1981 XT 250 Jetting after mods

Hey guys,

I have an 81' XT 250 that I have been trying to get a little more "umph" out of. I removed the factory airbox and installed an open element K&N as well as fabricated an 1 1/2" header and free flowing muffler.

The bike currently has the stock Mikuni carb with a 17.5 pilot, 165 main, and the needle is in the middle position. What I'm looking for is an idea on where to start on the jetting changes. Thanks in advance for any and all help on the matter.


How is the bike running? How does the plug look after WOT and shutdown and pull the plug?

The bike hasn't been fired since the mods. I currently have the carb apart cleaning it and I was hoping to get a ballpark on the jets needed so I would only have to do some fine tuning once it is back together.

I was thinking I would start with a 20 or 22.5 pilot and possibly a 170 or 175 main. I was hoping someone had opened up an old XT or TT in a similar fashion and could give me some insight as to what worked for them.

Maybe I should just try firing it with the carb in stock form and see how it reacts. Honestly though, I have heard that these bikes were set lean from the factory so with the added airflow I'm kind of scared to try it without throwing some more fuel at it. Any insights are welcome.

Thanks, Pete

Pilot should be fine, maybe 5 up max on the main I would think but I would see with the stock main first and see if your air screw is maxed out.

Ok, that gives me an idea of where to start, thanks.

Don't assume you have to go richer. On my '80 DR400 and my '80 KLX250 I had to lean the mix on the needle and the main. Better breathing caused a stronger venturi effectively sucking more fuel through the same jets. Find a long steep hill. Go up it at full throttle and hit the kill switch. The plug should be digestive biscuit colour. If black it's rich and/or using oil. If it's white or grey it's too lean. Once you get a main jet that's right then try the same trick at half and 2/3 throttle . If you have a slide carb [non vacuum, no rubber at the top] dropping the needle weakens the mixture.

Hi all. I have an annoying problem with my 3Y3 1981 XT250 which i'm hoping one of you knowledgeable people might help me with. I've recently had a top end rebuild and she now runs as sweet as.... except it wont tick over. During normal service the running is fine - come to a junction or lights and its cuts out unless you hold it on the throttle. If I try to adjust the tickover speed on the throttle screw it races its nuts off :banghead: I've tried most everything with no success. I should add that the bike has always done this during my ownership and to try a fix it I had the carb ultrasonically cleaned, a Keyster rebuild kit and all the chambers etc cleaned through by the carb builder - essentially a 'proper' job. I have checked the inlet boot for leaks but there is none and a new air filter takes care of the downstream side. Anyone suffered similar problems?

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