Why Do We Mikuni? or, Did I Drink The Koolaid too?

[djelliott]

Seriously. Is it because they are that much better then stock? Did someone see them on a race bike and say "gotsta get me those"? Is it because you find them all over the web as an "upgrade" or "performance mod" pre packaged by various dealers (albeit with the wrong emulsion tubes and very ambitious jet selections)? I feel like I drank the Koolaid and dumped a bunch of cash into something that didn't REALLY add any performance. Especially for a street bike. I spent the better part of a year tuning these suckers. Dumping cash into different jets, needles, and various airfiters and velocity stacks. I'm sure I bugged the complete shit out of crazypj as well.


The sum of my exploits produced a bike that ran from OK to good to pretty good but still not quite right and down to awful. When the top end was powerful, good luck keeping it idling at the stop lights or running well in slow traffic. Figure out the lows to mids and your top goes to shit. I won't say the VM30's are bad carbs, I've just come to the realization that they are not the right carbs for a CB350 street bike.


The realization came to me a couple of weeks ago when my bike was in a mood of running like shit and a bunch of my crew wanted to take rip. My friend offered up a bone stock CB350 he had just picked up and sad about not being on my bike it was better then not going. It's been awhile since I've completely caned the shit out of a stocker and lord let me tell you, it just kept giving and giving. I beat the hell out of that thing and was able to keep in line with the Buell Cyclone, GS550, and CB750 I was with. Damn near cracked the ton on the little sucker! After a great ride I asked my self why the hell I've been fucking around with the mikunis when I can access instant fun with the Kei-hins. So after I got home, out came the stock carbs for a good cleaning and rebuild (plus a minor jet change). I had everything I needed right there so it cost me nothing. I popped them back on last night and the bike runs better then ever. Plenty of throttle response, pulls hard throughout the RPM range and idles great.


So, that leads me to the question that is the subject of this post. Why? I read dozens of posts on many forums about "Mikuni VM30 tuning" or "VM30 jetting". I have yet to find a single one that says "bike runs perfect". Nearest I've found is "got it close, gonna have to do". For racing I'm sure they're great. For street and daily riding I'm not so sure. Discuss.




P.S. If you really want them, they'll be up for sale in the Selling section of the forum soon.

 

[andoor]

I know with my current setup (pods and less restrictive mufflers) my vm30's run way better than the stock kei-hins. The stock carbs were garbage in midrange cruising and had hardly any throttle response.

I think many turn to mikunis for the availability/ tuneability. I chose them because of past experience tuning dirt bikes. I can't say that is was a cheap upgrade, I probably spent $150 just on needles, jets, and emulsion tubes to get it right.

I think my bike runs pretty good, but then again I have never ridden a stock 360 that was running top notch! Anyway I have fun on my 360, and don’t have any major issues.

 

[whataulooknat]

I'm still running the Kei-hins I personally like them now that i finally fully understand them. But they are a #$% to tune properly if you don't know what you are doing. When adjusted properly I don't see any point in Mikuni over Kei-hin but if you can afford Mikuni's they are Easier to keep in tune.

 

[RedPlastic]

if it ain't broke dont fix it! I wish some manufacture's would learn that lesson too....also I like my big vm's on my gs

 

[teazer]

The argument is that it's difficult to jet CB350/360 carbs correctly and VM carbs are so easy to jet, they must be an improvement. That's principally because CV carbs are not well understood than those particular carbs are a bit of a disaster.

That doesn't mean that they don't work. I'd like to see PJ offering a carb rebuild and modification service for those carbs. He understands them and knows what needs to be changed to make them work. Whether anyone is prepared to pay what that service would cost is a whole other matter. Let's think about that for a moment.

Carb strip and dunk in Berryman's
Ultrasonic clean
test all drillings
Test all jets including needle jets
Replace jets as required
Modify carb bodies as required
Install new air jets
Install new main jets
Supply additional alternate jets +2 up and -2sizes
Set float levels
Price say $180 - I'm making this up as I go along but at three hours of work at say $60/hour - sounds about right.
Replacement parts including diaphragms extra.
Plus shipping both ways.

People assume that they are better off fitting larger Mikunis and then spend a further $100- $200 trying to get the jets right and age prematurely in the process.

Or maybe PJ could supply jets and set up a 1-900 number so he gets paid for the time he's on the phone on tech support.

Just thinking out loud again.....................

 

[Frankmowthelawn]

I figure Honda had both carbs available when they built these bikes and they chose CV Carbs. The accountants had input too so the difference might have been a $1.00 difference in the cost of carbs. Once the accountants got their way (they always do!!!), the engineers spent a good amount of company time tuning these bikes (and carbs) for the best performance in the environment they expected the majority of their customers to occupy.

Enter DTT.

