Mikuni VM30's tuning for a 1969 CB350

jonnymedia

jonnymedia

Well-known member
Hi all. I have a lovely 1969 CB350 that I rebuilt and have ridden almost daily for the past five years. However, I've been having carb issues with the stock Keihins so I picked up a Mikuni VM30 kit.

I spent many hours trying to get them working as they came fitted, but gave up last season as the snow started to melt. The VMs just gave me white smoke pops out of the intake side, would die quickly, impossible to dial in, etc. At most I got it idling and able to move. But anything over a certain rpm and it would cock out. I put back on the Keihins and learned to live with them for another year.

This winter, I really want these tuned up and I could really use some advice to get me in the right direction.

Relevant bike specs.
  • 1969 CB350
  • Uni pod filter
  • Plugs: NKG 5422 BR8ES
  • Proper throttle cables.

Mikuni VM30s with the following:
  • Main Jets: 150
  • Pilot Jet: 20 (but I think I've seen people on forums write this as 2.0)
  • Needle Jet: 159 P-5
  • Needle: 6F4
  • Needle position: middle knotch

So I know getting a range of main jets and some fresh spark plugs are in order. But to avoid multiple or incorrect purchases, my questions to the community are:


  • What range of main jets should I get?
  • Any of the other jets/needles should I consider getting others sizes of?
  • Any other components that are connected to all this I should purchase?
  • Any advice or similar experiences?

Much appreciated.

jonnymedia
 
It will help to get an accurate description of exactly what it is doing and under what circumstances.

In the absence of any personal knowledge of that engine with those carbs, go back to first principles.

Find a setting of choke, throttle position, pilot jets, idle mixture screw position, and float level that will allow the engine to start. It sounds like you have gotten to that point.

Get the engine running and warm it up for a minute or two. What happens when you ever so gently start cracking the throttle open just the slightest bit? If you can get it up to 2000 - 3000 rpm no load, the pilot jet and float level are probably in the ballpark. If it starts stumbling and missing, let it go back to idle and then try snapping the throttle to whatever position you found before where it started showing signs of stumbling. If it pulls to those revs when the throttle is snapped to that position but it's rough when you try to gradually get to that position, it is probably rich on the pilot jet. If it simply quits when the throttle is snapped, it's probably lean. Another clue is what happens as it warms up. Carb engines need some choke off cold start, that's normal, but if the engine can be taken off choke almost right away but actually runs worse as it warms up, it's probably too rich off the bottom.

So try different pilot jets based on the above. Expect to have to change the idle mixture screw position. Ideally, the idle mixture screw should be in the middle of its travel (normal range 2 to 3 turns out on more modern Mikuni carbs).

Once you get the pilot jets so that it will run decently at no load up to moderate revs, now start doing the same with the needle position. This will require riding it at low speed. If it stumbles on takeoff (worse when cold), it's probably lean. If it just feels boggy and lazy (worse when hot), it's probably rich.

I'd suggest getting the main jet sized properly on a dyno. But you need an engine that at least runs, over some reasonable range of load and speed, or you are going to waste a lot of time there.

Not having an airbox is going to make your life miserable for setting this up. Might be worth playing with velocity stack length and diameter ...
 
Last edited:
fROM ANOTHER FORUM: Well it's been a long road, but well worth it. This old girl is finally running great after 4 weeks of testing, tuning, and waiting to receive jets a few times over. I've got some jetting numbers to share to feed the hungry baby birds out there who seek to run a bleed type needle jet. Keep in mind, these numbers work for MY bike in MY conditions. Perhaps it will get you in the ballpark and save you some cash.

Bike:
1972 Honda CB350 twin
Generic 12" shorty mufflers, freshly repacked, with additional baffle inserted before muffler</u> for a bit of backpressure and less raspy pop
NEW Mikuni VM30's and K&N pod filters
Fresh points at proper gap, condenser, plugs, valves checked, correct timing, clean and smooth timing advancer, good compression
87 octane fuel w/10% ethanol

Conditions:
Typical hot summer, sea level.

Jetting:
Main Jet:Large Hex 200
Emulsion tube/Needle jet: BLEED TYPE 176 series P-8
Slide: 2.5
Pilot jet:30
Needle: 5F3, lowest (richest) clip setting
Air Jet: 0.5
Air Screw: 2.0 turns out

So far I've got a few hours of spirited riding on this combo and it feels great from idle to redline at all throttle positions. Plug has a slight cardboard color so far and I'll keep an eye on it. The bike can idle for an extended period and still have a clean off-idle response. I'll eventually try an even richer needle to get the clip settings closer to the middle, but let's enjoy it for now. Hope this helps.
 
Taximan, Brian.

Great suggestion on where to start and how to move forward. Really appreciate your time. Thank you.
 
You must log in or register to reply here.
Back
Top Bottom