Mikuni carb on stock 250r

[gimmeamidget]

Tried searching for hours and no definite answer. Finally got my 85 250r put together and pressure tested and fired up last night. When I was cleaning the carb I checked the main jet and it was a 290. This thing runs great but my concern is it almost runs too good and I am woried its too lean? Throttle response is very good and it doesnt act like its starving for fuel. 290 seems like plenty big jet but I could find no reference since 250r had Keihin carbs. Does 290 sound right for those who have run Mikuni. Bike has stock pipe and cover on airbox.

[gimmeamidget]

I plan on doing plug chop but it got late last night and my neighbors already dont like me, just hoping someone could chime in let me know if im close. Also I live in Utah elevation around 5000 feet.

[RIDE-RED 250r]

There aren't too many guys running Mikunis on their 250r, and I hate to give you a non-answer. But the best advice I can offer is to take the jetting where the engine leads you. If you have concern about it possibly being a bit lean, fatten up the main until you KNOW it's overly rich and step down from there a size at a time till you zero in on a size that offers good performance without being too lean. Repeat the process for the pilot jet.

What machine was your Mikuni originally on??

Do you have an assortment of jets on hand to dial it in?

Is there any info out there that cross-references Keihin and Mikuni jet sizes?? That would certainly be helpful in determining a safe starting point....

[RIDE-RED 250r]

Hmm... I just did a quick search and did find a cross reference chart... But I'm wondering if different Mikuni carb models have different jet numbering sequence because the chart doesn't go as high as 290 for Mikuni... I had Ski-Doo sleds that had jets numbered high like your carb...

More digging to do...

[RIDE-RED 250r]

OK, I think I found it.. I bet you have a Mikuni with hex (Amal) type main jet. Those mains are numbered drastically different than Keihin, DJ, and even Mikuni round (slotted) main jets...

If this chart is correct, you are running equivalent to 147.5 main in Keihin sizing... Now granted, different makes and models of carbs will have different characteristics and jetting from one carb will not necessarily be spot on for another... But, with this info, I think we can better help you out...

For a stock setup like you are running, your jetting sounds to be pretty close. But I would get a few jet sizes above and below what you have for main and pilot jets so you can tune it in properly and KNOW your jetting is sound...

Here is the chart I found:

[gimmeamidget]

Perfect, thanks for your help. I have been at this all last night and this morning with no luck but this chart makes me feel tons better. It is running great.

[The_Steve_Man]

290 sounds about right for a stock motor.

[gimmeamidget]

Glamy its a 38 and i will let you know if I decide to swap it. I have heard good things about these old Mikuni's but I have alot more experience with Keihens.

[RIDE-RED 250r]

290 sounds about right for a stock motor.

You just jogged my memory, you are running a Mikuni aren't you?

Is 250r.org not working for you too lately? My anti-virus thinks their page is a threat and wont let me do anything there

[The_Steve_Man]

You just jogged my memory, you are running a Mikuni aren't you?

Is 250r.org not working for you too lately? My anti-virus thinks their page is a threat and wont let me do anything there
I run the TMX off of the CR250. Absolutely the best carb I have used.

250r.org works fine for me. I use tapatalk and don't have any problems.