CB350 Mikuni VM30 jetting results
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CB350 Mikuni VM30 jetting results

This is a discussion on CB350 Mikuni VM30 jetting results within the Technical forums, part of the Caferacer.net Forums category; 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 ...

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  1. #1
    Junior Member mystahagy's Avatar
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    CB350 Mikuni VM30 jetting results

    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,[u] 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.
    \'01 Honda RC51, \'02 GSXR600 track, \'72 CB350

  2. #2
    Senior Member Teazer's Avatar
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    Thanks. That's most useful. I was surprised that the Main jet ended up so large and air Jet was so small, but that's a great place to start.

  3. #3
    Junior Member mystahagy's Avatar
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    Glad to help. These numbers were the result of trial and error throttle feel with an emphasis on staying away from any lean conditions. Over time I may need to tweak it a bit based on plugs.

    I also considered compensating elsewhere in attempt to get away from such a small air jet, but so far the bike's telling me it's happy so we'll see what the long term reveals.
    \'01 Honda RC51, \'02 GSXR600 track, \'72 CB350

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  5. #4
    Senior Member Teazer's Avatar
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    Isn't that needle the richest one in the 5 range already?

    I suspect that the small Bleed nozzle is why it needs large mains and small air jets. Ideally, I'd expect/guess that it would need a larger needle jet but they are not generally available in 176 series. It would be interesting to see a set of dyno charts for the gas analysis at 1/4, 1/2, 3/4 and 100% throttle opening to see how the mixture looks.

  6. #5
    Junior Member mystahagy's Avatar
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    I was also thinking that I needed to go big on the needle jet, and P-8 was the largest I could find so far. There's most definitely improvement to be found in the details.
    \'01 Honda RC51, \'02 GSXR600 track, \'72 CB350

  7. #6
    Senior Member Mc69's Avatar
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    Thanks for the update, I've been doing some tuning as well on my CB350, but I'm still using 32mm mikuni's (I have both 30's and 32's, but 32's are working better for me)

    My motor is a 362, bigger mild cam, exhaust etc, and carbs have k&n filters on them. Here's my specs right now, it's running really well, but I'm prob going to be changing my needle today to a 6F4 to richen up my 3/4 range or I might just have to step up the main to a 195 or 200.

    - 190 main jet (hex style)
    - 2.5 slide cutaway
    - 6f5 Needle, Richest setting (needle lifted up, with clip on the lowest setting)
    - 159-P-6 needle jet primary
    - air screw--1/4 turns out
    - Air Jet Removed
    - 20 pilot jet
    - Standard float height

    From what I've learned, by removing the air jet it leans out the mid range so you can use a 159 primary.

  8. #7
    Senior Member Teazer's Avatar
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    Thanks for sharing.

    6F5 is a really lean needle from 3/4 to 100%. 6F4 is much richer all the way through the range. If that doesn't work for you, try larger needle jets. O-2 or maybe even O-6 to richen it all the way through.

    Taking out the air jet basically changes from 2.0 air jet to say 3.5 and tilts the fuel slope ie it leans out the top end (revs) proportionately more than it does lower revs - for any given throttle opening. So at WOT, it will be much leaner at high revs without an air jet than with one. It's not so much throttle dependent, like most jets, but is load (gas velocity/engine speed dependent). It has more effect when gas velocity is high and pressure differentials are high (high engine revs).

    If you did a full throttle run on the dyno with no air jet and with say a 1.5 air jet, the two A:F curves should move apart as revs rise - both at WOT. It's a great device to fine tune wide open throttle across the whole rev range and has much less effect at lower revs.

    #20 pilot jet and 1/4 turn out on idle air screw indicates it's too lean at the bottom and would benefit from a larger pilot jet - say 30 or maybe even 35. If that makes it too rich just off idle, it needs a leaner cutaway (bigger number/gap).

    But what this proves is that there are many ways to compromise and to cover the holes in the fuel curve.


  9. #8
    Senior Member Teazer's Avatar
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    I should have added something about Bleed vs Primary type nozzles. At constant throttle opening a Primary type will be much richer at the top end IF the bottom end is right. Conversely if the top end is correct, a BLEED needle jet will provide more fuel at lower revs than a Primary type.

    That makes a Primary type great for a piston ported motor. Reversion tends to make them over rich at lower revs, so a primary nozzle tends to lean that out somewhat.

    If we use a Primary type on a four stroke with little reversion through the intake, it will tend to run really rich at the bottom end if the top end is close to correct. Using a smaller nozzle can clean that up a little but at low revs and large throttle opening it will still be rich if it's right towards the top.

    Air jets, height of the primary tube and the use or Primary Vs Bleed types are more RPM dependent than throttle position dependent effects.

  10. #9
    Senior Member Mc69's Avatar
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    Good stuff Teazer. I experimented with a larger pilot (30's and 35's) but it was super rich. I think yer right, the 6F4 is a good needle for this motor due to its consistency in the taper.

    So I'm not sure if anyone else has run into this but my 32mm mikuni's are not threaded for an air jet. My 30mm's are but the 32's (even though they have the same port) don't have threads to accept an air jet. Anyways, I have jetted it to work around the issue. The bike will be going to the dyno/tuner so I'll post those numbers when I get them.

  11. #10
    Junior Member mystahagy's Avatar
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    Great information in here, thanks guys. Mc69, I'll mention that my efforts started with a primary type needle jet like yours. Idle, off-idle, and midrange performance were perfect, but I couldn't get away from eventually fouling plugs even with the air jet removed. I cut my losses and chose to focus on tuning with the more preferred bleed type needle jet.
    \'01 Honda RC51, \'02 GSXR600 track, \'72 CB350

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