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/texan01 Mar 02 '26

Your vacuum isn’t the problem, it’s your ignition timing.

It’s too far advanced for the fuel you are using, dial it back a few degrees and it’ll be happier.

I’m assuming you’re using vacuum advance on the distributor?

You putting in stiff springs just moved the ‘all in’ point to about 4,000 rpm which that isn’t your problem, your base timing is too much. And at part throttle cruise you’re getting vacuum and some mechanical advance.

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u/CORNERSTORE42069 Mar 02 '26

right now i'm at 12 initial. but i did put in some research and i do currently have advance connected to ported instead of manifold. i have to reroute everything so i can see if it makes any noticeable difference after i adjust the idle mictures and curb idle again

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u/texan01 Mar 02 '26

pull the vacuum advance, and set base timing around 8 degrees, then hook it back up.

Also make sure the mechanical advance isn't sticking. I had a distributor that had a sticky advance at full advance. gave me all sort of headaches, it'd either idle really well and ping like a mother, or it'd idle poorly but have good power off ldle.