Cafe Racer Forum banner

1 - 20 of 31 Posts

·
Registered
Joined
·
472 Posts
Discussion Starter #1
Hello gents,
I have a '73 CB350 twin. I have a set of Mikuni VM32s for it, and I'm hoping someone with a CB350 with similiar mods could point me in right direction on jetting and such. Here's what I have:
-the motor is internally stock except for an early "Type-1" cam
-K&N pod filters
-stock head pipes with short, LOUD glasspack type mufflers

Anyone have any insight?
 

·
Registered
Joined
·
1,112 Posts
Sid,

Welcome to the forum.


Did you get them new from Sudco? Like, after you told them what you have and they were supposed to "set them up" for your bike?

I got some from them (350 w/ a 362 kit, mild porting, K&N's, stock cam, 2-1 exhaust) and they were WAY over-jetted. They came with 180 main jets. I've got 130's in there now and I'm close but not quite right.

You'll also have to deal with the needle setting.

Get a few sets of clean plugs, an assortment of jets (can be ordered from Sudco--like $30), a 6mm socket, a place to do some test runs and an afternoon. You're going to have to do some trial and error testing. (They make these carbs so easy to adjust for a reason).

Like I said, I have 130's in there and my needle is set to the second notch down from the top.

Here's a link to tuning info from the Sudco manual:
http://www.lafishmag.com/CarbTuning.html

And the Sudco page:

http://www.sudco.com/mikunijets.html


It's kind of a pain in the ass, but not impossible. Or maybe I'm just a hack...
 

·
Registered
Joined
·
166 Posts
This is what worked for me. I have a lot more motor work than you, but here we go.

PJ=3.0
NJ=P8
MJ=175
NDL=6F5 (top position)
AJ=2.0
Slide=3

Thinking about using a NJ=P6 and NDL=6F4, or 6F9, or 6DP7.
I've got to lean mine out in the middle and make it a little richer up top. These carbs a really easy to swap jets. Just turn off the gas, get the beer can that you just finished and cut it in half to catch the gas coming out and walla. Messing around with the needles make a huge difference!

Anybody got any different ideas about needles and needle jets, I'd love to hear your setups too.
 

·
Registered
Joined
·
2 Posts
Hello,

I found this post with google :D... and this is my first time here [:I]

I have a question related to this old post and I hope I will finally get an answer (I searched internet hours and hours and could not find anything about Mikuni till I found this forum! crazy)

The question is:---> what are the advantages or disadvantages in terms of performance [}:)][}:)] to replace the stock carbs on a CB350twin for the Mikuni VM 32 set??? it's for road use on a stock CB350!

Thanks in advance,

Enzo
 

·
Registered
Joined
·
347 Posts
imho, a vm32 is overkill for a stock 350. the advantage would be slim to none. if you were to go to a vm28 or vm30 the advantage would be better throttle response as you aren't waiting for the vacuum to build in the slide of the stock carb. unless your stock carbs are totally [email protected]#ked, i'd leave them on the bike until you build up the motor.
 

·
Registered
Joined
·
1,112 Posts
The biggest advantage to Mikunis is the ability to adjust them to suit your engine. Racers like them because they can change the main jets and set the needle height without removing the carbs. They also work better under sustained load (a.k.a. long bouts of WOT) than the stockers.


I suspect another reason many 350 guys have gone with Mikunis is a torn diaphragm in their OEM CV carbs. These diaphragms can be gotten now, but for a long time they were unobtanium.


Mikuni round slides are a bit different than CV's in throttle response. Without the vacuum assist from the CV diaphragm, you can't just blip the throttle in traffic without getting some stumble. You've got to roll on the throttle.
 

·
Registered
Joined
·
2 Posts
Thanks for the explanation... after reading some posts here jetting those carbs doesn't seem to be easy for everybody!

quote:unless your stock carbs are totally [email protected]#ked, i'd leave them on the bike until you build up the motor</u>.


