302 engine studders during high RPM

[Dieter123]

Apologize in advance for this lengthy post. In the past I have seen where the initial post may not have covered everything soooooo, I figured I will provide as much as I can up front. I have been driving my MK4 for almost 3 years. Have 4200 miles. The last couple years I have noticed a problem that hopefully someone can provide some guidance. When I am going through an aggressive shifting sequence (say 3500-4000 rpm) my engine sputters. No back fire, just will not maintain the RPM’s commanded. Sorta like when I get on a highway “on ramp” doing 60-70 mph and shifting hard and fast is when this is noticed. Normal shifting around 1500-2500 rpms the engine acts normal. The only thing I have done (with no luck) is the following. Replaced the inline fuel filter rated at 130 gph (located between the mechanical pump and the carb)….Replaced the stock fuel pump with a Speedmaster PCE146.1013 (115 gph). Some build facts>>>> Mild build SBF 302, Holley 600 w/elect choke, Edelbrock Performer RPM intake, Comp Cam 35-349-8, GT40P heads, Pertronix PNX D130710 with Pertronx Coil 45001. Folks on this forum have tons of knowledge and “do this” as well as “do not do that” comments. Not sure if it is a fuel issue at this point. Thanks for reading and I appreciate any input/advice anyone can provide.

[J R Jones]

Fuel delivery problems are usually longer time events than spark issues. Fuel flow vs the speed of light.
Starting with easy first, higher compression challenges spark events more than low compression. Your spark plugs may not tolerate higher RPM/compression.
Cleaning is not IMO an option. Good new plugs are a place to start. Everyone has a favorite brand, I like NGK & ND. Your milder build likely does not require cold plugs that foul easier when driven moderately.
jim

[gbranham]

Have you checked your plugs? What do they look like? I'd put the best plug I can buy in there and see how it runs. It's a 15 minute project that may fix your issues.

Greg

[Jeff Kleiner]

Since you have a carb I think it is more likely an ignition problem; coil, center button in the cap, plug wire breakdown…

Jeff

[Dieter123]

I do have NGK plugs but I have yet to tackle this issue to be an ignition issue. I wanted to poll the forum first. From your comments, it appears I should redirect my troubleshooting to ignition. Thanks for your input(s).

[GoDadGo]

Start with the basics and I know you'll figure it out.


Check the plugs, wires, rotor and cap first to make sure all is well.
After that it may be an issue with the carb bleeds or possibly the power valve.
Is your Holley a four (4) corner or two (2) corner style idle circuit style carburetor?
Do you have a timing light and a meter to check the wires to make sure they are good?


Again, start with the basics and I know you'll figure it out.

[Dieter123]

Start with the basics and I know you'll figure it out.


Check the plugs, wires, rotor and cap first to make sure all is well.
After that it may be an issue with the carb bleeds or possibly the power valve.
Is your Holley a four (4) corner or two (2) corner style idle circuit style carburetor?
Do you have a timing light and a meter to check the wires to make sure they are good?


Again, start with the basics and I know you'll figure it out.

Yea, I was attempting to eliminate fuel starvation first. I am not familiar with carb bleeds, power valve, or 4 vs 2 corner. Admitting to my ignorance on anything to do with the carb cause it is a "hostile" environment for me to venture off in. I plan on checking wires and plugs and as well my PerTronix D130710 Flame-Thrower Electronic Distributor/wiring. Yes I do have a timing light.

[AC Bill]

Prior to building my roadster I went for a cruise with a fellow, who had built an FFR with the SBF 302 equipped with a Holley. He had an issue very similar to what you describe. He believed it was caused by the g force under strong acceleration causing the fuel in the float bowl to slosh to the rear of the float bowl. This he summarized, caused the engine to be starved for fuel. He felt a different carb was the answer, as he had heard of others with the Holley having the same issue. At the time I never thought to ask what model Holley he ran.
He was quite a knowledgeable fellow, and at that time was highly respected in the FFR builder's scene, so I presumed he knew what he was talking about. I avoided that and other carb related issues and chose to build mine with EFI..:)

[StangRacer]

What valve springs are you using?

[Dieter123]

What valve springs are you using?

Guessing you are talking about the carb and not the cyl heads. I have no idea. I just took the carb out of the box and "plug and play" of course adjusting idle.

[StangRacer]

Guessing you are talking about the carb and not the cyl heads. I have no idea. I just took the carb out of the box and "plug and play" of course adjusting idle.

No, I did mean the valve springs on the cylinder heads... I assume the Comp Cam you mentioned is a hydraulic roller...

[Avalanche325]

I can't imagine he is floating valves at 4000 rpms.
So you didn't tune the carb, but it ran fine (don't tell the RFI guys - LOL), now something has changed. As most people are stating, check the ignition first. You might be shocked at the condition of the cap and rotor. They don't tend to seal all that well and can get corroded.
If you have a rev limiter, check to make sure it isn't set crazy low.

Also pull the plug wires off of the cap and check for moisture. I have seen that cause issues.

