400 backfire problem
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400 backfire problem
After re-sealing my intake, changing the plugs,wires cap and rotar, the 400 is starting to run decent. It has good off idle responce and acceloration. The problem is that when you accelorate hard it will crackle and pop. Not real bad like it did before I sealed the intake but enough to really slow it down. The carb is a holley 2300 500cfm. The 2 jets are #73's. Do I need to jet up a little? Or does it need a new power valve? OR???? I dont think that it is the timing, I know that it will backfire with or without the vacume advance hooked up. What do you guys think? Where should I look first?
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I have an 84' 351 and am having the same problems. I also have a really strong smell of gas comming from the exhaust. Its also had to start, and gives off eye burning smoke for about 3-5 seconds after starting. I replaced the power valve and it made no real change. Any Ideas? I did remove the Cat converter and made real dual exhausts into flow masters. It has brand new plugs in her. HELP.... I scare children every time I shift or downshift from the backfires and Popping!! I have the carb lean dials turned all the way to lean trying to get her strong gas smell out of the exhaust.
Thanks for any help guys an gals!
Thanks for any help guys an gals!
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99% of the time if it backfires through the carb, it is timing related....either your cam timing is off or the distributor is off...thats about the only thing that will make one fire through the carb. the intake valve isnt closed when the fuel mixture fires in the combustion chamber....
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Nope...not in the carb, if it is I havent heard it. The exhaust is really where I hear it. WHat about the RICH smell if gas in the exhaust? I have the lean screws all the way leaned out....??? If you rap the exhaust at a light or sit still it has a tendency to try and stall. Its a little ball-less for a sec when you take off, then it runs pretty strong. I'm going to pull the carb and have it rebuilt. Someone told me I needed to have a H-pipe instaled to balence out the exhaust back pressure. Do you think this would help? I cant see how it would but if so please explain.
Thank you again...
Thank you again...
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personally, I dont see how a h pipe would cause that....I know my mustang has a h pipe to balance the back pressure to help the cylinders clear faster and it makes it sound better....I had a truck that backfired everytime I clutched to shift, it actually blew flames out of the pipes, the carb was messed up on it. it went out about 2 months later....
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From: 3rd Rock
Originally Posted by jpbronco
Nope...not in the carb, if it is I havent heard it. The exhaust is really where I hear it. WHat about the RICH smell if gas in the exhaust? I have the lean screws all the way leaned out....??? If you rap the exhaust at a light or sit still it has a tendency to try and stall. Its a little ball-less for a sec when you take off, then it runs pretty strong. I'm going to pull the carb and have it rebuilt. Someone told me I needed to have a H-pipe instaled to balence out the exhaust back pressure. Do you think this would help? I cant see how it would but if so please explain.
Thank you again...
Thank you again...
…..See you on the boards.
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Joined: Mar 2006
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This truck sat for several years before the lines and tank were changed out as with the power valve. Everytime you gave it gas there was a leak of fuel leaking out of valve. I replaved it however I didnt rebuild the carb. By the way... DOes anyone know much about this Holy 2 barrel carb. Its an 84' with some weird electronic assist built in it? I'm gonna rebuild the card unless someone else has some imput. Thanks for all your help guys!
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Drove the truck to work today, it ran ok. When I got home I backed my timing off to 5* ATC, and I changed my #73 jets to #78's. Made a huge difference. The backfiring under a hard acceloration was greatly reduced, and the engine really smoothed out. However it still lightly stumbles and looses power when you give it full throttle. I might put in a set of #80's and see if that helps. Also I am going to replace my power valve with one that fits my vacume range. I will let you guys know what happens.
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