off idle miss

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Ironmike

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Well I've exhausted everything I can figure, so I'm throwin' it to you guys.

2 weeks ago I developed this annoying, just off idle miss. I believe it's a miss right between the idle circuit and the main. Doesn't matter what speed or what RPM, its a throttle position thing.....I think. Very irritating cause criusing at pretty much any speed, I'm always "in the miss". Give it a little pedal, it stops, let off it stops.

Electrical is ruled out. All good there. The only unknown is my MSD Digital 6, but I think they either work or they don't. I'm 99.999 percent sure it's a carb issue.

Took carb off, went through everything. Air bleeds, jets, powervalves, etc. Spoke with Holley, they suggested closing secondary blades so I could open up the front to get closer to the slots. Did it. Didn't help.

My last thought is maybe it wants more main jet, but why would that change all of a sudden?

I'm running a HP950 with a pretty stout solid roller, Super Victor intake. Now have 78's front and rear with 4.5 powervalves.

I'm open to any suggestions!
 
Something is clogged up in a passage,disassemble and blow out in reverse flow all passages.try another carb?
 
Have you checked the vacuum advance for a leak ? Off idle the ported vacuum advances the timing. If the dashpot leaks, the timing will be retarded, along with vacuum leak both together a miss or hesitation. Letting off shuts port, getting on it drops vacuum so timing would be retarded normally and engine fine, except for very slight lean condition.
 
Just as a curiosity and a possible quick answer to what it might be, if you have a choke (electric, manual or whatever) try blocking it half way closed and see if that miss is gone. (just drive it down the street and back)

If the miss is gone the carb is going lean right there in the throttle.
If it happens to get worse it's going rich right there. (this is pretty rare)
If the miss doesn't change then it may very well not be the carb at all.

This will tell you real quick what is going on, and then you can figure out why.
 
I've got a similar miss on my car sometimes. Just found a leak in a vacuum hose, but haven't driven since fixing it, but that might clear it up.
 
Well, no vacuum advance...no mechanical for that matter. Distrib locked at 33 degrees. No vacuum lines to leak either. HP series carbs have no choke.

I'm really wondering about a plug wire. Can one be bad SOMETIMES? Test and tune tonite and didn't lose anything at the strip. But the rotten miss is still there.....just off idle. It's actually miserable to just drive, cause it's always in that throttle position cruising.

I'm gonna put a spare set of wires on and see what happens. I have a feeling though, that ain't it. Just "feels" like a carb thing. Other than jetting, I'm no carb expert, though.

Thanks for the ideas, guys! PLease keep em coming. Someone HAS to have had this problem before me.
 
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