mukini carbs

Taiji_Dat · Posted: May 31, 2005, 3:04 pm Post subject: mukini carbs

What is the difference between the different mm mukini carbs? What are the benefits of going to a different size? Are there any? Like to hear your experiences and knowledge.
Thanks, Taiji_Dat

verticaltwin · Posted: June 22, 2005, 12:03 pm Post subject: Re: mukini carbs

I'm no carb guru, but I understand that larger carbs will perform better at high rpm's and smaller bore carbs will operate better at low rpm's. Something to do with the amount of air flow it takes to aspirate the fuel. The area of the bore increases greatly with each mm increment, so it's usually a good idea to stay close to stock unless you are racing. Trials riders who need low rpm torque generally keep the carb bores small so they get good aspiration at lower rpm's where they do their thing.

y 500cc bike does better overall with an original 26mm carb than with the 30mm it came with, but it did lose some top end power in the trade.

VT

kingwj · Posted: September 16, 2005, 7:50 am Post subject: Re: mukini carbs

Are Mikuni's worth the price? What are the benefits of the modification? Is the jetting correct if you buy the upgrade package from Ebay? I am thinking 34 mm for my stock bike but don't want to waste my money if the benefits aren't significant.

Bill

'79 XS650F

grizld1 · Posted: September 16, 2005, 8:10 am Post subject: Re: mukini carbs

The worth of a modification depends on your needs and ability to tune. If you're tuning for all-out race performance, 34 or 36 mm straight-through carbs improve flow and response; they have no butterflies to impede flow and slide control is direct. Shell Thuet recommended the stock CV units for the street. Don't count on "correct" jetting; there's little to be gained from performance carbs unless the rest of the engine's breathing freely, with performance filters and exhaust, a little port work, maybe a performance cam, and no one can predetermine "correct" jetting for every possible combination of components and changes. If you get into performance mods you have to tune for them yourself, regardless of the vendor's claims.

Phred · Posted: September 16, 2005, 3:05 pm Post subject: Re: mukini carbs

Quote::

I understand that larger carbs will perform better at high rpm's and smaller bore carbs will operate better at low rpm's. Something to do with the amount of air flow it takes to aspirate the fuel.

Generally true. The size of the carburator venturi is where it is really important to consider the *airflow* of the engine. Air volume used by the engine translates to air velocity through the venturi. A large engine that operates mostly at low engine speed can use a smaller venturi than a small engine that runs wide open. The air velocity through the venturi creates vaccuum, which is what all carbs need to function. If an engine is "undercarbed" it will starve for air at high rpm/load, if it is "overcarbed" it will idle poorly, have poor throttle response (especially at low speed), and be unpleasant to drive (remember the typical high school kid in his Camaro that barely idles and has to be revved up to leave a stop light-OVERCARBED!)

Carb selection can be very hit-and-miss, especially if you don't know the actual c.f.m. numbers for your engine (volume of airflow at various engine speeds and loads). If you are building a street motor I would recommend selecting a carb that is easy to tune and reliable, if you want more power a size or two larger would probably work. If you are building for maximum performance you will be spending big $$$$ buying carbs to try unless you can find someone to help you with their experience.

I like the Mikuni design as it is adjusted with jets, which don't require adjustment-once you get it tuned it stays that way!

grizld1 · Posted: October 20, 2005, 10:26 am Post subject: Re: mukini carbs

OK--I can speak to this with a little more confidence now. I just finished dialing in a pair of VM36 Mikunis on my stock '82 engine (MAC 2-2 exhaust and Uni dual-element filters). I'm redoing a cammed and ported engine with 10.5:1 compression on 700 cc bore-in pistons that I built some years back on stomp-only cases (mistake--a leg injury left my right boot a little too light to fire it without a struggle) on pushbutton cases, and figured I'd get the big pots baselined for that, then hang the BS38's back on (figured the stock mill would be overcarbed).

Not so. The carbs came from sportingforless.com and the pair was under $250. I hung them on a pair of Shell Racing billet mounts from a project bike, measured out cables and had 'em soldered up, fabricated a padded alloy shelf for rear support, stepped the pilot up one increment (22.5 as delivered to 25) and the mains up four (190 to 230) and got good idle, low-end, and transitions without playing with the midrange; IMHO that's an extremely close out-of-the-box setup. Mileage under hard riding is in the neighborhood of 48 mpg, which is slightly better than what the BS38's I had on the scoot delivered. The VM's won't be coming off.

If you're considering a change to VM or other custom carbs, get Sudco's Mikuni Tuning Manual. Jetting's a process, and that book explains it well.

Gham · New Member Joined: Nov 06, 2005 Posts: 1

grizld1-Igot my vm36's from sportingforless and they came with 35 pilots!
I'm rich off idle with some gurgling around around 3k rpm.I dropped the needle one notch and they're great at highway speeds.I'm not calling you out but my milage dropped to 35mpg with k&n filters and aftermarket pipes.
Taij-I worked to get them stock carbs right for a year!I started missing to much riding time and sleep thinking about jetting.I tossed the old cv's in a box and never looked back.If I had it to do over I would get the 34mm carbs as I have not done any engine mods and the 36's take some tweaking.
Gham

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

The problem with carbs from SFL is the their jetting is way off the correction factor for the XS and I mean way off. Who ever does their jetting needs to find another occupation or maybe just stop selling them period. Call 650 Central for your mikuni needs and proper jetting. You should be using 17.5 to 25 pilot jets for either the 34 or 36 round slides..........Jack

grizld1 · Posted: November 7, 2005, 10:44 am Post subject: Re: mukini carbs

Kee-rect Mr. Jack! A #35 pilot is just about cylinder-scouring rich. Looks like SFL may have learned a bit by the time my carbs were shipped. Gham, I'm also running relieved intake and exhaust (Uni 2-stage pods and a Mac 2-2 system); getting the pilots right will help your fuel consumption. I've moved the mains up to 250's and still get fuel consumption in the low 40's under hard running. The only remaining issue is a lean spot at around 3/4 throttle; I'm going to try 6N1 needles after reconfirming float levels. BTW, the VM34's also take tweaking; I have a pair in the stash that I ran for years.

