Hey, guys I need help. The only thing that is keeping me from finishing my car is a light intake backfire under take-off and a missfire under moderate to heavy loads. If I set the idle under 1100 rpm I have an occasional miss at idle. The distributer, cap, rotor, plugs, and wires are new. The timing is set at 15 degrees advance with the vacum advance plugged. I have been using 87 octane gas. I run a Holly 1850 600 carb with a Mallory 3 port regulator and the fuel pressure set to 4.5 psi. I don’t even know if it is an ignition problem or a carb problem. ANY help would be very greatly appreciated. Thanks!

 

change ur ignition control moduale. i had those same problems when my car would get warmed up too a hot condition and after about 300 bucks it comes down to a 20$ part and 10 mins swaping them out. and after that ive had no problems but ya change ur ignition control module becuz like u stated about having it backfire on and load or heavy load , i had the same problems. so thats ur first thing to check.

 

Thanks for the reply. The module is new- less than an hour run time. I know its possible to have a new bad part but I think the problem lies with the tuning. I think the timing may be a little to advanced for the 87 octane gas and stock cam. I'm going to retard it slightly and see if there is an improvement. Also I talked to a couple of friends one is a tech on Holleys help line and the other is just very knowlegdable about Holleys and Thirdgens. They both agree that my fuel pressure is too low and should be about 7 psi and to raise my float level slightly. They feel my miss is due to fuel starvation. I think it is possible my timing could be fine but too advanced for the amount of fuel. Any other ideas?

 

I'm not familar with that carb, but is it electric choke? If so, make sure the choke is set properly.

Also, check every plug wire, even though they're new, I've had so many burn by laying on a header.

 

A backfire through the Intake is Charactoristic of a Lean Condition.

 

Did you pull the window plugs in the Holley and check the float levels?

 

Thanks for all the reply's! I was told the Holley 1850 is basically a 4150. The number 1850 just means it was meant for GM applications. Yes the choke is properly adjusted and works well. The more I read and talk to people the more I feel it is a lean condition and and the problem lies in the fuel pressure and float levels. I have checked the float levels and they were both about 2-3 mm below the sight plugs. I have two different suggestions on the proper float level. One is it should be just below the site level and the other is it should just trickle out. How does everyone here set thiers up? Thanks again for the responses!

 

I found my problem! While checking for an arching spark spraying water on the plugs, wires, and cap I found that the #3 and #7 headers weren't getting very hot compared to the rest of them. I pulled the wire off the plug with an insulated pair of plug wire pliers and found the spark seemed weak. I pulled the wire off #1 and got the **** shocked outa me. Guess I need to replace my NEW plug wires.