Alright let's start from the beginning i bought an lt 500 year 88 with all stock parts except for fmf exhaust. When the engine gets hot and i personnaly rev it high it will stay high. I can get it back down sometimes by loosening the throttle idle screw not all the time though. This only happens when it gets hot. P.S. i am using standard green coolant with standard water and a BR8EG plug with everything else to the manual stock. Also when i look into the crank during a new rebuild i find oil in the crankcase i don't know if it is two stroke or engine oil.

Now i have rebored the engine twice over
I have had the crank trued
I have had the cylinder head resurfaced
I have replace the engine case seal
I have replaced the crank seals
I have replaced the head gasket and base gasket
I have changed fuel numerous times
I have rebuilt two dif 43b10 carbs only reusing the main and pilot jet and the needle pin but the main jet and needle pin is brand new. i have not checked the float level height though
I have done the wd40 leak find test
I have done a two stroke leak down test and held 10 for 10 which is as long as i let it
I have checked to make sure that my carb is not getting hung up
I have wiggled the crank
I have done all this and there has been no sign of problem

It seems to only happen when it is hot
Please everyone who reads this if you have an idea of what might be wrong please post it i will check this daily

 

oh i also have an lt 250 that i can literally pour sand into the gas tank and it will run and a kx250 that i rebuilt and it runs like a champ. I am not an idiot and have a lot of experience with small engines of this kind i have been working on the lt500 for about a year now and just want to take it out for one succesful ride thanks for everyone's help

 

I would do a compression test to see if you are way high.... wonder how much they took off the head resurfacing it...shouldn't be more than like .005 of an inch. I would think...

 

i will but what would be wrong if i was way high ast it is right now i weigh 240 and it will stop me if i am trying to kick it and it is compressing at the time but will do thanks any other ideas

 

I don't know much from 2 strokes but could the reed valve be causing this problem by any chance ?

 

Whenever I hear of what I call a "High Idle" or resistance to idle back down to normal tells me that the motor is lean. (As long as your positive the slide is falling all the way down, and the carb hasn't been bored excessively allowing air to leak past the slide)

Since your confident that you don't have a 'leaker' and have done your leak down properly, it must be jetting. Step up a few sizes on the pilot and see if that helps. Since it's worse when the motor is hot, that might indicate you still have a leaker when it's warm (Expansion of materials with temperature).


I'm sure Doc will jump in here and offer some additional wisdom eventually.

 

You have to be missing something. That sounds to much like an air leak to me. Maybe you installed the stator side crank seal wrong? DId you try spraying carb cleaner or wd 40 behind the flywheel while its started?

 

alright to answer everyone's ideas so far

1. Yes i have checked the reed valves and no sign of wear or damage and they seat completely closed.

2. A larger pilot jet will only help me idle easier but i don't see how any jetting on earth can make my quad recieve much more to compress also when the High idle will happen it does not appear to be burning much fuel.
Also i think you are right with a minor leak that will not show up when the engine is cold but how do i do a leak test and heat the engine at the same time. I thought about pouring boiling water into the radiator and heating the engine up that way.
Question what if my float is too low and the bowl runs out of fuel will i just start sucking air right through the breather hoses with no impidence?

3. When i ripped apart the engine to do a complete rebuild The internal engine case had no sealant on it so i redid it with yamabond i coated both sides so i could see no metal and bolted them together then i did the pressure test and ensured everything sealed completely I did 10Lbs for 10 min. I also had the crank trued and replaced the stator side crank seal I don't know how you could install it wrong but when this happens it does run very lean I mean no exhaust smoke will come out, i have tried the wd40 test but no not on the stator. I am too afraid to let the machine run like that for too long. How long do you think i could run it like that before it seizes especially with a very lean mixture.

These are all great ideas and i love the feedback please if anyone has an idea please post it it will be much appreciated

 

Hi,
Duneaddict is right on the money..............................it's lean on the pilot/idle circuit.
LT500s were f-ed up w/the jetting from the factory.
PM me w/ALL your EXACT info. (what pipe, reeds, compression, carb/cyl mods, current jetting,etc) and I'll help you get thru this.
$.02
RK

 

I had the same problem with MY LT250R. The carb slide wears and it sucks in air through there, also you needle and seat it more than likely shot too. I bought a new keihin carb all my idle and other problems are gone.