Well I rebuilt my son's quad a few months ago because it had filled the cylinder up with gas on multiple occasions due to the wonderful vacuum petcock. I honed the cylinder and put all new rings in bearings in when I got done it fired right up and ran great for like three minutes and then it died. Went out the next day fired right up and ran great for like three minutes and then dies. So it has sat in the shop for a few months now and when I decided to try to get it going today in the 105 degree weather it won't start. So my question is what do you all think would cause to run better than it ever has for three minutes give or take a few seconds and then it just starts dying down almost like it is running out of gas and then it is dead for several hours. Compression is good I don't remember for sure what it was but that was what I though the first time but when I checked it it was fine, has good spark, and seems to be getting plenty of fuel it is actually slightly rich running but not enough to foul the plug. I have about had it with this thing I have already put $400 dollars into a $500 four wheeler and am not going to put much more into it before I scrap it.

 

I'm certain you have more experience than I but I'll give my $0.02.

It sounds to me like the bowl is not filling with fuel at the same rate the machine is burning fuel. I suspect either the float valve or the vacuum fuel valve is not functioning properly.

I suspect the bowl is filling during those several hours the bike is parked. Since fuel should not flow while the machine is not running, I suspect the problem is the vacuum fuel valve.

Do not take this as a professional diagnosis as I'm only a rookie.

 

I am actually glad you brought that up, that valve is the reason I decided to go through it. When it went bad we filled the cylinder up three or four times with fuel. I have already bypassed that valve and replaced it with a regular on/off petcock. I have tried the new petcock half open, all the way open, a third open and all that has accomplished is figuring out that half open is not enough fuel at wot and that a third open is not enough period.

 

The problem is in the float needle and seat. Maybe wore there.
You rebuilt it when you had a problem but did you clean/check the carb then?
You mentioned bearings..Main/crank bearings?
And on that line, how about the crank seals? Maybe a problem there but it does seem to be fuel related.

 

I took the carb apart and everything looked like new, but that doesn't mean I could see a weak spring or a seat that is worn just a little. I think I will try a carb kit just to rule that out. If it doesn't work I will be back for some more ideas. And yes I changed all of the bearings but did not mess with the crank seals. What exactly will bad crank seals or case seals cause? I am a machinist and an ASE certified tech but for some reason I always have problems on small engines.

 

Bad seals will leak air in the motor. IMO, it can cause what your explaining.
If you take a valve cover off a car, do you use new gaskets?.
Seals are like 3 bucks each and are always replaced when doing bearings.

 

do yourself a favor and change that fuel hose to clear just to see how the level is I have a clear line on my sons LT80 and it helps to trouble shoot quickly at times even on the trail, you can eliminate it being outta gas pretty quick lol. or if it's flowing or not. or keeping up with demand. but yours sounds like a carb problem not a flow problem. if it is flow, check the fuel filter pos in the tank