Sled engine problems

[Stomper]

I didn't where to put this so I put it here. It's a snowmobile engine question, but its still in the sled not a buggy. Thought I would ask you guys because you know lots about them.
I just completly rebuilt the engine in my 91 polaris iny 400. I cleaned and adjusted the carbs twice. It idles and runs great up to 6500-6800 rpm then bogs. I can't get it to go past 7000rpm. The primary engages at 4200rpm. Now I know very little about clutches, but is it possible that the engine is getting the fuel to go faster but the clutches are keeping it from doing so and there for flooding it. 

[Admin]

Did you adjust the clutching?Does it bog with the throttle wide open, or just when it hits that rpm?

[Stomper]

It bogs at that rpm and with the trottle wide open it won't go past 7000rpm. The manual says that the operating rpm is 8000. And no I haven't touched the clutches at all for three years except for lubing them.

[Admin]

May not have enuff fuel? Spec whats your thoughts here?

[Nutz4sand]

First off why did you rebuild it? Just weak or a bad cylinder?

Did you test the crankcases for sealing? Positive and negative testing is best.

How long was it down during the rebuild? Was the pipe on the ground somewhere? Mouse make a nest in it? Check the airbox too if its got one. They got me in the airbox once.



 

[Stomper]

I had a bearing go on the crank and a bad piston.(just after I was bragging how good of engine it was with over 7000 hard miles on it :-\). I had all the crank bearing replaced cylenders bored out and new pistons. It was only apart for 3 weeks from the time I blew it up to the time of the first ride. I cleaned the air box before I put it back on and the pipe is ok too. I havn't checked for leaks yet, but was told to spray a small amount of starting fluid around the seal,gaskets and carb boots. If the engine revs up there is a leak.

[Admin]

was the same base gasket installed etc, Port timing may be off...

[Nutz4sand]

Is this 7000 rpm peak riding it or with the track held aloft as its stationary?

[Stomper]

All new gaskets and seals. and the 7000rpm is with me riding it.

[SPEC]

WOW 7000 MILES
That's a long time between rebuilds...
Those 400 were good little powerplants...
It's IMO starving on top I'd rebuild the fuel pump or buy a new one,
Was the piston burnt? did it score the cylender wall...I really think it's fuel starvetion tho

[Stomper]

7014 miles to be exact and yeh it's been a great sled. It's taken everything that I have thrown at it. Yeh the mag side piston was burnt a little and the cylender wall was scared. I had both cylenders bored out 10 thou over. it is cheap to rebuild or buy a new fuel pump so I will give that a try. It just seem strange that it never had this problem before the rebuild.
Ive had people tell me that it is clutching problems, but I have to agree with you spec, and think it may be a fuel problem. Iwill do all of what I have been told though, clean clutches and rebuild or but a fuel pump. after all it cant hurt, right.

[SPEC]

Ya,
 

[Admin]

Are the floats adjustable? Maybe the float bowls are sucking dry, that would cause it to run lean tho...

[Stomper]

rebuilt the fuel pump and cleaned the clutches. Works good now. Don't know which one it was, but it works now. Thanks for the help guys.

[b.c.bugger]

good to hear you got her running right, I'm curious what you mean when you say you are lubing the clutches though? Sled clutches do not require lube. Some apply a light amount of lithium to moving parts and claim it's never hurt, but any professional sled mechanic will tell you not to lube your clutches