tuning 20mm VM Mikuni carburetor

is there a manual that actually describes how to tune these things?

my air/mix ratio is rich at idle and doesn't get much better at WOT, blue/white smoke regardless of how far the throttle is open.

im not double stroking though. acceleration is smooth, idle is decently smooth, and after warming up at idle, the plug is Black and shiny. Soon as i get Spark plugs, I'll be able to do proper plug chops

i think my 99 Tomos Targa A35 has a big bore kit, as opposed to an after market head. The head looks exactly like the other stock heads I've seen. But my acceleration and top speed tell me its not stock

Re: tuning 20mm VM Mikuni carburetor

i'm impatient. i cant tinker on the 'ped to figure this out because its raining its ass off outside

Re: tuning 20mm VM Mikuni carburetor

Once you get the idle set at the proper RPM leave it. Sounds like you just need to lean out the mixture screw a bit. Adjust the mix till you get a nice resposive first 1/4 throttle snap. Then do some plug chops to see where your at. Just isolate each throttle position when doing the chops.

Re: tuning 20mm VM Mikuni carburetor

so i just took my moped on its maiden voyage. Runs great once its warmed up.

things i noticed:

WOT at idle very slow = die

once above like 10 mph, no bogging, no stalling

3/4 throttle produces the same top speed as WOT, only difference being the engine sounds a little different (the "more throttle but not faster" lower frequency sound)

i did some sudo plug chops while i was out. Spark plug went from black and shiny(at idle) to a nice chocolate brown (at WOT). soon as my box o plugs gets here i'll do real ones.

on level ground, it averages 38 mph at WOT, sometimes faster when the throttle was at 3/4. topped out at 42mph going down a small hill

I'm pretty sure there's an exhaust leak on the head at the muffler. always leaves a little (non dripping) oil spot just below there

speed measured with a cyclo-computer, not sure how accurate it is. Its the type that you set the diameter of the wheel its measuring

Re: tuning 20mm VM Mikuni carburetor

so i know how to do a plug chop

what are the proper RPMs?

(i don't yet have a way to measure those)

what throttle positions do i need to test?

1/4 throttle snap as in 0 -> 1/4 (very quick like) = good response from engine?

Re: tuning 20mm VM Mikuni carburetor

Wrap a piece of tape around the throttle grip and mark the throttle casting somewhere. Now mark the tape to correspond to the baseline on the throttle casting for closed and wide open. Then divide that area with two more marks to get 1/4 throttle increments. Now you can ride it and know where the problems are. 1/4, 1/2, 3/4 or full throttle position.

Go to this site and dwld the Mikki manual and track down the area that needs tuning, choose the parts, or jets and have at it.


Re: tuning 20mm VM Mikuni carburetor

you only need 1 plug to do a plug chop man!

Re: tuning 20mm VM Mikuni carburetor

so i've read the manual 3 times now. it does a decent job explaining how the Carb works and how to adjust the Carb for idle and WOT tuning. Mid range is kinda vague.

is needle size the only adjustment? or is there somthing they didnt mention?

i like to figure these things out before i start taking stuff apart

Re: tuning 20mm VM Mikuni carburetor

The needle taper is the most common part to change for mid-range other than the pilot and main jets. You don't need to change the needle jet unless you can't get the proper needle taper to do the job. There are some different slide cutaways available too, but you generally don't need to change them if it picks up well off of idle. Mikuni slides can be a bit pricey at about $30 a pop. Jets and needles are more reasonable.

Sudco is a big dealer but they mostly sell quantities to dealerships. motorcyclecarbs.com handles small orders but you pay the price.

not finished...

im pretty sure i know what im doing, just want to make sure...

so i bought new plugs and managed to finish the WOT plug chop before my rear tube popped.

Its Definitely lean, ceramic was mostly white with some very light tan and gray coloration on a small area. At least it wasn't pitted. There also was this very thin, flaky, gray layer on the little arm of the plug. I think this hints at too much oil in the mix

so I'm gonna try adjusting the air screw in (I'm assuming now that I've seen 3+ other sites that all say the same thing about Mikuni carb air screws)

here's a site i found that shows me where all the important things are on my carburetor: http://www.teamcalamari.com/dox/tekcarb20.html

Re: not finished...

Im happy with my tuning although there is more to be had at top end.

Ive went through main jets 120 to 165 in increments of 5-10 and 145-155 seems best.

Idle screw out 2-2 1/2 turns(pilot jet definitely too big)

Timing advanced slightly, boyesen dual stage reeds(still not 100% sure about these yet).

Re: not finished...

145-155? do you have a 70cc kit?

mine's running lean with a 180 in it...

Re: not finished...

King Drunky JCams /

Lean with a 180 makes me suspect an air leak someplace.

Nice bike foxboss ; )

Re: not finished...

Thanks, finally pulls my bigass ahead of traffic

Re: not finished...

rode around doing plug chops while only adjusting the air mix and idle screws.

air screw ALL the way in at WOT = rich w/ double stroking

1 turn out at WOT = lean ( not sure how much, was kinda dark and street lamps suck)

so its currently at 1/2 turn out and i was able to adjust the idle screw for any air mix position to make it idle nice n slow

guess i'll go ahead and remove muffler, drop the engine, completely disassemble the carb, remove the intake, take off the head, and then check to see if its a stock piston/cylinder/head. Its supposed to have a 70cc kit, but this wouldn't be the first time i was screwed by CL. If Its not kitted, I'll either buy a kit (potentially lots o fun) or put the stock carb back on(not potentially fun).

but hey, i manged to get my funky side stand setup working

Re: not finished...

180 main jet... that is absurd. I'm running a bit rich with a 140, and that's with a kit, techno circuit pipe, and no air filter (waiting for one to arrive).

Re: not finished...

i'm running a 210 in one bike, a 185 in two others with vm20s. 11000+ rpm ftw

Re: not finished...

Weak-Ends Jamie /

this post is from last summer....

Re: not finished...

140 main jet sounds about right for a low revving techno circuit pipe.

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