• Husqvarna Motorcycles Made In Sweden - About 1988 and older

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Jetting for 1979 250CR with 38mm Mikuni

crew67lm

crew67lm

Husqvarna
AA Class
Mikuni 38mm VM carb,I assume the original.
Pilot jet is a 45
Slide cutaway 2.0
Needle jet is a R-2
Needle is a 6DH3
Main is a 430
Carburetion is way rich almost throughout the rpm range.Burbles coming out of a corner,cleans out in the upper middle,then back to rich on top. What is anybody else using for jetting from sea level up to 1000 feet? Thanks
 
Here is a quote from an old post:


"Here is the reccomended jetting for the 1980 250/390 CR from ProCircuit for Southern California elevation 0 to 3000 ft

Pro Circuit knew Huskys back in the day information is a direct quote from ProCircuit

250 CR Husky 1980
38 mm Mikuni Round slide
Stock Needle 6DH3 change to 6F15
Stock Main jet 430 change to 480
Stock R2 Needle Jet change to R0 or Q8
Go to a 60 Pilot
These changes from the stock factory bike will result in cleaning up the bottom end and make the top end a lot fatter."
 
crew67lm said:
Mikuni 38mm VM carb,I assume the original.
Pilot jet is a 45
Slide cutaway 2.0
Needle jet is a R-2
Needle is a 6DH3
Main is a 430
Carburetion is way rich almost throughout the rpm range.Burbles coming out of a corner,cleans out in the upper middle,then back to rich on top. What is anybody else using for jetting from sea level up to 1000 feet? Thanks
Click to expand...
I'm running a 35 pilot and 360 Main in my 78 which is the same motor as yours I would go to a 390 Main first and see if it still blubbers on the top,the last time I rode mine it was 95 degrees out so the leaner jetting worked ok for me.
 
I wouldn't recommend the Pro-Circuit jetting specs. No way will the bike run with a 480 main jet.
I seized my 77 250 with a Q-8 needle jet. Confirmed after a new top-end, it can't get on the main jet with a Q-8 (Should have done that first).
I also confirmed an R-0 with a 6DH2 needle is still to small.
I tried an R-2 needle jet with the 6DH2 needle and it ran good, but is it a bit to rich, even with the clip at the top.
Haven't had a chance to try it yet, but I think the R-2 with the 6DH3 needle (a bit fatter profile) is going to be perfect. Probably 4 slot down on the clip.
I'm running a 40 pilot with a 2.5 slide and a 430 main jet. Air screw about 2 turns out.
A 35 pilot with a 2.0 slide would probably work the same, but I think it starts easier with the 40 pilot and the 2.5 slide and really pulls clean off the corners.
FYI, I ride at about 4000 ft elevation, but you should be close, without fear of seizing.
Ron
 
Ron,what slide were you using with those specs? I seized my 79 WR250 20 yrs ago with:3.0,45 pilot,6F15,R-0,410 main jet,had 2 inches added to the pipe like 1980,Answer silencer,Old Wiseco on 2nd overbore,sharpened transfer dividers,milled head...flew until it melted !
 
When it seized it had,
2.0 slide
40 pilot
Q-8 needle jet
6DH2 (middle clip)
430 main jet
It seemed to run great, but when I road it harder than normal at the Elsinore GP, I seized it climbing a nasty hill from a standing start in first gear. Made it up the hill, but seized about 50 ft later, heading uphill on a fire road.
 
Thanks for all the info. I agree,the Pro-Circuit specs are not the right direction. Since my jetting is so close to Ron's,I am thinking maybe the choke is somehow letting excess fuel in all the time. Anybody ever had that happen?
When I got this bike a couple months ago,it hadn't had the best of care.Thanks again.
 
Now I am not sure what to think. I took the 36mm Mikuni off my '77 250CR & put it on the '79. The '77 had been raced several times,and ran fine. After the swap the '79 ran like a bear,way better than ever.
The jets in the 36mm are:
300 main
Q-5 needle jet
6FJ6 needle
2.5 slide cutaway
45 pilot jet
Didn't have a chance to do a long run wfo,but idle to where I did shut off was pretty darn smooth.
Looked at Sudco's website,and a box-stock 38mm isn't jetted that much different from this.
A 250CR really needs a main jet 100 numbers higher than stock?
Thanks again.
Martin C.
 
