My '90 LT500 runs very well through all the gears but when I am in 5th at the very peak it acts like it is running out of gas. This only happens well into the higher rpms of 5th. I have had this happen one other time at the top of 4th on a big hill climb as well but I was pretty well reved out. Any ideas?

 

offroader247

Is this something new that just started? Did you just do any new mods?
Are you sure it is the motor that shudders? Does shudder mean vibrate? If so then maybe you should check cases where crank bearings mount.Many LT500s wallow out the bearing boss in the cases. vibration is one of the signs of this problem.

"Running out of gas" is a totally different thing.
Does the motor cut out and then come back or does it die and have to be restated?

I will try to help as much as I can but without more info it is very hard to help. Kind of like shooting in the dark, you might hit it or you might miss. Please shed some light on your problem and I am sure you will get more responses.
Give us more info. The more information the better and more accurate we can help.

 

You might want to take a plug reading, for starters. If your plug is old, change it. See if that makes any difference in how it runs. Then run WOT in 5th untill the point where you notice the problem. After a few seconds at this speed, pull in the clutch and hit the kill switch. Keep the clutch in and bring the bike to a stop. Remove the plug and check the color of the center electrode. It should be a dark brown. If it's all wet or really black you're too rich. Too light or almost white--too lean. All this assumes that the electrical system is functioning properly, though, but a good place to start.

 

I just recently bought the quad so it is new to me. This bike was hardly riden in it's time and is running on the original piston. The previous owner said that the bike has less than 100 total hours on it from new off the showroiom floor and it definitely looks it. I have not made any changes to the quad other than change the plug (NGKB8ES). It runs fine except when in fifth WOT after nearly maxed out it fades as if it runs out of gas, then I let off the gas and hit it and it is good again until i get to nearly maxed. It actually runs at max for a couple secs and then starts to do it. I have also hit this same thing in 4 th climbing a pretty demanding hill. Other than that the bike runs flawlessly and is very responsive. Let me know what other info you guys might need and thanks for the responses.

 

Something similar happened to my 500. The screen on the fuelvalve inside the tank was partially clogged with a white hard powdery substance. A compressor had a hard time removing it. When in need of lots of fuel it would stall. I removed the little screen and added an in-line filter. I'm not saying this is your problem but it may be something to look at.

 

offroader247

It runs fine except when in fifth WOT after nearly maxed out it fades as if it runs out of gas, then I let off the gas and hit it and it is good again until i get to nearly maxed

So I take it that this is what you meant by "shudders".

If you have an inline fuel filter check it for partial clog or change it.
There might be a screen on the fuel petcock pickup assembly, check it and or clean it.

When you are sure your fuel system is unrestricted check piston wash for a lean condition.

Piston wash is the way to go. Get one of those bore lights bend a light that you can shine down the plug hole and move the piston up till you can see the top than down to see the edges. Or take the head off.

piston wash guide

Check the top pic by Eric Gorr...it is perfect jetting

Some tuners use “trial by error jetting". If you do, START rich and work your way down. I never had good luck with that type of tuning; maybe I am just not smart enough.
Anyhow it is not recommended for top performance or no melt downs.
If you are using leaded race fuel plug chops are useful.....but with pump gas unleaded, plug chops can be very confusing and are not a good way to tune IMO.

Piston wash tells you exactly what is going on inside your motor, use it for top performance.

 

stormnorm

You beat me to it!

 

Thanks for the hints. It sounds logical that the bike can be starved for fuel by a blockage. I will check it and let you guys know. It's good to have all these lt500 experts out there to give a helping hand. I hope to be able to return the favor.