Riding lawn mower carb

G

Garys 68

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Sep 22, 2008
Messages
476
OK, dont laugh, not exactly a "car" but at least carb related.
I picked up a 96 craftsman riding lawn mower, free. It had been sitting for years but ran fine before.
It has a 15.5 hp Briggs and Stratton with a Walbro LMG carb that was gunked up. I rebuilt the carb, cleaned all the passages, and replaced the float, needle/seat, jets, etc. and it started righ up. Idles great, responds to the idle air screw, throttles up fine and runs smooth. But even a light load at full throttle will bog it down and kill it (engaging the drive belt even in neutral).
All the belt rotate easily and it will backfire when it bogs, so I'm thinking the main jet is lean. There's no adjustment and no optional jets that I know of. The governor also seems to be working fine and will try to throttle up when it bogs.
I've rebuilt Q jets, Carters, Holleys, even Webers and this Walbro is about the simplest I've seen. Anything I could have screwed up???????
 
Last edited:
Twin_Turbo said:
could be timing related
Click to expand...

AFAIK, most of them are fixed timing, these are very simple engines...just a point cam off the camshaft...one or two lung....

I would make damn sure the gas lines are clear and clean, wash out the tank but GOOD....fresh gas, obviously....

make sure the air cleaner is clean...or pull it off to check operation...

:bump:
 
Garys 68 said:
I rebuilt the carb, cleaned all the passages, and replaced the float, needle/seat, jets, etc. and it started righ up. Idles great, responds to the idle air screw, throttles up fine and runs smooth. But even a light load at full throttle will bog it down and kill it (engaging the drive belt even in neutral).
Click to expand...
Are you sure you have the right size jet (emissions tube) in it? That is the setting of high side on that carb. There are different sided tubes depending on what the carb is for. I would put the original back it if you still have it.

If you want to make it adjustable you can drill the tube out to a larger size and get a $15 adjustment needle to go in the bottom of the float bowl and drill out the tube.

That is the carb I used to run on my racing mower. You can squeeze a lot of extra power out of it.:devil:
 
Twin_Turbo said:
could be timing related
Click to expand...

Not really a timing adjustment on that engine. The coil bolts on in one place and is triggered by the flywheel. The only way to really adjust the timing is an aftermarket flywheel or an offset flywheel key. Plus normally when you sheer a key the engine won't even run.
 
I compared all the old parts to the new and they looked right. But I threw away the old stuff.
It's got the anti-run on solenoid on the bottom with a fixed jet off the side. I disconnected the the solenoid ( no power leaves a needle valve closing off the emulsion tube) and it would start but die out. So I think that circuit is ok. It really does seem as if that high side jet is too small. The old one was so gunked up it was hard to compare.
But am I on the right track looking at the high side? There's just a jet in the bowl, feeding the emulsion tube, then fuel goes right into the carb throat, right?
Fuelie74 said:
Garys 68 said:
I rebuilt the carb, cleaned all the passages, and replaced the float, needle/seat, jets, etc. and it started righ up. Idles great, responds to the idle air screw, throttles up fine and runs smooth. But even a light load at full throttle will bog it down and kill it (engaging the drive belt even in neutral).
Click to expand...
Are you sure you have the right size jet (emissions tube) in it? That is the setting of high side on that carb. There are different sided tubes depending on what the carb is for. I would put the original back it if you still have it.

If you want to make it adjustable you can drill the tube out to a larger size and get a $15 adjustment needle to go in the bottom of the float bowl and drill out the tube.

That is the carb I used to run on my racing mower. You can squeeze a lot of extra power out of it.:devil:
Click to expand...
 
Fuelie74 said:
Twin_Turbo said:
could be timing related
Click to expand...

Not really a timing adjustment on that engine. The coil bolts on in one place and is triggered by the flywheel. The only way to really adjust the timing is an aftermarket flywheel or an offset flywheel key. Plus normally when you sheer a key the engine won't even run.
Click to expand...

If you want to adjust the timing, all you need to do is slot the two screws that hold the coil in place and advance timing under the flywheel. This is an hour job with a rat tail file. It will allow you to advance the spark, but probably not a solution to your problem.

As an after thought, have you tried manually overriding your engine governor, with your finger controlling the linkage? Before engaging the blade, when it stalls, try manually controlling the governor and see if it still stalls. You may have a weak part in that system that is producing the problem, but I'm just guessing at a solution. As mentioned, these engines leave plenty of power on the table.
 
Last edited:
Garys 68 said:
I compared all the old parts to the new and they looked right. But I threw away the old stuff.
It's got the anti-run on solenoid on the bottom with a fixed jet off the side. I disconnected the the solenoid ( no power leaves a needle valve closing off the emulsion tube) and it would start but die out. So I think that circuit is ok. It really does seem as if that high side jet is too small. The old one was so gunked up it was hard to compare.
But am I on the right track looking at the high side? There's just a jet in the bowl, feeding the emulsion tube, then fuel goes right into the carb throat, right?
Fuelie74 said:
Garys 68 said:
I rebuilt the carb, cleaned all the passages, and replaced the float, needle/seat, jets, etc. and it started righ up. Idles great, responds to the idle air screw, throttles up fine and runs smooth. But even a light load at full throttle will bog it down and kill it (engaging the drive belt even in neutral).
Click to expand...
Are you sure you have the right size jet (emissions tube) in it? That is the setting of high side on that carb. There are different sided tubes depending on what the carb is for. I would put the original back it if you still have it.

If you want to make it adjustable you can drill the tube out to a larger size and get a $15 adjustment needle to go in the bottom of the float bowl and drill out the tube.

That is the carb I used to run on my racing mower. You can squeeze a lot of extra power out of it.:devil:
Click to expand...
Click to expand...

My .02....sounds like its running lean to me. I say tear the carb apart and go thru it again....
 
Yep, governor is working fine, and I tried that. When I over ride it and rev the engine higher, it still stalls.
Guess I'll take it apart and clean the passages again.
Red said:
Fuelie74 said:
Twin_Turbo said:
could be timing related
Click to expand...

Not really a timing adjustment on that engine. The coil bolts on in one place and is triggered by the flywheel. The only way to really adjust the timing is an aftermarket flywheel or an offset flywheel key. Plus normally when you sheer a key the engine won't even run.
Click to expand...

If you want to adjust the timing, all you need to do is slot the two screws that hold the coil in place and advance timing under the flywheel. This is an hour job with a rat tail file. It will allow you to advance the spark, but probably not a solution to your problem.

As an after thought, have you tried manually overriding your engine governor, with your finger controlling the linkage? Before engaging the blade, when it stalls, try manually controlling the governor and see if it still stalls. You may have a weak part in that system that is producing the problem, but I'm just guessing at a solution. As mentioned, these engines leave plenty of power on the table.
Click to expand...
 
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