I have a newly rebuilt 250 trail boss. Starts fine and idles fine, when at an idle if I open throttle all the way it will bog down and die. If I feather the throttle slightly it will run fine up to about 3/4 throttle. If I open up the throttle all the way while I am going it will bog down and die. Any suggestions?

 

welcome to the forum. is th engine still stock except maybe for a bore=ing of the cyclinder? has the carb been cleaned and adjusted? you say it idles fine, but when you take off does it seem doggy and has no power(or like starting in a high gear if it were a standard shift car? if this sounds like the issue it might be the buttons in the secondary clutch need replacing as this is a common issue that makes it seem like the bike has no power. just a couple of things to check.

 

Motor is stock except for .040 over piston. Carb has been cleaned and I checked the clutch and it seems to be fine, thanks for the reply

 

Try without air filter for a second. If she runs better air fuel issue.

 

Did you do a good clean job on the carb, here is a good how to link Disassembly and Cleaning of a Mikuni Round slide Carburetor

Its important to get the "pilot air jet" back into its exact original condition.

 

Sounds like fuel issues for sure. What about the fuel filter, been changed? Lots of smoke, could the choke be partially on due to being missadjusted when the carb was re-installed after cleaning. Choke cable too tight.

There is a air screw on the side of the carb. Make sure, as mentioned above that the setting is correct but that should only be on the idle circuit if memory serves me right. Jetting on the carb sets the tone for how it runs at high rpm.

Let us know what you find.

 

main jet/circiut is for say 1/4 throttle and up. if he says it's idling fine then i'm guessing it would be on the main jet side of carb would be the issue if it's the carb.

 

Oops I meant to say position not condition.

From link

 

After checking and rechecking everything, jetting up and down I discovered that when I played with the choke it pepped up. So I adjusted the cable until I got it running pretty good. I'm not sure its 100% yet buts its pretty close.

 

Adjusting the choke is reasonably simple. You need to remove it first by unscrewing it from the body of the carb. Reason for this is to look at the base of the choke plunger to make sure that the little rubber stopper is in the base of the brass fitting that holds it. It seals when the choke is in the off position and if it is missing the engine will run rich.

If it is intact then place it back in the bore and tighten. Then adjust the cable by making sure you have a small amount of play in the choke cable with the choke in the off position. You should be able to pull up on the cable near the coke plunger 1/8 to 1/4 inch max, any tighter and you may have the choke on partially.

The choke lever should then toggle easily and you should be able to kill the engine when you pull the choke on while the engine is running. If this works and you now have the proper main and pilot jets back in the carb you should be good to go.

These engines are cold blooded and need some warming up after being started so I always set my choke very light and can let in run in the 1/2 on position for a few seconds to help it run when 1st started.