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Discussion Starter · #1 ·
Hello All:
There seems to be a growing number of members converting to the Mikuni VM Roundslide carburetors.
This Stickey is for posting FINAL tuning specs once you have dialed in your bike. Please use the general threads for tuning discussions.


VM tuning general guidelines
VM Roundslide Synching
Two Mikuni round slide carbs are simple to balance and synchronize perfectly with no carb sticks or other vacuum gauges.

• Insert the smooth end of a drill bit from the intake side under the slide throttle valve cutaway and set the idle thumb screw so that it just drags on the smooth end of the drill bit. Use the same drill bit on the other carb and set it the same as the first. If the motor idles too high, use the next smaller bit, or if it idles to low use the next larger drill bit etc. Do not do this with the motor running.
• Once both idle positions are set, you can easily set the slides to lift together at the exact same time, and be at identical slide position heights by using two popsicle sticks. Put one popsicle stick into each carb about the same distance under the slide then simply twist the throttle and watch when each popsicle stick begins to move. Now adjust the throttle cable adjusters on the top of the carbs so that you can see both popsicle sticks begin to move at exactly the same time.

Setting Pilot and Idle Air Screws
To properly tune the VM's it's critical you get the Pilot and air mixture set properly at idle before you do anything else. You'll need a box of fresh plugs to do this properly...at least 3 or 4 sets. Once the carbs are synched, ride the bike around for 10 minutes or so to get it fully warmed up. Then set the idle and mixture screws so you get a steady idle.

• Turn both idle-air screws (the smaller ones, not the larger idle speed screws) all the way in and back both out 1, 1/4 turns. Screwing it out will lean the mixture while screwing it in will richen it. If turning the screw between one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or smaller one. While adjusting the pilot screw, turn it 1/4 turn at a time and test run the bike between adjustments. Adjust the pilot circuit until the motorcycle runs cleanly off of idle with no hesitations or bogs.
• Install new plugs and start the bike WITHOUT ANY CHOKE OR OPENING THE THROTTLE AT ALL. Let the motor run for 3 or 4 minutes like this at idle NEVER TOUCHING THE THROTTLE. Then pull the plugs, they should be very light tan at the base of the porcelain cone and white at the tip. You're checking idle-air mixture only here. After you've let it run at idle for a few minutes, turn off the motor and pull the plugs - fouled = open the idle air screws up another 1/2 turn, white = turn them in 1/4 turn. Repeat with fresh sets of plugs until they look white at the tip and tan at the base of the porcelain. Repeat this test with NEW PLUGS EACH TIME until you get the proper plug readings. If you want to save money on plugs, start with the leanest pilot jets first...they won't color the plugs and will let you reuse the plugs till you get a reading on them

Setting slide cutaways
To properly set slide cutaways…

Setting Main Jets
To properly Mainjets…

Setting needles and needle jets
To properly set needles and needle jets…



Mikuni VM 30s on Stock CB350 K4 motor with aftermarket 2 into 1 exhaust and Uni filters
150 Main Jet
2.5 Slide cutaway
3rd Jet Needle = 6F4
P-5 Needle Jet = 159 P-5 (primary type)
1.5 Idle screw
2 Air Jet
3.3 Fuel inlet valve
20 Pilot Jet
22 Float Height
 

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Killer writeup so far!

I'll post what I know from my setup. If you can help me figure out the other things I'll do what I can to tell you!

Bike runs great and doesn't foul the stock B8ES plugs.

1974 CB360G
Mikuni VM32s on Stock motor with aftermarket 2 into 1 exhaust, Uni filters, and electronic ignition
90 Main Jet
?.? Slide cutaway
?.? Jet Needle =
?.? Needle Jet =
? Idle screw
? Air Jet
? Fuel inlet valve
? Pilot Jet
? Float Height
 

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That was great! Thanks I'm looking to get a pair of those soon and this will be a great help when getting them dialed in!!
 

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New to the boards, but have been lurking for a couple months!

I bought the DCC kit and have been attempting to make it work.
I have aggregated a text document of all the various suggested tunings I have found using google. I don't know what kind of stock bike they expect it to run on, but it won't run to well on my CB350.

About my bike:
CB350K1 Fair compression - 145 + 155 with valves adjusted perfectly. Timing is spot on advance is working. Dyna coils

Currently
Main Jet: 150 Hex
Pilot Jet: 30
Air Screw 1 turn
Needle: 6F4 clip full lean
Needle Jet: 159 (Primary type) P5
Air Jet - none

Runs great on the pilot circuit from 0-30% throttle. The bike is surprisingly powerful even at these low throttle settings! Plugs have a nice brown toasty color when ridden in this range. However the mid-range dies if you get on the throttle fast. Generally it won't take much throttle unless you really let the RPMs build. It has no tolerance for lugging the engine at all. My feeling is that it's generally too rich. Raising the needle and/or richer main jets seem to make it worse. My current jet inventory jumps from 150 to 190 though....

