Ive been wanting to try different things with my JM 250 Sport. I'm looking on doing a couple of things to get a little more power out of it. For the type of riding i do here its a little bit of everything. But i personally like go up some decent size hills that have kind of soft dirt.

I believe my JM has a 15tooth sprocket in front and im not sure on the back one. But i have noticed that i need more lower end torque like in 2 and 3rd gear. I recently took it out and hit some decent size hills with soft sand was able to go up some and others i would get to the top almost clearing the hill but would die before i got over it.

I know JM has a 14 tooth but was wondering if i could go lower than 14tooth and would that increase my lower end torque. Im really not intrested in top speed at all cause very rarely do i use 4th gear when i ride but i really want 1,2,3 gears to have some more power.

What else can i do to my JM 250 sport to get more power i was looking for a better carb maybe a mikuni carb but it seems like JM only has for 150 i believe anyone have any ideas that would help i would greatly appreciate it

 

I have changed my Jet 250's to a 14 tooth front and still has the original 40 tooth on rear and I still wish I had a lower gearing for climbing. You could go a 13 front and the 40 rear or swap out both to a 15 front and a 46 rear like the Vbike 250's, either would help low end but I'm not sure if one would be better then the other. I think you would need to replace the chain in order to get the extra length needed for the 46 rear.
I am thinking about going a 13 front if I can find one because it would be a lot cheaper to do and easier.

 

Hi outofline, Well i believe i have a 15 in the front and a 46 in the back but ill have to verify that tomorrow. Mine is the batch with the nerfs from Dec of last year. I was looking and found a previous forum saying the front sprocket can be matched to a JTF-259-XX being the teeth and the the back are JTR 802 on JTSPROCKETS.com. I also was thinking about going to a 13. Where you ride are there hills with soft dirt and rocky terrain? Well from what JM told me today is that for every 1 tooth in front you should have 3 in back bit not sure about this and how that works cause another buddy thought it was 4 teeth for every one. I just need a better 2 and 3rd gear and i think i can tear it up here a little better on the soft dirt hills. Do you know if there are any better carbs for these biles? Thanks for your response

 

It would be interesting to see if Jet put a bigger rear on yours, to my knowledge they didn't change it though. They started out putting a 17 tooth front and 40 on rear when they first came out then later changed to a 15/40. John at Raceway sent me two 14 after I had tried the 15 and still was not happy with the bottom end.
I don't have any performance upgrades but if you read back on the Raceway threads someone I forget just now who it was had worked his 250 over a bit and claimed to have gain some power.

 

Ok i checked the back and its a 42 tooth i still think they have a 15 tooth in the front but didnt get chance to take cover off and screws are really tight. Check that website and it has a lot of info jt sprockets. I think it was biskit or someone bored the head or something. Well im going to do more reasearch and order some sprockets next week.

 

Just saw your pics for the first time, looks good but wheres the Nurfs? Let us know how the new sprokets turn out! Looks like Jet after discovering from the first batch (mine) that the gearing was way too high and changed to a 15/42 which still would be higher then my 14/40.
John (raceway) is there a 13 tooth available yet? I would love to swap my 14 out!

 

Those were the first pics and i didnt put them on at first. I have them on know and will post some up soon just have been forgetting but im going to wash it and hopefully take pics tomorrow. ? does your sprocket and chain get full of dirt alot? Well im going to try a 13 tooth perhaps as well. Do you what would be the cause for my exhaust backfiring when i dont have the throttle applied is that cause im running to lean or rich. I did get the new jets from raceway when i first purchased it and adjusted to 1 1/2 turns out. But jetting takes a lot of time and the screw is really hard to get to.

 

It kind of seems one of mine developed a back fire and found it was an exhaust leak at the head. I tightened the two pipe nuts and the back fire as well as what I thought was a valve clacking went away. I don't think there is any way around the whole dirty sprocket and chain.
I installed the new jets, it really was not that hard and took only a few minuets. The new jets sure helped on the cold blooded start up, now I only need to choke for a min or so where before it was 10 or so before it wanted to run good.
You may want to loc tight these following bolts before you lose them, out of my 4 Jets these have all worked thier way out and lost them at one time or another!

Front and Rear Gas tank bolts
All black body bolts
the 3 exhaust pipe to Muffler bolts

I'm sure there are others but these you will lose! And if your riding aggressive be sure to reinforce your rear shock mount! Even just welding a thick washer at the eyelet on the swing arm would do the trick!