Mikuni VM18 Carb Tuning on Puch

85 main jet, 17.5 idle jet, stock needle (not sure if adjusted clip or not). Treats Reed 70cc kit. The bike runs super fine and fast and relatively cool with no bogs anywhere on the upswing. However, when I let off the throttle, the engine just immediately begins to almost engine break quickly on its way down to zero mph, and it does not sound like the motor is spinning at all (no sound of motor idling). It's either something odd with the clutch or it's not getting fuel on the way down? I was thinking maybe the idle circuit wasn't providing fuel on the way down from WOT. I just don't want to blow my bike up. Any ideas?

Re: Mikuni VM18 Carb Tuning on Puch

We run a 60-65 main around these parts..

Re: Mikuni VM18 Carb Tuning on Puch

When did you jump that high on your main? I run a 70 on my Hero kit, and I have a 70 in Ben's NOI Polini as well, both run no airfilters too

Re: Mikuni VM18 Carb Tuning on Puch

It may be too high. It runs fine WOT. But that shouldn't affect the fact that when the throttle is disengaged, the engine just dies until it falls back to idle at 0 mph? (edited)

Re: Mikuni VM18 Carb Tuning on Puch

Is it forward facing?

Re: Mikuni VM18 Carb Tuning on Puch

Faces out to left and back.

Re: Mikuni VM18 Carb Tuning on Puch

you should also mention the fact that your running an HPI

Personally Id say since your running that ignition with a mega lightened clutch that there just isnt enough rotating mass on your crank, the weight of say points or a cdi set up would have more mass and would continue their momentum where as your mega light set up does not? Just a theory

Re: Mikuni VM18 Carb Tuning on Puch

I think it's just too much fuel, once you close the slide your getting swamped with a last burst of super rich mixture and causing serious bog, I feel like a mikuni can run super super rich without really noticing until you start dropping jets and noticing higher speed without a spike in tempature.

Re: Mikuni VM18 Carb Tuning on Puch

^ I've been feeling the same way.

Re: Mikuni VM18 Carb Tuning on Puch

always downjet! mikunis are easy. watch that temp and blast

Re: Mikuni VM18 Carb Tuning on Puch

NB0tt aholetechbro /

check for an airleak......after a long ride last week a similar thing started happening to me (same set up too with hpi etc) and it turned out my intake loosened up on the cylinder and had a major leak. Tightened it back up and all was well again in the land of the mikuni.

Re: Mikuni VM18 Carb Tuning on Puch

Thanks dudes. Will check down for leaks and then down jet and see if that helps.

Re: Mikuni VM18 Carb Tuning on Puch

So I got around to doing a shake down. No leaks. Downjetted the main to 75 and that really helped with the engine breaking effect. However, tuning for that BK100 is a little tough on the top end because it wants to pull so hard at WOT and then temps shoot up like crazy. Carb is prolly a little small for the pipe methinks. The timing on the HPI is set to "set it and forget it." Maybe a little retarding may help? Or maybe a needle that can suck up a larger main jet? (edited)

Re: Mikuni VM18 Carb Tuning on Puch

When I owned a bk100 I had a similar issues with a tm24. After about 10 seconds of wot I would hit another powerband with some spicey speed and temps to go along with it. I purchased a power jet kit to mod the carb to see if it would help but sold the bike befor I got to try it...

Re: Mikuni VM18 Carb Tuning on Puch

Giovanni Giarratana /

Das Riot, do you have any updates on this matter? I just got a vm18 for my 70cc kstar and a proma circuit pipe, HPI as well no reeds thou and seem to be noticing a lot of the same attributes, 70-75 seems to work 70 better then 75, 80 way to much but I was afraid to go to 65 bc with the 70 the plug was pretty white, idk if this is normal or not, do keep in mind it was 37 degrees out and unfortunately I didn't have a temp gauge on, was trying to beat the sunlight

Re: Mikuni VM18 Carb Tuning on Puch

VM18s rely heavily on the needle, and not so much on the main unless you are basically wide open. I ran a 75 main but was rich as hell. The sweet spot with VM18s appears to be getting the right needle setting and the air mixture screw adjusted correctly. Needle settings on the stock needle make a HUGE difference -- you will notice if it is too rich or too lean right away. Definitely get a temp gauge.

Re: Mikuni VM18 Carb Tuning on Puch

Giovanni Giarratana /

Wow that actually makes sense, bc I leaned the needle once and didn't put it back and even when I up jetted it still showed lean on the plug. but with the stock needle setting and a 70 jet seemed richer on the plug then a 75 jet and a leaner needle setting, makes a lot of sense now gunna have to try moving the needle

Side note I've looked around for different needles, are there other ones out there for the VM or do you just run the stock needle? I know there are different ones for the dellortos don't know

If this is true for the Mikuni

Thanks!!