Not being the types to leave things alone, bikers like DTTers start trying to find things to match their bikes to the environment that they personally occupy by changing things around...like carbs, stacks and pipes!

Just like a fur coat matches a cooler environment, CVs match the average (temperate) environment and others (Mikuni) might match a different environment. They are still gonna need to have some amount of engineering done to match the carbs to the bike AND the environment (AND the owner's version of kick-in-the-pants).

Both will work best in the specific environment they are tuned for IF they are properly tuned AND the flow through doesn't overpower or underflow the bike's engine.

Get it right and it's Pegasus. Get it wrong and it's a pig. Funny thing is that goes for both types of carb!!!

Personally I'm of the opinion that you should dance with the one that brung ya.

My $.02

FMTL

 

[IndieSol]

I think in the case of the 360, it's just easier. I've seen PJ mention that even the air correctors on the stock Honda 360 carbs need modification for pods or stacks. Also, good luck finding a replacement primary main jet for a CJ360. They were a proprietary Keihin jet that I believe is no longer available.

At least with Mikuni's, you can find success stories of people tuning them. It seems like every person running stock Keihin's on a 360 with pod filters has a hiccup or burp or something when cruising at about 4k RPMs, and that seems to be about the best they can get them.....Unless they're PJ, of course.

Hey, PJ, since you're in the job market, and it aint great, how about a 360 carb tuning service ?

I jest.....kinda.

 

[Sonreir]

No need to jest...

The real point is, you don't need to spend $300.00+ on VM 30/32 when you can make stock carbs work. I fitted a set without replaceable air jets, now I know it works, I'll modify anyone's CB360 stock carbs, even those with pressed in jets (for a fee )

 

[outobie]

I've put Mikunis on 4 of my 350s and they run noticibly better than perfectly tuned stockers under all conditions...they just make more power and have much better throttle response. the main reasons manufacturers went to CV carbs in the first place is because of emissions rules and the CV's ability to operate under slightly more varying climate conditions. Jet the Mikuni's properly for where you ride and the improvements are very noticeable.

if you are having trouble tuning them I'd suggest taking your bike to a reputible shop with a Exhaust Gas analyser equipped Dyno. Assuming the rest of your bike is in a good state of tune (fresh battery, good compression, good exhaust, good carb boots and air filters, properly adjusted cam chain and valves, properly set ignition timing, clean fresh unrestricted fuel supply and properly vented tank) and assuming that an assortment of jetting options are available during the process, they can have you carbs dialed in perfectly in less than 2 hours.

 

[crazypj]

Well, I spent a bunch of time with the stock carbs on 360 and they work about as good as it's possible to make them work.
Idles fine, pulls clean through mid range and top end, works at cruising speed and, I still haven't found out if it's pulling max rpm in 6th gear (I'm going to have to fit a rev counter )
Because I got them working so well, I haven't even messed with the VM32's I bought years ago. (originally for my XS650)
VM's may work slightly better for racing, but, I'm not even sure about that anymore?
I know how/why VM's can improve acceleration on track but they will waste a bunch of fuel on the street
Before fuel injection made them obsolete, plenty of sportbikes used CV carbs, even when racing.
I think the main problem is, most people don't really understand what they want and then don't know what or how to fix problems.
Without a gas analyser and dyno, it's incredibly time consuming (and it's not a 5 minute job even with a dyno/gas analyser)
I've spent so much time and effort on 360 carbs I'm not prepared to give the information away.
Even though the CB350 carbs are similar, they are not the same as 360 carbs (they are slightly easier to work with though)
Using 'two stroke' VM's on four strokes, may also need slide changes, with dyno time and jet/needle changes it's going to virtually double the cost of carbs.
Very few dyno operators understand whats going on with a smaller engine,and many of the newer operators can only 'do' Harley's or fuel injected bikes
Because of racing, more information is available for older model inline 4's (Suzuki GS, Kawasaki Z/KZ, Yamaha XS/FJ/XJ)
What works on a GS1000 won't swap onto a CB350/360 though

Just went through a few more posts, I may have to do something like that, modifying carbs, and selling home brewed kits.
Not a very big market though so going to be difficult to make a living, need to find something for the long term (shelf stacker a t Wal-Mart ;D )

 

[Kanticoy]

I feel your pain Dustin. I fought with them...for 2 years...seriously.

I was almost to the point of putting the stock carbs back on, and then I got so fed up, I put Gretta on the shelf. She stayed there for half a year, collecting dust in a corner while I chased my infatuations with the cb400f. Now that I finally got the little four running and for the time being finished, I started messing with my bitch baby again. She was in need of some love. the seat had gotten cracked when she got blown over in a tornado, there was an overall maintenance problem (still not running right), and a general headache looming.

