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Old 05-29-2013, 12:22 PM
NO6YHY NO6YHY is offline
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Backfiring thru Carb - long saga

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...
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  #2  
Old 05-29-2013, 05:55 PM
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itselliot itselliot is offline
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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?
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Last edited by itselliot : 05-29-2013 at 05:57 PM.
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Old 05-29-2013, 11:47 PM
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Dog_Wagon Dog_Wagon is offline
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I'm far from an expert but, I've always been under the impression that backfiring through carb was valve timing related....
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Old 05-30-2013, 01:20 AM
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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.
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Old 05-30-2013, 02:08 PM
NO6YHY NO6YHY is offline
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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.
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  #6  
Old 05-31-2013, 04:37 PM
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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.
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  #7  
Old 05-31-2013, 04:51 PM
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bwwhaler bwwhaler is offline
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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.
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Old 06-01-2013, 07:11 AM
Ristow Ristow is offline
 
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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?
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  #9  
Old 06-01-2013, 04:50 PM
NO6YHY NO6YHY is offline
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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.
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Old 08-03-2013, 09:11 PM
NO6YHY NO6YHY is offline
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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?
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Last edited by NO6YHY : 08-03-2013 at 10:35 PM.
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Old 08-03-2013, 09:15 PM
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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.
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Old 08-04-2013, 06:24 AM
69J3700 69J3700 is offline
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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.
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Old 08-04-2013, 07:23 AM
NO6YHY NO6YHY is offline
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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.
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Old 08-04-2013, 11:59 AM
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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.
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Old 08-04-2013, 12:40 PM
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Mike D Mike D is offline
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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 ...
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Old 08-04-2013, 01:36 PM
NO6YHY NO6YHY is offline
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Okay, now I have to retract what I said earlier. Started running it today with the wire pulled on the low compression cylinder, and after running great for 90 seconds, it started backfiring again! So much for the valve being the cause, I guess. Also pulled the valve cover and the valves on that cylinder were moving up and down like normal, at least as far as I could tell.

Needless to say, huge frustration. When I primed it and started it, it ran great for a minute, then all the problems came back and it would hardly run at all. Finally I decided to pull the top of the carburetor off and there was about a centimeter of gas in each bowl. It didn't quite come up to where it enters the little brass ports (jets?). So maybe it's just been starved for fuel, and backfiring when it leans out?

One thing I had to do last summer is replace the fuel pump when I got the car out of storage. Maybe the new fuel pump isn't putting out enough fuel? I might to get a fuel pressure gauge and check it out. Can anyone tell me what range fuel pressure should be in?
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Old 08-04-2013, 04:22 PM
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All the readings are low, but I assume you did the test without inserting oil into the cylinder, and did not wait 3-4 strokes to ensure the gauge was pumped up as far as it will be. If not, you have a very tired engine, because you should be getting around 145.

Regardless, you clearly have a problem with #6 low compression, and more than likely it's a leaky/cracked/chipped valve. If this was the only information I had, I'd be ripping the heads off mine without further ado. Any other info or symptoms is interesting, but the #6 thing is reason to get a valve job.
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Old 08-04-2013, 04:38 PM
NO6YHY NO6YHY is offline
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Thanks for the replies. I did wait 3-4 strokes for it to get up, but I didn't open the throttle at all, and I only took one spark plug off at a time instead of all off. And no, didn't squirt any fuel in. I assume this is why they are low.

I just did a fuel pressure check and the meter only read 4 psi. Service manual states it should be 5-6.5 psi. And when I did the volume test, it only filled maybe 4 ounces in 30 seconds instead of a pint. So maybe that's my problem. Of course, the fuel pressure gauge was designed for fuel injected cars, and I had it in front of the fuel filter instead of at the carburetor like the service manual says (but that should make it read higher rather than lower anyway). But the reading seemed low regardless.

I think I'll postpone pulling the heads off until I have money and time, but that's good advice too. Again, thanks for all replies, I appreciate the help.
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Old 08-04-2013, 09:22 PM
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Fix the fuel issue first. If you don't have a filter in the line before the fuel pump, I suggest that first to keep crud from the tank from jamming the valves of the pump open. The PO of you vehicle "may" have dropped the wingnut for the aircleaner down the carb. If you know someone with a bore scope, you can "peek" through the plug hole to see the cylinder and valve condition (just don't crank it with the scope in the hole)
Last time I used a Harbor Freight inspection camera to do mine. Saves a lot of gue$$ing.
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Old 08-05-2013, 12:22 AM
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I'm thinking that you've got to get the fuel tank squeaky clean first. Otherwise, it may just continue to export problems to the rest of the system. Drop the tank, dissolve the varnish with acetone and a steel chain as a scrubber, treat the entire inside tank with muriatic acid, and go from there. If everything else is getting gummed up, then get rid of the gum first.
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