I ordered some new jetting from my local shop including:
159 P4 primary needle jet (one leaner)
176 P8 bleed needle jet (I read that the bleed jets are much leaner than the primary type. P8 is the richest bleed type for the VM30 and somewhere in the range of a P5 or P4 primary type)
140 and 145 main jets
1.0 air jets to try with the bleed jet
 

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Update:

The 176P8 bleed type needle jet didn't do much for me. I tried with all sorts of jets and needles with no luck.
Current Jetting:

140 Main Jet
2.5 Slide cutaway
3rd Jet Needle = 6F4
P-4 Needle Jet = 159 P-4 (primary type)
1.25 Idle screw
Removed Air Jet
25 Pilot Jet

Pulls great from 0-75% throttle - have to play with main jets some more. Bogs just a tiny bit if you roll on too fast. WOT is bogs unless RPMS are high. Thankfully, I have become friendly the mechanic who I ordered some of the jetting through. He is sympathetic to my cause and will allow me to swap out parts that I have for what he has in his on-hand inventory.

I do believe that my lower than ideal compression is causing some 'low signal' issues. I do believe I am close to having it pleasurably rideable; until I can do a top-end job and have to rejet it again. I have around 70 miles on it with various 'not pleasurable' jetting scenarios.
 

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Quick update with my current jetting. Still a little bog if i'm in too high of a gear and snap the throttle open, but it's feeling really good. Probably a little rich, but the 6F5 needle seems too lean...


145 Main Jet
2.5 Slide cutaway
1st(full lean) Jet Needle = 6F4
P-4 Needle Jet = 159 P-4 (primary type)
1.25 Idle screw
1.0 Air Jet
25 Pilot Jet
 

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Discussion Starter · #7 ·
what do the plugs look like
 

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Well, I haven't done plug chops at various throttle settings like I know I should. I will eventually! However, at the end of a general day to day ride the ground straps are light brown like the color of toast, and the outer ring of the bodies are more blackish. Fuel economy is not super - Guessing 40-45mpg. Hard to say since I haven't run a full tank of gas with out draining the bowls at least twice.

I don't think I'm done tuning these yet. However, I may not go too much further. I do see a top end job in my future, so I'm going to have to do all this again...
 

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200 is the jet that a lot of places ship it with. In my case it was way way way too big.


I'm pretty close to as good as it's going to get, which seems like where a lot of people stop. My compressions are not great: 130 on the left 150 on the right, so I'm not buying any more jetting unless I do something about the compression. Here's where I'm at. With this current jetting, I am getting around 42mpg around town. I run it really hard, shifting at red-line often. If I took it easy I might get 50mpg. I've had it up to 80mph, probably would do a little over 90mph given enough time/cajones.

140 Main Jet (If I had 135's I might try them)
2.5 Slide cutaway
1st(full lean) Jet Needle = 6F4
P-4 Needle Jet = 159 P-4 (primary type)
1.25 Idle screw (not sure about this anymore - might be a bit richer than that now)
Removed Air Jets
25 Pilot Jet


Plugs looks just okay, not great. The barrel of the plugs looks dark, rich idle. However if I don't run the idle fat, I'm a little lean at mid cruising speeds say 10-25% throttle. (Burbles and runs a tad hot) Ground electrode is brown/grey, indicating that at power I'm in close to the ideal range. It still bogs a bit if you snap the throttle too fast. If you roll on smoothly to 2k, then it will wind out to red-line and beyond.
 

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Discussion Starter · #11 ·
you'll need to do plug chops at each throttle opening to be certain but from your description it sounds like...

go smaller on pilots
raise needles
go smaller on mains

but you are correct...you'll need to do some more fine tuning once you rebuild your top end to get compression back but it should be very close.
 

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I hope i'm not hijacking anything here, but I was going to post my settings and hopefully get some advice.

150 Main
25 Pilot Jet
2.0 Air slide
159 P-5 Needle Jet
2.0 air jet
6F4 needle on top most clip setting
Idle mix screws out 1.25

I had the bike running and it is really strong at WOT but midrange is really choppy. I was moving the needle lower and the midrange (cruising) got better. I left it set on the lowest setting (top most ring on the needle). I left the bike for a few days and tried to ride it and it would crank fine and idle but die when you turned the throttle. It got a little better when it warmed up.

I'm kind of clueless as to what to change. I think my bike is possessed :?
 

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Discussion Starter · #13 ·
what are your compression numbers?
have you set the valves properly
have you adjusted the timing chain properly
have you dynamic timed the points
clean gas tank, petcock, and new fuel lines
clean air filters
have you synched the throttle cables
have you synched the idle stop screws
what exhaust do you have

all those things need to be perfect before you start tuning. and it helps to have a whole box of new spark plugs
 

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Thanks for the reply!

I had to check the compression but everything else on your list I have addressed. The carbs are synchronized, I was VERY careful on getting the slides the same. I used the smooth end of a drill bit as a gauge. I had the valves adjusted at a bike shop. I have new fuel lines and a new petcock. The timing chain is good and I have the pamco ignition and set the timing with a timing light. I have brand new UNI filters and factory exhaust. My compression readings were good at 150
I found out when I was doing the compression test that my battery was low. I didn't even try the starter when I had the trouble yesterday so I don't know if this could have had anything to do with it. Even if it was the problem, it still seems that I need some more adjustment since I have the needle in the lowest position.

