Stalling/Backfiring

[Mgicm]

Hope this isnt a problem to post here but have a 73 GMC stepvan with 350 sbc and auto trans. Keep having an issue with occasional backfiring and sometimes stalling out. Replaced fuel pump, changed to electronic ignition, new carb, plugs, wires, and starter while we were in there.

Truck runs well most times. We noticed that when it is hot out (90+ degrees) this seem to happen most often and usually in traffic. I am assuming that the massive amount of heat in this small engine bay is part of the issue. Has a fan on the radiator...not sure if a clutch fan .

My question is..what would cause a backfire through the carb out of nowhere after an hour of driving? Could the gas be boiling while we are in traffic and have something to cause the stalling? How could we move more air in engine area while sitting in traffic to help cool it down? That is assuming it is a heat related issue.

It is an older truck of course so the mechanic is starting to blame other things like engine components but i dont see how thats possible when the truck runs fantastic most times..very smooth, nice response. it is just at times it decides to stall out or even hesitate like no gas is getting there.

yes..we changed fuel filter 2x.


thanks! GP

[geezer#99]

Assuming you have a quadrajet, then you need a heat spacer like this to help keep your carb cool.
http://www.tdotperformance.ca/mr-gas...FUqSfgodu6gIPQ

Also, what's your initial timing?
And with the distributor change did you use a dedicated 12 v power wire to replace the factory resistance wire?

[Mgicm]

thanks for the reply.. its a holley 650 i believe. Would there be something needed for this..and what exactly does this do? Sorry for my ignorance but trying to understand...

I dont think mechanic redid any wiring that i know of. What would this do if not changed? Not sure on the timing..will ask.

[geezer#99]

If it's a holley you need a heat shield instead. Like this.
https://www.holley.com/products/fuel...s/parts/108-70

Either the shield or the spacer's job is to absorb and deflect heat away from the float bowls in the carb. Keeps the fuel from heating up and starting to boil.
The holley bowls hang out over the intake so needs a shield.

Without a dedicated 12v to the distributor the coil heats up and doesn't work to full capacity. You get a reduced spark voltage. Results in rough running and misfires.

What do you have for an air cleaner set up?
Could you post some pics of your engine?

[Mgicm]

Have the normal black snout air cleaner pointed sort of to front of truck to get some air...

Let me get some photos and post here this am.

How hard is that install of the carb spacer..just unbolt and rebolt or is there now a need to redo fuel line as it sits higher?

I will look at distributor and grab some photos... sure appreciate.

[geezer#99]

Heat plate is just unbolt/rebolt job. You gotta watch for any linkage hanging up on it though. Especially on the choke side. Depending on fuel line, you might not have a problem. Heat plate only raises carb 1/4 inch.

[Mgicm]

Ok.. kit reviews say some trimming may need to be done so i will be aware. You think this little bit would really help? I guess they wouldnt make it if it didnt help.

Getting some photos ..to show what I got going on here. if my phone would work right lol.