I can't tune my Mikuni VM20 and need help

Hey everyone!

Once upon a time I was going through some shit so I built a moped cause it made me feel good. Well, here is where I have come. And yes before the sharks come to feast, I am a noob! So I built this E50 magnum that has a non ported non case matched k star with one base and one head gasket, stock clutch, vm20 with MLM intake and foam/mesh air filter, Simo pipe, 16x40 gearing, timing is 16 degrees from TDC.

I know that the kstar with a simo pipe isn't very wise…. And I wasn't wise when i paid for it because I didn't know any better than a 5 star rating on treats and liking the way it looked.

So now, I am having a horrible time tuning it. It is constantly 4 stroking as soon as I hit the throttle. I have ran through almost every jet size from 185 down to 115 mains and 17.5-22.5 for idle. The thing idles ok but boggs and four strokes as soon as the throttle is touched, I did get it to break 30 mph with what I believe was a 140 main jet but as soon as I added my heavy chain in my bag the thing would not move and just 4 stroke.

My concern is:

-Is there some major problem I am missing? I have checked for leaks. Could it be some leak the bottom end of the E50?

-Should I try a different needle jet? The bike idles fine and the main jet was small enough that the motor cut when I went straight into full throttle. The issue is constant 4 stroking anywhere in the middle throttle range that worsens with load

-If I get a low end pipe will this problem will go away??

A couple side notes:

The motor did soft seize on me once.

The bike 4 strokes at more throttle positions as you increase the load. When its on its stand, it 4 strokes less.

I am 230lbs

I have been trying to figure this out for a while and have been getting frustrated so I appreciate all the feedback.


Re: I can't tune my Mikuni VM20 and need help

Try raising the needle clip up near the top or the needle

Re: I can't tune my Mikuni VM20 and need help

^ def agree with moving the needle clip. in addition to that, have you played around with the air mixture screw at all? start with a 20 idle and 180 main, and do a half turn out and see if there's any improvement. if there is, that may be your culprit, and you can dial it from there using quarter turns. if not, i'm guessing air leak of some kind, (though probably not the bottom end) or your carb internals (float or slide) are stuck somewhere.

Re: I can't tune my Mikuni VM20 and need help

Hey guys,

thanks for getting back. I forgot to mention that the needle is in the leanest positions. I am not sure if there are any other needles I can purchase that will allow me to go leaner.

I also tried to dial in the air mixture screw every time I changed the idle jet. I tried to get it set where it runs best without being more than three turns out.

at some point i did see a really small leak in my exhaust coming from the place where the manufacturer welded it together ( got it new from treats) which I took to a weld shop and payed 20 bucks to fix.

Other than that, I checked for leaks all over the top end and didn't find anything. (edited)

Re: I can't tune my Mikuni VM20 and need help

Russell Huddy /

Sounds like a lean bog. Try going richer on the needle.

Re: I can't tune my Mikuni VM20 and need help

Do you still have a stock crank?

Re: I can't tune my Mikuni VM20 and need help

hey Russell,

I tried that and it gets worse if I put it in a richer needle clip setting

Re: I can't tune my Mikuni VM20 and need help

It has an upgraded crank and new seals that was installed by the person who had the E50 before me. The bottom end was sitting in the trunk of my car for a while with no top end attached.... not sure if it rolled around so much that it knocked something lose or anything like that..... really thinking about anything that could be the culprit to why I'm having such a hard time getting this to run right.

Re: I can't tune my Mikuni VM20 and need help

Overpriced Parts /

Remove topend and check for ring, piston and or cylinder seize damage, get rid of that pipe for it’s a 8k to12k rpm pipe so you need to jet it for that RPM and you can’t even get it to that RPM, I’d pitch restrictive air filter too,

I’m sure if you didn’t install the treats kids it’s not prepped, spaced, matched, squish nothing is probably done right on it to begin with either but If no cylinder seize damage and all looks OK then run a estroil or circuit type pipe, being a noob I’d run an easier to tune 14 mm bing carburetor probably first,

You will find the beginning set up of an easier tune carburetor and metal mesh filter and a low end pipe will give you if you great satisfaction of being able to use your bike instead of messing it constantly with a race pipe and carb and never being able to get it dialed in correctly

Re: I can't tune my Mikuni VM20 and need help

Thank you Ken,

I really appreciate your reply. I did take off the top end and everything looked fine. I couldn't see any scratches or damage to the piston, rings or cylinder.

I was thinking about swapping out that pipe for a tecno boss- any thoughts on that pipe? - trying to be more economical since this cost a lot more than initially expecting.

Do you think that if get that pipe put on the vm20 would be tunable? right now its not... if it does become tunable I wouldn't mind trying to tune this mikuni. I had a maxi prior to this with a 12mm bing that I tuned myself so I'm not completely new to tuning carbs. (edited)

Re: I can't tune my Mikuni VM20 and need help

Simo is a great pipe, big old ripper, like the biggest badest pipe out there. Ive had it on 50 and 74cc set ups and works great, its just a bit hard to tune in. It hits late so low gearing is a must to get to the pipe faster. I think your problems are ignition related, try cleaning up the points and setting them at .45mm or .017". Usually if a bike is 4 stroking (no matter what jet you put in it) under heavy load its a case leak. Also most piston port set ups are much more forgiving to tune in with out an air filter.

Re: I can't tune my Mikuni VM20 and need help

Hey Daniel,

I did clean the points and set them when I did the timing. The motor was working ok with the stock 49cc top end on it however I do recall it being more prone to having a fouled plug when I changed the trans oil- maybe from a leak? I am really hoping its not a case leak, that would suck. Is there a method you use to diagnose a case leak? just read on MA that you can hook up a pump to the intake and seal off the exhaust port and spray the case with soap and water while pressurizing the inside of the motor with the pump.

