# Backfire Through Carberator



## tufcat (Mar 9, 2020)

Hi Everyone,

I have a Craftsman LT1000 with a 21 HP, V-Twin.

I was mowing today, engine running fine. I turned the mower off to move some vehicles and clear the next area for mowing. 

I tried to start the mower it would not start/run. I replaced the spark plugs and the engine will start now, but runs poorly and backfires through the Carberator.









Can anyone point me in the right direction to analyze/fix this issue? The mower was running great this morning.


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## pogobill (Jan 31, 2012)

Did you try the old spark plug again and see if it starts and runs? Have you removed the cooling shroud just for this photo?


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## andyvh1959 (Jul 1, 2015)

Fuel shutoff solenoid? If your engine has one, there would be a wire running to the bottom of the carb float bowl. If that wire has come off, or worn through to ground, the carb will not get fuel.


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## tufcat (Mar 9, 2020)

pogobill said:


> Did you try the old spark plug again and see if it starts and runs? Have you removed the cooling shroud just for this photo?


Hi Bill, 

Thanks for the reply.

I probably should have been clear about "running". The engine will only operate at a low RPM, kind of a pop, pop, pop, barely running kind of idle. It won't throttle up. I'm not an engine guy, but it,s almost seems like a timing issue.

The engine was running/mowing fine (no issues). It didn't stop running, I turned it off. When I went to restart the engine, these issues started. Kind of a Twilight Zone thing.

Yes, cooling shroud, air filter, etc. were just removed for trouble shooting this issue.


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## tufcat (Mar 9, 2020)

andyvh1959 said:


> Fuel shutoff solenoid? If your engine has one, there would be a wire running to the bottom of the carb float bowl. If that wire has come off, or worn through to ground, the carb will not get fuel.


Hi Andy, 

Thanks for the reply. 

I don't think this machine is equiped with a fuel shutoff solenoid, but I will check it out.

I'm not much of an engine repair guy and I was hoping there might be a simple fix. I'm just about ready take it to a repair shop.


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## bbirder (Feb 26, 2006)

Remove that large nut on top of the flywheel and see if the key still lines up with the shaft. Even a partial shear will throw the timing off.


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## FredM (Nov 18, 2015)

check the fuel shutoff solenoid under the carby bowl like Andy suggested, there will be one there, unless modified, and another suggestion, water in the carby bowl will cause popping too, one drop of water will not vaporize through the main jet, let alone the idle jet.

and check the flywheel key as bbirder suggests too, if the key shears, then the timing goes into retard, how long since the valve clearances been checked?, if your engine kicks back on the starter when trying to start, then an adjustment is needed.


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## Bob Driver (Nov 1, 2017)

A sheared timing key is usually not the problem on a V-twin mower. The blades are belt driven and not direct drive like a push mower. If you hit a stump with a direct drive, it will shear the key. If you hit a stump with a belt drive, the belt usually just slips in the pulley. 

I'll bet if you run a leak down test, one of the cylinders is really bad. One of the intake valves is not fully seating at the time the ignition sparks and the combustion flame is going back out the intake manifold rather than being contained in the cylinder.

Check your push rods and run the valves. If it still has a problem with leak down. The valve seat may have come loose and an intake valve is stuck upon .... you'll need to pull the head


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## captain001 (Oct 29, 2019)

I had a similar problem on my 26hp. I believe it was the valve guides had moved and was causing the engine to have very low power and noisy. I had already taken it into the shop but I did a search and saw it was a pretty easy fix. Might want to do a quick search and like I said it was an easy fix.


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## wjjones (May 28, 2010)

Yes it does sound like a valve adjustment issue.


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## Busted Tractor (May 22, 2018)

Hope you weren't running the tractor with the blower housing off. The engine needs air directed over the cylinders and especially the cylinder heads. Often a rodent will build a nest in the head area which restricts the cooling of the head. When that happens the valves and guides will overheat and often the guides will move in the head. The guides move "out" of the cylinder head and not allow the valve to open which then causes the push rod to bend and fall out. Usually this is the exhaust valve and being the cylinder builds pressure because the intake valve is working. You get a pop, pop, pop, pop out the the carburetor/air cleaner. Quickest way to find out is to remove the valve covers and inspect the valve train, checking the rocker arms and push rods. Looking thru the springs look at the top of the valve guide and compare to the rest of them. The most sure way to repair is to install a new head. Some of them have been improved for better sealing of the head gasket. As Dirty Harry would say "Do you feel lucky?" you could try and drive the guide back in place but it may not stay and you would have to replace the head anyway. BTW When Briggs sels ahead it is complete head, gaskets, valves, and other hardware but no headbolts.


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## John Liebermann (Sep 17, 2018)

This is an abrupt change therefore component wear or adjustment is probably not the issue. I would pull that flywheel and consider something about timing before further tearing anything else down.


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