Backfiring through carb, won't run...

joejax

Hello all, new here, I've got a '93 GXT804 will not run right. I replaced coil, good spark, getting fuel, timing marks lined up, compression is 175 and 135 on driver's side.

I'm a little worried about the 135, valves are set at .004, it backfires through the carb and I can take 1 plug wire off and it will run (badly) and it doesn't matter which wire I remove.

I'm really stuck here, thanks in advance for any help.

BGW

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joejax

Update: went out and worked on it some more, found my valves were too tight, must have not been paying attention, adjusted, now my compression is 180/160.

I also did the old air hose trick; blow through spark plug hole and see what happens. Seemed to be OK, not blowing through the valve stem seals or anything.

It seemed pretty tight, the air pressure wanted to turn the motor,I don't have a regulator, so it's full blast or nothing.

I'm beginning to wonder if it's an electrical issue, not firing at the right time, I've had this motor out a couple of times, replaced pistons and rings, valves looked good at the time.

Any help or comments are welcomed, this seems like a cool board.

BTW, it's a '93 with an '86 Marathon front (if anybody was wondering)

victorjohn

The difference in compression should not exceed 10%. A 16 psi difference is tolerable but not ideal.

COURTESYCART

so it still backfires with valves adjusted?

If so you prob have a fuel issue. check you linkages for binding. Make sure they are move freely. Sounds to me the carb does no close all the way when you let off the gas. this will cause a back fire.

scottyb

Interesting that it stops back firing when either of the plug wires are removed. Sounds electrical or timing. Have you had the timing belt off?

joejax

Hey, thanks guys, it backfires through the carb as I'm trying to get it started, I'm sure I've got fuel, bowl seems pretty full when bleed screw is loosened.

Linkages look good, everything moves OK.Carb removed, cleaned, all passages seem clear. Good fuel flow from pump (all over my shirt ).

Yes, it is really weird that it will start to run (badly) when 1 plug is pulled, even when I reattach the plug wire it will sputter down to nothing and start to backfire. I've tried to spray cleaner in while cranking, spits fire out, ignites puddled gas in air box, loads of fun.

Timing belt in lined up really close. Maybe 100 hours on rebuilt motor (need to verify that) new pistons, rings, gaskets, fuel pump etc. I've had zero issues until now. Coil went bad, was sparking on 1 cylinder.

I replaced coil, seemed OK for while, my Niece decided to go on a 7 mile tour of the neighborhood, next day having problems that only got worse.

I'm wondering if it is not sparking at the right time, or is that even possible, maybe that sensor thing near the fan has gotten out of whack?

popatop

sounds like valves not adjusted correct to me

crash12888

I would recheck the valves, just for fun run the intake valve out to .006.
You might also start it up at night or with the engine compartment covered (really dark) and check for spark bleeding to ground.
Electrical things can look a lot different when it's dark.

joejax

Thanks, I had adjusted the valves, checked compression, was down, valves were too tight, took my time the next adjustment, compression was up OK.

I will definitely check them again though, loosen them up and try the lights out trick. In the past, a valve adjustment made the cart run better, with seemingly more power.

An adjustment came when power was noticeably down or during routine maintenance such as cleaning plugs, oil change, etc.

Now I'm beginning to wonder if my cam lobes are worn on one side causing the valves to open too soon or something. I'm checking clearance on the bottom. Is that even feasible?

Thanks again for all the help, Joe.

joejax

Well...here's an update, adjusted valves way out, no luck . Did some more research on the "sensor thing", I'm assuming it's called the Pulsar Coil, mounted on wheel behind the fan.

I wiggled the wires while starting (attempt at own risk) got it to turn over and run a little, I think I'm on the right track. Ordering one soon...