Tuning a vm18 Mikuni

Jay Rivett /

Jul 15, 2020 08:40PM

Sooooo...

Yeah I am trying to step up my carburetor game. Don't know why I chose the vm18 over a phbg, but I did. So here we are.

I have not changed out the atomizer or the needle. I have changed out the main jet and the idle jet trying to get the bike to run better with the throttle more than 1/2 open.

I started at a 30 idle jet with a 100 main jet.

The bike started easily with the 30 idle, but after heat cycling to break in the kit, the plug had a lot of carbon build up so I figured that was too big so I went down to a 25.

The 100 main jet with the needle in the middle position bogged down and killed the engine at any throttle position. I dropped the needle to the top position and the bike gained some throttle responce. It would still bog and die at WOT though.

Now here is where I am still trying to understand things.

By dropping the needle down it closes off the fuel, leaning out the mix. So that says to me that I would want to down jet my main.

I down jetted to a 95 main and kept the needle in the same position. The bike has some more throttle response than before but still bogs at WOT. I did take the bike for a spin and just had to half throttle up to speed, hit 38mph but it started bogging when trying to open up the throttle all the way so I backed it off.

I checked the plug after my run and it looks a bit too lean. I have not played with the needle just yet, but will it richen things up enough?

Is there any difference in feel or sound between a lean bog vs a rich bog?

My temp gauge never got up over 310, but I was not running for very long.

Setup:

70cc tccd kit w/low comp head

Mlm peoples pipe

Vm18

Re: Tuning a vm18 Mikuni

Emil Kniemel /

tpr950h

Jul 15, 2020 09:41PM

Your bog is probably four stroking. 25 is still pretty rich. People are usually closer to 15.

Re: Tuning a vm18 Mikuni

Mike Boyd /

Go to branch

boyd

Jul 15, 2020 09:52PM

edit: vm18 has an air mixture screw rather than a fuel mixture screw so i had to reverse what i posted:

Tune the idle first. Start with a warm engine and the air mixture screw 2.5 turns out from flush (a full turn is 180 degrees. Adjust it by 1/4 turn at a time, blipping the throttle each time.) Turning it IN richens the idle mix, turning OUT leans it out.

If you can close the air mixture screw IN to 1 or less turns from flush with no difference, then you need to go DOWN a size on your idle jet cuz you're too rich. If the engine "hangs" after you blip the throttle it is too lean. You will know you have the right size idle jet when it idles smoothly and returns to idle smoothly after a blip, and the air mixture screw is between 3 and 2 turns out. (edited)

Re: Tuning a vm18 Mikuni

Jay Rivett /

bjaockx OP

Jul 15, 2020 11:41PM

Thanks for some answers and ideas! I am going to get into it some more this week!

Re: Tuning a vm18 Mikuni

Jacob Fulton /

jake5857

Jul 16, 2020 12:50AM

For reference, the setup on a 70cc tomos i kitted with a VM20 and pipe I had a 22.5 idle, 85 main, leanest needle position. Felt it could even be a little leaner still.

Re: Tuning a vm18 Mikuni

Jay Rivett /

bjaockx OP

Jul 16, 2020 08:07AM

This is all very good info. If anyone else can share their numbers especially in their vm18 that would be super helpful as reference points. I appreciate all the help, I know I can get this guy set up right!

Re: Tuning a vm18 Mikuni

Graham Motzing /

Go to branch

grahammotzing

Jul 16, 2020 08:37AM

Yeah your main is way huge... VM18 runs 75, 80, or 85 on almost all moped builds I've used them. The idle is usually 17, 20 or 22.

Re: Tuning a vm18 Mikuni

Jay Rivett /

bjaockx OP

Jul 16, 2020 09:01AM

Graham!

That is a huge help and I have all of these jets!

Gonna mess with it some tonight I hope. I think the monsoons are coming and finally giving us a little break from this heat.

Re: Tuning a vm18 Mikuni

Jared Rut /

jrut

Jul 16, 2020 12:27PM

za50

vm18 on tccd70

Techno circuit pipe

High flow filter.