I just ordered a boss exhaust hoping and I am hoping that that would magically solve this horrible 4 stroking issue..... really hope I don't have to mess around with the case or seals etc.

Re: I can't tune my Mikuni VM20 and need help

Its really not that hard, dont be daunted. All said and done it takes about a 6 pack of beer and a couple special tools; the pullers, a strap wrench and maybe an impact to make your life easier. Tons of videos on the interwebs.

Re: I can't tune my Mikuni VM20 and need help

Haha thanks for the encouragement! So I guess I will wait till the boss pipe comes in and see how that goes. If Im still 4 stroking like that still ill pull apart the motor and redo the seals. too fat for beer but whiskey sounds nice. Since it did have a upgraded crank put in with new seals and bearings placed by the owner before me I think ill just replace the 3 seals. thats pretty much it, right?

Re: I can't tune my Mikuni VM20 and need help

Gotra pull a bearing to replace the tranny side seal, you might damage the bearing getting it off. I would grab a new 6203 nr just to be on the safe side.

Re: I can't tune my Mikuni VM20 and need help

I appreciate you bro! thanks!

Re: I can't tune my Mikuni VM20 and need help

updating anyone who cares lol. I ordered an estoril pipe yesterday and gonna see if that jives better with my kstar kit. hoping the 4 stroking goes away....if not then ill order some new seals and see if its a case leak thats the culprit. I really am hoping the pipe fixes this issue though.

Re: I can't tune my Mikuni VM20 and need help

Mikuni wrote a tuning manual for their carburetors. Stop guessing, read the manual, memorize the manual, then tune the carb properly. Download the manual for free.

Do the same with your ignition. Then re-tune the carb again.

You may also wanna run a temperature sensor.

Re: I can't tune my Mikuni VM20 and need help

Only guessing going on is you guessing I haven't read the manual, which I have. lol.

ignition also addressed earlier in the thread. I'm sure someone reading this thread one day will appreciate the link to the mikuni site tho.

Re: I can't tune my Mikuni VM20 and need help

Tri-ped Dave O.D.B. /

4 stroking eh? What color is the plug? All the above speculation is just that, until you check the basics. Like a plug chop. 4 stroking is almost always rich not lean. First off VM20 is way too much carb for a stock case making tuning problematic. I have had best results on not matched kits with a 15mm. But you need to know which direction to go with jetting and the plug should speak to you

Re: I can't tune my Mikuni VM20 and need help

I haven't been able to run it WOT to do a plug chop but I was going thru plugs fairly quickly. I also went through a ton if jet sizes but the four stroking persists.

On the tuning spreadsheet I saw a bunch of people running PHBG 21mm carbs so I figured that the mikuni was a solid carb.

lastly, I value everyones input. feeling like I am getting some good learning points based on the symptoms that I presented. Definitely has helped me figure out a game plan vs being frustrated.

Re: I can't tune my Mikuni VM20 and need help

Tri-ped Dave O.D.B. /

Even without being able to run WOT the plug still speaks. Is it wet?

Re: I can't tune my Mikuni VM20 and need help

Alright here is an update. Made some crazy good progress so I'm excited AF.

installed the estoril pipe (Thanks Ken). was trying to get it dialed in. started with a 185 main and 22.5 idle.....worked my way down to 130 main and 17.5 idle ... was still super rich and boggy,

Took off the air filter and tried to start it but it wouldn't start till I pulled the choke lever. took it for a ride with the choke pulled and no air filter and that thing RIPPED HARD. blasted me to 37 mph with amazing acceleration till I eased off the gas.

so it seems that the air filter was choking off all the air. I put my mouth up to it and it was pretty hard to blow air through it.

So my next question is- can I just use panty hose for an air filter ? has anyone tried this? I know its ghetto but as far as function is there anything that will be detrimental ?

rather not wait or spend money on a new filter if the panty hose will work fine. please let me know what you guys think.

I will go back up on my jetting and rejet the idle and main jet with a panty hose filter just because I want to make sure I am not running lean since the bike only ran on choke.

thanks very much! Very excited...would tune more tonight but its past midnight and my neighbors will hate me.

Re: I can't tune my Mikuni VM20 and need help

Panty hose has been used b4....but it won't stop much dust and will contribute to wear....I'd find a freer flowing filter.

Then again pantyhose prolly stops more than those wire mesh Sha filters...

Don't gotta worry about pine needles and rocks tho. Ha

Re: I can't tune my Mikuni VM20 and need help

I've used velocity stacks and fine metal mesh filters a few times. Expect to inspect & clean more frequently, but the performance is killer (though maybe not the best for longevity...).

As a generalized, non-scientific observation - I find that tuning issues like these crop up more frequently on bikes heavy on bolt-on upgrades. It's like half the bike is set to blast hard, the other half thinks it's stock, and finding a jet that spans the range is impossible. But glad you're making progress; you'll get it. Great looking bike.

Re: I can't tune my Mikuni VM20 and need help

+1 on using the manual

i had the vm20 on a sachs and it took a lot of tuning, but the manual finally pointed me in the right direction.

also - it made me re-examine air leaks and found that the gasket between the intake and the head had a tiny leak - that made everything suck

also - have a tone of jets for that thing - buy them all lol

Re: I can't tune my Mikuni VM20 and need help

Get the timing set to 18 and that you dont have sparks coming from the points...

then i would get a different pipe just about anything will be better on that kit with your weight

then start jetting

115 mains and 17.5 for idle needle in the middle

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account