Chevy 350 off-idle hesitation - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-02-2007, 01:59 PM
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Chevy 350 off-idle hesitation

I'm sure this hasn't been asked before...

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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Old 04-02-2007, 03:04 PM
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First off, a 2 bbl carb is now probably too small for the engine and cam selection. The 2bbl, as you found out, was set up to be "emissions friendly". Actually, having the holes in the throttle plates IS a good idea when you have low vacuum from too much cam. All in all, the carb will probably never work well on your engine. The enrichment setup is probably way too lean and when it "comes in" will be way off too, all due to the cam.
You might try removing the copper wires and check the accelerator pump operation too. The bog during the shifts is most likely the enrichment circuit being way off and the engine goes way lean during a vacuum change. (the enrichment valve opens and closes at a pre-set amount of vacuum signal.)

That carb was designed and set up to work on a specific emissions curve. That's all gone now. My suggestion is to install an Edelbrock 2101 intake (checker...100 bucks) and try to find a decent Edelbrock 600cfm 4bbl carb. (Even a good working Quadrajet would be good too.)
It's a very good "out of the box" setup for a mild engine/cam combo. The carb new is a bit expensive but in my opinion, a good choice for a pretty much hassle free carb that runs good year after year.

As far as the timing goes, I have a Crane 276 duration cam (middle of the road) and run my base timing at 14. Personally, I think 6 is way too low even for a stock 350. Try 10 and work your way up and listen for any signs of pinging. It may help some but the carb setup still needs to be addressed.

Mark

Last edited by Jmark; 04-02-2007 at 03:10 PM.
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Old 04-02-2007, 03:19 PM
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Got any thoughts on ways to compensate for the cam with the existing carb? I'm actually trying to sell the truck and don't want to throw money at the problem if I don't have to.
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Old 04-02-2007, 03:27 PM
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You could try removing the copper wire plugs your dad put in, up the timing to around 12 and see if that helps any. Worth a shot I guess. You may need to richen up the idle mix (2 screws at the base/front of carb) some too.
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Old 04-03-2007, 11:07 AM
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I went for the simple, quick fix and cranked the distributor around to 12-degrees of advance. That helped a LOT. 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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