71 390 backfiring through carb
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71 390 backfiring through carb
Hey guys, wondering if some genius has an answer for me, my 71 390 has just had a new edelbrock 600 cfm carb and manifold fitted, new headers, otherwise standard, timing is at 12 degrees.
Under hard acceleration or higher rpm it sometimes spits out of the carb and loses power, any ideas?, rear 7 cylinder has lower compresion than the rest at about 95 psi, the rest are between 115 and 120.
would that be the cause though? Thanks.
Under hard acceleration or higher rpm it sometimes spits out of the carb and loses power, any ideas?, rear 7 cylinder has lower compresion than the rest at about 95 psi, the rest are between 115 and 120.
would that be the cause though? Thanks.
Logistics Pro
Joined: Aug 2015
Posts: 4,093
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From: Lind, WA- Eastern WA
Welcome to the Forum! I wouldn't think that the compression in # 7 cyl would cause it to backfire. Most of the time it's caused by 1. the accel. pump on the carb not working, or 2. the carb is not jetted correctly. Or 3. it may be a timing issue. Which may be caused by a dist problem. The dist may not be getting the advance it needs, or if you have points, it could the an adjustment. Or the dist plate and/or shaft bearings worn. CHECK THE EASY THINGS FIRST! I would check that the accel pump is working ok first. With the engine cold and OFF, remove the air filter and the metal housing( breather). While looking into the carb, pull the throttle back quickly. You should see two streams of gas shoot into the base of the carb. IF that is OK I would check the numbers stamped onto the carb jets. One of the guys that knows more about which jets size you need should chime in after you post the jet number. FYI- You said that it has headers- many times you have to go 2 jet numbers higher when headers are added. How high above sea level are you driving?
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Welcome to the Forum! I wouldn't think that the compression in # 7 cyl would cause it to backfire. Most of the time it's caused by 1. the accel. pump on the carb not working, or 2. the carb is not jetted correctly. Or 3. it may be a timing issue. Which may be caused by a dist problem. The dist may not be getting the advance it needs, or if you have points, it could the an adjustment. Or the dist plate and/or shaft bearings worn. CHECK THE EASY THINGS FIRST! I would check that the accel pump is working ok first. With the engine cold and OFF, remove the air filter and the metal housing( breather). While looking into the carb, pull the throttle back quickly. You should see two streams of gas shoot into the base of the carb. IF that is OK I would check the numbers stamped onto the carb jets. One of the guys that knows more about which jets size you need should chime in after you post the jet number. FYI- You said that it has headers- many times you have to go 2 jet numbers higher when headers are added. How high above sea level are you driving?
Tuned
Joined: Dec 2004
Posts: 340
Likes: 8
From: Rock Hill, SC
Try testing #7 again with a squirt of oil in it and see if it comes up or stays at 95psi. I assume when you changed intakes you had to pull the push rods? If you did maybe the intake on #7 is to tight. Just a thought because I am not an FE guy.
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Yeah seems to be good, we set it at 10, now its better, but cant rev much past 4k it just stops, is that normal?
Lead Driver
Joined: Jan 2003
Posts: 9,893
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From: Ohio
No, it should rev freely.
Sure sounds like it is lacking fuel. I would pull the lid off the carb and make sure there is no gunk in it. Not the first time a "new" carb was screwed up.
Sure sounds like it is lacking fuel. I would pull the lid off the carb and make sure there is no gunk in it. Not the first time a "new" carb was screwed up.
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Ok what should I remove? doesnt seem like there is anything in the main butterfly
Logistics Pro
Joined: Aug 2015
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From: Lind, WA- Eastern WA
I think that "Freightrain" was talking about removing the top so that you can see into the fuel bowl (where the float is). Double check to see if the float is set at the correct height. On the two barrel carb, the float should be about the thickness of a quarter from the top. If it's set too low, the engine will starve for fuel. Of course if it's set too high, you'll have gas flowing everywhere. Do the checking and setting the float on a cool engine! Have a fire extinsiher nearby when trying to start the engine- JUST IN CASE.
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