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Tech501
Tech501, Auto Service Technician
Category: Car
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Experience:  38 years exp, ASE Mastertech with L1 Advanced Engine performance
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Ford: back fires..trottle..changed carb, plugs,wires and coil

Customer Question

ford small block back fires when you crack the trottle fast.i've changed carb, plugs,wires and coil
Submitted: 11 years ago.
Category: Car
Expert:  Tech501 replied 11 years ago.
 Does this thing have any "cam" in it? What kind of carb is it?
Customer: replied 11 years ago.
Reply to Tech501's Post: i bought the block from PAW ,it not a stock but it idles smooth.its in a 67 mustang and it set up for about a year. i had a holley 600 and just put on a 750 ,both with vaccum sec.it run smooth till you get in it hard.as you probally can tell i'm not a mechcanic but i'put together about six small block motors for myself and friends.

thanks for help,
Ray    
Expert:  Tech501 replied 11 years ago.
 You may have a little too much carb for the cubic inch you have. It just may not have enough down draft. Remember, bigger is not always better. Matching the CFM to the engine is very important. What you can do to try to overcome it is first, try advancing the timing a little at a time to see how it runs. The second thing I used to do to overcome bogs like that is to increase the accelerator pump volume somehow. I have a micro drill kit and I drilled out the squirter nozzels a little. I think they also make a 50CC pump kit for that carb to increase accelerator pump volume. They make different plastic cams that drive the accelerator pump that have different degrees of sweep on them so changing to a different sweep would use the accelerator pump earlier which may help. You may also look at which manifold your using. Some have their power range at different RPMs and have to be matched with cam duration and carb CFM size. I don't know what your using for a distributor but some type of advance kit should be used. The last thing to look at is gear ratio. If you get too many of the acceleration handicaps going, the gear ratio will be a big factor in acceleration.
Customer: replied 11 years ago.
Reply to Tech501's Post: the motor ran great with the 600cfm carb till it set up for a year. thats when i started changing things.i put a rebuild ket in the carb and then tried the 750 carb a friend had.oh i also changed fuel pumps because it seemed to not have much fuel in the see through fuel filter.i guess i should look at the pressure with a guage.
Expert:  Tech501 replied 11 years ago.
 That's a possibility also. One of the rubber hoses could have deteriorated and cause it to suck air instead of fuel too. If your feeling any surging under acceleration, that would indicate fuel starvation.
Expert:  Greg replied 11 years ago.
Is it a "true" backfire (out the exhaust pipe), or a bog down sound from the carburetor?
Expert:  Tech501 replied 11 years ago.
 Backfires don't only come from the exhaust and in this case is more likely from the intake.

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