79 YZ 125 midrange problems

[fatboy570]

Been fighting this bike all year. New top end, new reeds, all seals were replaced at the beginning of the year. New 34 mm round slide Mikiuni. The problem is, after the engine gets hot (normal race temps) it will seem to lose power thru the mid. Does good off the bottom and on top end, but struggles and misses thru mid. Ive changed needle jets Q-O, P-8, P-4, O-8. Changed needle clip up and down every time I rode, trying to nail it down. Changed main jets, every step from 390 to 330. Needle is 6F15, about the only thing I havent changed. Could the problem be in the coil/electrical system? Thanks for any help

 

[2 strokes for life]

fatboy570, never heard of the problem befor but i am willing to bet a different needle might clear things up. Not sure witch one to go to thoe.

 

[zig06]

Try going back to stock.

I personally think that you are running too lean on the needle. But go back to totally stock (assuming that your using a stock carb) and go from there.

 

[CRazy250]

make sure all hoses and vent tubes are routed correctly and in the right spot

 

[fatboy570]

Thanks for the replies. It is a 34 mm carb, stock is 32, so I may jet the 34 with 32 specs, that will give me a good starting place. Thanks again

 

[pryor]

Hi fat boy can you try a stock carb on it? jetting the big carb the same as the stock one may cause a lean condition.

 

[fatboy570]

Ill have to dig thru my carb pile to see if I have 32mm. If not I can jet the top end rich

 

[2strokerfun]

I could be wrong, but when you say you changed "needle jets," from the numbers you list, it sounds like you are actually changing the "jet needle", not the "needle jet". The needle jet is the hollow brass tube the needle goes into on one end and the main jet screws on the other. When changing carburetors, I often have had to change that needle jet when I can't dial it in with changing the needle alone. Between the needle and the needle jet, you have a very wide range of options for dialing it in.
The VM34mm normally comes stock with the 159Q2 needle jet. Try going one up on the needle jet, then play with the needles again and see what you think.

 

[fatboy570]

Thanks again. I have Sudco's phone number, so I will call them Monday to ask some questions. I need to get a new float bowl for another carb since it is cracked as well

 

[79yz125]

i have a 79 yz 125 and it kept braking up about half to full, so i re-oiled my airfilter and it ran fine, sounds simple but try that with some regular jets and it was a quick fix for me