• Husqvarna Motorcycles Made In Sweden - About 1988 and older

  • Hi everyone,

    As you all know, Coffee (Dean) passed away a couple of years ago. I am Dean's ex-wife's husband and happen to have spent my career in tech. Over the years, I occasionally helped Dean with various tech issues.

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1976 WR250 with VM34 Mikuni

Marcgregory

Marcgregory

Husqvarna
C Class
I picked up a '76 WR250 about two months ago and it was already fitted with a Mikuni carb. I figured it would have been a VM36 or VM38 since i feel that is the usual swap from the stock Bing. Turns out it is a VM34. I have not ran the bike much since I have been freshening it up and getting it ready for riding when the weather gets better, just a few rides up and down the driveway. Is a VM34 too small for this bike? I feel that VM34s are usual spec'd on 125s and 175s. I am just afraid that the bike isn't getting as much fuel as it should? I believe the pilot is a #35, starts good and idles good. Haven't got the bike over second gear yet, cannot remember what the main is. So far the spark plug looks good in color, but like i said, haven't gotten through all the gears yet. Thanks, Marc
 
The carb size moves the power up or down. My 83 250 has poor low end and Sudco recommended a smaller carb to help. I now have a 36 on it.
Once you get a good ride on it and feel how it runs that may tell you more. If the plug reads good, the jetting is good.
 
Marcgregory said:
I picked up a '76 WR250 about two months ago and it was already fitted with a Mikuni carb. I figured it would have been a VM36 or VM38 since i feel that is the usual swap from the stock Bing. Turns out it is a VM34. I have not ran the bike much since I have been freshening it up and getting it ready for riding when the weather gets better, just a few rides up and down the driveway. Is a VM34 too small for this bike? I feel that VM34s are usual spec'd on 125s and 175s. I am just afraid that the bike isn't getting as much fuel as it should? I believe the pilot is a #35, starts good and idles good. Haven't got the bike over second gear yet, cannot remember what the main is. So far the spark plug looks good in color, but like i said, haven't gotten through all the gears yet. Thanks, Marc
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it wouldnt be a question of how much fuel it gets, more how much air it can flow. to a point, the smaller carb will boost bottom at the expense of top altho im sure theres a point where too small will hurt bottom too.
 
the bore size of the carb and the engine vacuum control the velocity that the air moves over the jets. small carb means higher velocity at low engine speeds, better atomisation of the fuel thus better initial throttle response and low end performance. at higher revs, the small bore carbs hit max flow and the engine starves for air/fuel mix as the carb just wont let and more through. too big a carb and the low rev velocity is so slow the fuel is poorly atomised and motor feels like its bogging at wider throttle openings...performs better at higher revs where the velocity through the carb is faster. much better top end as it can supply all the air the motor needs ...shimplessss
 
Isn't that what those venturi splitter plates supposed to do- atomize the fuel better at low rpms for large carbs?
 
Eurofreak said:
Isn't that what those venturi splitter plates supposed to do- atomize the fuel better at low rpms for large carbs?
Click to expand...

they help to a point but theres much more than atomization issues when talking about which carb size to use.
 
I know of several people who have 34mm Mikunis on 250s of various brands and they work fine especially if you want the power to be focused in the low/mid range. My 76 250WR has a 38mm and it runs flawlessly. I would ride the bike and see what you think first. If I had a Bing on my bike and was going to buy a carb I would probably go with a 36mm in spite of how good mine runs now with the 38mm.
 
Thanks for all the good replies, I learned a lot! I am mainly going to use it for hilly trail riding so it looks like it will work out good because i will be needing extra low end power. Below is a pic of the bike. It was a mess when I got it. No compression, '78-'80 WR tank, mismatched petty fenders, blown Curnutts (Scott Tremblay is currently rebuilding them), crap aftermarket handlebars, all cables shot, stuck clutch. The usual for a low priced craigslist Husky. I have been making pretty good progress, luckily I had acquired a lot of new parts over the last year for a '76WR360 project that i haven't got to yet.IMG_7954.jpg
 
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