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Old 03-14-2013, 09:44 AM   #76
Beater
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I think you can make any carb work with enough fanagling. But here is my thoughts ... Think extremes.

A HUGE carb on a small piston. At idle you can make it run perfectly. Once you turn the throttle, you almost immediately move completely through the sweet spot mix of gas/air, and now you're going to be giving way to much of both to the piston.

A tiny carb on a HUGE piston. At idle the piston is not getting enough. As you give it throttle you move toward the sweet spot for idleing, and little more. You never even reach the sweet spot for acceleration / performance.

Lesson: You need to have the carb and piston balanced. Seems logical, right? Now where is it? You will get MANY people telling you it's from 32 to 40mm. Bing, Mikuni, Delorto ... Jet sizes, idle, main, needle ... the skys the limit. Seems very daunting. Don't let it get to you. Ask yourself this question:

1. What do I want out of the engine? Performance, Racing, Reliability, Gas milage? What?

For performance top end, or low grunt? Racing distance or sprint? Reliability?

I have a R100 engine that has huge low end. Mikuni 32's, 18" rear wheel, 37/11 drive. I had an R100 that had a better top end, but the balance of the engine wasn't as good ... 32/11 final drive, 18" rear wheel, and Mikuni 34's. For milage obviously a miserly approach is best ... but where's the fun in that?

For lower end city riding, I would go with the 32's. For top end freeway I would go with the 34's or 36's. But in the big picture, I really don't think it matters. We're talking about smaller differences between incredibly variable attributes. Why, the whole formula goes out the window if you go from sea level into the mountains.

For me, the 34's are a bit more rare in the world of motorcycles. The 32's were on MANY of the Japanese bikes, therefore you can get cheaper. WFO on the Dobber will lift the front wheel and have you praying for a better grip on the handlbars. I can cruise at 80mph ... and there is plenty more umph there ... but that's enough for me.

In any case ... get what you want, and we'll titrate it work great. Think of it this way. BMW put the 32's stock on R75s through R100's. Not that they did everything right, they were just able to titrate the jets to make it work well. We can do the same with yours.

Quote:
Originally Posted by blaine.hale View Post
That brings up a good thought. Am I lazy enough to buy the pre-jetted 34 kit from this Amazon purchase or should hunt down some 32mm kit somewhere. Do you think it would be a big noticeable difference on the r90? I've seen a lot of people run the 34's on r75s.
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Beater screwed with this post 03-14-2013 at 09:52 AM
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Old 03-14-2013, 09:48 AM   #77
Uncle Pollo OP
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Quote:
Originally Posted by Kt-88 View Post
Not to flood this post, but I'll relate my freeway riding tonight.

At the above mentioned settings and jetting, I can't redline 4th or 5th gear. First, second, and third feel pretty good, but WOT on the last two just never produced results. I mean, I was still doing 80 easily, but 2 k short and had ZERO acceleration capabilities. This leads me to think the mix is STILL rich on the main jet. I'll try lowering the needles tomorrow, and post results.
OR LEAN

mine does that when i jet it too lean
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Old 03-14-2013, 09:52 AM   #78
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Originally Posted by Kt-88 View Post
Bet you're right. Bill was stocked out of 180s so he sent 185s. ...probably need 175s here, anyway.
cycle gear will order anything or might have it in stock

jets are us is the best bet
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Old 03-15-2013, 08:33 AM   #79
blaine.hale
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Just asked Bill with Rocky Point about a jetting for a 34mm VM on an r90:

Slide: #2.5, #3 may be OK
Needle: 6DH3
Main: 210
Needle jet: 159 P-0
Air jet: 2.0

I told him I'm running hoske pipes with a little more flow than stock..so he may have adapted those jettings for that factor.
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Old 03-15-2013, 09:18 AM   #80
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Quote:
Originally Posted by blaine.hale View Post
Just asked Bill with Rocky Point about a jetting for a 34mm VM on an r90:

Slide: #2.5, #3 may be OK
Needle: 6DH3
Main: 210
Needle jet: 159 P-0
Air jet: 2.0

I told him I'm running hoske pipes with a little more flow than stock..so he may have adapted those jettings for that factor.
I may go back up on main jet size. Raising the needle actually helped.
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Old 03-15-2013, 09:28 AM   #81
blaine.hale
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Anyone know about the different slides?
I was just going to order 2 of these and figured I'd only need to buy jets and maybe a needle:
http://www.amazon.com/Mikuni-Round-S...ds=34mm+mikuni
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Old 03-15-2013, 11:04 AM   #82
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Maybe they mean 2.5 slides? I'm not clear on the needle jet size there, either.
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Old 03-15-2013, 05:33 PM   #83
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Quote:
Originally Posted by Kt-88 View Post
Maybe they mean 2.5 slides? I'm not clear on the needle jet size there, either.
There are 26 needle jets in the 159 series and they increase in size by some arcane formula I don't understand but can regurgitate. The letters increase by 0.050mm, so O-2 is 0.050 larger inside diameter than N-2. The digits are increments of 0.005mm, so N-3 is 0.005mm larger inside diameter than N-2.

The arcane point I don't know enough about is the lack of uniformity between the letter series: 5ea N, 6ea O, P, Q, 4ea R. So the diameter makes a bigger jump between sizes at the lean and rich ends than it does in the middle ranges. Haven't found an explanation for that yet.
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Old 03-16-2013, 02:12 AM   #84
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It seems to me to be strange that people have been fitting Mikunis to airheads for at least 20 years and there is no consensus on what the jetting should be.
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Old 03-16-2013, 05:31 AM   #85
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Quote:
Originally Posted by chasbmw View Post
It seems to me to be strange that people have been fitting Mikunis to airheads for at least 20 years and there is no consensus on what the jetting should be.
I believe the whole point of this thread is to build a knowledge base for that exact reason.
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Old 03-16-2013, 06:13 AM   #86
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Originally Posted by oldroadie View Post
I believe the whole point of this thread is to build a knowledge base for that exact reason.
There are so many combinations .... Better to have as many as possible
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Old 03-16-2013, 06:30 AM   #87
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Quote:
Originally Posted by chasbmw View Post
It seems to me to be strange that people have been fitting Mikunis to airheads for at least 20 years and there is no consensus on what the jetting should be.
Different engine combinations, altitude, etc. Even Bings, which compensate for altitude, were jetted differently over the years.

I'd venture to guess most combinations here aren't 100% optimal, including my own, unless a wideband and dyno were used.
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Old 03-16-2013, 06:58 AM   #88
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Agreed, dyno results and wide band plus idle sniffers would really flesh out the discussion. All the same, we're talking carbs so humidity, air temp and altitude are always going to interfere with perfect results.
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Old 03-16-2013, 10:10 AM   #89
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There should really be baseline jetting, that you could adjust for altitude silencers etc .

Not having baseline info, persuaded me to fit dells about 7 years ago and it seems that things have not got better since.
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Old 03-18-2013, 01:18 PM   #90
blaine.hale
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Well my Mikuni's are on the way.
I'll be sure to post results.
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