I am currently working on a stock 98 or 99 Trailboss 250 for my wife's Grandpa. He let it sit a while (6-8mos) without any fuel stabilizer or anything. He started it one day last week, let it idle for a while and revved it. It was fine. Another day, last week, he started it and coudn't keep it idleing without the choke. I took it for a spin and found that full throttle would not work at all (tried no, 1/2 and full choke). I figured it was a dirty carb from sitting. I cleaned it all out and found the main jet plugged. I cleaned everything with carb cleaner. I dumped all of the gas and put in new fresh gas. I took every jet and screw out and blew every hole out with carb cleaner. I put it back together and the problem changed a little. If I am using 1/2 choke, it will go like normal in all throttle positions. If I shut the choke off, I can only use no to half throttle. Full throttle without the choke acts like you switched the key off (But spark is fine). The paper filter is new and the prefilter is freshly cleaned and oiled. I would think that maybe the jetting is off, but why all of a sudden? Grandpa has owned this since new and always had the dealer do the service. Also, I checked the intake manifold and made sure the bolts were tight. I checked compression, it is about 115psi. I tried to adjust the idle mixture screw and could run it all the way in without affecting the idle! What is happening? What could be wrong? What can I check? Help!

 

well i know whats wrong. im 15 and have a 1972 yamaha dirt bike with the same problem. it was sitting for 5 years with no stabilizer. i finally took it to a place to have them fix it and they told me thats the jets and needles get clogged for sitting there with gas in them. you will either need to replace them or have them cleaned. hope this helps

 

Like I said, I cleaned the jets and ALL passages and everything is fully open and clean. What else can it be?

 

Sounds to me like the jets are plugged up again. It takes awhile for all the crap to come out of the tank, and itll keep replugging the carb, until the tank is totally clean, or you put in an inline filter.
Are you running it with the air box lid installed? If its off, that wil lean it out.
You should be able to just drop the float bowl, and pour the contents into a clean paper towel. and see if any new crud is in there.

 

sounds like its lean fatten it up a bit

 

I took it apart a 2nd time and cleaned everything out again. Also, it already had a fuel filter installed. It has an in line oil filter as well. When I took it apart the 2nd time, everything was still clean (No chunks or goobies in the float bowl). Main jet was clean. Nothing was clogged. I was running with the airbox lid on (Every time). What else?

 

air filter clean? Track the fuel system, and make sure lines are ok........new spark plug??

 

check and see if the petcock is working properly. See if you have steady fuel flow

 

Air filter is new, pre-filter cleaned and oiled. Petcock works fine (That is how I drained the tank). Flow is good. A new spark plug is on tonight's to do list. Anything else?

 

Did you blow something out of the carb? I've heard that if you blow at the passages the wrong way, you can really screw up the carb.