Mike (VonYinzer) had sent me a set of Mikuni's off of a snowmobile from the 70's. I got to comparing them with the Sudco versions that I had and started noting some differences. The Sudco's have a ventilated float bowl that exits on two little nipples on the side of the carb (they have pink vacuum lines on them when you buy them). I noticed that the vintage VMs didn't have these. I also noticed that the actual slide chamber (that the slide moves up and down on) was significantly smaller on the vintage VMs. 8mm smaller in fact. The biggest difference I noticed is that the new Miks have a build in velocity stack on them. The flange on the intake side is somewhere in the neighborhood of 55mm. The vintage carb is a straight plenum with no "funnel" going into the intake slide measuring 43mm. The air circuit is significantly different with 5 individual air holes in the front of the bell mouth. All things considered, this is a totally different carburetor. So I tried them. Holy shit, is it different. After about 4 hours of raiding my Mikuni jet stash, I got them synchronized, and fired her up. A few minutes later, it was idling happily at 1200RPM, and the throttle response....hot damn. It's insane. I dunno what it is, but the older VM30s are so much different than the new ones. I almost gave up, but after blasting her around 2 days ago and feeling the rear tire start to spin in 1st-3rd gear, I am sold. It takes patience, but it does pay off. The kicker is though, no two bikes are the same. It's just that game that we call tuning and we all must endure that right of passage to make it to the point of running well. I dunno man, looking back, I feel accomplished and more knowledgeable as a result. What a ride. If you're in for a crash course, buy the Miks, dig in, and wait for the day that it all pays off.

BTW as a recount....

Initial modern VM32s.....fail
Vintage 32's....fail
Modern 30's.....near success...
Vintage 30's....thank god, she is happy!

That was an expensive endeavor.

 

[djelliott]

Enjoying the insights and experiences. Thanks guys. Kiley, that is some really interesting info. I'll have to check those old VM's out.

 

[djelliott]

Here's a thought I may follow up on if I find a set cheap enough. The Kei-hin round slide carbs from the SL350. The engineers at Honda obviously thought they were a good match for this motor. Same mechanical principle as the Mikunis. Anyone have any experience with them? Pros/cons? Might try to hunt a cheap set down and do some experimenting. Thoughts anyone? (cough. cough. PJ. cough ;D )

 

[Tim]

For me it was simple.

I bought a crap (CRAP) XS650 (zero compression) which had a couple sets of the stock CV carbs in boxes.

To rebuild the carbs would have cost as much as buying 2 VM34's (I got them cheap).

Dead simple, I can adjust needle position at the side of the road (never had to do it), change jets etc. all easy. No o-rings, no diaphragms, no problems.

My XS runs like a clock. Starts with one kick, pulls strong, idles nicely. Carbs are easy to sync (I haven't bothered with gauges yet - do it by feel / sound).

I installed some on my BMW last fall and haven't had a chance to ride it much yet (wheel bearings etc) but the first run down the road was stronger and more responsive than the stock CV.

I'm planning on using a VM34 on my SR500 next. All the same carbs, all the same jets/needles/slides... makes life easy.

 

[djelliott]

Glad they're working out for you Tim. I Probably should have put more emphasis on the CB350 application with the Mikunis.

 

[bscaferacer]

I've got an early pair of mikuni vm17-18 carbs that I need some parts for and they look great nice and easy to work on the only problem is I can't seem the find rebuild kits for them.... my floats bad in one of them and I'm not the most motorcycle savy person on here

 

[ndromb]

I am currently debating the pros and cons or either rebuilding the stock Kei-Hins or buying a set of pre-jetted VM30's from Dime City.

The problem is, The only good parts of my carbs are the bodies themselves and the slides. Other than that everything needs replaced--floats, diaphragms, o-rings, float valves, etc. Pricing out everything it will cost me almost as much to re-build the stock carbs as it would to buy Mikunis. The only thing that worries me is the mixed reviews of the Mikunis.

 

[whataulooknat]

If you have to sink that much money in the stock Kei-Hins by all means buy the Mikuni's. I was just saying I personally like the Kei-Hins now that I fully understand them. But if they are that far gone there is no reason other than originality to keep them. The Mikuni's will perform very well when jetted and tuned properly and they are easier to keep in tune.

 

[lingo]

I know most of this discussion has been about 350's, but there are several differences between the 350 and 450 stock carbs. Of those of you who have swapped to Mikuni's, have you done it to a cb450? Was it worth it?

 

[Kanticoy]

Here's another little food for thought....if you've ever really messed around with both sets, ever notice how heavy the CV carbs are in comparison to the VMs? Pretty wild.

And for all of you guys running relatively stock 350s, the problems you are having, I would bet money, all lie in the needle jet you are running. That is where the secret for getting the carbs "there" lies.

Dustin, I looked up the SL carbs.....very interesting.