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post #1 of (permalink) Old 07-03-2009, 10:31 AM Thread Starter
Tim-E's Avatar
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misfire diagnosis w/ vacuum gauge


i was reading the junkyardgenius page on using a vacuum gauge to tune my carb, and some questions came up.

I've been using a vacuum gauge to try and get my mc2100, HEI 258 to run a bit smoother for me. She's got a random misfire that's been plaguing me for a couple of years.

Just at idle for example: it'll idle smooth, but every 5-10 seconds it'll have slight miss that I can hear in the exhuast....crawling in first gear gives me a rough, stumbling, surging....the same thing at most rpms, though its less pronounced at higher RPMS...

fuel pump is giving me 3 psi.
plugs are only a month old
HEI dist. is new
Idle is about 800rpm or so...
Timing set 8-9*

So heres my vacuum gauge questions:

1. when my gauge is not connected, the needle rests at at 7in HG, not 0. is that normal?

2. when i've got the gauge hooked up to the port on the rear of the intake manifold, i get about 24in HG at idle, but it fluctuates(vibrates) between 22.5" Hg and 24" Hg
Isn't this vacuum too high? what to do next? Retard the timing?

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