LT80 carburetor issue

[sicivicdude]

I'm not new to ATV's, 2 strokes, or repairing them but this one is a little new to me (I'm used to adjustable carbruretors!)

I bought a project on CL. The PO's demonstrated that it could start and run for me before I purchased it but I didn't run it any length of time. I got the other "bugs" worked out but the carburetor is still giving me fits. It starts but won't idle without an insane amount of idle screw (like bottomed where the slide is halfway open). When it revs is very hesitant and sputters at high rpm. I've tuned plenty of 2 strokes and if it were anything else, I'd drop pilot jets until the air screw was 1.5 turns out and do a plug chop on a leaner main until it screamed. Obviously this isn't an option on this carburetor.

My main question however, is whether the choke knob should actually seal the dip tube or not. I've run across a Mikuni or two that were having the same issues I'm describing and after much head scratching, I finally found a bad choke plunger seal. A new replacement choke plunger isn't terribly expensive and I'll splurge if needed, I was just wondering if anyone had any insight into the CORRECT operation of the plunger on these carburetors before I go "throwing parts at it".

[sicivicdude]

Well, it's a good thing I wasn't waiting with baited breath for a reply and forged ahead.

I received a "shindy" carburetor kit in the mail and removed the emulsion tube from the carburetor. Of course it was covered in crud from sitting and was clogged. I reassembled the carburetor using all of the pieces in the kit and sprayed the rubber tip on the choke plunger liberally with xylene based carburetor cleaner (causing it to swell slightly) to make sure it would seal if it was supposed to.

The quad would start right up but wouldn't take past half throttle. I got down in front of it with a powerful flashlight and looked down the carburetor throat. I could see the pilot circuit spraying fuel like crazy but not a drop coming up the emulsion tube (main jet circuit) so I pulled it back off. In the kit there were two small o-rings. One fit on the idle adjustment screw to seal it off and the other, even smaller o-ring, appeared to go under the main jet. There was a tiny piece of rubber like material that LOOKED like an o-ring under the main jet when I disassembled so I put the new o-ring under there. Unfortunately, that o-ring was blocking the fuel flow through the main jet.

Anyone else know if there's supposed to be some sort of "seal" between the mainjet and the emulsion tube? Mine does not currently and is running like a scalded dog (hauling my butt around the yard is no easy task!)

I also adjusted the idle circuit to a reasonable speed. The slide is up farther than I'm used to (on larger PWK and PJ carbs) but seems to be happy there as throttle response is crisp and the idle is stable under the clutch engagement speed.

I see people talk about putting the 100 main jet in instead of the 92.5 mainjet. I have the oil injection blocked off and I'm running 32:1 premix (I like Klotz supertechniplate, that's what I run in all of my 2stroke toys). Is the 100 mainjet the most likely mainjet size needed now? I've got a completely stock engine with the exception of oil injection blocked off.

[LT80]

I don't know where you heard of installing a 100 jet.
You can go down to a 90.

Yes, there is supposed to be a o-ring under the main jet.
I've had a cpl swell and shut the fuel flow off.


"Well, it's a good thing I wasn't waiting with baited breath for a reply and forged ahead."
Ya...good thing...:blah:

[sicivicdude]

Originally posted by LT80
I don't know where you heard of installing a 100 jet.
You can go down to a 90.

Yes, there is supposed to be a o-ring under the main jet.
I've had a cpl swell and shut the fuel flow off.


"Well, it's a good thing I wasn't waiting with baited breath for a reply and forged ahead."
Ya...good thing...:blah:

Just the person I was hoping would chime in. Your reputation preceeds you....

I've done a LOT of reading about the carbs and saw it somewhere else. Do you recommend a smaller than stock mainjet with oil injection blocked off? I haven't done a plug chop yet but from the amount of blue smoke it's belching I would suspect that the stock mainjet is sufficient or maybe even a little fat. Just want to make sure I'm in the ballpark before I unleash the youngin' on it and it fries the top end from running lean.

With 20 views and no reply with "my choke seals" or even "attaboy, keep up the good work" I was starting to get a bit worried. I feel like I'm on the right path now thoug. It's still a little cold natured (it needs to idle on choke for about 2 minutes) but seems to run like a top once warmed up.

Just for everyone's information.... the lower tube doesn't seal on this carburetor even though the choke seems to be functioning as intended. I figure there's some sort of mixing apparatus that allows air to escape when you blow up the tube (with the float bowl off) but apparently that won't allow fuel up.

[LT80]

yes, some sure can be cold blooded.
You're correct, they are jetted fat stock. Some ppl like going to a 90 main.
32:1 is what I recommend. "atta boy"...LOL :D

"I figure there's some sort of mixing apparatus that allows air to escape when you blow up the tube (with the float bowl off) but apparently that won't allow fuel up."
Don't know about all that..The choke works on another lil hole. I'm no scientist on the carbs tho. :)

I was out of town or I would have chimmed in sooner.
Don't feel bad about not getting any kudos in this forum.
The LT80 is a red headed stepchild in here.
A guy got quad of the month and not even his buddies could give him one once of congrats.:(
Anyways.......keep up the good work! :)

[sicivicdude]

Originally posted by LT80
yes, some sure can be cold blooded.
You're correct, they are jetted fat stock. Some ppl like going to a 90 main.
32:1 is what I recommend. "atta boy"...LOL :D

"I figure there's some sort of mixing apparatus that allows air to escape when you blow up the tube (with the float bowl off) but apparently that won't allow fuel up."
Don't know about all that..The choke works on another lil hole. I'm no scientist on the carbs tho. :)

I was out of town or I would have chimmed in sooner.
Don't feel bad about not getting any kudos in this forum.
The LT80 is a red headed stepchild in here.
A guy got quad of the month and not even his buddies could give him one once of congrats.:(
Anyways.......keep up the good work! :)

I appreciate your time. I understand you are a busy fella and don't have time to sit and chat :D

Leaving the stock mainjet may be leaving some power on the table but it'll keep my mind at ease. I'll probably just leave it at 92.5.

I run premixed 32:1 premium gasoline in my other toys so I've always got a can mixed up. I understand that the stock oil injection pump is stout on these little machines but just as soon use the same gas can to fill all of the 2 stroke toys at once...

I'm not a pro at carburetors either but know basically how they work and can investigate their operation. This one seems to be functioning properly (it runs without pouring gasoline out of the tailpipe!) and I can blow up the choke dip tube with the choke knob closed. It's not blowing out of the hole into the downstream side of the throat though, I think it's blowing back out of the inlet side of the carburetor. Air only blows through into the downstream side when I pull the choke knob open (which makes it easier to blow through the tube). I'm definitely able to blow through the tube with little resistance with the knob closed which had me worried before.

I'll take and post some pictures of the little monster (my 4 year old) and the atv when I get a chance. So far it's been a great $225 investment.

Just for future reference:

I blocked off the oil injection and plugged the hole in the carburetor.

I installed a yamaha blaster petcock and blocked off the hole for that.

I installed a new belt.

I removed 2 rollers from the front variator movable sheave.

I shortened the wiring harness and installed a ATO sized weathertight fuse holder into the "battery box" area as well as shortening the wires for the starter relay and other fuse holder.

I installed a new moose racing air filter and carburetor inlet boot.

I cut the footpeg assembly in half and welded a tube inside the right side assembly to access to the variator assembly. (good advice on that mod BTW!)

I installed a new battery and new starter.

I am waiting on a keyswitch before I can bolt the hood piece back on and it'll be all done.