Martin,good stuff! My first Husky back in the mid 80's when I was 14, was an 75 CR250,it had a 38mm Bing carb on it,probably off a KTM.I couldnt get the bike to run right,so I brought it over to a local shop,where the owner pulled the Bing off after riding it,sold me a nice used 34mm Mikuni off an 81 CanAm 250,with a new throttle cable to match.I installed it right there in the parking lot,then took it for a ride,what a difference!The shops owner then showed me one of his flat trackers,a full race 360cc Bultaco Astro,with a 38 Lectron,explaining to me how it just about makes use of a carburetor that big,with close to 50 horsepower! Sure learned my lesson though,put the 38 Mikuni right back on that WR250 five years later!
 
On the same bike, a bigger carb will need bigger main jets because it will "see" less vacuum. Vacuum signal and air flow are two completely separate things and any change of carb size on the same bike or engine size with the same carb can not be directly compared.

I'll give an extreme example. The '82 Huskys all had the same 38mm carb from the 125 to the 430. The 430 has 440 main stock and the 125 had a 470, and the 470 in the 125 is SUPER lean, it needs a 530. Why does the 125 need 100 sizes bigger than a 430? Its not because the 125 needs more fuel or flows more air, its because the vacuum signal the 125 engine sends to the carb is so weak it needs a much larger main jet to draw enough fuel.

In the same way, a 36mm and a 38mm carb jetting even on the same exact bike can not be compared because instead of the example above where the size of the bike is different, the different size carbs "see" different vacuum signals.
 
I agree,Kartwheel68,BUT my point is that a 2mm change in carb size changes the jetting requirement THAT much? Most of my tuning experience has been on V8's with 4-barrel Holleys,and within reason,out-of-the-box jetting is pretty close.
I think what I will do is swap the choke assembly out of the 36 to the 38,and see if that helps. My fear is that some "tuner" has drilled out the main jet,etc.
Looking at Ron's post again has me thinking that IF the carb hasn't been "tuned",it's going to be something silly like the choke isn't shutting off all the way.
I'll let you know.Thanks.
Martin C.
 
Martin:in Holley 4 barrel terms,that 38 on a stock 250,is like putting an 850 double pumper on a dump truck with a 8-1 compression 350.
 
Yes, 2mm can make that much difference. I'll give another example, I have a '79 Maico 440 and an '82 Husky 430, both use identical 38mm Mikuni's. The Maico runs a 330 main and the Husky a 440-450. 1cc difference in engine size, virtually no difference in peak HP, similar bore and stroke, but one is piston port and one is reed valve.

I'll use my '82 125 in another example. It came with a 38mm, stock main 470 and I run a 530 and even that 530 is really not rich enough, the carb is so big it just cant draw enough vacuum. When Husky went to a 36mm on the 125 in '84 it used a 370 main, exactly 100 sizes smaller than with the 38mm. I put a 34mm Mikuni on it once and ran a 230 main.

Some times jetting can be completely counter-intuitive.
 
Hey Dave,it really gets fun when you go the other way-put a 390 carb on a 400 inch plus V8. Talk about drawing a vacuum!
Martin C.
 
Martin,A few years ago one of the Nascar teams got disqualified, drilling 1/2 inch holes in the bottom of the intake manifold runners! I always thought Husky put 38's on the 125's and 250's of the air cooled era,because they either saved money and simplified parts inventory by using 1 carb across the model range,or because the Japanese bikes of the same era had them.If you were comparing specs buying a new motocrosser and the carb was 2-4mm smaller than the competition,you would assume it would not have as much HP as the other brands.
 
And Kartwheel,if I may say so,you have done an excellent job of explaining in understandable terms,the difference in airspeed and pressure drop across a venturi,and how it relates to fuel flow/jetting!
 
I try, but its not as easy to understand as it would seem on the face of it. It took me a long time to really get my head around it since like I said, its actually sort of counter-intuitive. It is one of those weird things in physics that kind of works the opposite of what your first common sense guess would lead you to believe.
 
I know this is an old thread, but I'm looking for some high altitude jetting guidance. I just moved from sea level to 5280 (Denver), and I'm not sure what to go with. The track I ride is actually at about 6000', and the current jetting just ain't gonna cut it. Any suggestions would be greatly appreciated.
 
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