VM18 issue

F

fischer550

Member
So I got my vm18 and my pipe from mzmiami. I live at 5,000 feet elevation. I have a 60 main jet, and 25 pilot. The air screw is turned out one turn only. Needle clip is on leanest setting.

It 4 strokes a tiny bit at very little throttle. I runs great throughout the power band, just one issue. When I open the throttle from 0 to full quickly, it hesitates, like 2 little pops. Then engine doesnt die, but it hesitates just for a second.

Any idea on what I should do?
 
Hey Fischer,I would try turning the air screw out another 11/2 turns and see if that helps.You might need to go down a size or two on the pilot jet,I think Treatland has a 22.5 or a 20.I'm trying to sort out my Mikuni to after installing that same pipe. Are you running a stock cylinder?I am and I am wondering if that isn't part of the problem,not sure.
 
No the cylinder is ported, it runs really good except those 2 things. I need to do a plug chop to see if the main jet is right. Wouldnt going to a smaller pilot cause it to be leaner? Cause going from 0 to full with hesitation sounds like a lean problem no?
 
On another note, I hit 42mph without even trying. On my nt speed carb and sbp pipe, It was really hard to get it over 36ish. This thing's gonna be sweet once it's dialed in!
 
yeah you might very well be right on that.I thought maybe with the higher elevation your at it could be a tad rich,you will know when you check your plug. Mine seems like its running way rich but the plug was just a little wet,it ran really good with a speed carb and a 66 jet.
 
One of the things you must consider when going to a bigger carb is that they can be bogged down if there's not enough rpm and you crack the throttle all the way to full from a really low speed or from a stop.
This isn't really a problem once you get used to feathering the throttle until you get a little speed built up.... try this... do a quick take off but crack the throttle only about half way then roll into it after the bike gets moving some.
What happens is when you go full throttle there's a huge amount of air going in but it takes the fuel just a little time to catch up and these carbs don't have an accelerator pump to give it that little shot of fuel when cracked wide open suddenly. But if you crack the throttle to like 1/4 to half throttle first, then roll into it it won't bog or hesitate. It takes a little getting used to, but after riding for an hour or so you'll get the feel for it quickly.
 
Well, I will say this pipe is too loud. I will look into a kx style pipe soon.

For the tuning. I moved the needle clip one spot richer. This fixed the 0 to full throttle hesitation. It 4 strokes alot at very little throttle, hopefully messing with the air screw and pilot jet will fix this. I will experiment more tomorrow, my neighbors were glaring at me through their window haha.

I also had a #60 main jet in, which ran really well but after a plug chop, the porcelin was white so I put a 70 in. It 4 strokes on top, so I have a 65 on the way.
 
That pipe is loud isn't it,I put a 1.75x4.00 inch baffle in the tailpipe on mine and it took some of the high pitched rapping out of it,still loud though.It sounds like your getting it close to running well,keep us posted.
 
I got it off eBay,it was less than 10 bucks shipped.I had to pound it in though and didn't quiet it to much.Right now I'm at Milan drag way here in Michigan,a lot of old gassers running today,it's bad ass!
 
Glad you got it sorted... some of the other sites recommend the 65 main and 25 pilot jets to make the vm18 run right in these, maybe a little tweaking with half sizes to fine tune with the pipe and if there's any porting done. You can always get individual jets in all sizes and half sizes from motorcyclecarbs.com if needed.
 
I replaced the 25 pilot jet with a 22.5, and weirdly its 4-stroking more. Maybe its a lean condition i've just never experienced before? It sputters on start-up and take off, but it doesnt feel like lean hesitation
 
No it's a genuine mikuni jet I wouldnt think so. Ill keep experimenting. I have the 30 pilot that came in the carb I'll try that
 
It could be possible you need to go the other way with a 27.5 and adjust it down with the idle mixture screw... Basically if you can't get a lean stumble at idle with the idle mix screw before it's all the way to soft bottom, then the idle jet is too big, same as if you can't get a rich stumble out of the idle mix screw before it stops responding to change, then the idle jet would be too small. The needle position also plays a big role in the off idle and part throttle cruising, then the main jet plays it's part in conjunction with the needle position and the idle jet as well as the idle mixture adjustment.
Other things will also factor in for what jet will run best in that carb... How restrictive the air filter is, or the lack of restriction from the air filter, and how restrictive or free flowing the pipe is will also determine which jet will do best.
A good pipe will need a little more fuel going into the engine and so will a less restrictive air filter.
Try adjusting the idle mixture and make sure you can get both a lean stumble and a rich stumble when adjusting the idle mixture screw and jet accordingly, then set the needle in the middle position and see how it performs at part throttle, raise or lower it accordingly to get it to behave at it's best, then try a full throttle run and see how it reacts, you may need to go up or down a size on the main then go back to your needle settings to get it to dial in. It's definitely a tedious job depending on what upgrades you got and your altitude, then there are other ways to fine tune the off idle by either changing or modifying your slide...
Here's a little more info on these carbs if things get too tricky...http://www.iwt.com.au/mikunicarb.htm
 
your right davezilla on a 2 stroke. lean or fat act the same. who can argue with a truckload of holed pistons. on piston port motors i always liked to put a long hose before the air cleaner to help with fuel blow back soaking the aircleaner just like most 2 stroke motorcycles before reeds. i remember the racers with gas soaked leathers from spitting out the stacks, crazy now? not so much back then.
 
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So I did a little test with the same 22.5 jet, I just took off the air filter. This air filter had just been cleaned and oiled. Without the filter, it was much better, 4 stroking was nearly gone on the bottom, still a little bit though. I have a 20 pilot, that will probably be perfect for no filter. Just wondering how much i'll have to down jet to compensate for the filter. It's weird because this filter is rated to be the least restrictive, maybe i'm using too much oil.
 
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