3hp power loss

[RvK]

Hi all,
I picked up a 3hp 3MHSQ (92) in great (seemingly) condition for my little tender.
She starts 1st pull (every time!) and runs/sounds great with full power for about 5 minutes...then starts to sputter and loses power till she can only idle in gear (kind of slow!). An increase in throttle is an instant stall at that point. Fuel tank & filter are clean. Carb's rebuilt. Just put in a new impellor. Not a steady stream from the tel-tails, in fact only light spitting at all rpm, so I pulled the thermostat cover and no thermostat. Ran it for a few seconds w/o the thermostat cover and saw plenty of water coming up.
So any advise? It's hardly worth taking to a pro.
Thanks,
Robert

[Ken]

Robert,
I am not familiar with that 3 hp, but sounds like fuel starvation. When the motor stalls after running at high speed, shut it down, remove the hood, and take a look at the fuel filter bowl. It it is not full, then problem is from there on back to the tank - check for proper venting. If bowl is full, then problem is from there to the carburator - mabe needle vave of carb sticking.
Good luck,
Ken K

[RvK]

Hi Ken,
The bowl's full, so the fuel is gettin to the carb. I put the carb back on and started her up; ran great again until warm, then lost power and would stall unless I backed down to idle. Same as before. I then increased the choke about 1/2 way to restrict air. This immediately helped and the motor would throttle back up. Then w/choke off I sprayed some carb cleaner on the carb exterior. This helped too. So she's must be only running lean when warm. My conclusion is a vacuum leak, but why only when warm? The motor runs great cold w/o choke. Kind of opposite than what I would expect from a vacuum leak. Shouldn't a motor w/a vacuum leak run BETTER as the rpms increase?
Maybe I'm way off and it's a blocked jet, but why only when warm?
Thoughts?
Thanks,
Robert

[Ken]

Robert,
You say the carbs (more than one for that 3 hp?) were rebuilt? When you choke it 1/2 way, it picks up in rpm - that tells me it's starving for fuel. Does that little Yammie have a fuel pump, or is it gravity feed from a local tank?
If it has a fuel pump, then that would be suspect. If not, then sure sounds like carb is messing up - I would tear down and install a carb kit and adjust pilot screw (low speed jet) per the manual.
Good luck and let us know how you come out.
Ken K

[RvK]

Hi Ken,
One small carb with a little fuel pump mounted on it. Since the bowl's full, and your thinking it's a lack of fuel (and not too much air), the problem must be in the jets/needle valve.
I'll check them again, try it out and if it still stalls, I'll rebuild the carb.
I'm still scratching my head as to why this happens only when the engine's warm.
Thanks,
Robert

[Ken]

Robert,
Yeah, does make you wonder why the problem doesn't occur until the motor warms up - but clogged up carb can give you weird symptoms. It may be something as simple as carb float valve sticking after warmup and not keeping the bowl full. Since rebuilding that single carb should be fairly inexpensive and simple, I would do it. Examine the jets carefully - some have a small o-ring, I understand, on the very end, and if it is mis-shaped or split, that can give you problems.
The carb kit should have all gaskets and seals and o-rings in it.
Good luck, and let us know how you come out.
Ken K