Ok, well actually my carbs are fine , I just thought I ll get better acceleration with the Mikunis [8D][8D]. You say build up the motor... ok then do you know what should I do to get more HP (still for street use)[?][?]

Thanks
 

·
Registered
Joined
·
720 Posts
Open wallet, throw money at people.
 

·
Registered
Joined
·
472 Posts
Discussion Starter #10
Yes, the 32s are too big for a basically stock CB350. Firsthand knowledge. Someone who I used to consider very knowledgable recommended them to me and I foolishly took it at face value. If I could do it over again I'd use VM30s. In fact, I'm keeping my eye out for some 30s (see my post in parts wanted if you're interested in a trade). The bike still runs pretty good with the VM32s and I ride the hell out of it, though. The low end response is not what it could be, and I just know that the bike could run better overall. I still consider the swap the best mod I've made to my bike, though. I never liked the stock CVs, they were always giving me problems. Sometimes the bike was nearly unrideable because of them. There was never any good explanation for it. I know my way around a carburetor, and I tried all my tricks. One day it would run like hell, and the next day it would scream. Very strange. I really admire the Mikuni VM roundslide. They're well designed, good looking, infinitely tuneable and man, do they run sweetly when properly set up. The problems arise because since they are nearly infinitely tuneable, there are a lot of different ways to make them NOT set up properly. I would strongly recommend swapping your CVs for some VMs. There are some good tuning manuals out there, and the concepts are not terribly hard to understand.
 

·
Registered
Joined
·
4,721 Posts
wow! I must say it almost shocks me to read anyone on the net suggest there is such a thing as too big a carb for a given engine.
 

·
Registered
Joined
·
4,721 Posts
quote:Originally posted by krapfever



The biggest advantage to Mikunis is the ability to adjust them to suit your engine. Racers like them because they can change the main jets and set the needle height without removing the carbs. They also work better under sustained load (a.k.a. long bouts of WOT) than the stockers.


I suspect another reason many 350 guys have gone with Mikunis is a torn diaphragm in their OEM CV carbs. These diaphragms can be gotten now, but for a long time they were unobtanium.


Mikuni round slides are a bit different than CV's in throttle response. Without the vacuum assist from the CV diaphragm, you can't just blip the throttle in traffic without getting some stumble. You've got to roll on the throttle.

I cannot say I agree with any of that in blue other than there being a perceptible difference in throttle response but when both types are equally optimized/tuned.... the difference is pretty much only noticeable on a shaft driven bike piloted by a very seasoned rider. Optimally sized and tuned mechanical slide don't care one bit if you roll into the throttle or stab it to the stop.
 

·
Registered
Joined
·
2,495 Posts
I swapped out BING 40mm CVs on my BMW R100 for a set of used Mk VM38s I bought from Paul Heid in Johnsburg, NY. I thought they were going to be too big for the R100, since a lot of folks run VM34s. Paul knows his BMWs and he tuned them up for me as well. I notice the difference. The BMW is a shaft drive and I push it hard...(see Hacks comment)... Never had them on a CB350...so I digress.
 

·
Registered
Joined
·
1,229 Posts
Ditto
VM38 on my BMW 750. They rock! Very responsive (for a BMW) especially in comparison to Bings,

Wish we could use Mikunis for FCB!
 

·
Registered
Joined
·
5,379 Posts
the 32's are great on my cb350... of course i have a built motor.
If you want to jet your carbs properly... go and tune it on a dyno.

A bone stock cb in top running order ( blueprinted )... you may get 28 hp... the 32mm carbs will not add anything to the mix... when you change something... like a carb setup... you need to change the entire formula... so.. bigger carbs, forged pistons, race cam, flowbenched/ ported head, race exhaust... get the picture.

good luck.
 