I think I settled around 17.5 idle and 75 main, I'll check to be sure when I get home I usually write down whats in there

Re: Tuning a vm18 Mikuni

Dirty30 Dillon /

hydra

Jul 16, 2020 12:43PM

> Graham Motzing Wrote:

> -------------------------------------------------------

> Yeah your main is way huge... VM18 runs 75, 80, or 85 on almost all

> moped builds I've used them. The idle is usually 17, 20 or 22.

Yeah, I was always so surprised how small my VM18 jets ended up in comparison to the VM20. on similiar setups.

Re: Tuning a vm18 Mikuni

Jay Rivett /

bjaockx OP

Jul 16, 2020 02:19PM

Super helpful info for those of us just starting out on there kinds of carbs! Thanks everyone!

Jared, how do you like that tccd on the za50? It is where mine will end up once I get the za50 rebuilt that I have.

Re: Tuning a vm18 Mikuni

Graham Motzing /

Go to branch

grahammotzing

Jul 16, 2020 02:48PM

> Dirty30 Dillon Wrote:

> -------------------------------------------------------

> Yeah, I was always so surprised how small my VM18 jets ended up in

> comparison to the VM20. on similiar setups.

Yeah, they are totally different carbs, mikuni almost should have just named them something totally different. All the tuning parts besides the main jet are totally different.

The other thing I was gonna say... These can be tricky to get right because they don't four stroke or run funny like a phbg if the jetting is slightly off... That's nice for getting an initial tune and getting it running, but annoying for fine tuning.

The way I have found works best is to go with a small pilot jet and a big main. Work your way down on the main until the temp starts going up, the plug chops are correct or it starts to 'feel lean' if you can feel it. After a few years you can feel when it's lean but for starters the head temp gauge is the best. I usually wait until after break in before doing a plug chop, and sometimes if it's running good I don't even do one, I just check the plug and go 'yup'

Every time you change main, you will need to adjust the screw but leaving the pilot jet too small, you will notice right away when it's lean. Then start going bigger on the pilot jet until your screw is between 1 and 2.5 turns out. Turn the screw listening for the idle to raise, that will get you within about 1/2 turn off being right, then go quickly from 0-1/2 throttle repeatedly until it has the quickest and cleanest sounding rev up. You will also need to move the needle up and down. If the spot where the screw wants to be at idle, and the spot where the screw wants to be for a clean rev up are 2 different places, that will tell you your needle needs to move.

Once you understand the logic of these carbs they are super easy to tune and they really run well. If you are just randomly flipping parts around until it works, you will be frustrated.

Re: Tuning a vm18 Mikuni

Jay Rivett /

bjaockx OP

Jul 16, 2020 02:59PM

Graham this is exactly what I needed. I had read thru the mikuni vm tuning guide I found on the wiki but it is definitely for bigger carbs on multi gear bikes. It gave me some ideas on some tuning aspects but it was still pretty greek over all to me.

I do have a basic grasp of what is happening when I make adjustments and how to follow, but I also suspected I would not get the same "four stroking" and other indications that I get from a sha. So I am hoping that I will be able to get this little guy tuned up real nice. Perfect would be great, but not seizing is all I really need. With it over jetted I already hit a good speed so I'm not trying to get too much more out of it.

Re: Tuning a vm18 Mikuni

Jared Rut /

jrut

Jul 16, 2020 04:30PM

Jay, I think those are great kits for the money. I have 1 on a za dirtped and one on an e50 maxi. The za is geared pretty low but I wanted torque not top speed. It'll lift the front when I want it to and tops around 38.

Re: Tuning a vm18 Mikuni

Emil Kniemel /

tpr950h

Jul 16, 2020 05:20PM

> Graham Motzing Wrote:

> -------------------------------------------------------

> > Dirty30 Dillon Wrote:

>

> > -------------------------------------------------------

>

> > Yeah, I was always so surprised how small my VM18 jets ended up in

>

> > comparison to the VM20. on similiar setups.

