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Backfiring thru carbs while cranking

  
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Backfiring thru carbs while cranking

 
Tooneyman Tooneyman
New User | Posts: 8 | Joined: 01/13
Posted: 06/17/13
06:40 AM

Have a hi-rise tunnel ram with 2” tall spacers and two 600 Holley 4-bbl carbs on a 350 Chevy crate motor. Just installed everything a couple of days ago and started the motor yesterday. During cranking, sometimes get a very loud pop (backfire) through the carbs (enough of a “pop” to blow off a plastic vacuum cap on a carb). Have a HPV-1 electronic ignition on the motor. No distributor, just four separate coils with two spark plug leads each). The ignition has four separate dial settings where you can manually set the spark advance and other things. Despite the occasional backfiring out of the carbs during cranking, the engine will start and does run real good. Have not taken it out for a test drive yet, but will soon. (note: the motor is in a modified Harley motorcycle frame). My question is the backfiring a timing/ignition issue or a carb issue? I have played with the INT advance setting for the ignition using several different degrees settings (from 4 to 16 degrees), but that didn’t seem to make any difference with the backfiring. I know that having two 600 Holleys on this motor is excessive for the motor, but when it is running it seems to be very good then. Just trying to figure out why it’s popping through the carbs during cranking. Any help would be appreciated. Thanks. ...... (see my profile avatar for a pic of the motor in the motorcycle frame)  

redneckjoe69 redneckjoe69
Addict | Posts: 2337 | Joined: 03/10
Posted: 06/17/13
06:48 AM

perhaps wiring your starter and ignition seperatly would help?   get the motor spinning first with the starter, then turn the ignition on and light the fire.  

68scott385 68scott385
Guru | Posts: 1990 | Joined: 10/09
Posted: 06/17/13
10:54 AM

Have you checked the valves? Sounds like pressure coming back up the intake tract, could be a valve ajusted slightly too tight or hanging open.  
68scott385 68scott385 68scott385

skyeking skyeking
Addict | Posts: 2738 | Joined: 08/09
Posted: 06/20/13
05:10 PM

Hi there Gang,
Simply take a compression test to check for sticking lifters etc
as suggested bu the young Brigade. All the best to all of the people.  
skyeking

Tooneyman Tooneyman
New User | Posts: 8 | Joined: 01/13
Posted: 06/24/13
08:09 AM

Set the "INT" timing to 26 degrees, reset the carb idle mixtures screws to about 1 turn out max and fixed a small vacuum leak at the bottom of one of the carbs. No backfire anymore. Will test ride the bike this week to see if all is ok. Thanks for the responses guys. Appreciate it. (See my avatar profile pic for a pic of the motor in the motorcycle)It's a beast!