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Chevy 350 off-idle hesitation

10K views 2 replies 1 participant last post by  Yardstick 
#1 ·
I'm sure this hasn't been asked before... :rolleyes:

The 350 in my 1975 Chevy truck has the stock intake and exhaust manifolds on it, and it has the original 2bbl Rochester carb. Originally it had a hole in each throttle plate that caused a lean condition at idle (for emissions compliance, apparently) making the truck run very rough. My dad fixed this back in the 70's by putting a piece of copper wire through each hole to block them off. Ever since it has had a bad off-idle stumble/hesitation/sag.

Fairly recently I had an Edelbrock EFI setup on the truck and it ran beautifully. I took it off to sell seperately (or keep for a potential future project) and now that I've gone back to the carb, the hesitation is back. It happens the worst from a dead stop when trying to take off at a normal (even slow) pace. If I stab the throttle and back off it picks up and goes. To me, that suggests a fueling issue. The problem is, I also notice a pretty bad lug as it shifts from 1-2 and then again from 2-3. That makes me think it might have something to do with the initial timing.

The engine does have a recent cam in from when I had the EFI on it. It is a Summit cam (Edelbrock Performer copy). I plan to check vacuum tonight. It idles great, so I don't really suspect a vaccum leak. Should I be running more base timing than the factory 6-degrees? What else should I look for?
 
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#3 ·
I went for the simple, quick fix and cranked the distributor around to 12-degrees of advance. That helped a LOT. :thumbup: Vacuum at idle went from around 14 in-Hg to 17 in-Hg! It could probably still use more fuel in the off-idle circuit because it still has a bit of a hesitation.
 
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