Q: I have a Chevy 350, bored .060-inch over. It has TRW forged flat top pistons, 230-degree duration (at .050-inch lift)/.560-inch lift COMP Cams roller cam run straight up, and iron heads with 64cc chambers, 2.02/1.60-inch valves, and ported intake runners. The intake is an Edelbrock Torker II with a Performer 750 cfm vacuum secondary carburetor. Fuel delivery is via a high-volume mechanical pump with regulator. I’m also running a Mallory Unilite distributor, 1 7/8-inch primary tube headers with a full 2 1/2-inch dual exhaust, and a 10-bolt rear-end with 3.73 gears.
The problem is the motor builds power to about 4,000 rpm, then starts dropping off. I know the car runs too rich because I have to turn the idle screws all the way in to get the highest vacuum (11 in./Hg), but the plugs are always fouled. I have to advance the timing quite a bit to get the motor to run. It seems to be around 26-30 degrees, but the engine will not run with the mark anywhere near the timing tab. When I bought the heads, I was told that I would have around 10.25:1 compression with my pistons. Still, the car runs on 92 octane pump gas with no spark knock that I can hear.
I didn’t index my roller cam. But if the cam is retarded for some reason, why am I losing power at about 4,000 rpm? If it is advanced, why is the carburetor running so rich?
A: The first problem we see is those 1 7/8-inch primary tube headers. That’s a big piece of pipe for your combination; a 1 5/8-inch primary tube is much better suited to your engine. As for your carburetor running too rich, it sounds like you need to change the step-up springs and metering rods. You will have to do some experimenting to find the right combination. You should also check to see if your fuel line is kinked, or if it is large enough to adequately feed your 350. Fuel starvation could be part of the reason you’re losing power above 4,000 rpm.
As for your timing, did you replace the stock harmonic damper with an aftermarket damper? If so, check to see if the timing mark is in the correct location for your timing tab.
Judging from your combination (64cc heads and flat top pistons), you aren’t running 10.25:1 compression. The compression is probably closer to 9.0 or 9.5:1, which is why the engine runs fine on 92 octane gas.
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Also, if you are having to turn the idle screws all the way in, but are still fouling plugs, it is likely that you have the primary throats opened too far to get it to idle, and you are actually running on the transition circuit rather than the idle circuit. (I had exactly this with my 355 / 650 Holley Vac Sec combo).
Close the primary throats right down, then start opening the secondary thraots (on mine was a small screw set into the base plate on the non-throttle side, a right bugger to get to. 1/4″ drive ratchet with small flat blade screwdriver attachment works best.
Set primaries a half turn of idle screw, set your idle mix to 1 1/4 turns out, open up the secondaries a turn or two, see how it idles. If it still bogs / dies, then add more secondary until you get a smooth idle. Then play with idle mixture.
First off you will require 17 degrees initial timing with 36-38 degrees all in by 2800. You need to set the timing with the vacuum advance disconnected. Use a good timing light, you can tune the distributor easily for this. I’d also suspect that the fuel filter could be installed before the mechanical pump which won’t work. The fuel filter needs to be after the pump, you would be surprised how often this mistake happens so thought I would mention it.
The older gm carbureted cars i owned with mechanical pumps had a inline filter before the pump near the gas tank and one in the carb.
???? Really, name some of those gm cars. Some had a sock type filter in gas tank on end of gas line but i never seen one near the gas tank but a lot of them had them in carb
I had a similar problem with a 400 SBC. Comp XR282HR cam. I changed the valve springs to the ones recommended by comp cams and now revs to near 6k
check vac advance and mechanical advance rates
make sure you unhook the vac advance when you time it…
find i way to measure the fuel pressure as you drive it… under load
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but you have a rich problem…. you need to get it to idle on the carb first…… then fix other issues
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its either advance curve…
fuel restriction
or the valves are floating