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

Kanticoy said:
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.
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Me too Kiley, interesting indeed. As stated, I'm going to try and get a cheap set to run some tests. Was shocked to see they're 24MM. I read some old SL350 reviews as well and the general consensus was that the bike ripped. Of course that's 40 year old ripped, but none the less. I know a bit had to do with different sprockets and gear ratios too.
 
I had no idea that they were that small....makes you wonder about plenum size doesn't it. I know we fitted a set of VM20's on my buddy's CB200, and it is a monster. Makes you wonder if 150cc's makes you need such a huge jump in carb size. There is a math equation to figure this out, but I don't know it. This thread really makes me want to experiment.
 
Interesting thread but all over the map.

OEM carbs are usually manufactured from a zinc based alloy known as Mazak or Zamac. It's a cheap die casting alloy ideal for manufacturing in high volumes. Aftermarket carbs, like the VM series, are cast in Aluminum which is much lighter. OEM VM type carbs were also cast in zinc alloys. Check out a stock carb from say a T500 or early GT750. They look like a regular VM series but are heavy zinc and the air passages are different and they are not good performance carbs.

24mm Keihins are nice but rather small on a 350. We use a pair of 26s from a CB77 Superhawk on a CB160 (180cc) race bike and it has good mid range.

Those 24's are great on the SL because they are small and create high gas velocity at low revs so they are responsive and make the bike feel fast, but it lacks top end.

And that brings us to what size is ideal for a street 350 and the answer is 26-28 for great low end and maybe 30 for someone that gets it to wind wide open. If most of your riding is around town, keep the airbox and stock carbs. If you have to have that naked midriff look and already tossed the airbox, get PJ to rejet your carbs. If your carbs are also junk, get a set of 26 or 28mm VM series (I'd go with 28s)
 
Thanks for the info teaser. That's good stuff. Would a change in jetting help the top end at all or is the bore just to small?
 
Bore is too small for max rpm power but around town it will be great fun and probably surprise a bunch of people ;D
I've been recommending 30's for max power but always pointed out I had my 391cc '250' up around 120 with 28mm carbs off an RD350YPVS (with a few mods and re-jetting ;) )
The 30mm Chinese knock off with oval bore on e-Bay will probably be about optimum for street use (it's a Kei-Hin mechanical slide carb with Mikuni badges, guess that's how they get around copywrite?)
I always liked the oval bore, good gas velocity at low throttle openings and a reasonable flow at max opening.
I have bored them 'pear shaped' so flow is higher at max rpm (bit of a PITA though)
 
crazypj said:
Bore is too small for max rpm power but around town it will be great fun and probably surprise a bunch of people ;D
I've been recommending 30's for max power but always pointed out I had my 391cc '250' up around 120 with 28mm carbs off an RD350YPVS (with a few mods and re-jetting ;) )
The 30mm Chinese knock off with oval bore on e-Bay will probably be about optimum for street use (it's a Kei-Hin mechanical slide carb with Mikuni badges, guess that's how they get around copywrite?)
I always liked the oval bore, good gas velocity at low throttle openings and a reasonable flow at max opening.
I have bored them 'pear shaped' so flow is higher at max rpm (bit of a PITA though)
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Thanks PJ, I only bid $20 on them so if I win them I may experiment. Was wondering if you know if it's possible (or even logical) to open up the bore a bit. 2mm off all around would pop it up to 28mm. I know some guys that have done it but not with these particular carbs.
 
crazypj said:
Bore is too small for max rpm power but around town it will be great fun and probably surprise a bunch of people ;D
I've been recommending 30's for max power but always pointed out I had my 391cc '250' up around 120 with 28mm carbs off an RD350YPVS (with a few mods and re-jetting ;) )
The 30mm Chinese knock off with oval bore on e-Bay will probably be about optimum for street use (it's a Kei-Hin mechanical slide carb with Mikuni badges, guess that's how they get around copywrite?)
I always liked the oval bore, good gas velocity at low throttle openings and a reasonable flow at max opening.
I have bored them 'pear shaped' so flow is higher at max rpm (bit of a PITA though)
Click to expand...

Any experience with jetting the knock offs?
 
I don't have the dimensions of a PW24 but teh 26mm are 27mm tall and 24.0-24.5mm wide with a slight figure 8 shape. That works out to the equivalent of a 26.0mm round carb.

The slide is only 25.9mm diameter, so the bore can only be opened up slightly to say 24.9 mm with straight sides on the oval and could probably be raised 1mm to get a slight increase in area but not much. We remove the choke slides and epoxy that slot and clean it up for about an 8% improvement in flow.

Compare those numbers to a VM26 Mikuni which has a round 28mm bore and a 30mm slide.

24's would probably be similar dimensions but everything smaller.

We also have to keep in mind that old CV crabs are really bad from a flow perspective and their effective diameter is much less than we might think. BS40 on a GT750 is supposedly equivalent to a 32mm VM type and it's probably not that good.