Re: Mikuni VM18 Carb Tuning on Puch

I am no expert on carb tuning by any means, but the one benefit of VMs is that they have fairly wide latitude for error. They are motorcycle carbs that are designed to work in a variety of atmospheric conditions without constant user fine tuning. There are a variety of needles available for the VMs, but they are not something that the moped shops sell -- more motorcycle shops. At the same time, I am not aware of many folks who have swapped the needles out on a VM18 for mopeds. Maybe someone who has can chime in?

70 should be about right for the main jet. If you find it overheating running wide open with a temp gauge, you can try going higher. Also, it is important to properly tune the air screw as well. I found that was a quick and dirty way to richen or lean out the range without having to rejet or move the needle. (i.e., turn the screw in a half a turn to richen, the other way to lean it). The Mikuni VM manual tells you how to do this, and there are better explanations if you search around on MA.

I found my VM18 to be a gas hog, quite frankly. Riding hard, maybe 30 mpg on a reeds kit e50. It was also rich, so that may have been a contributing factor. (I am in the process of switching over to a ZA and plan to see how to VM18 performs on a lower RPM set up).

Re: Mikuni VM18 Carb Tuning on Puch

Giovanni Giarratana /

Yes I noticed that adjusting that screw did make a big difference on my buddies maxi that I road for a while,we were adjusting it to help the idle and it ended up helping it all around, and now I'm trying to make the same setup bc his setup was unreal but he sold the bike a little while ago so just trying to figure it all out on my own, I read the vm tuning manual i just wasn't getting out of it what I think you do on here

This is all coming together now, all makes sense, can't wait to get home and give it all a whirl

One last question meant to ask before I think, any idea what pilot jet I should be running, carb came with a 30 and I put a 17.5 in it, not sure I was just trying something I have a 15 and a 12.5 and 10 on hand

Thanks! This was very helpful

Re: Mikuni VM18 Carb Tuning on Puch

No followup on what jetting you ended up with ?

Re: Mikuni VM18 Carb Tuning on Puch

Giovanni Giarratana /

Jetting that comes with the vm18, I think the needle height is even the same

What the problem was, the vm18 needs a more agressive cut away on the throttle slide, there were 3 available at one time 1.0 2.0 and 3.0, which adjusts midrange (higher number leaner) without adjusting your top end, by just adjusting the needle for mid range adjust your top end too and I was noticing that my top end was perfect but mid was shitty and when Id move the needle it did the opposite,

Fortunately my buddy had a 3.0 slide (no longer avail) and I sanded mine down to match his and the bike is tuned mint, idled nice and pulls so hard in the middle

I use this same exact setup on a 70cc kstar and 76cc arisal

Re: Mikuni VM18 Carb Tuning on Puch

So, you ran the 30 pilot that came with it ?

Re: Mikuni VM18 Carb Tuning on Puch

Giovanni Giarratana /

Yup I ended up back at the stock factory settings besides the angle of the slide cut away and the thing pulls so hard in the mids, I also found a lot of torque in a boss pipe by modding it a little using the material that is already there, two things you can do, mod the pipe one way so you get a ton of low end torque, or the other way and lose some low end and gain a ton of hi end rpm and top speed

Re: Mikuni VM18 Carb Tuning on Puch

George Cruz /

im running a vm18 with a 70 cc dr kit 120 jet 17.5 idle with oyesen dual stage power reeds, opened the reed holes and running awesome.....

Re: Mikuni VM18 Carb Tuning on Puch

RingringBANG ! Wrote:

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> We run a 60-65 main around these parts..

Nailed it on one of the first posts. Your all to damn rich, I'm at 60-70 on all my vm18 powered bikes, as I stated above, and tbh I'm at 60-65 on most of them, I only use a 70 on my polini v1's mind you those are 73cc's compared to the 64cc kits most if you are referring to in this thread

Re: Mikuni VM18 Carb Tuning on Puch

I run 60 Main and whatever is factory for for the rest of the jets. needle clip is one up (lean) from middle slot. this is on a 50cc TCCD kit. doin 50 woth a 50 and runs like a charm.

Re: Mikuni VM18 Carb Tuning on Puch

Emil Kniemel /

what is everyone using for shims on their intake? the one it comes with or something else.

Re: Mikuni VM18 Carb Tuning on Puch

Shims on Intake ? What ?

Re: Mikuni VM18 Carb Tuning on Puch

Giovanni Giarratana /

Lol shims? I put a different angle on the slide of a vm18, to me moving the needle affected wide open to much so to lean it out in the mid range instead of adjusting the needle I just "adjust" the angle of the cut away (steeper cut away leaner mids) there is a thread out there that I read about this, plus my buddy's bike came with a different slide then mine with a more aggressive cut away can't get them any more so I sanded mine to match his, I'll take a pic of the comparison and post it here

Mikuni used to carry different slides so you don't have to mod them but they don't any more so you have to sand them down your self

Re: Mikuni VM18 Carb Tuning on Puch

Emil Kniemel /

yeah the white shim that is on the clamp portion for the intake.

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