I bought a year 2000 lt80 for my daughter a couple of years ago. It had been ridden hard before and had a flat spot at the top end, at full throttle, other lt 80s would walk away from it. The bike is stock except for the pipe which is a DG. Last week I had the jug bored 1 over and installed the corresponding piston and rings. It starts and runs hard for about 20 - 30 seconds and then flat out dies. It then will start right back up and do the same thing. I think it is starving for fuel due to the greater displacement. I'm not sure. Plenty of spark. Should I re-jet the carb, if so, any suggestions?[img]i/expressions/face-icon-small-confused.gif[/img]

 

I am almost positive that the jetting is the problem.

It isn't getting enough gas. Grab a bigger jet, and adjust the airscrew accordingly.
Hope this helps!!

Z

 

Thanks for the reply. I believe your right. There isn't an air/fuel mixture screw on the mikuni vm 16sh. The carb has a starter, pilot, needle jet,jet needle, and main jet. I have lowered the clip to the lowest position on the needle jet to try and help out the main jet. No effect. I wonder if I should go up on all the jets in size?

 

Oh, haha. Yeah, I'm not a very fond of the "miniquads" i guess.
I think you should try to go up on the jet sizes. Give it a try, and update me on what all happens!!



Does anyone else have any other suggestions for the fella? Although, I DO think that the jets are too small. It makes sense.

Z[img]i/expressions/face-icon-small-happy.gif[/img]

 

Did you get the hoses hooked up correctly? It takes vacuum to open the fuel petcock.

 

I finnally got it running like a scalded dog. Baseballroxmasox was right. Re-jetting was the answer. In case anyone else has the same problem with the same bike, I will list the combo that worked for me. Bored the cylinder 1mm over w/ matching piston and rings, this brought it from factory 82.4cc to 89.2cc. Changed the main jet from #92.5 to #97.5, third position from the top on the jet needle, went to a hotter spark plug NGK bp6hs, 2.25 turns out on the idle screw. It runs smooth through the rev range and is a heck of alot faster. It will even pull the front end up a bit on asphalt. Thanks for the replies guys, it really helped point me in the right direction. Through the jetting process I have now become a master at the removal/installation of a mikuni carb on an lt80. Thanks P.S. I am at sea level, differant altitude may require diff. jets.

 

Hahaha, that's great news! Yeah, I am the master at getting my carb out and back in too, lol. I can take the airbox off, then the carb, put different jet in, put the carb back on, and replace the airbox in under 2 minutes!! YESS! lol.

I think my jetting is still off, because I'm running a small #128 while Piper told me to be running a #134 with the air screw 2 1/4 turns out. But I have it with the #128 at 1 3/4 turns out. It runs good... but sometimes idles really low, or dies. Hmm, I am thinking about posting a thread. [img]i/expressions/face-icon-small-happy.gif[/img] Well anyways, glad your quad is running right!!

Z

 

Interesting..
1 over (1mm) is the 2nd bore over and will produce 85.798 cc's. You must have went hog wild and went to the max of 2mm over (52mm) which gives you 89.196. and a Weisco piston. Did you need to go that big?
Seeins I'm 10,000% positive that the jetting could not be the problem (they are jetted way fat stock) I think another problem it was. I think maybe you crushed the lil black rubber piece that covers the jets up (I've seen this many times~the 20-30 second run time tells me this). OR the stock jet was plugged solid. FYI: next time in the carb, leave that black rubber piece out.
You'll prolly find the plug getting black in the near future.
I bet it'd run great with a 87.5 or a 90 main jet. [img]i/expressions/face-icon-small-smile.gif[/img]

 

Your right. My mistake, I used 52mm instead of 51mm. The piston is a Suzuki 1mm over. The black rubber boot is installed correctly. What does it do? What does getting rid of it do? Before I took the jug off to bore, I cleaned the carb real well, removing the jets and cleaned every thing with cleaner and compressed air. No jets were blocked when I re-installed the jug.
It seems to run great now with the bigger main jet. It starts quick and idles smooth. But you've got me suspicious of it now. It does'nt add up. Now that I realize that I am at only 85.7cc the factory jetting should have handled that. Do you know what the compression should be on that motor? I need to investigate further. Thanks for the help.

 

Once seated, it should have 130 +/- compression.