Backfiring thru Carb - long saga

[NO6YHY]

Had to leave the jeep sitting for two years while I was out of country. It ran well when I left it in storage, and I have spent a year now since being back trying to get it running again!

Latest problem is persistent popping through the carb. This happened last summer, then I somehow fixed it, and now it's back. When it happened before I changed out the plugs, wires, cap, and rotor. Also siphoned out and replaced the gas, which had gone sour, and new fuel filters and fuel pump. After that it ran but not well - wouldn't go over 40 and kept wanting to bog down and die (maybe varnish left in the fuel tank was poisoning the new gas?).

Rebuilt the carb and found one of the bowls dry and some gummy residue from the gas. Put it back on and car ran better - up to 55 mph! - but still bogged down.

Bogging down is worse now and backfiring through carb has returned. One theory I had is that when the car bogs down, the unburnt fuel leaves carbon buildup everywhere and this is causing the spark plugs to act funny and detonate at the wrong time, causing backfire. I took the plugs out and sure enough they are all black. Sprayed them with carb cleaner but it couldn't get all the black off.

Tried running seafoam to clean everything out and the engine actually ran GREAT - WHILE THE SEAFOAM WAS POURING OUT. Then when the seafoam was gone, the backfiring came back!

At a loss right now...

[itselliot]

Backfire

If you search here, you'll find lots of info about backfire related power valve failure.............And lots of differing opinions.

Personally, if a backfire "pops" thru the carb and there is a following loss of power , I am of the opinion that the power valve has been damage.

Couple this with a fuel bowl that is empty after sitting idle for a few days.....and I am convinced that a power valve has failed allowing fuel to drain out of the bowl during the down time.

As to the cause of the backfire..........not sure...........timing? Sticky valve? Distributor advance weights not moving smoothly?

[Dog_Wagon]

I'm far from an expert but, I've always been under the impression that backfiring through carb was valve timing related....

[Carnuck]

Usually on a vehicle that sat a couple years a valve or two sticks partially open (sometimes from rodent seeds and sometimes broken vacuum hose chunks are sucked in) Do a compression test and see what is happening. Compression lower than others is often stuck intake. Compression higher than others can be bent exhaust pushrods.

[NO6YHY]

Thanks for the replies. Compression test is a good idea, I hadn't thought of that.

Still confused as to why the backfiring would have stopped for a long time after I replaced spark plugs, fuel filters, etc. And why the backfiring wouldn't have happened when the Seafoam was in the engine.

[Rich88]

Backfiring thru the carb can only be intake valves (burnt, not seating, cracked) or uber-hot residue causing a glow-plug effect.

If its valve related it will be consistent, and only get worse.

The fact that it went away with sea-foam and again with new plugs points to having a hot-spot problem. I expect a compression test will confirm valves are OK. Can't hurt.

Otherwise I agree with possible power valve rupture causing rich-running.

By the way, when in engine is off, many valves are left in the open position, and now exposed to humidity. I resurrected an engine that sat for 8 years and was smart enough to pull the heads first. Good thing I checked because all the open valves had rusty surfaces and so were the seats. So I had the shop clean it up, and was good to go.

[bwwhaler]

I had some of the same issues, backfire through carb when cold, but I also had a lot of valve pinging at speed. I ended up changing the power valve with a 6.5 (it had a 9) and adjusted my timing, and tweaked my carb, air-fuel screws and got it right now. I would check timing first, but all other suggestions are great also.

[Ristow]

popping thru the carb is classic lean symptoms. weak ignition can do it too. the bog and popping suggests lean. the black plugs suggest weak ignition. timing is verified?

[NO6YHY]

My carb is a Carter AFB which doesn't have a power valve, so at least that variable is eliminated.

I had thought of the hot spot problem, but it seems like running seafoam through would have cleaned it out?

Timing has been in the same spot both when it wasn't backfiring and when it was. I might try wiggling it around a little if nothing else works though.

I think my first step when I get back to my car (unfortunately it is 4 hours away) will be to replace the spark plugs. It helped when I replaced them before, and they were black then just like they are now. The underlying problem of course is the bad gas coming out of my tank, which makes the engine run erratically and thus turns everything black inside.

[NO6YHY]

Okay, FINALLY got a chance to work on my car again. Did a compression test today (when I did the test I didn't know anything about holding the throttle open, taking all spark plugs out, etc., and I couldn't warm up the engine because of the backfiring, so numbers may be low) and here are the results:

1 - 115
2 - 115
3 - 122
4 - 116
5 - 108
6 - 82
7 - 109
8 - 100

So it looks like #6 seems to be the problem, hmmm? So I pull the wire from #6 and run the engine again and ta da - no more backfiring. Engines shakes a little, but no backfiring.

So it seems the intake valve on #6 isn't closing. Is there ANY way to fix this without pulling the head??? Or am I looking at a valve job?

[grand_wag_85]

Could be a bent pushrod but I think you'd hear other noises too.

I had one that did the same thing but it ended up being a bad cam.

If it were me I'd pull the valve covers and spin the engine by hand. Take extra care to note the range of motion of the valves and the condition of the pushrods.

[69J3700]

Never been inside one of those engines but I wonder if a sticky lifter would make sense? Are they hydraulics or solids? Could have gummed up while it was sitting. Could also come and go.

[NO6YHY]

I guess a sticky lifter would mean having to remove the intake, at least. Never dealt with this sort of thing. I think they are hydraulics. I'm going to pull the valve cover today to see what it looks like.

[Carnuck]

Pull the valve cover. You probably have a bent pushrod or broken exhaust rocker on #6. Fuel/mix goes in, fires, but exhaust doesn't open so flamefront goes back into the intake.

[Mike D]

carnuk ... wouldn't a broken exhaust rocker -- make it backfire 100% of the time??

i guess the pulling of #6 wire and backfire goes away is a good indicator that its the offending cylinder ...