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Engine Diagnosis

General engine tech -- Drag Racing to Circle Track

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vortecpro
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Engine Diagnosis

Post by vortecpro »

Some time ago Ibuilt a engine for a customer, it was dyno tested and ran fine, the engine was installed in the car and would run for about 5 minutes good and then start running rough, the plugs would come out and look like, see picture.

1. Dist changed
2. 13.4 volts to the new HEI
3. Plenum clean
4. Carb not running a high float level
5. All 8 plugs look like this
6. Owner did assemble new MSD plug wires before installing the engine


Any help would be appreciated........
IMG_3434.jpg
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KnightEngines
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Re: Engine Diagnosis

Post by KnightEngines »

Buggered power valve?
Power valve too high? Like a 8.5 power valve with a big cam.

Plug is sorted, but not wet, so it's either a weak spark (that seems to be covered) or pig rich at low load.

Got a known good carb to throw on it to confirm or rule out a carb issue?
Monza355
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Re: Engine Diagnosis

Post by Monza355 »

Does the HEI have the coil in the cap or is he using an external mounted coil ?
vortecpro
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Re: Engine Diagnosis

Post by vortecpro »

Monza355 wrote: Fri May 13, 2022 8:44 am Does the HEI have the coil in the cap or is he using an external mounted coil ?

He has tried both.
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vortecpro
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Re: Engine Diagnosis

Post by vortecpro »

KnightEngines wrote: Fri May 13, 2022 8:22 am Buggered power valve?
Power valve too high? Like a 8.5 power valve with a big cam.

Plug is sorted, but not wet, so it's either a weak spark (that seems to be covered) or pig rich at low load.

Got a known good carb to throw on it to confirm or rule out a carb issue?
The cam was 228 @ .050 and I believe we tested the same carb on the dyno, 770 avenger.
Racing a NA NHRA stocker should be mandatory before any posting.
KnightEngines
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Re: Engine Diagnosis

Post by KnightEngines »

vortecpro wrote: Fri May 13, 2022 8:52 am
KnightEngines wrote: Fri May 13, 2022 8:22 am Buggered power valve?
Power valve too high? Like a 8.5 power valve with a big cam.

Plug is sorted, but not wet, so it's either a weak spark (that seems to be covered) or pig rich at low load.

Got a known good carb to throw on it to confirm or rule out a carb issue?
The cam was 228 @ .050 and I believe we tested the same carb on the dyno, 770 avenger.
Yikes, horrible carb, slap a 750 square bore pumper on it.
rebelrouser
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Re: Engine Diagnosis

Post by rebelrouser »

From time to time I have issues similar to this engine. First thing I do is slap a vacuum gauge on it, with a big cam bring up until the lope is gone, tells a lot about what is going on. Bouncing gauge valve train issues including flat camshaft, low vacuum retarded ignition timing, or just a poor running engine. then it gets a leak down and compression test to verify the base engine before I start trying other stuff. I also have and use an ignition scope also rules out a lot very quickly. Plugs look gas fouled so it is either rich, not enough compression to fire the cylinder properly, or very weak ignition. You never know what people have done. I built an engine for a guy, it sat for over a year waiting for the car to get painted. Called up said it would not start. He had put tape over the exhaust ports to keep out critters, BUT it left it on, and bolted the manifolds over the top of the tape. It simply would not pull enough air through the engine to start.
allencr267
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Re: Engine Diagnosis

Post by allencr267 »

Too much fuel pressure, regulator return line restricted/clogged?
GerryP
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Re: Engine Diagnosis

Post by GerryP »

It starts and runs fine cold. It turns bad once the engine gets some heat in it. I'd be looking at stuff that changes with heat. Maybe a stuck heat riser. Maybe heat in the ignition module. Something is changing. Even valve lash.
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Re: Engine Diagnosis

Post by PRH »

Any gas smell in the oil?

I’m also on board with trying another “known good” carb, and/or ignition.

Anyone checked the voltage at the ignition?
Somewhat handy with a die grinder.
V12MECH
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Re: Engine Diagnosis

Post by V12MECH »

Good voltage and a coil swap done, if it has 4 pin module, get another, maybe a napa TP45, check pick-up coil resistance hot, about 800 ohms, if memory serves, the old rule about carb problems turn out to be ignition may apply here.
V12MECH
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Re: Engine Diagnosis

Post by V12MECH »

Also don't forget heat compound under module, and ground strap/ wire to in cap coil if that applies.
HQM383
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Re: Engine Diagnosis

Post by HQM383 »

Back to basics - plug heat range?
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1980RS
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Re: Engine Diagnosis

Post by 1980RS »

What carb and with what bleeds in it?
HQM383
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Re: Engine Diagnosis

Post by HQM383 »

Check metering block gasket is sealing the squirter hole to carb body. If that’s not an effective seal fuel will go into the pv vacuum chamber in the body. Cold might like the extra fuel but not warmed up. Just a thought.
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