# Engine Shudder at High RPM



## wbnapier (Jun 7, 2016)

Hello,

I have a '65 GTO with a 455, 48 heads, and a '65 tri-power manifold. I run 100% race gas.

I'm new to the mechanical arts, but trying to learn.

Friday I changed the oil (20W-50 + 1 qt Marvel Mystery Oil), belts, spark plugs (ACDelco R43S gapped at 0.035), and ran a can of Sea Foam spray through the center carburetor. I took the car out after this, and no problems. Tonight, halfway through the drive, when I punched it and hit 3,800 - 4,000 RPMs, the engine would shudder and come in and out of power. When I say shudder, I mean total power loss then back again. Jerking we forward and then back. From idle to 4K, no problems. At home, I parked and brought the car up in RPM and it shuddered at 4,800 - 5,000 RPM. When I open the hood and toggled the throttle I don't see or hear any problems.

Any ideas? I've owned the car for 1.5 years and had no problems like this. All sorts of other problems, but nothing like this.


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## PontiacJim (Dec 29, 2012)

fuel filter, or fuel pump going bad?


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## wbnapier (Jun 7, 2016)

Thanks for the reply.

The fuel filter is about 1 year old. The fuel pump, I have no idea. I will try to disconnect the line from the carburetor and route to a can. Crank the engine and see if it pumps fuel into the can.

I just did another test drive, and the engine stops at 4K. It's like there's a rev limiter / governor. Really strange. I did a visual inspection of my MSD box, 6AL, part nr. 6420. It has a rev limiter at 6K. I can't imagine this box going bad.

In your experience, do fuel pumps give out 1/2 way like that? One day, no problems below 4K, but above that, they can't keep up?


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## wbnapier (Jun 7, 2016)

Fuel tank vent tube cleaned, fuel pump tested, tri-power linkage tested, new fuel filter and hoses installed. All good. 

Test run performed, and the same issue, I hit 4K and shudder. More troubleshooting ahead! Could it be my new spark plugs? AC Delco's R43S gapped at 0.035. The improper gap for my compression?

Really strange. 

When I changed my plugs, there was some oil on the plugs from cylinders 4 and 6. Maybe I'm losing compression at higher RPMs? I didn't have a problem before, and I can't imagine I suddenly have an issue.


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## PontiacJim (Dec 29, 2012)

OK, several more things to consider..........

"Jerking me forward and then back. From idle to 4K, no problems. "

This still sounds to me like a fuel/carb problem as you hit higher RPM's and the engine demands more fuel. When the carb runs out of fuel, it will nose over, then the fuel will catch up and slightly fill the carb bowl, pick-up power/rpm's, and then nose over again once the fuel has been used up again. Using the Sea Foam you may have dislodged something and it got lodged somewhere in the carb internals. Not saying I am right, but it could be a possibility at this point. *BUT....*. 

Those plugs look bad, but it could be excess oil, carb running rich, or plugs not firing correctly. The R43S is a "cold" plugs and may not be the correct ones for your application. You might want to try a "hotter" plug like the R44S or possibly the R45S. The R43S is the safer bet only because fouling is a much more desirable "problem" to have than detonation. Fouling can cause misfires or poor running and a hotter plug may be necessary. If you're running too rich or burning some oil, the hotter plug can sometimes compensate for this as long as your engine isn't knocking or pinging away on the road under load. If the engine does begin to knock or ping on you and you cannot compensate for this through a small timing adjustment, then you will need to stick with the R43S. So you may need a hotter plug to help burn better. Try the R44S plug instead and this may eliminate all the sooty looking appearances of the plug and may help with your problem.

.035" - 0.40" gap is fine. I prefer a tighter gap at .035"
*BUT.....* 

You did not tell us what type of distributor you are using? With the engine losing power near 4,000 RPM's, the plugs fouled, ...........might it be an HEI or aftermarket electronic ignition? If so, these units need a 12volt power wire. The factory ignition wire to the original points type distributor used a resistor wire to supply about 7 volts to the points so they do not burn. If a 12volt wire has not been used to jump/eliminate the factory resistor wire, you will not get enough voltage for the electronic ignition to work/fire properly and _you would have the same problems you describe.
_
I assume you have a good coil as these can go bad? Anything with regards to ignition may cause a poor spark at higher RPM's. A bad MSD can't be ruled out. Can you change your wiring around to eliminate the MSD from the ignition system just to test it? 

Other things to check would be the 5/7 plug wires. Have read that if they cross on top of each other (touch), they can cause problems.

Fuel pump could be pumping gas just fine at lower RPM's, but if something were amiss with the diaphragm, it is possible it could be causing issues. Is it a fuel pump made to work with the ethanol gas? Only way to really know would be a fuel pressure gauge. If low fuel pressure, then you can back track from the carb. Fuel pump, the sock https://www.opgi.com/chevelle/CH28637/ on the end of the fuel pick-up collapsed or plugged up, or something floating around in the gas tank.


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## Goat Roper (Oct 28, 2014)

First I would dump the oil and filter and get rid of the mystery oil and go with 10-30 and ZDDP additive.
Mystery oil is designed to seep to unlock frozen parts and can blow by rings and seep past seals.
It sounds like fuel starvation, I had the same problem and replaced my tank, sending unit and used ethanol fuel resistant hoses.
I would change out filters and blew the sock but that would last for a hundred miles or so then reappear.
I found a small crack in one of the hoses, that can also suck in air that won't be noticed until you hit higher RPMs as the pump pulls harder.
I may have been able to just flush the tank and replace hoses but since it was 50 years old and I had done a bumper to bumper mechanical restoration I just replaced the whole enchilada.


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## wbnapier (Jun 7, 2016)

Agreed, I will dump the oil in the near term and run straight Kendall Liquid Titanium (GT-1 Competition) 20W50.

Why do you suggest 10W-30? I think the extra viscosity of 50 weight would help my issues.

I replaced the hoses from the pump to the carb, you are right, I should replace the others to ensure ethanol resistance and integrity.

Thank you for help.


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## wbnapier (Jun 7, 2016)

Thanks Jim. 

I am running an MSD 6AL CD Ignition Part Number: 6420. 
The distributor is MSD, but I don't know the part.
The coil is MSD, but I don't know the part.

All is circa 1992.

I will check the wiring, but I don't think it is routed directly to the battery. 
I will check to see if any plug wires are crossed as well.
These dirty plugs were R43S, and I replaced all 8 with R43S.

All of the above electrical/ignition issues are logical, but why now? Following a spark plug change?

If fuel related, following the seafoam intake cleaner use?

What about a vacuum issue?

More troubleshooting tomorrow if time.


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## wbnapier (Jun 7, 2016)

Problem solved!

It was the MSD ignition coil. It went bad above 4K RPMs. The replacement is on, and it is running smooth as high as I want to go. 

I'm really happy. I wanted to thank you guys for taking the time to help me troubleshoot!

Regarding the oil on the plugs. I will do a wet / dry compression test in the coming weeks and see what's going on there.


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## PontiacJim (Dec 29, 2012)

Glad it was that simple. When you have problems such as yours, about the only thing you can do is a step-by-step elimination process. If you had all the "extra" testing equipment, it would be much easier, but if you are like most of us, checking the system piece by piece is how you do it even if it costs a few dollars to replace parts you may or may not have needed.

I rather sink $500 into my car and have a few new parts that I might not have needed than to take it to a shop and get the bill for $450 of labor and a $50 part. Plus, you learn a lesson or two, know your ride better, and if problems ever repeat in either this car or another one down the road, you know exactly where to start looking - that's how I learned. :thumbsup:


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