I have a LT 80 (year unknown) that a friend gave me in pieces. I rebuilt the top end, cylinder bored 1 over, new piston/rings, rebuilt carb (stock rebuild kit) everything else seems to be stock. The bike runs great (but smokes a little) all the way up to full throttle. At full throttle it'll run real good and hard for about 15 sec then it bogs down and smokes like crazy. Let off the throttle for a few seconds then nail it, then again it'll run fast then bog down and smoke. It does this even with no one on the bike just the rear wheels up in the air. To me it seems to be running rich?? The circlip on the needle jet is set on the top (leanest position). I've tried all the other positions it runs like crap and won't get up to speed. It has a new air filter, spark plug and fuel filter. I've tried: disconnecting the air filter, running with petcok in prime position, replaced main jet with the original, and cleaned the carb several times (it wasn’t dirty). I checked the petcok vacuum side by sucking on it, didn't get a mouthful of gas, I assume that's correct. When the engine is bogged down and I put my finger over the vent hole on the carb it'll speed back up. One last thing, I tried duct tapping the spark plug wire down. I'm pretty much lost at this point. Should I get a smaller main jet? I didn't think boring 1 over would require re-jetting the carb. Any help would be extremely appreciated. Jason

 

Howdy Jason, welcome to the forums.
Being in pieces, do you know if it ran corect before it was taken apart?
Stock Zuki piston Wiseco?
I agree that rejetting shouldn't need to be done especially as I consider them jetted fat in stock form.
The finger/vent thing sorta indicates a lean condition. For a few bucks I suppose I'd try going bigger and smaller on the jets.
Got a mouse nest in the top of the airbox (under the tank)?
Plugged pipe?
Loose ground from frame to motor? (I'd check this first)

If you have the wheels up and doing a WOT for 15 sec...That's alot..Careful.

 

Thanks for the welcome and the advice. The bike was not running when I received it. It was bored out .5 with a new stock piston installed backwards. It was quite a mess. The cylinder walls were a little beat up, it's now bored 1 over with Wiseco piston and rings. Compression test reads about 135 psi. No mouse nest in the air box. The exhaust pipe seems to be good. Are you talking about the ground that bolts up on the stator side, behind the plastic cover? It's hooked up and appears to be a good connection. Is there another ground? Will a lean condition cause the smoke? Is there a good place online to order jets?

Again thanks for all the help, Jason

 

Your LT80 is a 2 stroke machine....you should see it smoke, completely normal.

 

Well, I'm pointing the blame to the piston. I could be wrong. Wiseco's swell like my belly on a big beer night. LOL
They need a meticulous break-in IMO and I doubt yours is broke in yet..But it's working on it..By means of failure if we are not careful.
I'd like to know the exact clearance the shop gave the piston/cyl.
Want to do a test for me? I'll send you a new Zuki 1.0 piston, you try it and see if the problem persists. I'll buy the gaskets and shipping.

 

Here are the pics of my cylinder...looks like the piston could be the culprit. What do you think?

 

LT 80, it's Wholeshot again. 05 LT80 still running good with your help. Need some more advice. Gas pours out the exhaust when trying to start. After 30/45 sec after turning over and squirting gas it starts. No movement for a few minutes, then it catches and takes off and runs good. I think it is the petcock. Any thoughts?

 

You are correct, fuel is going by the petcock/float needle and filling the motor with fuel. Usually the plug needs to come out and you turn the motor over to get as much fuel out as possible. Then you apply full throttle till it cleans out and runs.
You'll need to clean the carb to make sure nothing is on the float needle/seat.
Yes, the petcock can be the problem.
Go to the auto parts store/lawn mower supply and get a in line filter and on/off valve. That'll help with the problem.

 

Thanks LT80. I almost purchased a replacement petcock from LT80 Hop ups that had the off selection in lieu of the prime. Your suggestion of the inline on/off is even better. I'll try that and report back. Thanks again.