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  1. #1

    Default Help: Mikuni Carb Tuning/Adjusting

    Just finished up my first buid, an 81 xs650, and I dont have a lot of experience with tuning or adjusting carbs. I have Mikuni vm36 and I can't seem to get them working properly. The bike starts up but only with the choke lever down on one of them and as soon as you lift the choke lever the bike stalls. The other carb seems to be working fine, you can start the bike with the choke up or down and the bike reacts as it should when the choke is down. But the other carb just stalls the bike when you lift the choke. I adjusted the screw numerous times on both carbs trying to get them both even and trying to start the bike on different number of screw turns but the one carb still has the same problem. Any suggestions?

  2. #2
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    Your build isn't finished if it doesn't run.
    Carbs need to be removed and thoroughly cleaned. Flush out all debris from inside the fuel tank. Reinstall carbs and synchronize them.

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    How well did you clean the carbs? Pull your main and pilot jets and make sure they are completely clean. It doesn't take much to clog them. You should also try dipping the carbs in Pinesol mixed with hot water for a few hours.

  4. #4
    Jetblack
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    Yeah a cleaning etc is in order; check out http://www.chopcult.com/forum/showthread.php?t=28816 for our carb tuts.

  5. #5

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    wow, thanks big time guys, I thought I should probably clean them but had no idea where to start, thanks for the tutorials, I'll clean them and let you know how it works out.

  6. #6
    Jetblack
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    No worries, to me it's one of those relaxing grab a oven pan and crack a beer deals.

  7. #7
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    I second that Jetblack

  8. #8

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    That definitely sounds like a good idea jetblack. So following what you said,"then put some cleaner in the jets etc. then soak the carbs...and reassemble check float height," I'm not actually scrubbing anything, just spraying carb cleaner and soaking? I only ask cause I don't want to scratch anything or break anything.

  9. #9
    Jetblack
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    Here's my cleaning routine...

    Items:
    large sheet pan with a lip
    Small glass jar with a lid
    Acetone an amount of this goes in the small jar no need to fill it, but cap it or it'll evaporate quick.
    Pinesol
    Bucket
    Hypodermic doesn't need to be the needle type but it helps
    Alcohol the drinking kind

    Tools...flat head & phillips drivers that fit the screws and jets prefectly, slop will strip and scar, I replace those with cap head screws so an allen wrench can be used in the future hrd to strip one of those. A tiny wrench to remove the emulsion tube(s)

    Crack into the alcohol have a gurgle

    Once the carbs are off the bike, and shop rags put in the intakes, to keep bugs moisture and other curious critters out. Plop the carbs on the large sheet pan the lip so stuff doesn't make a break for it initiating a game of hide and seek. Then start dismantling, all the exterior screws go to one corner of the pan you can put a magnet in the corner to keep them from being all over the place. Best to leave multiple carbs on the bracket for the first time... or you're going to have to bench sync them all from scratch, so it's easier to just keep them on the bracket. Rubber such as gaskets in another corner.

    Ok, I guess order will help, I do the bowls first then the needle jet diaphragm part last. Those are the basic and really only two areas of a carb you need to deal with taking apart.

    Once the bowls are off, screws in one corner gaskets in the other, you'll see the float and main jet first. There's a pin that holds the floats on, a small nail will help shove the pin out to remove the float, watch out for the float needle it just hangs on a small tab, if it's rubber tipped don't acetone jar it. The main jet sticks up in the middle of the bowl area, get out your flat driver and unscrew it, if it's in there good it's varnish welded grab your hypodermic get some acetone into it from the small jar and squirt it on all the brass you see in the carb, it knocks the varnish welds out instantly, then remove the jets use the small wrench on the emulsion tube, it also doubles as the main jet holder. If you have not found the A/F (pilot screw) then it's most likely under a small round tamper proof seal, get a wood screw or sheet metal one and screw it right into that little tamper seal you don't have to go far just far enough in to bite the seal then pull it out. And remove the pilot screws, there will be a spring a washer and depending on carb an O-ring once that comes out.

    Here's a good image of where the hidden A/F screws are; if your don't have any seals someone's been tweaking them before and maybe changed out the jets from stock size already:


    Now remove the top part of the carb, it's a jack in the box so anticipate a large spring popping out of there. Once that's set aside, stick your finger in the middle hole if it's a diaphragm type and pull it out don't grab the rubber diaphragm at all, if you tear that ya done fucked yerself.

    Ok it's all dismantled how easy was that?

    Now drop all the jets and brass colored shit in the jar with the acetone recap it and give it a good swirl and set it aside. Do not put rubber in there... at all just the brass.

    Now get some water piping hot in the bucket and mix in the pinesol, 50/50 mix is good 25ps/75h20 is fine too. Dunk the carbs in making sure they are completely covered, on carb banks over two, a plastic rectangle storage box saves the day instead of the bucket. Now just leave everything to sit til the same time the next day if you can. Keep curious animals and small children away from the pan, to avoid accidents or losing stuff, out of sight out of mind so move it if you need to instead of guarding it with asshole threats.