On CB900s the hot trick is to fit GSXR carbs which are much easier to jet and are much more free flowing than old Keihins on Cb350s or 360s, but PJ has got his bike working really well with modified CV carbs. So we know it can be done. Typically if you want to improve throttle response on a CV carb, the air entry to the upper chamber needs to be larger than stock, On a GT750 that's next to impossible to achieve because of the design. I may try drilling the slide next.

Or I'll just fit that set of TM39mm flatslides and call it done....

So who here has flatslides on a CB350/360? Maybe a set of 28 flats from an RGV or KRIs. Time to spend some cash for flash and see what works.
 
Nope, but they will need around 107~115 main jet , needle and needle jet/emulsion tube should be OK as is,
Not sure of slide cutaway, 2.5~3.00 should be OK with oval bore.
Don't know what pilot jet is in there but it may not need changing? (maybe go up a couple of sizes are they are jetted for smaller cylinder size)
djelliott, I sent you PM
 
Agreed. PW26 on a stock CB77 needs 135 mains. On our 175 racer the same carbs run with 115/120 mains probably because we need less fuel and air. I would expect that we have lower gas velocity through the carb, but we have a shorter stroke and higher peak airflow, but longer duration cams in which to get the gas in etc ...................

On a mild CB350 you may find that the small 24mm carb would need a smaller main jet because it's sucking hard and that raises gas velocity.

The beauty of those oval bores is that they act like a smaller carb at small throttle openings and that makes for a nicer ride and more linear thorttle response.

AFAIK, all slide Keihins of that era used the same 2.60mm bore needle jet. There are at least three different heights but otherwise they are all the same bore and same external dimensions. We use taller CB750F needle jets which are 2mm taller to change fueling at the top end, but I don't need to tell anyone about that so forget it was mentioned......
 
I'm running FSR39s on the SuperIII.....they're neat! I love the extra 'presonality' at idle. Anyone who's been in the pits at a road course will instantly recognize the flatslide slap.
That and the throttle response.....and the top end.......and the midrange.

As for the VMs, to really set them up you need more than a series of plug chops and rips up the street and back. I use a shitty fuel/air gauge and some bosch oxygen sensor to get close, in truth with patience you can get quiet close to 'there' this way.
A wideband and some time on a dyno are what you need to really get them dialed in to 10/10ths.

For reference sake:
My 81 Yamaha 850 special, Jardine 3-1 with an old kerker can, mild regrind cams, cooler plugs and the stock BS38 carbs, hogged out airbox and a K&N dialed in on the dyno made a solid 92hp at around 9000rpm. That same engine with nothing but 3) vm34s and crappy generic pods made 102hp, pulled an extra ~1000rpm and had a much fatter torque curve, something like 6-7ftlb between 3500 and 6500rpm.

Yeah I'm a believer. Are there better carbs out there? Hell yeah, the TM series or the Keihin FSR series come to mind, but bang for the buck......VMs are tough to beat.

I think one problem guys get into, pointed out here by Teazer and corroborated by Kanticoy; Bigger MUST be better. Right? ....not.
If you're running a big carb, the reason they really only work way up in the upper rpms is because the flow over the jets is slow, thus draw thru the jets are either low or erratic. Unless you're building something really bonkers, keep your carb throat diameter within sane limits.
The last cb450 I sold was really fun to ride with a pair of vm30s. It was running 2mm overbore pistons, final static compression was right at 10-1, Dyna (gl1000)iginiton and xs650 coils and a crap mac 2-1. It made great torque, spun freely up to 9000rpm or so and would lift the front end pretty easily in 2nd.
with 32's is worked pretty well but were boggy off the line if you were heavy handed and didn't seem to make much difference up top though it used more gas once you got the rpms up.

Carb tuning is a black art. It's voodoo(sic), scientiology and comet worship all rolled into a pile. There's is no blue pill, it's a matter of taking the time and earning the bloody knuckles.
 
They are simple oval bore Keihin carbs. Nothing you can't deal with. I'd be very interested to see inside those and measure and document them. They should work great around town.
 
teazer said:
They are simple oval bore Keihin carbs. Nothing you can't deal with. I'd be very interested to see inside those and measure and document them. They should work great around town.
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I will certainly do that. Photos and stats to come after I get the carbs. My dad scratch builds huge model trains and has a nice lathe/mill set up. Probably employ his services. I'll post some before photos and measurements first for a team opinion on how far we can take these.
 
sometimes I feel like i should dump my carbs and fit a fuel injection system...
 
Hi. I recently fitted some pre-jetted Mikuni 30s on my cb350 from dimecitycyles (Jet size: 25 Pilot, 200 Main, 6F4 Needle, 2.0 Air Jet, P-5 needle) and switched to uniflow pods. I understood they were specifically 'pre-jetted' for cb350s with pod filters (stock everything else) - a simple switch - back on the road in no time.

I'm having trouble even getting the bike idling well (let alone actually rev without serious spluttering or cutting out) and have begun to wonder whether buying 'pre-jetted' carbs was worth it or whether I'm going to have to pay out more $ to embark on a long tunning process, which makes the whole notion of buying pre-jetted carbs kinda redundant...

Ah, hindsight...

Now, pass me that Koolaid.
:)
 
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