Engine Hickup, backfire?

[Heimeken]

The engine on the J10 ('77 360) has been hickuping on takeoff, or hard acceleration. It's like it backfires through the carb. Started happening when the weather got cold and progressively getting worse. I checked/adjusted the timing yesterday and tuned the carb (stock 4 barrel motorcraft). No difference. Today was a hot day, no difference. I need direction before I go and take the carb apart. Any ideas?

Thanks in advance.

[carrotman]

If you still have the Prestolite distributor, I would change to an HEI or Duraspark before messing with the carb. My Prestolite would do the same thing and also completely die. People may argue about it, but changing the distributor cured the vapor lock, hard starting, backfires, dieing, and stalling. I too had blamed the carb, and also the fuel pump, fuel filter, gas tank and gas lines, until I got the HEI.

[Treeshark]

Vacuum advance not working. Bad powervalve on carb. Clogged fuel filter. Just some ideas.

[Its all about the Jeep]

It's moody. Mine does that when my wife washes it. Garbage Truck afraid of soap.

[Heimeken]

Yes I do have the prestolite and I was thinking of an upgrade, though I'm not sure what is a good upgrade for it. Is it less complicated to get an MSD setup than it is to upgrade to the Motorcraft?

I'll check the easy stuff, like the fuel filter and advance first.

[FSJ Thing]

I could be completely off base because this doesn't jive with what the others are saying, but if you're back firing through the carb, in my experience, 9 times out of 10 it's because an intake valve is sticking open for some reason. If it's been fairly cold where you are, it could be that oil is taking too long to get to the valve guides on start up and it's causing one to wear too quickly, or maybe the crappy gas that they like to sell in the winter is fouling a cylinder and carbon is keeping a valve from closing right...I don't know, just some thoughts...Hope you get it figured out!

[carrotman]

J-truck, you are right. The intake valve has to be open while a spark is on for a backfire up the carb. Two ways it can happen. Valve stuck open during normal ignition or ignition at wrong time (with intake valve open). Hot carbon, hot edge, or crossed wires can spark the mixture at the wrong time, or the (Prestolite) ignition can fire at the wrong time. In three years, I haven't had any backfire since changing to an HEI.

[Heimeken]

Great info J-truct and carrotman, though not sure how to attack this problem. we do have the oxigenated gas until next month. Think there is something I can pour in there to test? I've been thinking about an ignition upgrade anyway, just not sure what is good.

[fulsizjeep]

I found my #1 exhaust lifter was collasped causing this. I started by pulling plug wires on each cylinder until I found it was #1. Pulled valve cover and found that rocker was not moving. It was also clicking in the top end, which gave me a hint that it was lifter problem...

[Heimeken]

Great info guys, now that I think about there is a slight clicking coming from the right side of the engine, sounds a bit more sharp than an exhaust leak. Guess I have another project on the list smile.gif