    Ok, next day swirl the jar of brass, then go pull the carbs out of the water/pinesol take them to a sink and rinse the shit out of them use a old tooth brush to help remove all the crud the pinesol pulled off if you need to.

    Go back to the pan with them, and the jet jar and hypodermic, open the jar fill the hypo with acetone and start slowly squirting the acetone through all the little holes in the carb, do not stop squirting in a hole until you see acetone come out of some other hole, do that to all of them. Good all those little ports clean. The bodies clean, jets clean from hanging in the acetone...

    Reassemble, make sure your float height is spec (check a tut for important detailed info on how) put all the brass back in, A/F screw order: Spring then washer then O-ring, set the A/f screw(s) 2 turns out as a good starting point. Two turns etc. think of a clock face and the slot is the hand, starting point of the slot light seated all the way in is at 12:00, turns: 3:00 1/4, 6:00 1/2, 9:00 3/4 back to 12:00 is 1 turn, or a complete 360 degrees. For accuracy you can make a small line at 12:00 on the carb body if you're not very spatial.

    Special notes: if you saw tears in the diaphragms you need to replace them, holding them up to a light will reveal if they have pinholes, pinholes not as bad as tears but it screws up the carbs vacuum so they are best replaced if you find any. Those fucking bowl gaskets are expensive when that's all you want and one is like 20 bucks and in reality it is a .00000002 cent piece of rubber... fuck that. Get a saucer pour in some acetone drop the gasket in it and watch, let it swell about 2 to 3 times it's normal size only takes about 30 seconds max, yank it out and set it aside to dry, don't rub it or pull on it even though I said yank be gentle, acetone out gases at nearly 100% after about 1/2 hour of out gassing; that gasket will shrink back to normal size and look good as new even if it was squashed fucking flat, it'll will go back to it's original manufactured shape not the smooshed flat shit it was and hey you just saved 20 bucks per carb.

  10. #10

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    Ok so, cleaned the carbs, just like you said, put everything back as careful as can be and put them back on the bike. Choke problem fixed, but now I have a new problem. Bike is idling very low and stalling after about 15 seconds. The bike isnt responding to the idling screw at all, when before I cleaned the carbs the bike was responding to the idling screw just as it should, the more turns in the higher the idle, the more turns out the lower the idle. I have a feeling it has something to do with the throttle cable being adjusted somewhere but everything is basically where it was before I cleaned the carbs. Maybe a combination of adjusting the nut the connects the cable to the the carb? Maybe adjusting the throttle at the handle? I think the air mixture flow screw is out correctly but I'm not sure of that either. I had two amateur bike builders at my house trying to help but they were stumped as well. Any suggestions?

  11. #11
    Jetblack
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    Once a bike has fired it should die when choke is applied so it's doing it's job unless it dies when you are shutting the choke off. The choke is a enricher to help starting...usually only used after sitting to help prime it or in cold weather because of the air difference. Once it's fired you shouldn't need it at all... if you do need it to start; that's where the tuning comes in, try your mixture screws at 1 turn out, then set your throttle stop screw(idle) after it's warmed up to full operating temperature or you'll be chasing the settings for a good while on a screw top type of carb. Leave the choke alone, though and make adjustments. Can't recall what the idle rpm is supposed to be on it, my guess is somewhere between 1200 and 1400 rpm.

    This will help you dial the mixture in exactly:
    http://www.chopcult.com/forum/showthread.php?t=29958\

    And this is all you'll ever need to know about VM34 and 36 carbs. It's a PDF file that downloads so heads up.
    http://www.amckayltd.com/VM34-36.pdf

  12. #12

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    I am able to start it on first kick without either choke applied. The problem is the idle should be higher/faster. Its running really slow and no matter what I adjust I cant get it any higher. The bike is running as if the idle screw is turned all the out, even if it's all the way in. I should be able to get it to idle higher and steady without it stalling or without blipping the throttle. It's really weird.

  13. #13
    Jetblack
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    The idle screw I believe on these should be on the side of the carbs, if you have to hold the throttle open to keep it running, then you need to either adjust the slide or cable like you said at the bar cable or if it has one on top of the carbs, I haven't really studied the VM layout at all, but it's typical on screw top types for the idle to be on the side, it adjusts the slide height which is the idle, not sure if that's the screw you are talking about or not... aka slide stop screw. The other screw on it to adjust is the A/F mixture screws those from a quick glance around the webesphere says they should be set at 1 turn out 2 turns out max and 1/4 turn increments for any changes because it's a finicky carb. Typically you have to wait about 10 seconds between turn changes on A/F screws, before the new setting makes a difference and kicks in btw... it's not instant.

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