1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Mikuni pilot jetting

Discussion in 'Norton Commando Motorcycles (Classic)' started by Mwood7800, May 6, 2019.

  1. Mwood7800

    Mwood7800

    Joined:
    Jul 26, 2017
    Got 1500 miles on a fresh motor, valves set, new 12v coils, Boyer, timing set. New vm34. I’m running a 230 main, needle on 3. My pilot jet is a 35, to lean out the air screw I have to turn it all the way out, I tried a 30 pilot and it solved the air screw problem but did not like to crank. Im guessing I might order a 32.5 pilot unless anyone else has any better ideas. Bike is running rich at low speeds.
     
  2. Craig

    Craig VIP MEMBER

    Joined:
    Dec 20, 2005
    You will have to provide more info on your bike ( yr.,model, engine size, modifications etc.) and all the components of carb in use plus air filter type .... been so long I set mine up I couldn’t recall exactly what I used other than basics and that worthless info unless you have same bike as I do .... I’m sorry ,right now just don’t have time to tear mine apart for you , I should have made notes for future but .....
     
  3. eskasteve

    eskasteve VIP MEMBER

    Joined:
    Sep 24, 2012
    I'd go back to the 35 and drop the float level a bit. That's what it took for mine. Stock 73 850 at sea level. It worked well all the way up to 6,000ft. I'd also get in touch with the kits' supplier for a recommendation for your area.
     
    Peavey Jeltz likes this.
  4. Craig

    Craig VIP MEMBER

    Joined:
    Dec 20, 2005
    Try “Bushman’s Carb Tuning Secrets” has a section at bottom for Mikuni VM on Brit Twins .... that where I started ....
     
  5. Peavey Jeltz

    Peavey Jeltz

    Joined:
    Jan 12, 2019
    I like eskasteve's suggestion, check the float level. For your reference, here are my VM34 settings for a 750 Commando at 1000 feet.

    Main 240
    Pilot 35
    Needle clip in pos 3 (1 is top)
    Mix 1-3/4 turns out
     
  6. 1up3down

    1up3down

    Joined:
    Jan 12, 2011
    stock 850 my Mikuni 34 settings
    main 230
    pilot 35 - don't bother with a 32.5 as too small of a difference
    needle middle clip
    mixture screw 1 and a quarter turns out

    but you say everything is fine other than it does not like to crank
    what do you mean by this, are you saying it is hard to start as in many kicks with choke on?
    If so then did you take the plugs out, maybe they are simply fouled from leaving the choke on too long?
     
  7. Craig

    Craig VIP MEMBER

    Joined:
    Dec 20, 2005
    PeaveyJ’ .... as an example: I have a ‘72 750 ...elevation is from 1’- 1000’ here in Nova Scotia .... pretty sure my main jet is 220 , slide is 2.5 ,needle clip is one above middle ... I run racing gas mix about 100 octane .... to get good advice , need accurate bike info from Mike .... when it provided I will have a look at my carb , if similar .... where is Acotrel when ya need him , eh?
     
  8. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    You have said the bike is running rich at low speeds. What is important is how much the throttle is opened. The pilot and air jet usually only affect the operation at less than a quarter throttle. It is unusual to be riding your bike using such low throttle openings. The pilot system is used to get the motor to pick up cleanly from idle, for normal riding the needles and needle jets should probably be the determining factor. As long as the motor idles and picks up cleanly as you open the throttle, the pilot circuit has done it's job. The rest is needle and needle jet.
     
  9. RoadScholar

    RoadScholar VIP MEMBER

    Joined:
    Dec 28, 2008
    The #35 pilot (idle) jet is a good starting point for a 34VM, I'd expect to see the air screw right around 1.5 turns out from seat, maybe a shade less. A 6DH3 needle compliments the transition, but there are many ways to skin this cat.

    What most newbie tuners can't get their arms around is the high degree of interplay between the air screw and the speed screw (slide height). What the idle circuit is designed for is providing an adjustable pathway to a smooth take-off from rest and a seamless transition to the needle. The idle circuit should be set a tad rich; so, as you open the throttle the mixture goes from rich to ideal, but the needle comes along before the idle circuit starts to become lean. You set the idle mixture a bit rich because as the throttle opens, usually rapidly, air with it's lesser inertial property enters the engine more rapidly; if your idle mixture is set statically to ideal or on the lean side it will become lean or become more lean because the fuel hasn't caught up. So, you set the idle mixture a bit rich and when you take off you get a mixture that starts off rich heads towards ideal, which gives you a boost in power in addition to the greater volume of A/F.

    Where do you begin? Select pilot jet that is appropriate for the altitude you ride most in, mot ride at or near sea level, so a #35 or a #40 are good starting points. Set the air screw at 1.5 turns out from seat, start the engine and set the slide height screw to establish an idle RPM around 1000. As the engine warms the idle will go up. Go for a ride with a small screw driver. When you are confident that the engine is in the operational range try a few take-offs with the twist grip action that is hardwired inside you. If the engine bogs the idle mixture is too rich, open the air screw (I like 1/8 turn increments), lower the slide to about 1000 RPM, keep up the direction until the engine starts to pop (after fire), now go in the reverse direction until your motorcycle can be put into motion on flats and up-hills, Your done.

    Except: the Mikuni is more sensitive to ambient temperature then the Amals, so you may want to tweak it when the temp swings.
     
    MichaelB and nortriubuell like this.
  10. Mwood7800

    Mwood7800

    Joined:
    Jul 26, 2017
    More info. I’m trying to dial in my 70. 750. It’s the same mostly as my 69. 750. Both are fresh motors. My 69 starts easily with a 30 pilot and a 240 main, 1 1/2 on idle screw. Needle on #3.
    The 70 with the same set up will not start after warm up. It will start but takes too many cranks. Same timing, pistons, valves, I ing, gas, I’m missing something just not sure what
     
  11. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    If you back the throttle stops right off. the motor should still idle. I usually adjust the mixture screws towards lean until the motor coughs, then back off slightly. That way you should get a smooth idle with jetting which is not too rich or too lean. If you drop the slides to rock-bottom, the motor should not stop. If it does stop, I usually wind the stops up until I get a moderately quick idle. You need to make sure that when the slides are at rock bottom, there is still slack in the cables, and the slides are level at full throttle - if you have long bell mouths, you cannot check the slides with your fingers to see if they are level, so I usually do it by eye when the throttles are almost fully open.. I've seen some bikes with carbs in which the throttles never reach fully open. Sometimes you can wind the twist grip to it's end, but if the cables are too long, you end up with some slide still in the venturi. I don't like junction boxes.
     
    Last edited: May 7, 2019
  12. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    I have never known any bike to idle with the throttle stops backed right off ever
     
  13. RoadScholar

    RoadScholar VIP MEMBER

    Joined:
    Dec 28, 2008
    With the slide sitting on the floor of the bore the engine shouldn't start. When I set up the single Mikuni I use a 6mm drill bit like a feeler gauge between the apex of the cutaway and the carb body and set the slide height such that the drill bit just moves in and out; this will permit a start, but usually results in a high idle at OT, which is easily changed.

    Tuning a fresh engine is like shooting at a moving target, you have to keep on it until the valves and rings settle down. I jet on the rich side for fresh engines because they don't provide a full or consistent vacuum signal to the carb; I carry spare plugs during the run-in period, but, so far, haven't needed them; more of a "magic feather" factor. A #40 and the #240 sounds like a good set of jets to run-in with, but these suggestions are moot because I need to know what slide number, needle jet and needle you have to make a "system" level recommendation.

    For an engine that is fully run-in a #30, in my opinion is too small; I suggest a #35 or a #40. A #240 main, in my opinion, is too large; I suggest a #230, but no smaller than a #220. Again, need to know what needle jet and needle you have as well as the slide cutaway number.

    Best.
     
    Peavey Jeltz likes this.
  14. Bob Z.

    Bob Z.

    Joined:
    Mar 30, 2012
    Does fuel octane number or ethanol content affect the jetting?
     
  15. Craig

    Craig VIP MEMBER

    Joined:
    Dec 20, 2005
    Glad I didn’t go to effort of stripping my carb for info ...
     
  16. Mwood7800

    Mwood7800

    Joined:
    Jul 26, 2017
    What if when running a 35 pilot jet the screw has to be 4 turns out. With the 30 only 2 screws out but it is hard to start. Same jets in both carbs. My red bike has a 35 and everything else is the same and it the screw turned out only 2 turns and starts fine. Not sure why carbs are different
     
  17. Craig

    Craig VIP MEMBER

    Joined:
    Dec 20, 2005
    You missing something .... more jets to compare yet before you can say same ...
     
  18. eskasteve

    eskasteve VIP MEMBER

    Joined:
    Sep 24, 2012
    I don't mean to beat a dead horse but carb tuning is a lot more involved than what jet to use. Craig had an excellent suggestion in an earlier post when he suggested that you consult Bushman's Carb Tuning Secrets. Also, you should have received a tuning manual from your Mikuni kit supplier. You say that you have two bikes both with single Mikunis and one runs well and the other doesn't. Same jets are easy to check and compare. Do they both have identical needle tapers, slide cut away, needle clip position, air or fuel passages plugged, Float level off (which is my guess) plenty of areas that can impact you fuel delivery.
     
  19. Mwood7800

    Mwood7800

    Joined:
    Jul 26, 2017
    Carbs are identical. Slides, needles,jets, float level. New manifolds. I guess I’ll just swap them and see what’s up. Only issue is to many screws on air screw
     
  20. Peavey Jeltz

    Peavey Jeltz

    Joined:
    Jan 12, 2019
    Not my thread, but thanks to all that replied on this one. Especially RS for the recommended minimum main jet size of 220. You've motivated me to do something about my VM34 jetting which I knew was rich, but ran well enough and I just couldn't be bothered. Main jet reduced from 240 to 230 (smallest supplied in the kit). Seat of the pants dyno says it pulls redline quicker and a little less boggy on roll on. I'm intrigued enough to consider sending off for a 220 main.

    To the OP, I recall a thread on the new Amal Premiers with the advise to check the jet passages for swarf. Sometimes manufacturing defects do happen.
     

Share This Page