C4 Tech/Performance L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

Does This Vacuum Seem Healthy

Old 06-09-2019, 01:00 PM
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Default Does This Vacuum Seem Healthy

Hello All,

I have been been having trouble tuning the small block and am afraid that I may have internal issues.
I set timing to 34 total at 3000 RPM and it ended up at about 15 initial.

I keep getting popping in the exhaust every so often at idle and it get worse if I hold the rpms at 2000 or so.
When trying to adjust idle mixture I don't get much reaction from the vac gauge I am using.
Tried the card test over the exhaust and I get an infrequent suction back in, maybe every 2nd or 3rd cycle of the engine.
I do not seem to be missing on any cylinders and all header pipes get hot at the same time.
I did a hot compression test on the 4 cylinders I can access with the headers on and 2,4,and 5 were all 140-150, #3 was 120... yikes...

Engine seems to rev fine at idle and will pull through the power band but seems dead below 3500 rpm or so.

I took some videos of idle and vacuum readings hooked up to the back to the crab, what do you guys think?
Internal engine issue or can I not tune?

Engine specs.

355 block with dished pistons
305 416 heads, Valve lash at 0+0 to try and find sticking valve
New HEI, NGK Plugs, Wires
New Holley 650 Vac secondary, idle screw 1 1/4 out
Cam spec 224/224 at .05", 114 lobe separation
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Old 06-09-2019, 06:24 PM
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Backfiring many times on SBC is valve adjustment. No mention of valve lift. New springs to match cam?There can be more than to setting up Holley than idle. The are susceptible to power valve failures due to intake back fire. Jetting / float level or both is probably of leading to lack of low speed performance. I think you need someone local to help you.
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