>

> Yeah, they are totally different carbs, mikuni almost should have just

> named them something totally different. All the tuning parts besides

> the main jet are totally different.

>

> The other thing I was gonna say... These can be tricky to get right

> because they don't four stroke or run funny like a phbg if the jetting

> is slightly off... That's nice for getting an initial tune and getting

> it running, but annoying for fine tuning.

>

> The way I have found works best is to go with a small pilot jet and a

> big main. Work your way down on the main until the temp starts going

> up, the plug chops are correct or it starts to 'feel lean' if you can

> feel it. After a few years you can feel when it's lean but for starters

> the head temp gauge is the best. I usually wait until after break in

> before doing a plug chop, and sometimes if it's running good I don't

> even do one, I just check the plug and go 'yup'

>

> Every time you change main, you will need to adjust the screw but

> ut leaving the pilot jet too small, you will notice right away when it's

> lean. Then start going bigger on the pilot jet until your screw is

> between 1 and 2.5 turns out. Turn the screw listening for the idle to

> raise, that will get you within about 1/2 turn off being right, then go

> quickly from 0-1/2 throttle repeatedly until it has the quickest and

> cleanest sounding rev up. You will also need to move the needle up and

> down. If the spot where the screw wants to be at idle, and the spot

> where the screw wants to be for a clean rev up are 2 different places,

> that will tell you your needle needs to move.

>

> Once you understand the logic of these carbs they are super easy to tune

> and they really run well. If you are just randomly flipping parts around

> until it works, you will be frustrated.

Do you count a "turn" as a full turn (360) or half turn (180) ?

Re: Tuning a vm18 Mikuni

Mike Boyd /

Go to branch

boyd

Jul 16, 2020 06:16PM

I fucked up and typed 180 in my earlier post, but i meant 360. A full turn is always 360. i just suck at circles

Re: Tuning a vm18 Mikuni

Emil Kniemel /

tpr950h

Jul 16, 2020 07:03PM

> Mike Boyd Wrote:

> -------------------------------------------------------

> I fucked up and typed 180 in my earlier post, but i meant 360. A full

> turn is always 360. i just suck at circles

haha i was thinking that i count 180s as half turns. so when i am setting carbs it will be like 3 half turns vs 1.5 turns. i dunno why. easier for me to count that way.

Re: Tuning a vm18 Mikuni

Graham Motzing /

Go to branch

grahammotzing

Jul 16, 2020 07:35PM

Haha yeah I do the same thing, that's how you hold a screwdriver! I do the math in my head because all the service manuals specify 360 deg full turns and it's easy to communicate that way

Re: Tuning a vm18 Mikuni

Andrei Stefan /

puttputtforthefunofit

Jul 16, 2020 08:09PM

I saw this posted the other day and saved it.. Maybe it will help you out?

http://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html

Re: Tuning a vm18 Mikuni

Jay Rivett /

bjaockx OP

Jul 16, 2020 08:11PM

I always count a turn as 360. But I always count in 1/2s when I'm turning too. Thanks again for the helpful info!

Re: Tuning a vm18 Mikuni

Jay Rivett /

bjaockx OP

Jul 16, 2020 09:07PM

> Andrei Stefan Wrote:

> -------------------------------------------------------

> I saw this posted the other day and saved it.. Maybe it will help you

> out?

>

> http://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html

Reading thru this. Good stuff. Brain melting info. This chart tho...

Re: Tuning a vm18 Mikuni

Satan 666 /

pepilapeau

Jul 16, 2020 09:50PM

> Mike Boyd Wrote:

> -------------------------------------------------------

> edit: vm18 has an air mixture screw rather than a fuel mixture screw so

> i had to reverse what i posted:

>

> Tune the idle first. Start with a warm engine and the air mixture screw

> 2.5 turns out from flush (a full turn is 180 degrees. Adjust it by 1/4

> turn at a time, blipping the throttle each time.) Turning it IN richens

> the idle mix, turning OUT leans it out.

