backfiring through carb
Thread Starter
On The Tree
Joined: Dec 2006
Posts: 108
Likes: 0
From: dayton,oh
backfiring through carb have a 73 nova with sbc 350 have a msd hei distrubtor wires and 750 carb car has a cam specs unknown. it will idle fine and rev fine in park but once you drive it at 2500 rpm it backfires through carb. timing is set at 14 deg BTDC with vacum pluged and 36 deg when reved to 3500 with vacum advanced hooked up its at 47 deg. total.it idles in park at 1000 and in gear at 650.have check timing gears and there dot to dot with #6 at tdc, also checked lash on valves and it seemed good,the double valve springs also looked good. i also installed a vacum canister for the brakes and a adjustable transmission vacum modulator. the car did not run when bought. any help is appreciated.
Staging Lane
Joined: May 2008
Posts: 75
Likes: 0
From: Florida
Sounds like you are running lean with the fuel mixture. If it is a holley carb try backing out the idel fuel screws 1/4 at a time or better yet get a vacuum gauge and set them so that you have the highest vacuum reading on the gauge,you will want to keep both screws close to the same revolutions out from seated. I also try and set them in gear you will notice less of a drop in RPM when the car is put into gear. If you still have a problem look for a larger accelerator pump nozzle.
verify your timing! take #1 plug out and bump the motor with coil unplugged till the piston blows your finger out the hole.just put your thumb against the hole, then look and see where the dist rotor is pointing.if its' close bump the motor where you want initial timing to be[14*] .turn the ignition on , take the coil wire off and hold it about 1/4" off the block, turn the dist until it the coil fires. tighten the dist down your initial timing is now set.
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TECH Enthusiast
Joined: Dec 2005
Posts: 576
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check the timing if the problem still there you have an intake valve leaking I had the same problem with my old set up I opened the heads and I found out that it was leaking water and the valve was hitting the piston I tested the heads and they were in bad shape so first check your timing and make sure the mark on the timing chain is set properly
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Staging Lane
Joined: May 2008
Posts: 75
Likes: 0
From: Florida
The pop or back fire when trying to accel is from the carb beeing to lean on the accel pump shot. And for the original poster have you tested to see how much fuel pressure you have? Also is the ign cap in good cond along with the coil.
TECH Enthusiast
Joined: Dec 2005
Posts: 576
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ok I had that problem before but it was tuned by a racer he did the same thing I saw in Holley's videos in their CD how to tune carbs and the pressure was 4-5 PSI and the distibutor had new coil and cab and rotor but the weights and everything inside it wasin very bad shape so I guess it was from the distributor
Thread Starter
On The Tree
Joined: Dec 2006
Posts: 108
Likes: 0
From: dayton,oh
havent tested fuel pressure or changed anything yet. my 95z decided to commit suicide last week.should have it put back together this weekend. but keep the ideas coming. you can adjust the accel pump cant you.
