GM GM V8 Running Issues

cult45

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Nov 30, 2011
Messages
3,503
Location
Harbour City, the New Southern Wales
Hi all,

I'm in Australia, but the info should translate as I'm running a GM engine. It's an early 80's cast iron V8 with a CDI ignition and Quadrajet carb. All factory fitted. Comps are 120psi wet. 40 psi at idle. No leaks. Engine is new to me but used. Has been rebuilt at some stage in it's life but specs unknown. Seems to have a bit of a cam. New plugs, leads. Old dizzy, cap, rotor and coil but those parts are only 12 months/10k miles old. No visible exhaust leaks or audible sounds.

The issue is that it idles fine but under load but its down on power. Seems to be making like 10-20% of what it should. Won't even do a hill start. Instead, it backfires regardless of timing, both through the carb and out the exhaust - though mostly out the exhaust.

We have:

Checked and rechecked timing over a dozen times across two dizzies [factory CDI and the MSD unit that came with the engine] and two carbs [my rebuilt Quaddie and the Holley 450/465 Economaster that came with the engine]. This was done from base [#1 at TDC and rotor at #1 lead]. We're 100% sure it's timed correctly.

What could this be?

Thank you.

Daniel
 

whitey45

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Joined
Oct 18, 2007
Messages
3,741
Location
San Gregorio
Hey mate,

First thought is your dizzy advance working properly? But you have swapped out for others, so second thought is the ol cam lobes, flat cam will cause your issues. Compression is ok due to slow speed cranking, as soon as you get up in rpm’s timing cannot keep up when the valves don’t open when they should! Crack her open and put a big stick in er!

Cheers,

Wannabe Aussie out in Cali!
 

cult45

SILVER Star
Joined
Nov 30, 2011
Messages
3,503
Location
Harbour City, the New Southern Wales
Hey mate,

First thought is your dizzy advance working properly? But you have swapped out for others, so second thought is the ol cam lobes, flat cam will cause your issues. Compression is ok due to slow speed cranking, as soon as you get up in rpm’s timing cannot keep up when the valves don’t open when they should! Crack her open and put a big stick in er!

Cheers,

Wannabe Aussie out in Cali!
Whitey!

Damn son am I gonna visit your ass when I'm in town! Thanks for the feedback mate. I was initially leaning towards electrical? I've since recalled the dizzy shaft has 2mm of endplay and the alternator has taken an oil bath for the last four months. Was thinking it was breaking down under power. But good thought on the cam. I've parked'er up and have come the 1000km home via plane. The Catskinner is going to help me fix'er whilst I'm afar.
 
Joined
Nov 9, 2012
Messages
7,469
Location
Olathe, KS, USA
Whitey!

Damn son am I gonna visit your ass when I'm in town! Thanks for the feedback mate. I was initially leaning towards electrical? I've since recalled the dizzy shaft has 2mm of endplay and the alternator has taken an oil bath for the last four months. Was thinking it was breaking down under power. But good thought on the cam. I've parked'er up and have come the 1000km home via plane. The Catskinner is going to help me fix'er whilst I'm afar.

Did you ever get this resolved?

I was going to suggest the same thing. I had a 1970 350 that was bored, ported, polished and balanced, and I had the #7 exhaust lobe go flat on the cam. That would cause backfiring and low power issues, as well as some valve ticking. I attempted to adjust the valves and succeeded in not allowing it to breathe and it blew the head gasket, then flooded the cylinder, and I hydrauliced a rod. I had a mild towing cam in it, so it had plenty of low end torque and wasn;t made for high-revving power. It was a workhorse.

The Gen 1 350's had an oil galley issue that was due to casting and the cam oil would leak through one of the other casting ports in the block on the rear plenum seal on the left side of the block, right near where the oil pressure port comes out of the block. So, #7 exhaust lobe going flat from lack of oil is a relatively common thing.
 
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