And kee-rect again on 650 Central, Jack! I did my first jetting job on a Mikuni carb in 1965 when I hopped up a 50cc Suzuki 2-stroke and have lots of jets, needles, etc. on hand; but folks who are new to the process will have a better time if they avoid the bargain basement, pay a little more, and get the excellent tech support that comes with carbs from 650 Central; and those who don't have a stash of tuning options already on hand will likely break about even on price.

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

You here so many people complain about the jetting of the mikunis from SFL and their replies are the jetting is correct for the XS and It was'nt my attention to bash this company but hey, someone had to speak up to bring about these jetting issues cuz to me, it sounds they're objective is to gather all the greenback they can and ignore the real issues here,that there's a problem with the jetting and instead of screwing their customers by having to purchase extra jets, they might just sell a few more if the customers could get off first base.................Jack

grizld1 · Posted: November 8, 2005, 4:50 pm Post subject: Re: mukini carbs

I'm divided on this whole issue, Jack. I didn't care if SFL's jetting was right or not (though my carbs came surprisingly close) because I had a drawer full of Mikuni stuff to play with. On the other hand, SFL's advertisements imply bolt-on problem solving, and given the variations just in the exhaust systems folks are running, that sort of spin is going to raise some unrealistic expectations. Another factor is the buyer. I hate to sound like a snob, but guys really should inform themselves on the basics before they think about custom carbs (or exhausts, or air filters!) and equip themselves with a manual before they hang 'em. You get what you pay for, and most guys would do well to understand that, pay a little more, and get Dwayne's good tech support.

grizld1 · Posted: December 3, 2005, 10:25 am Post subject: Re: mukini carbs

OK, folks, here's a baseline setup for the VM36 Mikunis that's doing what needs to be done (raps out to redline, fine transitions and rolloffs, cold starts on the button and the scoot's own juice in 30 degree weather with 20/50 in the crankcase) on one particular stock '82 engine with relieved filters and exhaust operated at around 400 feet above sealevel. All components except main jets are as delivered from sportingforless.com (who appear to have listened to feedback and changed their jetting).

ains: 230. Pilots:22.5. Jet needle: 6DP1, raised 1 notch from center. Needle jet: 159-P-6. Slide: 2.5. Air jet: 2.5

There's a jinx that come with new performance goodies. That which was working fine will break down upon installation of the new component, giving the installer a fine puzzle to sort out. I went chasing a little rolloff breakup for some time--till my "heavy duty" Chinese coil broke down far enough to get me looking at ignition. No, Gertrude, 1.5 ohms across the primaries ain't right! Stuck in a 5 ohm Dyna; my old Boyer Micro Digital loves it.

kingwj · Posted: December 3, 2005, 11:52 am Post subject: Re: mukini carbs

As the owner of a 1959 MGA I have learned the hard way the 95% of carburetor problems are electrical. Just be glad we don't have Lucas electrics and Amal carbs on our bikes. I'd hate to tell a youngster to tickle his carbs, he'd think I was crazy. Thanks for the info, it may come in handy if Santa got my list.

Bill

grizld1 · Posted: December 3, 2005, 9:05 pm Post subject: Re: mukini carbs

Hope Santa acts on that note, Bill. You sure know how to jump-start evil memories though! I bought a new Trunch Bonnie back in '68. The thing ran like a stripeassed ape for a couple hundred miles--then it was back to work on timing, synch., and all-around tighten-up to buy a little more prime-time operation. And wasn't it grand how those Amals behaved heeled over? If the modern tire compounds had been around back then, that could have been a serious problem. The most fun I had was replacing the pushrod tower O-rings (they switched the O-ring composition in the '68's and they all went bad--usually, like mine, just past the warrantee mileage).

I'm betting Lucas' claim to the Prince of Darkness title is going to be seriously challenged by the stuff China's turning out these days. Think the Brits will sink to the occasion?

xs650mike · Posted: December 4, 2005, 10:46 pm Post subject: Re: mukini carbs

Never had the slightest trouble with my Amal Monoblocs. The Lucas "dynamo" was another matter entirely. Fortunately the Matchless iggy was a magneto so you could still get home if it was daylight!

grizld1 · Posted: December 5, 2005, 11:08 am Post subject: Re: mukini carbs

Agreed, Mike; the Monobloc was a nice straightforward unit, and I should have made it clear that I was referring to Concentrics, which were notorious for level-related mixture issues when heeled over hard, and produced many good aftermarket sales for Mikuni.

xs650mike · Posted: December 5, 2005, 2:52 pm Post subject: Re: mukini carbs

Britbike Jim, one of my best friends and one of the few left of the old Frisco "tavern racing" fraternity, was a big proponent of Amal Concentrics when they were current "state of the art" britbike carbs. Back then he never complained at all about any difficulties when "heeled over" which he was a lot. I'll ask him about what his fixes were. I notice that the Amals on his current fire breathing '65 Bonneville are actually licensed Mikunis though!

RidgeRunner · Posted: January 2, 2006, 2:21 am Post subject: Re: mukini carbs

Ok so have you gotten your carbs?
And if so did you get the 36 or 34mm carbs?

What Happened to 650rider.com?

Yamaha xs650