>

> If you can close the air mixture screw IN to 1 or less turns from flush

> with no difference, then you need to go DOWN a size on your idle jet cuz

> you're too rich. If the engine "hangs" after you blip the throttle it is

> too lean. You will know you have the right size idle jet when it idles

> smoothly and returns to idle smoothly after a blip, and the air mixture

> screw is between 3 and 2 turns out.

Super confused about your turns out, haha. Should be a flat blade screw driver and turn from dead stop out 180 equals half turn. So if you turn the blade 180 three times it will be at one and a half turns. You say three turns out and that is to much haha. I always say half, half, half. Haha fuck it.

Re: Tuning a vm18 Mikuni

Mike Boyd /

Go to branch

boyd

Jul 16, 2020 10:00PM

Yeah read in "flat view" and see my previous post, i just spaced and wrote 180 rather than 360. sometimes i do wish you could edit old posts on here, now i've made another contribution to future confusion ugh.

Re: Tuning a vm18 Mikuni

Jay Rivett /

bjaockx OP

Jul 17, 2020 08:50AM

Someone posted this page up yesterday and it has been a good read on the understanding of how these carbs work. I have skipped over some of the history and basics, there is a lot of info about the carb. This, like other mituni tuning items I have read is primarily working with 24mm+ carbs and on multi geared bikes, but the understanding of the jets is all the same.

mikuni tuning

Re: Tuning a vm18 Mikuni

Andrei Stefan /

puttputtforthefunofit

Jul 17, 2020 09:08AM

That drove me to the bottle. If only somebody made a TL;DR for it. I thought my knowledge of carbs was limited before reading this write-up, now I realize it's beyond that. Apparently replacing Honda and Stihl lawn tools over the years was not a great precursor to precise tuning.

Re: Tuning a vm18 Mikuni

Brandon Weiss (Detective brandon to you) /

mopedallday

Jul 17, 2020 11:39AM

Just here to say that on the contrary to all I've read and experienced with vm18, I'm blown away by how large of a jet my 65cc metrakit pyramid Reed engine wants with it. It wanted a jet in the high 130's low 140's. Leakdown tested, nothing amiss.

Re: Tuning a vm18 Mikuni

Emil Kniemel /

tpr950h

Jul 17, 2020 12:12PM

> Jay Rivett Wrote:

> -------------------------------------------------------

> Someone posted this page up yesterday and it has been a good read on the

> understanding of how these carbs work. I have skipped over some of the

> history and basics, there is a lot of info about the carb. This, like

> other mituni tuning items I have read is primarily working with 24mm+

> carbs and on multi geared bikes, but the understanding of the jets is

> all the same.

>

> mikuni tuning

Damn that is a lot of reading. I always thought that the "choke" on mikuni was fuel enrichment and not air choke (eventhough its clearly labeled choke). guess i was wrong.

Re: Tuning a vm18 Mikuni

Satan 666 /

pepilapeau

Jul 17, 2020 12:14PM

> Brandon Weiss (Detective brandon to you) Wrote:

> -------------------------------------------------------

> Just here to say that on the contrary to all I've read and experienced

> with vm18, I'm blown away by how large of a jet my 65cc metrakit pyramid

> Reed engine wants with it. It wanted a jet in the high 130's low 140's.

> Leakdown tested, nothing amiss.

Mikuni jets are always up there in size compared to dellorto

Re: Tuning a vm18 Mikuni

Dirty30 Dillon /

hydra

Jul 17, 2020 12:20PM

Take this with a grain of salt, because obviously carb design plays such a large part in how a jet functions, but this is a useful chart for people trying to cross-ref jet sizing.

Re: Tuning a vm18 Mikuni

P D /

pdcomm

Jul 17, 2020 12:44PM

> Dirty30 Dillon Wrote:

> -------------------------------------------------------

> >

> Take this with a grain of salt, because obviously carb design plays such

> a large part in how a jet functions, but this is a useful chart for

> people trying to cross-ref jet sizing.

Have you WIKIed this chart ? It should be there .

MOPED ARMY

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