Vintage Dirt Bike Q & A

Just swapped a CR250 top end onto my MT. I'm having a hell of a time with carb selection & jetting specs. I have a Mikuni 38mm flat-slide that I'd like to use but it is running real rich. I also have a 34mm Mikuni round slide. The bike runs better with the 34 but it needs to have the choke engaged to run.

What size carb are you using & what are the jetting specs?

Thanks
~JM~

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

Nobody has any information to share on this topic? I have searched the board but really haven't found any answers. I now have the 38mm Mikuni flat-slide installed with a 280 main & a 45 pilot. The needle is dropped to the lowest position. The bike starts & runs but is still very rich plus it won't idle without dying.

Where should I go from here?

Thanks
~JM~

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

The 38 is way to big for your application. Most of the ported cr250m bikes I see only run a 36 and yes some have the 38 but yours is a stock cylinder. The stock carb was 32mm and the optional one was 34mm. I have a flat slide keihin pj on mine but I dont know what size it is but it is too small. I think its a 34mm carb but dont know for sure and it wasnt fun to jet. There is a lot more to tuning thses than just the jets my advice would be to read a lot about tuning carbs and that will make it a lot easier.

My cylinder has been bored 80 over & has been decked .040". It has also recieved some port work. The bike is ridden like a road racer. I had personally thought that the 38mm was too large, but have recieved conflicting opinions on this. I have read the SUDCO book on tuning the Mikuni. I am just trying to avoid having to buy a bunch of jets that I will be unable to use later.

Thanks
~JM~

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

Well I have swapped to a 34mm round slide Mikuni. The bike runs fairly well throughout the throttle range other than a pinging at the top end. I am now running a 40 pilot & a 260 main. This 34 doesn't hit anywhere like the 38 did. But the 38 only ran good at WOT.

Anybody have any other jetting experiences they'd like to share?

Anyone familiar with the differences in jetting a flat-slide vs a round-slide of the same size?

Thank you
~JM~

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

i have a cr250 r and i use a170 main jet 270 is like a straw i would try a 180

Dave,

Are you running a Mikuni or Keihin carb? What size is the carb & is it a round-slide or a flat-slide?

Thank you for the reply.
~JM~

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

The 38 is really only good for wot on a bike this size as its so big. I would stick with the 34 and get the jetting and needle jet and slide right. 280 also sounds way rich to me probably even on my engine and the ping is not a good thing what gas are you using. You need high octane with your deck job thats somthing I didnt mess with on mine, I like pump gas. Mine is still a touch lean on the pilot so when I take it off to up it one size I will check the specs. My carb is a keihin flat slide pj off of a newer cr. The guy bought a new carb as he said it wouldnt idle. Lol he just didnt know how to use it thats all.

When I had a Mikuni round VM34 on my MR250, I had best results with a 290 main.