in. No change at all.

I really feel like it is still an ignition issue. The most luck I've had was swapping plug wires. I had gotten it to where the miss was almost imperceptible. Then I swapped intake manifolds to get rid of a vacuum leak and now the plug wires do not seem to make any difference.

I've gotten it much more drive-able now by getting the timing right, but it just doesn't sound right. I have a new torque converter on the way. That will be a big project.

[mattfranklin]

Originally Posted by drew8mc

...Then I swapped intake manifolds to get rid of a vacuum leak and now the plug wires do not seem to make any difference....
.

I'm grabbing at straws here, but have you checked for additional vacuum leaks? If someone had suggested that to me I'd be like "What are you kidding? I'm very careful and methodical. That's very unlikely." But recently I discovered that I had left off a cap for a 3/8" hose barb hidden under the back bowl of my carb. Once fixed, my idle really straightened out.

[drew8mc]

Originally Posted by mattfranklin

I'm grabbing at straws here, but have you checked for additional vacuum leaks? If someone had suggested that to me I'd be like "What are you kidding? I'm very careful and methodical. That's very unlikely." But recently I discovered that I had left off a cap for a 3/8" hose barb hidden under the back bowl of my carb. Once fixed, my idle really straightened out.

I have not done a recent check for vacuum leaks since replacing the the intake manifold but wouldn't I get way less than 15 in of vacuum if I had a leak?

[mattfranklin]

Originally Posted by drew8mc

I have not done a recent check for vacuum leaks since replacing the the intake manifold but wouldn't I get way less than 15 in of vacuum if I had a leak?

That's a fair point. I was grabbing at straws. But sometimes in troubleshooting we need to revisit things we thought we knew for certain, especially if we haven't yet solved the problem.

[drew8mc]

The only thing left to do is to try the old distributor bottom half. After that, I think it's time to take it to someone more knowledgeable than myself (which isn't hard).

[drew8mc]

There's nothing I hate more than reading through someones thread where they have a similar problem and getting to the end and there's no resolution. So I'm pulling this thread from the grave and giving it some resolution.

This problem was entirely timing. I have had trouble getting an accurate reading with my timing light and the DUI distributor. I was also thinking that the TDC mark on the damper was incorrect. I bought a TDC tool to insert into the spark plug hole and confirmed the engine builder's mark is right on. I was looking at youtube timing and ignition videos when this guy plugged this distributor from Progressive Ignition.

This distributor gets rid of all the mechanical advance portions of the distributor and replaces that with an electronic version of advance controlled completely from your smart phone or tablet. It's a little complicated for me to explain but there are videos on the website which better explain it.
https://progressionignition.com/

The ability to change the timing curve on the fly is huge. I think what my problem was, besides not getting accurate timing, the mechanical advance was coming in too early (too light of spring) at partial throttle. This new distributor has completely cured this and it's actually pretty fun to play with the timing so easily on this thing. My only concern is that in 10 years is the company still around and making the app available on the latest hardware? Impossible to know. But for me, this cleared up my problem completely.

[cadillac_al]

I have heard some buzz about these new distributors that can be adjusted from the drivers seat. I have never had the urge to do that. I guess I'm too lazy. It would be pretty cool at the drag strip though. The old $50 Skppys have never done me wrong.

[Flmarine71]

Drew, just curious- did DUI/Performance originally tune the distributor on their machine specific to your truck specs? I am surprised the timing curve would have been that far off. 24 degrees of mechanical does seems like a lot though for a recurved distributor compared to some of the timing curves recommended here.

That kill switch feature on the Progressive distributor is pretty cool.

[drew8mc]

I gave DUI all my engine specs. 24 degrees at 3000 is etched right on the underside of the distributor. It very well could have worked fine if I was ever able to get it accurately timed. I have two different timing lights, and 3 sets of plug wires trying to get it right, and it just never flashed consistently. I never had any confidence that I had it exactly where it should be. It was maddening.

[geezer#99]

Now that it’s electronic, what timing numbers are you running now?
Timing numbers like initial, when does mechanical start and how much, mechanical all in timing and at what rpm.

[Kudzupatch]

Never mind