Back firing through carb at anything over idle

[Kaiserman]

My 230 (12v conversion) started back firing through the carb recently. If I slowly increase the throttle it won't, but if I just stab the gas it will back fire like crazy.

So far I've rebuilt the carb (actually a civi 1920), replaced the plugs (correct .030 gap) wires, points (correct .020 gap) and condensor. I also replaced the cap and rotor with a like new (maybe never used) cap and rotor that came with the truck.

Runs better and took care of my running rich problem, but it still back fires.

I'm down to valve timing or a valve. I've got the valve cover off now and am going to check the timing mark on the cam sprocket to see it that is off.

Anyway, just wondering if you guys have might have any thoughts.

On a plus note, I seem to have very good upper end oiling. Looks very nice and clean under the cover.

Also, how much (if any) slop do these timing chains normally have? I can wiggle mine around fairly well, but i figure being such a long chain it might have some play in it.

Thanks.

[brute4c]

Chain slack spec...from the -34 manual, Chapter 3, Section 2

Check the chain for excessive wear or stretch. When the chain is installed measure the distance between the chain sides at the narrowest point. This distance should not be less than 3.38-inch. If the distance is less than required, the chain has stretched or is worn excessively and the chain and sprockets must be replaced.

[Kaiserman]

Yeah, I've got tha in my manual as well. Only problem is you have to pull the timing chain cover to mesure that. If I'm going to oull the cover I'm just going to replace it. I was hoping someone might know how much play there is just by feel on a good chain.

[hiker]

A quick check of VALVE timing can be done by taking off the valve cover, setting the crank to top dead center (using the timing mark on dampener pulley). If all is correct, the #1 lobe on the camshaft should point straight down. If so, I would concentrate in other directions and not mess with the timing chain.

As far as backfiring out the carb, that indicates a lean mixture during acceleration. I'm guessing an accerator pump problem in the rebuilt carb. With engine OFF, look down the carburetor throat while quickly opening the throttle. You should see a strong squirt of gas into the venturi if all is well.

[Roy]

I have seen coil problems cause similar symptoms also....

[BlessedM715]

Mine had a similar problem and would only run with the choke on, when I touched the gas pedal it would bakefire and try to die. We found out that my carb had sucked up a bunch of crap from my fuel tank. After a carb rebuild and installing a NAPA spin on fuel filter Part#4101 at the tank all was well!
You should have seen the look on the face of one of my nehibors as I sat out in front of her house with a back firing Army Truck! I think she thought she was being invaded!!
LOL
Thanks!
Mike

[Kaiserman]

I had thought about the coil. I've got one on order, should be in tomorrow.

I'm not sure the carb is the problem. The old one was running rich and it was popping, andthe new one *I think* I have dialed in pretty correctly. i will play with it a bit more though. I know my wet fuel level is a hair low.

Pretty sure nothing was sucked up into the carb. I've got a cheap clear filter in front of the carb, and I installed a super clean (in and out) gas tank behind the seat with almost all new lines.

I'll check the valve timing. I'm also going to adjust my valves. They are a bit noisy and feel a bit loose.

I'll check ignition timing and run a compression test as well.

Who ever swapped in the 12v dist. installed it 180 degrees off, but reset it right on the dist. side (as in found TDC of No. 1, pulled the cap and that wire became No. 1). Anyone think that may have something to do with it?

Thanks guys.

[amphi]

Who ever swapped in the 12v dist. installed it 180 degrees off, but reset it right on the dist. side (as in found TDC of No. 1, pulled the cap and that wire became No. 1). Anyone think that may have something to do with it?

Not likely as long as the correct firing order is maintained.

I would try opening the point gap just a smidgen. This will retard the timing ever so slightly but worth a try.

There is a lot of theory on dwell and coil saturation. I wont go into details now.

Just give it a try. If it's worse – close the gap some and see what happens.

[Kaiserman]

I really should have measured the gap on my old point. I wonder where they where set.

This all started when I plowed with it for the first time. Never back fired before.

[randyscycle]

Have you tried swapping in the old condensor?

I've created more problems for myself by "just replacing" a condensor that was functioning fine. Cheap new knock-offs have been known to be bad from the start. They will cause some really nasty backfiring and driveability issues.