Help required before I go mad.Can any of you guys tell me what settings you are using for a 36mm mikuni on a 74 250 hare scrambler? I have tried numerous combinations but this thing runs hopelessly rich off the bottom, has a midrange hit like a 125 on steroids then goes flat on the top end.Also, is there any way to convert to a five speed box on the early black case motors?
Only Mikuni jet sizes I have are in a 250 HS with a reed valve. I believe if you do a search here that there is a lot of info on Mikuni on 250 HS though.
Larry P
What jets are in it?
Mick
Mick, do you mean in his Mikuni?
Larry P
27.5 pilot, 310 main, 6DH4 needle, Q6 needle jet, 3.0 slide. Running a K and N filter with a brand new repro pipe from pollino diron. Cylinder has original porting with a new 72mm wiseco piston abd the correct twinplug head. Ignition timing is spot on, rewound motoplat, non resistor llug cap, new crank seals!
yes, the Mikuni. I'll try to get to mine soon and report on my jets.
Mick
In my 250 MC 5 with a 36 Mikuni, I ran a 400 main with a 35 pilot.
1976 MC 5 Original Owner
1976 Penton 175 XC
1985 20' Hi Point trailer
A 36mm without a reed valve is on the big side, so jetting is going to hard to dial in. I suggest you try other needles if nobody gives a solid reference. What most don't realize is the needle is the only 'jet' that changes as the throttle position is changed. If you have the wrong needle you will chase your tail forever. My only experience is with a reed valve motor.
So, you say a 36 is too big for a 250? Then what size would you say is better, a 34mm? Interesting.....
Quotequote:Originally posted by JP Morgen
A 36mm without a reed valve is on the big side, so jetting is going to hard to dial in. I suggest you try other needles if nobody gives a solid reference. What most don't realize is the needle is the only 'jet' that changes as the throttle position is changed. If you have the wrong needle you will chase your tail forever. My only experience is with a reed valve motor.
1976 MC 5 Original Owner
1976 Penton 175 XC
1985 20' Hi Point trailer
They came with a 36mm Bing though JP.
Larry P
In reading this forum for several years, I have read where some have used 34mm Mikunis primarily because they fit near the pipe better, a little more room to get the carb body set correctly. I think some one said that you can bore a 34mm Mikuni out to 36mm in order for it to flow better and have the best of both worlds. I would like to see or read how that is done. But some one in recent history has posted basic Mikuni settings for the 250, all you have to do is search and find it. What do plug readings tell you? Idle jet is for idle RPM up to when the throttle is cracked. The needle and it's jet commands up to 3/4 throttle and the main is for flat out top end. At least that is what I'm remembering from the old two stroke tuners books. I would think a reed doesn't make much difference in what an engine needs, it just make delivering that fuel supply more precise and efficient. Just my 2 pennies worth.
Type at you later,
Dan McEntee
I have done a search on this site and the only settings I can find are 30 pilot, 290 main, 6dh4 needle, q2 needle jet, 2.5 slide. I have a q2 needle jet but as yet havent had the chance to try it. Maybe I have a Friday Afternoon Mikuni because this thing is truly awful, especially off the bottom where it will gas up at every opportunity. I was expecting a fairly useable power delivery
or are they all basically unrideable? Mine is the only 250 HS being "raced" in the south of England and its proving to be a bit of an embarrassment when it should be an unfair advantage in the pre 74 class Looks lovely sat on the box though.
My reed valve 250 motor uses a 34mm Mikuni bored to 35.5mm so the airboot and body would fit.
Larry P
That is what interests me, how that was done. Don't plan on doing it any time soon, just real curious as to how the carb slide still seals after boring?? Or did you order it that way from Sudco or some one else?
Type at you later,
Dan McEntee
They do not bore the part the slide is in but down the intake throat. That is where the size of a carb is measured. Bored bigger it flows more gas. You have to have it done at a machine shop.
Larry P
So that is on the air filter side? Or the cylinder side? I'll have to grab a carb body and look at it to get a good visualization.
Thanks a lot,
Dan McEntee
They bore straight through to make entire opening more mm.
Larry P
400 main jet? You must be running on methanol! This thing will start from cold with no choke at all. I have gone as small as 22.5 on the pilot and it STILL goofs up.I am rapidly appproaching resolving the issue with a bale of straw and a zippo lighter as it seams impossible to get some ballpark settings to work from.:(
Just put a Bing on it like it came with with a good slide and a NEW needlejet and use the tickler to start and it will be fine and will flow more gas per mm than a Mikuni. The only trouble people have with the Bing is when they have a worn slide or a worn needlejet. Make sure those are in tiptop shape and you will pull no more hair. Don't look at theneedlejet put a new one in and make sure slide has no smileyface.
Larry P
This is the first article I could find with reference to standard Carb specs for a 250 Harescrambler. There is a wealth of information in the publications that were so painstakingly scanned.
http://www.pentonusa.org/publications/keepingtrack/No.%2015%20%20June-July%201974.pdf
When all else fails contact Chicago Jerry or Gary Ellis....
Dale Fisher
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Unfortunately I purchased the bike as a box if bits and there was no carb with it. I would have lut a bing back on it if I could have found one. The price of a new 36 bing would make your teeth bleed, hence the mikuni.
So Larry, where did you get the 35.5mm slide to fit the new bore?
Quotequote:Originally posted by Larry Perkins
My reed valve 250 motor uses a 34mm Mikuni bored to 35.5mm so the airboot and body would fit.
Larry P
1976 MC 5 Original Owner
1976 Penton 175 XC
1985 20' Hi Point trailer
I guess I am not explaining well but will try again. When a carb is bored the throat is bored from intake side to aircleaner side. The orofice the slide runs in does not change so the same size slide is used. Most all Mikuni slides for bigger bikes is the same size slide in diameter. There are different cutaways on slides that change the running characteristics but the size is the same. It is a bigger opening from front to rear that flows more air which flows more gas. Hope that explained it.
Larry P
Sorry to rain on your parade but mikuni slides are most definitely not all the same size. A 34mm carb is physically smaller than a 36mm so the slide is smaller - trust me. Going back to the original question, can any of you guys tell me what you have INSIDE your miks. Pilot jet, main jet, needle, needle jet and slide
Thanks Larry! Being of Polish decent sometimes I need a bit more detail than others!
Quotequote:Originally posted by Larry Perkins
I guess I am not explaining well but will try again. When a carb is bored the throat is bored from intake side to aircleaner side. The orofice the slide runs in does not change so the same size slide is used. Most all Mikuni slides for bigger bikes is the same size slide in diameter. There are different cutaways on slides that change the running characteristics but the size is the same. It is a bigger opening from front to rear that flows more air which flows more gas. Hope that explained it.
Larry P
1976 MC 5 Original Owner
1976 Penton 175 XC
1985 20' Hi Point trailer
Have you tried the search function? Here is one of many on the subject [?]
http://www.pentonusa.org/forum/topic.asp?TOPIC_ID=14244&whichpage=1&SearchTerms=mikuni%2Cjetting
Dale Fisher
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