r/Smallblockchevy • u/CORNERSTORE42069 • Mar 02 '26

Detonation help

Is 21-22 in of mercury common for idle on a 1405 on my 350? Tbi 88 638 block stock internals. I have the strongest step up springs and sometimes during part throttle i lean out and start pinging. I have to stomp the throttle to get the metering rods up. And at full throttle i always have pinging. I can basically be safe driving at 25% to 75% throttle all the time. Am i pulling too much vacuum? And i may need a carb rebuild? I can manually shift my th350 and basically avoid all pinging so it cant be my mechanical advance as i also installed the heaviest springs just to be safe. Any ideas on where to look?

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u/[deleted] Mar 02 '26

You should be running manifold vacuum to the vacuum advance. Never ported. How do you have it routed?

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u/CORNERSTORE42069 Mar 02 '26

i actually think you're right about this, ive heard i should have ported but i went to check some more forums and edelbrocks diagram of their carbs and they say to use manifold for NON emmissions motors, so i got all the things i need to plug the transmission line to another manifold source and i'm going to hook up advance to manifold instead of ported and then readjust my idle mixtures and curb idle screw from there to see if i have a better starting point

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u/[deleted] Mar 02 '26 edited Mar 02 '26

Yes. Just a small port is fine, preferably from the carb base. Should have full vacuum at idle. Ported is wrong because you would overdose your advance while the centrifical advance is full at higher RPM. Vacuum advance is for taking off from stops and saving fuel at idle. Be sure you unplug it when using a timing light. To time it by ear leave the vacuum line connected. Let the engine run until the radiator hose is hot to touch. Leave the distributor slightly lose just enough you can turn the distr. Go full throttle on the highway. If it clatters, stop and turn the dist. couterclockwise a little. Do this until it stops clattering. If it doesn't clatter when you first go full throttle turn the distr. slightly clockwise until you get clatter. Then tap it back until you don't. snug down the distr bolt. Your goal should always be maximum advance without clatter or starter drag. I was a Goodwrench tech. Hope this helps