My newly built 440 has a carb backfire problem. Seems to only happen before reaching operating temp. Does not happen unless feathering the carb a bit too much untill it will idle on its own. Any suggestions?

 

It could be too lean ? Do you have a working choke ? or Maybe run it at night See if a plug wire is arcing when it is cold causing a misfire. Check the plug wire routing.

 

thanks for the tip. I will look at it. No choke. Motor is fairly well built. Here is what i got to work with.

440 motor 15 over
Indy dual plane intake match ported to heads(max wedge config)
Indy Heads with cnc max wedge intake ports
1.5 rollers with behive springs
10.5 to 1 comp
msd 6AL ignition and wires with blaster 2 coil
comp hyd roller cam int 244/.600 exh 252/.600 lobe sep 108 inst strt up
12degrees advance at 1000 rpm idle with a total of 34degrees total timing
carb is a holley 950 HP built by diamond back racing in Texas(no choke)
i THINK that the primarys are #72s but dont recall, or think that makes a difference with the idle circuits. let me know if i forgot something.

motor runs PERFECT when warmed up, and was tuned hot on a vacum gauge. when cold i have to feather the throttle to keep it alive till its warm, and if it stumbles too far and i add too much fuel..POP backfire through the carb. I have recently thought that because of the cam overlap that the backfire is due to the low rpm while tring to keep it alive and back over 975+ at idle. Stumped at this point. Any more suggestions??

 

Also, motor is NEW. Very little time and doubtfully broke in. Only 2.5 street miles driven on it.

 

Mopar69, welcome to the forum. Without a choke on that carb,it will do that, learn to live with it or get a carb with a choke. Do you have a vacuum advance on the distributor? Where are you located?, temp will have a lot to do with it.

john

 

Thanks for the welcome. All mechanical timing. Can change springs for timing curve, but thats it. In so-cal. Allways like 70 out here

 

Vacuum advance is needed on a street car !!!! Helps with your gas mileage. Did you at least run it to break in the cam ?

 

Like TVLynn said, Get a distributor with vacuum advance, or with the MSD box, you can get a adjustable timing control MSD#8680 to advance the timing at idle while still cold.