·
Registered
Joined
·
1,112 Posts
quote:Originally posted by HackAsaw

quote:Originally posted by krapfever








Mikuni round slides are a bit different than CV's in throttle response. Without the vacuum assist from the CV diaphragm, you can't just blip the throttle in traffic without getting some stumble. You've got to roll on the throttle.

I cannot say I agree with any of that in blue other than there being a perceptible difference in throttle response but when both types are equally optimized/tuned.... the difference is pretty much only noticeable on a shaft driven bike piloted by a very seasoned rider. Optimally sized and tuned mechanical slide don't care one bit if you roll into the throttle or stab it to the stop.

Okay, ima gonna hijack here a little (any info may help the original poster anyways)...if this stumble is not an inevitable problem with Mikunis, what do I need to look at in tuning them further? I've got the needles and mains dialed in to produce a consistent mocha color on my NGK's and the bike idles and pulls like a monster, but I've always had the stumble when I thwack it. It almost seems like a fuel starvation issue--so maybe mess with the needles some more??

I was originally led to believe by friends who race mikunis that to some degree this is just something you deal with.

I haven't dyno'ed the bike, nor have I had anybody special look at it, preferring instead to try to glean what I can from others and figure the rest out myself.

Looks like the weather finally broke here so I'll probably be breaking the bikes out this weekend.
 

·
Registered
Joined
·
4,721 Posts
there are many variables but a full race machine is often a cantankerous machine which seldom tractors away from idle and then goes onto develop thoroughbred wot power

two basics that may apply to your exact situation

reversion..... less restrictive exhaust will narrow the torque curve and create stronger reversion pulses which result in the carb actually metering (and enrichening) the mixture at certain operating ranges THREE times before it sees the combustion chamber

how you jet in those areas and if you do or do not allow room for the reversion slug to expand on the fresh air side of the carb will make or break your ability to dial in jetting that is actually consistent as well as effective

a sub point of this is exhaust volume (length and diameter both) as well as intake volume are the external variables which can determine how much reversion and where it occurs... even the shape of the exhaust tip... straight cut or slash cut.... megaphone slip on or nothing... well all that stuff and more comes into play and can be used to help determine which way you need to go with your mikuni selective tuning parts

also float bowl venting is inadequate on many machines and costs virtually nothing to increase it


it is rich or lean when you are experiencing hesitation? do you really know how to tell merely by the seat of your pants? if not you should because pulling the plugs after the fact will only reveal very little for non single cylinder engines even when performing a proper plug chop

if your carb is sized for max horsepower then logically it will have weak signal at the other end of the engine's range

but if a CV (which really isn't a vacuum assisted carb at all) can provide good running manners, so can a mechanical slide carb
 

·
Registered
Joined
·
347 Posts
ok, down to brass tacks here. if one were to have a built motor(362 kit), early cam, 2-1 race exhaust, and velocity stacks, where would one start with jetting?
 

·
Registered
Joined
·
4,721 Posts
generally I start with whatever is inside already and go from there, unless something is obviously grossly over or undersized

then I whip out the spare bits and pieces along with the drills and files if they don't thrill me

I'm sure somebody here running a similar setup can give you good numbers
 

·
Registered
Joined
·
3,466 Posts
quote:Originally posted by buckandaquarter

Open wallet, throw money at people.
Throw some money my way and I'll do you a 378cc kit



[/quote]Originally posted by krapfever
I've got the needles and mains dialed in to produce a consistent mocha color on my NGK's and the bike idles and pulls like a monster, but I've always had the stumble when I thwack it. It almost seems like a fuel starvation issue--so maybe mess with the needles some more??

[/quote]

Its too rich if plugs are 'brownish'
Fuel isn't the same as it was in 1970's, plugs should be light grey
You probably need different slide cutaway, needle position and main jet change. If its set up right, you may get very slight stumble but it shouldn't be too noticeable.
Don't forget, most racers only jet for 3/4 to full throttle so race jetting doesn't work very well on the street


PJ
 
1 - 20 of 31 Posts
Top