[Dieter123]

No, I did mean the valve springs on the cylinder heads... I assume the Comp Cam you mentioned is a hydraulic roller...

Yes. Hyd. roller

[StangRacer]

I can't imagine he is floating valves at 4000 rpms.
So you didn't tune the carb, but it ran fine (don't tell the RFI guys - LOL), now something has changed. As most people are stating, check the ignition first. You might be shocked at the condition of the cap and rotor. They don't tend to seal all that well and can get corroded.
If you have a rev limiter, check to make sure it isn't set crazy low.

Also pull the plug wires off of the cap and check for moisture. I have seen that cause issues.

Probably not... but worth exploring. If whoever built the engine used the stock valve springs on the GT40P heads it's possible.

[Dieter123]

I can't imagine he is floating valves at 4000 rpms.
So you didn't tune the carb, but it ran fine (don't tell the RFI guys - LOL), now something has changed. As most people are stating, check the ignition first. You might be shocked at the condition of the cap and rotor. They don't tend to seal all that well and can get corroded.
If you have a rev limiter, check to make sure it isn't set crazy low.

Also pull the plug wires off of the cap and check for moisture. I have seen that cause issues.

Plug and play on the carb. With 4K plus miles all I have had to do is the recommended fluid changes (engine oil and Tremec first 500 mile recom change). Guessing humidity.... cause the car has never seen rain. lol. Not sure about the rev limiter

[gbranham]

If you don't know much about carbs, I'd suggest taking it to a local tuner to have them get it running in tip top shape. I don't know beans about carbs and I'm a DIYer, but if I had a carbed car running poorly, I'd farm out the tune up to a pro.

[Mike.Bray]

Could be fuel starvation

Could be an ignition issue

Could be rev limiter

Need to plug in and log some data

https://www.imagecoast.com/images/MichaelBray1/34559423959684575766142651227595185626544093na.jpg

[Dave Tabor]

Do you have or can you put an air/fuel gauge on it?

Dave
Gen III Coupe #17

[Railroad]

I did not read what the engine does, with more throttle, during the issue.
Does the engine clear up and continue to accelerate, or continue to miss?
A power valve can go through a late or early opening point and cause some hesitation. Usually this occurs as the rpms pass through occurrence and the engine revs normal when the PV and fuel need coincide.
I guess this miss could be maintained with holding the throttle at this rpm.
Holley has general rules for sizing power valves, which is easily found on line.
You will need a vacuum gauge and the specs on the carb you are running.
Very interested in what the engine does above 4000 rpm.

[Dieter123]

I did not read what the engine does, with more throttle, during the issue.
Does the engine clear up and continue to accelerate, or continue to miss?
A power valve can go through a late or early opening point and cause some hesitation. Usually this occurs as the rpms pass through occurrence and the engine revs normal when the PV and fuel need coincide.
I guess this miss could be maintained with holding the throttle at this rpm.
Holley has general rules for sizing power valves, which is easily found on line.
You will need a vacuum gauge and the specs on the carb you are running.
Very interested in what the engine does above 4000 rpm.

Yes. It continues to miss until I let off the gas pedal. Then, when returning to normal (non-aggressive) pedal command....everything seems fine.

[Purple Venom]

Fuel pressure, what is it? For a carb and mechanical fuel pump, you should expect 6 - 8 lbs, NO MORE.
Fuel bowls - If you have a Holley, you should have sight glasses in the side of the bowls. Make sure these show no more than 1/2. I've adjusted mine with the engine running, but, this is dangerous with fuel leaking out on a hot engine. Do this at your own risk. Otherwise, adjust slightly, start engine and check fuel bowl level.
Timing - for a stock 302 at idle, you need to be around 10 - 12 degrees. At 3500 RPM, you should be in the neighborhood of 34 - 36 degrees total timing.
Vacuum Leaks - With your carb, make sure any unused ports have a rubber cap. For those that have a rubber cap, check them. They could be cracked and causing your sputter/hesitation.

If all of the above check out, share more info on your distributor. Mechanical or Vacuum Advance?

Good luck!

[Railroad]

If the engine will not rev past the break up rpm, under no load, sitting in neutral, I would look at a rev limiter on the ignition.
You can remove the air filter and make sure the choke blade is not doing something crazy.
good luck,

[MB750]

Do you have or can you put an air/fuel gauge on it?

Dave
Gen III Coupe #17

This. It's the cheat code for engine tuning (carb or EFI).

Watch the gauge to see if there's an air/fuel relationship causing your hiccups. If the AFR's don't change, it's probably ignition.

[wbulk]

Dieter123 send me a private message with your number so we can talk on the phone. I tune a lot of cars for friends and have been doing it for years. I just typed a long page of possible solutions and lost connection and lost it all. Be happy to help.

Wayne

[JMD]

Lots of good advice. Maybe check spark plug boots to make sure part of one hasn't melted against the header or flange. It may be melted just enough to cause disruption at high rpm but not anywhere else. Or if it's just one cylinder not getting ignition it may only be noticeable at higher rpm.