Vintage Dirt Bike Q & A

!st time on this forum, but have been on the Yamaha Road Star Warrior site for a couple of years. I need some help from any Elsinore guys out there. I have a 74 MT 250, just did the top end, bored one over with a single ring Wiesco piston. I have 150 lbs compression. I rebuilt the carb, which is a stock Keihin 28mm, I replaced the jets and needle. The slow jet is a #60, the main I have tried #120-#125. the needle is a D41, clip on the 3rd psotion down. when I start it up, fires up every time, purrs at an idle , when I start accelerating I get to about 4000 rpm, even tough to get there it flattens out, no power no revs. I have checked the points and the timing, they are spot on. My sons Yamaha PW80 will beat this legend of a bike. Any help, I would very much appreciate. Thanks

I also have a 74 MT-250 and have been looking for jetting data for a long time. Let me know if you find something. I have owned this bike since about 1980. The bike came with an expansion chamber. I think it had been rejetted in the 70's. Back then the bike ran rough at the top end. 20 years later I have rebuilt the lower and top end and added a very clean stock pipe. Just mounted a cylinder with a fresh bore once over Wisco. Before the bore I had purchased 2 carbs on ebay and a Keyster kit with jets hoping to improve performance. Those Keyster jets (60 pilot unmarked needle and 122 main) would barely run. The first ebay carb was in bad shape. The second looked like new inside and out. Amazing how that happens. Unfortunately I didn't keep track of which jets were in the original and the first replacement. I find it interesting that they are all a little different.

The carb I am running now (second ebay purchase) was from a 75 MT250.
The pilot is stamped with the Keinin symbol. It has a 58 pilot. A 271004 Needle. The Main is marked MX108. This carb runs the best of the 3 but is still a little rich. Clip is second from the top. I am located just slightly above sea level.

The other carbs contained
2 Keinhin stamped Pilots both 60
2 Needle jets 271004 and 271002 (No other markings)
2 Main jets. Keihin stamped 122 and MX 125.

I'm sure what the MX represents. Maybe after market?

I would tend to believe the jets should be ok unless someone made some changes before you got it. Does it sill have the baffle? If so remove it and burn the carbon off with a torch. Also check where the baffle fits in the pipe to make sure there is no blockage there.

Good Luck

Check for a plugged muffler. Pull out the insert and give it another ride. Let me know if this works.

Hey Rick and Jake, thanks very much for the info,very much appreciate the ideas, will give it a try and post my findings! Paul

I'm not sure I have this totaled right, something like 3 different carbs with various swaps of jets, different pipe, a top end rebuild, and the MT has the same symptons?

That would lead me to believe carburation isn't the problem. The chance of 3 carbs having the same exact problem isn't real good. I mean if maybe there was a systematic error introduced like you reset the floats on all three to the same wrong level, but I believe you are probably much better at it then that.

Did you change the capacitor in the ignition circuit? I have had similar problems when one went south on me. If it went bad, the points will also be taken out from the failure and need to be filed clean. I'd also try setting the advance just a pinch more.

Hey Ricortes, you got a bit mixed up with one of the responses, I just have the one carb, but thanks for your input. I spent the day today determined to get it going. I tried another exhaust pipe at the parts place to try out...this increased my rpm a bit. I went to the stock jetting, this also helped. I noticed that the specs call for a 3.0 throttle slide, the one in my bike was a 3.5. I changed it to the 3.0, also adjusted the float a little more. After I have the pipe hot tanked it should run a lot better. Im pretty happy with it now. Thanks again for all your input! Paul