Need help with off idle stumble
Thread Starter
Teching In
Joined: Mar 2005
Posts: 41
Likes: 0
From: Roseville, Michigan
Need help with off idle stumble Ok this is really driving me nuts. If I get in the gas even to part throttle it will fall on its face and sometimes backfire. Its so bad i cant even get the tires to chirp from a dead stop. When this happens it sounds like its only running on a few cylinders. It will rev real slow untill 2500ish RPM's and then take off real hard like it should and rev real fast. This is only right off idleand really noticable form a stop. Although it deos hesitate if your cruising and give it gas at those RPM's.
I think it might be a problem with the carb, 670Holley Street Avenger, but i cant seem to find out what it be. Im using one the heaviest springs for the vacum secondaries since its a heavy truck. Everything else is the way it came out off the box except float and idle adujustments.
its a 350 with 9:1 cr
vortec heads
224/230 .477/.480 xe268 cam
RPM airgap manifold
headers
its in a 88 2wd 1/2 ton pickup
Base timing is set at 16* not sure on the total timing.
Thanks for any help
I think it might be a problem with the carb, 670Holley Street Avenger, but i cant seem to find out what it be. Im using one the heaviest springs for the vacum secondaries since its a heavy truck. Everything else is the way it came out off the box except float and idle adujustments.
its a 350 with 9:1 cr
vortec heads
224/230 .477/.480 xe268 cam
RPM airgap manifold
headers
its in a 88 2wd 1/2 ton pickup
Base timing is set at 16* not sure on the total timing.
Thanks for any help
Teching In
Joined: Mar 2005
Posts: 22
Likes: 0
From: Nash TX
Does this happen even if your rpm is above idle and you step on it? It sounds like your getting way too much fuel from your accelertator pump or your primary jets are way too rich. But if it only does it when you first get on it and it backfires from the exhaust I would bet you need to do a little adjusting on the accel pump or go with a less aggressive pump cam on the carb. If it is backfiring through the carb then you are not getting enough fuel from your accel pump and need to do some adjusting as well.
Thread Starter
Teching In
Joined: Mar 2005
Posts: 41
Likes: 0
From: Roseville, Michigan
It will happen when ever the rpm's are below 2500 or so. No matter how you drive it or how fast you are going it still deos it. It will back fire through the carb sometimes especially if you get in it real hard and stay in it.
now that its metioned ill check the accelator pump cam clearance maybe its to loose, is there a spec for that?
now that its metioned ill check the accelator pump cam clearance maybe its to loose, is there a spec for that?
i have had a similar problem with a similar engine combo in an s-10 once. i went up a couple sizes on the main jets and it cleared it right up. i think from 69 to 72 size jets but mine was on a 750 vac. sec carb though
Thread Starter
Teching In
Joined: Mar 2005
Posts: 41
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From: Roseville, Michigan
Originally Posted by terr1bleone
that sounds more like a timing mistake, but when you punch it from idle does it just bogg and produce a suction sound?
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Joined: Mar 2005
Posts: 22
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From: Nash TX
Normally a bog and backfire through the carb means a lean condition which would indicate either the accel pump is too small or needs adjustment or a accel pump cam change plus it also sounds like the primary jets may be alittle too small but start with the accelerator pump first. That is normally what makes the off idle stumble it is what takes up the slack between the idle circuit and part throttle.
Check your timing out (base and total). What type of distributor are you running. If it is a MSD make sure that you have the 2 lead wire (the one from the msd box to the dist.) plugged in correctly. It WILL plug in both ways. Also if you have had a large backfire I would replace the power valve. Holley.com has a good guide for tuning the carb including power valve, jetting and acc pumps. Good luck.
Originally Posted by terr1bleone
get a smaller carb like a 600 cfm may loose a tiny bit of power but will get rid of any hesitation
Or he could get a road demon from barrygrant and have his drivabilty, hp and fuel economy!!!
Staging Lane
Joined: Apr 2005
Posts: 93
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From: Coastal NC
I have to agree with those that say its a lean condition. I have always fornd the cure for a bog off idle was getting more gas into it. I would start looking at the accel pump, the float level and go through the entire carberator rebuilding it if I was even suspecious of any part in it.
Thread Starter
Teching In
Joined: Mar 2005
Posts: 41
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From: Roseville, Michigan
I bought a barely used 650 Speed Demon and put it on it runs 10 times better without doing anything to the carb at all than the street Avenger that we had been foooling with for a while. All the drivability problems are gone and it just roasts the tires like it should.
