I'm starting to run out of ideas what is wrong with my 87 LT 500R. I recently rebuilt the complete motor with new bearing and seals in the bottom end and a new connecting rod and piston. The motor will run extremely lean as soon as a heavy load is put on it. It runs crisp in nuetral and runs good up too about 1/2 throttle. As soon as you crack the throttle wide open it bogs and runs lean. I've cleaned the carb and put in a 440 main but it didn't change . Any ideas on where this air leak could be coming from?

 

Did you replace your reeds? Also the 440 is to big for the summer I would suggest running a 380 to a 400. the 440 would be ok in the winter time. Also make sure your reed boot has a good seal. I am real familiar with the 87 the jetting is different on the 88 to 90's. I have 87,88,89 Zillas check out my picture page and let me know how it is after you change your jet.

 

I tried putting a 420 main in it and it made the problem worse. The motor seems to be running out of gas or an air leak. I've cleaned the carb and fuel petcock and it still does it. I raised my float level and but that didn't make a difference either. I checked my intake o rings and they are in good shape. I have new crank seals too.

I'm really lost on where to go now

 

It may or may not help, have you checked your gas tank vent? It may not be flowing enough air for the gas to flow.

You can use an unlit blowtorch to test for air leaks, but I'm not so sure how "good" it'd be for a 2stroke. Using an unlit blowtorch, w/ the valve open, just move it around the carb/intakes/jugs etc. If you have a leak you'll hear it when it sucks the propane into the combustion chamber. I wouldn't do it for more time than absolutely necessary to pinpoint the leak. WD40 may be more 2stroke friendly as long as you don't mind the mess, it'll also tell you where/if any air leaks are occuring. And when looking for air leaks check the exhaust system as well.

However, I would seek a second opinion on carb setting/adjustment just to make sure you're not looking in the wrong direction (ie. looking for an air leak when you only needed to go up two clips on the needle).
Good luck.

 

First of all, anytime you do engine work, it's very wise to do a leak-down test to ensure you did not compromise the integrity (Seal) of the engine. This is a fairly simple process, and basically a very good way to tell if you have a leak. Leaks = Lean = kaboom.
The starting fluid, WD-40, propane method only works on the upper seals you can get to... no way to check the crank seals, etc.

Check this out: http://www.off-road.com/atv/ptester.htm

Please disregard roosters advice about jetting! It is unwise to make jetting recomendations based upon what 'someone else' is running.
You could very well have jetting issues, and after making sure you don't have a leak, that's one of the next places to look.
All the jetting correction in the world will not help you if theres a leak.

There are many many factors in properly jetting a motor. It's not complicated (Rocket science), but you must fully understand what is taking place before you can make good decisions, and get it tuned right... Proper jetting makes a HUGE difference in performance.
Do some homework and learn what does what...

Mebbe this will help: http://www.eric-gorr.com/twostktech/carbtuning.html

I'm assuming your running the stock '87 carb?
You could also have float level issues if the motor feels its running out of fuel under heavy load.
Needle/seat, pingle, etc, etc...
The stock carb is sub-optimal, and you really should consider upgrading.

Be objective, look things over well... you'll find your problem.

 

I'll try doing that leakdown test this weekend. What kind of carb would you recomend for a stock LT 500 with minor bolt ons?

Appreciate all the help guys

 

Mpg the best carb for the zilla Is a Kiehin 39pwk. And when your done jetting. I will bet you have a #400 main Let us know.

 

PWK taperbored to 41.5 works great on a stocker... these motors need to breathe.
Great throttle response and better top end.

If you had port work and a good pipe, I'd say spend the $$ and get a Lectron (44mm +) depending on your intended use.

Let us know how your testing turns out?