16hp Vanguard - Runs Rough @ Full Throttle When Hot?

[AdamE]

Hi all, I've got a Cub with a 16hp Vanguard. It seems to run fine when cool, but after mowing for a while it starts to run a bit rough. I can hear little burbles and coughs and when I throttle down, they become more apparent.

I'm hoping it's a carb problem, but my head tells me it may be a valve issue.

BTW, there's about 700 hours on the engine.

Any ideas?

 

[motoman]

Adam, Easy check. Look at spark on removed plug at hot. Have you hit anything that would shear the flywheel key and allow the spark timing to shift?

 

[AdamE]

Adam, Easy check. Look at spark on removed plug at hot. Have you hit anything that would shear the flywheel key and allow the spark timing to shift?

I haven't hit anything out of the ordinary. Mostly sticks and a root here or there...

What would I be checking the spark for? Wetness?

Thanks.

 

[motoman]

Check hot plug out of engine and grounded against engine to see what the spark looks like at the tip. Is there a spark and is it regular as you spin the flywheel? If the flywheel has moved because the key is damaged the spark will not reach the piston at the proper time. It would be difficult for you to prove this. It is easier to remove the flywheel nut and look at the keyway (key slot) shared by the tapered engine shaft the flywheel sits on and the flywheel itself. I am winging this a little . Perhaps others will chime in here.

 

[AdamE]

Thanks for the replies and I'm sorry it's been so long to get back here.

I just wanted to report that I figured out what was causing most of my problems. It seems the choke was working it's way out as I mowed. As long as I push it back in every few minutes, it runs much better.

Go figure...

 

[pay1920]

I had a engine do the same thing, seems B&S thought it would be better to put plastic lobes on the camshaft instead of metal, the plastic lobe had worn down.