Vintage Dirt Bike Q & A

My new to me 79 rm125 seems to have the carb setup bizarre: 260 mainjet (250 stock) needle clip second from bottom (second from top stock). It also had the optional B8 plug. So far I put a stock B9 plug and put the needle clip back. It still sputters at past part throttle to full throttle. I should be able to pick up a stock jet locally.

My question is why? Was this the hot setup in 1979? Will I cook the piston of I don't run mega rich? I get 150 PSI compression kicking with running shoes on, so hopefully thats OK.

Comments?

-doug

76 370 Pursang in pieces
79 RM 125
86 KX 250

Doug,
With out knowing the history of the bike, it's hard to say about the jetting specs in your carb. It may have been at run at a higher altitude than where you live (just a guess). Sounds like the float needle may be worn. My '77 would start easily, but spit and sputter and bog at full throttle. Messed around with jets, needles, air screw, pilot jets, etc,,,, Put in a new float needle and she ran like a bat outta hell. Try that before making any radical jetting changes. If you do have to make changes, do them in small increments, then check the plug color. 150 psi is plenty of compression for a 125.