Tuning the Vm34 Mikuni

Rusty bucket · Sep 19, 2015

Fired up my MK3 after a year long span. I've never run the bike more than 20-30 miles when I bought it last year. I have removed the carb and cleaned the Jets and passageways, cleans the bowls and reassembled as it was un I toke it apart (at least I'm telling myself that!). The bike fired right off. Didn't want to idle, ran on the right cylinder and died. I check the plug wires and checked spark. Ran both cylinders in a manner than seems to resemble an old hit and miss engine.

I have to adjustments on the left and right, on the left a/f screw I assume I'm out 2-21/2 turns, right side brass screw, was 2 turns, backed it off to one.... Holy crap I thought the bike was going to over-rev and explode! Sounds like a overly rich two stroke running. Oil and sump is fine, fresh fuel, new ignition that was proven to work well last year. I can only blame the carb and the guy who rebuilt it, ME!

Is there a good tuning link available, my Jedi search skill here is poor. My plugs were fouled and heavy with gas so it's possible it's a float or needle issue I need to adjust, I just need to know how. Thanks gentlemen.

Craig · Sep 20, 2015

here you go good luck
http://www.mikunipower.com/Manuals/VM_Manual.pdf

Craig

mikegray660 · Sep 20, 2015

http://victorylibrary.com/MIK-BT.htm

concours · Sep 21, 2015

Rusty bucket said:
Fired up my MK3 after a year long span. I've never run the bike more than 20-30 miles when I bought it last year. I have removed the carb and cleaned the Jets and passageways, cleans the bowls and reassembled as it was un I toke it apart (at least I'm telling myself that!). The bike fired right off. Didn't want to idle, ran on the right cylinder and died. I check the plug wires and checked spark. Ran both cylinders in a manner than seems to resemble an old hit and miss engine.

I have to adjustments on the left and right, on the left a/f screw I assume I'm out 2-21/2 turns, right side brass screw, was 2 turns, backed it off to one.... Holy crap I thought the bike was going to over-rev and explode! Sounds like a overly rich two stroke running. Oil and sump is fine, fresh fuel, new ignition that was proven to work well last year. I can only blame the carb and the guy who rebuilt it, ME!

Is there a good tuning link available, my Jedi search skill here is poor. My plugs were fouled and heavy with gas so it's possible it's a float or needle issue I need to adjust, I just need to know how. Thanks gentlemen.

Do you have TWIN VM34's? Sounds like you have the idle SPEED screw way too far in. This one should NOT be seated, but rather, back it out until the slide bottoms in the carb body (feel it with your finger). After it's started, turn it in until it idles.

RoadScholar · Sep 21, 2015

I recommend that anyone who decides to rebuild their Mikuni carb purchase an idle jet (at least); they are virtually impossible to clean and cost far less than the equipment that can do that job.

If your idle jet has been correctly sized your air trim screw (the rearward most, at a 45 degree angle to the bore, the smaller of the two, the one with no visible spring) should be out from seat between 1.5 and 2.5 turns, any more and you should consider trying a smaller idle jet, any less, then a larger idle jet.

Motorcycle carburetors (any carburetor), in general, don't like to languish with a bowel of fuel, drunken fuel is worse. I would not presume to have the complete solution for preparing your motorcycle for extended storage, but I'd advise anyone who has questions about the subject to get answers before you end up with a clogged carburetor and a rusty tank in the spring.

Rusty bucket · Sep 21, 2015

Thanks agents,
Lots of good info here. After reading some of the manual per lnk above and reading the following posts I'm quite sure one turn out on the air mix was definitely the wrong way to go! I'll back it off to 2.5 turns and see if this helps. I do agree a rebuild kit should have been ordered but everything appeared to be in very good order with no visible wear, just some old varnish and water, likely from the recent ethanol gas used last year.

I'm going to try these suggestions and depending upon the result open the carb and replace a few common items as well as maybe the floats and a float adjustment. I'll post my progress later this week. I love the Forum, and it's become a part of my evening routine ,checking on posts and grabbing up all the norton action I can. Cheers Ken

acotrel · Sep 22, 2015

If everything is all sweet and lovely inside the carbs and your slides are opening and hitting bottom together, you need to find out whether the idle screw controls the air or the petrol. Wind them out 3 turns from fully home and start the bike. With any luck the motor will spit back through the carbs indicating it is running lean - all you need do is gently adjust the screws a bit richer, then set the throttle stops to a fast idle.

RoadScholar · Sep 22, 2015

acotrel said:
If everything is all sweet and lovely inside the carbs and your slides are opening and hitting bottom together, you need to find out whether the idle screw controls the air or the petrol. Wind them out 3 turns from fully home and start the bike. With any luck the motor will spit back through the carbs indicating it is running lean - all you need do is gently adjust the screws a bit richer, then set the throttle stops to a fast idle.

On a Mikuni VM carburetor the small rearward adjustment is strictly an idle air trim screw, no fuel. If everything is sweet, then a 3 turn out from seat on the air screw will blue your pipes to the first bend in less then 5 minutes, and as you point out the engine will spit back (pop) back through the carb. This situation will injure the carb and may start a fire if the engine stalls before it can suck in the last spit (pop) back.

Not always, but a good rule of thumb is that it the small trim screw is aft of the slide its an air screw, if its forward of the slide its a mixture screw.

Tuning the Vm34 Mikuni

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