31P777-0348-E1 loses throttle control when warm

[Psychlopath]

I have a recently installed 31p777-0348-e1 with an unknown history. I was told it has fewer than 10 hours on it but a cam lobe had gone flat so it was replaced.

About 18 months ago, I got it and installed a new cam and have been running it on my Snapper since.

Over the past two months or so, I've noticed that when it is warmed up, I can't use my throttle to change engine RPMs, and I'm not sure what could cause this.

I've adjusted the governor with the manual here and there's no change. I also verified visually that the throttle cable actuates the throttle lever and choke.

The engine seems to work great except for that one caveat:

o When the engine is cold, I can control the throttle just fine.
o When I engage the blade or transmission, I can hear the RPM dip then go back where it should be.
o If I stop the engine for 10 minutes or so, I can control the throttle again 'till it warms up again.
o The engine doesn't seem to be using more fuel than I feel it should.
o When warm, if I move the throttle from "Turtle," to "Rabbit," there is NO change (verified by tach) in RPM until I hit the choke, where the engine falters as it should.
o I don't notice any smells or lack of performance that would indicate overheating; the blower and fins were cleaned out in January when I serviced the oil and filter.

I'm really at a loss as to what could be causing this, so any suggestions would be appreciated.

 

[Luffydog]

Do you mean like reving up like no governor control an over rev. Over just no throttle control or both at the same time.

 

[PTmowerMech]

In my opinion, it would have to be something in the carburetor. That is, if the throttle cable is actually connected and moving the throttle plate on the carburetor.

When this occurs, is the engine rev'd up or down?

 

[Boudreaux In Eunice La.]

HMMMM 10 hours and a flat lobe ????? That's almost impossible to me.... I know those cams are bad from the get go though...

Where you at in Louisiana Mon Ami ????

 

[Psychlopath]

Do you mean like reving up like no governor...

No, that's the strange thing to me. When it warms up, the governor still works to keep it up to speed. I can grab the governor lever and change the RPM and if I engage and release the blade, I can hear the engine briefly bog down, then the governor kicks it back up to speed as it should. When the engine is cool, I can move the throttle control and change the engine speed as it should be, but just when it warms up, moving the lever back and forth does nothing until the choke starts to close at the very far end of the lever throw. Then it'll bog out like it should with a warm engine and a closed choke.

...it would have to be something in the carburetor......is the engine rev'd up or down?

It seems reasonable that a leak in the carb could cause this, I think, but it should flood out when sitting or at lower RPM, shouldn't it? I typically use a fuel valve to shut down the engine, but I think I'll go out and open it for the night and see if there's any leaking or flooding in the morning. This may be the right route to follow, at least for now.

The throttle plate opens and closes. I verified this by being able to control the RPM when the engine is cool. When this happens, the RPMs are at full, governed throttle. That is to say, not over speed or runaway.

HMMMM 10 hours and a flat lobe ????? That's almost impossible to me...

It does sound odd, but the guy had no reason to lie to me about the hours and the cam that was in it sure enough flat on one lobe. Rounded off, rather...you know what I mean. I got the engine for the cost of a trip to pick it up, so I couldn't complain even if it had more hours on the clock.

I'm in Shreveport.

I appreciate the effort in the replies!

 

[ILENGINE]

I am wonder if you don't have an intake leak at the manifold to engine interface, could be carb to manifold interface, but either way I think you are getting a heat related warpage someplace in the intake manifold that only leaks when hot.

 

[Psychlopath]

I am wonder if you don't have an intake leak...

Ya know, I didn't even think of a vacuum leak. When it warms up again, I'll check with a little spray cleaner around those areas and see if there's any change.

That's one of the most basic (and easy) things to check and I didn't even think of it 'till you mentioned it.

 

[Psychlopath]

Quick update, it was a vacuum leak. One of the gaskets between the carb and plastic manifold had seen it's last day.

Replaced that and I can control throttle again at all temps.

Thanks for the help!

 

[PTmowerMech]

Well shiver me timbers. Thanks for the update. That must've been on real bad leak.