what size carb should i run?

I'm almost done restoring a 1975 WR250. I have a 38mm mikuni but I'm not sure if I should run it or go with a 36mm? Anyone running this bike with a 38mm?

I have the same 1975 wr 250. Usually an increase in carb size nets more H.P. in the top and less power on the bottom. Think about trials bikes. 300cc and small carb. I am using a 36mm. Starts easy and runs very smooth until you hit about half throttle, then it rips. If I had a 38mm....I would try it. You can always order a 36mm. I got mine from Huskyjunk pre-jetted. I changed the slide, needle, needle jet, main and pilot. Yup...I used the air screw and idle screw. Jeff

Thanks Jeff, I just ordered a 36. I have a 38 but I didn't want to go thru the hassle of trying to get it jetted and then want to try a 36. I know they came stock with 36 so that's the way I am going.

I'm thinking of trying a 36mm on my 78 250cr. With installing a UFO I may have to change the needle jet to a learner one.

On my 1976 250CR I originally was going to try a 36 Mikuni round slide. Once I started looking into the cost of swapping slides, needles, and brass plus all the time to dial in the jetting I decided to go a different route. This was based on the positive reviews on this website... and it was something I wanted back when I was a kid but couldn't afford... A Lectron carb.

I used a later Husqvarna Mikuni manifold and air box boot to replace the original Bing parts, sent the dimensions to Lectron, and they sent me a 36mm carb they claimed was "dyno tuned" for my bike. (yeah, right)

I didn't really believe them when they said it should run good right out of the box but the "jetless" design and reports of clean running over a wide range of altitudes and temps had me thinking it wouldn't be hard (or expensive... no jets or slides to buy) to dial in.

I was surprised at how PERFECT the bike ran WITH NO CHANGES! I was out at El Mirage and it ran clean from idle up to full throttle. I even purposely rode around as slow as possible for several minutes to try to get it to load up. It didn't. Responded cleanly when I rolled on the throttle effortlessly lifting the front wheel.

Lectrons are still expensive at twice the price of a "pre-jetted" Mikuni. But I have to say that their claims are not hype. It was truly a "bolt on and run" experience. I could probably fine tune it a little but the plug looked great and it ran so good that I didn't feel the need to touch it.

It also might be my imagination but I seemed to get GREAT gas mileage... that's something people report and I'll have to check out closer when I get to ride it more...

I go with everything you say about the Lectron. The first one I bought for the 78 390WR was spot on perfect out of the box. I did try 1/4 turn richer and leaner from stock, but it was not as good. I run the stock settings for the metering rod and 1/4 richer on the power jet setting. Gas milage went from around 20mpg (UK gallons) to 35-37 mpg. Runs cleaner and does not load up with slower running. I had not run this 390 for nearly a year until this week. I dug it out (complete with year old fuel) and it fired up second kick sweet as !

Disonny .... if you can stretch the budget go for a Lectron. You won't be disappointed !

indeed, no hype. when you start looking at the cost of slides, needles, and jets, the difference isnt as big either. lectron isnt the cheapest solution for sure. they do work well. the taper bore and bell area show great machine work too.
my time is also worth something as well. i do enjoy tuning but the lectron is simple and is finely accurate to adjust. more time riding is a good thing. the performance gains in smoothness, metering, and top end pull is just a bonus!

The Lectron is great But.... Lectron is not vintage legal, if you want to stay vintage legal. I rode my WR yesterday and it runs great. I am not a jetting guru, I bought a lot of brass parts. Hours of jetting and I ended up with good running WR.

legal for 75 and up, as they were around then.

YEA THEIR LEGAL IF YOU USE ONE THAT IS PERIOD CORRECT

the mikuni vm is a great carb. easy to get parts for, and works well. no doubt there

Lectrons are not legal for AHRMA Vintage but they are legal for Post Vintage. My '76 250WR runs fantastic with a 38mm Mikuni. It has so much torque and will lug so low I thought it was a 36mm when I bought the bike but recently I took it off to see what jetting it had and its a 38mm.

When Husqvarna first went to Mikuni from Bings, they put 38mm VMs on both the 250 and 390 models. Only appropriate jetting changes between them. I am sure they saved a lot of money just buying 38mm carbs until they introduced the 500CR anyway

Hi Jeff- just got done with the original Bing on my 76 WR250- going to the 36mm Mikuni- would you tell me how yours is set up since you changed so much on yours? I am just dying to get mine up and running after spending months restoring it.
Thanks,
Terry

Sorry, I can't help. Craig Hayes custom tapered a needle for me. So, all of my jetting specs are set up for that needle, But, I don't think it was needed for the custom needle. That is what I have. How are you mounting the carb to the airbell?