Vintage Dirt Bike Q & A

Help Please

I am looking at placing a Mikuni on my 79 CR 250 and was wondering what is the best size 36 or 38mm and what setup for main, pilot, slide cutaway and needle and needle seat combo based on sea level conditions.

I can tweak from there once i have the general vacinity to work from.

Any help would be appreciated,

Paul

Mikuni VM36 Round slide
Air screw 1.5-2.0 turns out
Pilot 35
Air 200
Slide 2.5
Needle jet 159-P8
Needle 6FJ6
Clip #2-#3 from the top
Main 350

This is ball-park for Southern California. You would need fine-tune the air screw, needle clip and main to suit. http://www.sudco.com has full charts on alternative needles if you need them. Sam Alexander at http://www.jastco.com also has some ideas on radical set-up for these carbs. You will also need to grind down the bell of the carb to get it to fit between the rubber boots on the CR250R. The Mikuni is longer than the stock Keihin. I have a picture...

_________________
Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

Hey Michael what air filter are you using with this setup??
Thanks

I am using one of several NOS stock filters I obtained. They are oiled with Bel-Ray filter oil. The filter is secured to the metal filter cage with a large after-market hose clamp that I can tighten from the side once I have the filter cage bolted in place. I use a replica Fox aluminum ring on the front of the air box to secure the filter cage in place. Many air boxes have warped and/or the filter cage bolt holes are cracked.

I am only aware of two after-market filters; Twin Air and Uni and both have problems. The Twin Air comes with its carb boot attached and you feed it through the front of the air box and it clamps directly to the carb. The Uni needs to be zip-tied to the metal filter cage. IMO, both approaches are hokey.

_________________
Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

..it may just be the cooler months but I now feel the main above is too lean. I am currently running a 380. Clip in position #3 (middle).

_________________
Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

thanks I'm trying to figure which way to go on the intake (carb and filter)..Your premix ratio is 32:1 ??? Have you seen the filters by Vintage Iron that are supposed to be greaseless?? I wonder how they fit the CR?

I believe you can go either way on the carb; Keihin or Mikuni. You just need to make sure it is not worn (jets, needle etc.) and you have a good baseline from which to make very selected changes. The air filter is a wash from a performance basis; my concern was for fit and making sure no dirt ever got by the seal. I have voted against anything that isn't specific to my application. The pre-mix ratio has a very limited effect on jetting, so going from 50:1 (Bel-Ray) to 32:1 (Maxima) might be just about a jet size or a needle clip position. And of course the really rule is pick something and stick with it.

_________________
Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF