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Thread: VM/Mikuni carb...only starts after adjusting the air screw

  1. #21
    So what kind of adjustments would get more air for starting? Needle adjustment...smaller slow jet?
    Last edited by marklocklear; 10-11-2017 at 04:49 PM.

  2. #22
    Senior Member gcrank1's Avatar
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    What is in the carb?
    Pilot jet
    Main jet
    Needle jet
    Needle
    Needle clip position
    Slide cutaway
    If you dont have this noted from when last apart it needs to go into the bike's maintenance log.
    If you have to take it apart to find out I suggest you take up Solo2racr on the 'bring it by' offer and expedite the process.
    '81 Yamaha SR500 Caff Racer
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    In all things maintain your uprightness lest the road reach up and smite thee

  3. #23
    All of the above LOL. Life is not allowing me time for that kind of evaluation right now. Assuming it starts the way it does, I will check out the plugs to make sure I'm not too rich/lean and roll with what I have.

    Thanks all!

  4. #24
    OK....Mark brought me the carb about a week ago. Gave it a once over and it looked OK. Nothing clogged up. Just a bit of fuel residue here and there but nothing that would effect starting or running.

    Air screw was at about 1/2 turn out and idle screw had the slide to high. Brought the air screw out to 1.5 turns (nominal starting point) and lowered the slide down to about 1/8" (3mm) above the floor. That took around 2 full turns to get that low.

    Jetting looked pretty close. Although, the Needle Jet may be a bit large.

    Pilot - 35
    Main - 230
    NJ - P8
    JN - 6DH3
    Slide - 3.0
    Air Screw - 1.5T

    It may not be perfect and need some fine tuning, but it SHOULD start with these settings.

    Opinions???

  5. #25
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    Quote Originally Posted by solo2racr View Post
    OK....Mark brought me the carb about a week ago. Gave it a once over and it looked OK. Nothing clogged up. Just a bit of fuel residue here and there but nothing that would effect starting or running.

    Air screw was at about 1/2 turn out and idle screw had the slide to high. Brought the air screw out to 1.5 turns (nominal starting point) and lowered the slide down to about 1/8" (3mm) above the floor. That took around 2 full turns to get that low.

    Jetting looked pretty close. Although, the Needle Jet may be a bit large.

    Pilot - 35
    Main - 230
    NJ - P8
    JN - 6DH3
    Slide - 3.0
    Air Screw - 1.5T

    It may not be perfect and need some fine tuning, but it SHOULD start with these settings.

    Opinions???
    Agreed it should start,jetting is close.Pilot would be to big for me with my set-up and definitely the N/J. 25 or 27.5 pilot,P4 N/J would improve things I reckon,although I would add that we've all got our individual start up procedures...for instance if I opened up my throttle a 1/4 it would NEVER start.I like the home made lever idea on the tickover screw to make it easy to reach down and tweek it up for cold starting,I use that method for cold starts ONLY if the bike doesn't fire up within 2 or 3 kicks with the choke on,then its choke off.

  6. #26
    Mark had stated that it ran fine before, so I have to believe the jetting isn't bad. He does have a Supertrapp on it as well. He told me it has good spark and compression.

    I just now remembered that I forgot to tell him that I also lowered the float height a bout 2mm. That will lean it down just a touch overall. (just a bit more so on the lower revs)

  7. #27
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    Quote Originally Posted by solo2racr View Post
    Mark had stated that it ran fine before, so I have to believe the jetting isn't bad. He does have a Supertrapp on it as well. He told me it has good spark and compression.
    )
    Let's see what Mark says, I would agree with Andy about the idle jet, On my '80 SR with VM34 K&N and canister muffler I go back and forth between a 25 and a 30 idle jet, the 25 starts cleaner but I have trouble with off-the-line being a bit boggy. The 30 is OK off the line but once in a while I have to crack the throttle slightly to make it start (I know, you are NOT supposed to do that but HEY ).. I have tried the 35, and I could get it to work but not as well as either smaller jet and am sticking with the 30 right now.

    THE use of a 3.0slide with a larger (P8) needle jet is maybe something I might try, I use the 2.5 slide and P4 or P5 NJ now with good results.
    Last edited by gotsron; 11-15-2017 at 11:24 PM.

  8. #28
    The specs I have, that have been used by others (posted from the braigasen.com website) are as follows....

    VM36 250-main, 30-pilot P5-NJ 6F8-JN 3.5-Slide
    VM36 240-main 35-Pilot Q6-NJ 6F9-JN 2.5-Slide

    I have no idea what mods may have been on the bikes for which these specs were posted. But if I were to venture a guess, Mark's SR would start with either a 30 or 35 pilot. Gotta have it running before one can sort jetting. The funny thing is that he says it ran well before and now there is a starting issue. The carb is clean and even if the pilot isn't perfect, it still should start. If he still has problems starting, I have to believe it's something other than the carb.

    (edit) - Though a drop to a 30 pilot would for sure eliminate the carb.
    Last edited by solo2racr; 11-16-2017 at 09:08 AM.

  9. #29
    OK...went out yesterday and got the carb back on, but no joy!

    Same behavior as previous, it will pop on the first kick, but nothing after that. Ken, thanks for looking over the carb for me, at least we have ruled that out. Not sure what to try from here. I do have the stock carb, so for kicks I may throw it on and see what the result is.

    I will be sure to report back when I figure it out.

  10. #30
    Quote Originally Posted by marklocklear View Post
    Ken, thanks for looking over the carb for me, at least we have ruled that out. Not sure what to try from here. I do have the stock carb, so for kicks I may throw it on and see what the result is.
    No prob. But if it does start with the stock carb, then we are back to diagnosing why it won't with the VM36 when it has in the past with the current jetting.

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