[Dirt Hokie]

Just got a XR75, its a 79 frame with a 74 motor. Motor was professionally rebuilt, but the bike was reassembled by someone else, so the engine builders were not able to tune the carb.
So now I have it. Not sure if the carb is original or aftermarket, it looks newer then a 40 year old carb, but looks like the original based on the pics in the manual.
So this is were I am at, bike starts and runs, and will idle...but will bog when giving it a little bit of throttle, and wants to stall out, then if you get going faster and give it more then 1/2 to 3/4 throttle it will not gain power and sputter. The other thing it will do is not start with the choke on, even if cold, and will immediately stall if you flip the choke on once its running, not sure what that means. I tried setting the idle and air screw per the manual, but it did not seems to help.

thanks for any help

 

[davmo]

Sounds like a rich carb situation. If it starts without choke when the weather is cold, that is a good sign of richness. It could be one or both jets are too big, the pilot circuit could be set too rich, or the idle set too high and causing too much pull on the main jet. Set your idle screw so that it just contacts the slide and raises it just slightly. Set your idle screw to get the leanest setting that delivers a good idle (since the pilot circuit mostly affects the first 1/4 of throttle, I just hold the throttle at about 1/4 and look for the smoothest, fastest running setting on the idle screw.) Once that is set, move to the main jet if there are still problems. The first thing to try is moving the main jet needle clip (moving the clip toward the pointed end makes it richer, toward the blunt end to get leaner.) If needle setting won't do it, a smaller main jet would be the next step. TBoltusa.com has great carb setting info in their tech section.

 

[Darrie]

Davmo's advice as good as it gets !!!! Thumbs up ... that answer bang on the money.

The only thing I might add is be wary that the air box intake hasn't been modified or if an aftermarket pod is in use.... airflow velocity changes can disturb the factory carb setup with too much airflow, pods sometimes have so little resistance its like running the bike with no element. May not be your case, but my Z50 with std carb & 88cc refused to tune with the snorkel cut off the airbox & the airbox opened up as such. Would bog down on fast throttle twist no matter what clip setting. Added back a genuine air intake and issue immediately resolved. Small motor syndrome not sure, but just couldn't handle the air volume change. Just FYI if all else fails !