1978 RM 250 C2 jetting question

[midlifemoto]

just installed a Mikuni 36 mm from Vintage Suzuki. It came with a 310 main, 35 pilot (i put in a 45) Q5 needle jet, 6FJ6 needle in the middle clip seting #3, 2.5 slide, 3.3 needle valve and 2.0 air jet. I live in Portland, Oregon, about 300 ft elevation. Ran the bike and it runs like crap....don't even know where to start...nuthin off the bottom, slight performance in the middle and blubbering on top....fresh gas, air cleaner, reeds, plug, etc...didn't neglect the obvious stuff...any suggestions? thanks

 

[Enduro_Nut]

You might check the Yahoo boards, they might have one for the RM guys...
Where did you get the benchmark numbers from and do you know how to jet a bike properly via throttle chops?

 

[BigRedAF]

Great bike!

Call FMF and ask a tech to dig up some notes. I think FMF used to race the 78 RM's on their support team like Pro Circuit is doing with the KX250F's now.

 

[2strokerfun]

I've been dialing in my 36 mikuni on my 76 elsinore for a couple of months. Finally almost have it perfect, but I'll never buy another "spec" carb. It is just such a crapshoot. Some others I've talked to have bought a spec carb that works, but most people have to fight through a complete rejetting, including different needle jet, jet needle, main and pilot. Good luck. It helps to find someone with a similar bike and carb who doesn't mind sharing the numbers (assuming they wrote them down!)

 

[2strokerfun]

Looked in my book to see what I'm running and speculate what you might want to change. If you are blubbery throughout, here is what I would do: go ahead and step your main down to a notch to 300 for a starting point. It might still be rich, but you'll be closer dialing in your needle and needle jet.
Try leanest position on needle you have. If still blubbery low to mid, then go to a 6cf1 needle or even a 6f9 needle. If bike still feels rich, you'll probably need to step the needle jet down to a Q4 and start over with your stock needle, going through the needles until you get it close to right.
Once you have it running close to what you want, I would readdress the main jet.

The problem I had was I couldn't even get my main jet right until I had my needle jet right, so I had to go through three different needle jets, before I finally found the right needle and main jet.

I ended up with a Q4, 260 main, 6f5 needle and 35 pilot--but your bike should pull a whole lot more air than mine, so your final solution should at least include a richer needle and richer main, I'd expect.
Good luck, its a PITA.

 

[midlifemoto]

Thanks!