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Discussion Starter #1
The B&S V-Twin in my LA135 keeps backfiring and popping and it seems to be getting worse. It does it at all RPMs, but less so at idle.
Here's a video of it at WOT:

 

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I would start with a fresh set of spark plugs, drain the fuel tank and refill with fresh fuel,drain carburetor sediment bowl, and change fuel filter. Then go from there....
 

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Could be getting lean/hot. Could also be carbon buildup or possibly a burnt valve.


How long does it have to be running for it to happen?
 

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Discussion Starter #4
I would start with a fresh set of spark plugs, drain the fuel tank and refill with fresh fuel. Then go from there....
I should have mentioned, it does have fresh plugs and I've put 10 hours on it since I've acquired it in September, so lots of gas has gone through it. I even installed new fuel and air filters.
 

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Have you cleaned the carburetor?
 

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Discussion Starter #6
Could be getting lean/hot. Could also be carbon buildup or possibly a burnt valve.


How long does it have to be running for it to happen?
It almost smells like it's running a bit rich. When you engage the mower deck, the popping stops. I have the mower deck off now since I'm using it to sweep leaves.

It starts the popping only after less than a minute of running. It runs strong, other than the popping.
 

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Discussion Starter #7
Have you cleaned the carburetor?
My father said he did, since the fuel shut off solenoid was sticking and causing it to not start. I'm not sure how long ago that was though. I ordered a new gasket set for it, and that's my next plan, to tear apart the carb and see how clean it is.
Just wondering if you all had any other thoughts!
 

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I have an old B&S Twin on a Craftsman that does the same even after changing the carb, the intake manifold an plug and don't know what else. I think the cam is wearing.


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If you close the choke, partially or fully, does the popping go away?

At idle, take a can of WD40 or other flammable aerosol, and spray around the intake and carb, see if the idle changes. If the idle changes you have an air leak.

What kind of shape is the muffler in?
 

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My old tractor /sold would do that if after i run it at high idle and just stop it Try to let it run at low idle for 1 minute before you stop the engine I do that with all my tractors:unknown:
 

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Discussion Starter #12
If you close the choke, partially or fully, does the popping go away?

At idle, take a can of WD40 or other flammable aerosol, and spray around the intake and carb, see if the idle changes. If the idle changes you have an air leak.

What kind of shape is the muffler in?
I will try that and see what it does.

It only has 204 hours on it, I wouldn't think the muffler would be shot, but I'll inspect it.
 

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Discussion Starter #13
My old tractor /sold would do that if after i run it at high idle and just stop it Try to let it run at low idle for 1 minute before you stop the engine I do that with all my tractors:unknown:
It does this popping constantly, no matter the engine speed. I do let it idle for a minute before shutting it off.
 

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I will try that and see what it does.

It only has 204 hours on it, I wouldn't think the muffler would be shot, but I'll inspect it.
Maybe the carb is loose allowing air in through the intake track which would make the engine run very lean.

When you engage the deck you are putting a load on the engine which should cause the carb to increase fueling. The increased fueling may be enough to reduce the effects of being lean.

Either the carb has trash in it preventing all circuits to operate fully or you have an air leak along the intake track.....that is my bet.
 

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Discussion Starter #16
Update

The carb rebuild kit came in the mail today, so I took the carb off, tore it down, thoroughly cleaned everything and put it back together with the new gaskets. Once it warmed up, it started popping again. The choke does stop most of it, so it's definitely a lean condition. Also, one of the cylinders is running cooler than the other. One is hot after running and the other is warm to where I can put my hand on the valve cover and not get burnt.
I'm not sure where to go next with this! I guess I'll pull the plugs and see how they look, since they only have 10 hours on them. Any other ideas?
 

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Check compression. If it's ok you may want to check the valves springs and valve clearances. Also, use a spark tester to make sure the ignition is ok to each plug when the engine is backfiring.
 

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The carb rebuild kit came in the mail today, so I took the carb off, tore it down, thoroughly cleaned everything and put it back together with the new gaskets. Once it warmed up, it started popping again. The choke does stop most of it, so it's definitely a lean condition. Also, one of the cylinders is running cooler than the other. One is hot after running and the other is warm to where I can put my hand on the valve cover and not get burnt.
I'm not sure where to go next with this! I guess I'll pull the plugs and see how they look, since they only have 10 hours on them. Any other ideas?
With temps being much different between cylinders I'd check to make sure the cool cylinder is firing like it should.

This is also a good next step

Remove the bolt or nut and washer from top of flywheel and check the key. It may have sheared.
 

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Use a temp gun to check both cylinders. Take your readings from the cylinder head at the spark plug base. Also do the exhaust pipe right where it exits the cylinder head. This will actually tell you if one cylinders running colder. Pull the valve covers & check the valve clearance first. Then do a compression test. If the rocker arm clearance is off you won't get an accurate reading. If the compression is good do a leak down test.
 

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I know you've tried this but maybe it will help someone else. My LT155 did this when new and the dealer told me to slow the engine down to idle for a minute or so before cutting it off. That minute seems like an hour while you're sitting there waiting but it solved my problem.

Good luck.
 
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