Thanks again!
 

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Discussion Starter · #15 ·
the honda factory manual recommends compression of 150 and below indicates time for a rebuild

target compression is 170 to 190
 

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Outobie,

noobie here can't do a compression test right :oops: live and learn

compression 190

I did change my needle to a 6f5 in the middle setting, but the problem is still there at idle. I found out that some of the mikunis had 2 stroke needle jets in them. From the research I have done, my carbs have the 2 stroke NJ (159 p-5), could that be my problem? Sorry for the confusion on the comp test! I didn't think the engine was worn out since it has just less than 2000 miles on it.

Thanks!
 

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Discussion Starter · #17 ·
all the mikuni's ship with the 159 series needle jets...theoretically they are of a 2 stroke design but everyone I've ever known has used them on their 350 hondas with good success. once upon a time I intended to run some tests on their equivalent 4 stroke needle jets but since the bikes run so well as is, I never got around to it.

I'd recommend buying a box of new spark plugs and doing a series of plug chops at 1/4 throttle and 1/2 half throttle and report back on what the plugs look like.
 

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After recent Koolaid consumption I have come out with a result. Still to do some tweaking but I am pretty happy with current running of bike...

Started out with 176 p8 bleed type with a 6dh7 needle and 150 main. Gave me a really boggy condition mid throttle. Next I tried putting back my primary type 159 p4 NJ and stayed with the 6dh7. Put it in with leanest clip setting and no air jet. This gave me nearly 0 power at all. Couldn't even make it up the ramp out of the carpark. So put the air jets back in, thought it might be leaning out. Still was grumpy but slightly better. So I thought maybe if I shoot for richest needle setting and just see what influence that has. POW POW! Took it around the block and could not believe the way it rode... Like a different set of carbs!

So went for a 5km ride to a mates place and then returned home. Plug chop came out a bit darker than I would like (still learning to read plugs) but a nice mushroomy kind of colour. The only real area i am concerned with is WOT may be rich, but yet to confirm that. At least now I can ride the thing!

So the long story short, I would recommend that if vm30's are the path you wish to take, my setup is as follows:
CJ360 engine (rebuilt, new rings, no oversize, was just honed)
2 into 1 exhaust with open style muffler (reverse cone generic type cafe)
K&N pod style air intake filters
vm30's with this jet setup:
main jet: 150 (still to confirm top end)
pilot jet: 25 (about 2 turns out)
Needle Jet: 159 p4
Jet Needle: 6dh7 (richest clip position)
air jet: 2.0

Seems a bit like a repeat on others settings but I thought it best to divulge than to not. I am a little dissapointed that the bleed type didnt provide any answers for me... I did however not try the bleed type with the 6f4 needles. Maybe the 6dh7 is profiled to adapt to a primary so by using it with a bleed type may have been like a double negative so to speak. What I am trying to say is the primary nj with the 6dh7 leans out the 1/4 throttle, and so does a bleed type, so maybe a more linear tapered needle may work better with the bleed type nj.

Thats a bit convoluted but it makes sense in my head... :eek: :eek: :eek:

ha ha
 

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Discussion Starter · #19 ·
sounds like you're making good progress.
take good notes with each change
try to only change one thing at a time
your 150 mains may still be a bit fat from what you've described
do plug chops at various throttle positions with new plugs each time.
 

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Hi all; I've been reading this thread and a few others about Mikuni VMS while working on installing VM30s on my CB350. I think I've finally gotten to a point where I have some (I hope) not too ignorant questions. Here's my setup:

2 into 1 exhaust
Good compression: between 180 and 185 on both cylinders (just did a careful and thorough rebuild)
Pilots: 30
Mains: 155
Needle: 6F5 on leanest setting
Air screw: About 1.5 turns out
Idle screw: 3+ turns out

The bike starts without choke if it's sunny, with a brief choke if it's cool, and idles beautifully. If I roll of idle slowly, it runs great all the way through the range (standing still, of course - no front brake yet). With new plugs in (after it's warm), they get a nice light brown color (see photo; right plug is slightly fouled because I left the wire off it for a few seconds, allowing an excess of mixture in there).



If I open the throttle quickly, especially twice in a row, it bogs or even dies and makes a kind of coughing noise. Changing the air screw didn't seem to make a lot of difference either way. I'm guessing that I need to raise the needle a little to enrichen the mixture, and maybe go one step smaller on the pilots. Was in the process of doing all that when I lost one of the e-clips that holds the needles. Of course. I can order them from Sudco or wherever, but does anyone know what size they are? I feel like I should be able to find one at the local hardware store. As soon as I get that bugger I'll get back to it and report back. I already like these carbs so much better than the stock CV ones - they make so much more sense! Can't wait to actually try them out on the road.
 
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