VM18 issue

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.

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.

Wow thats pretty good,I hope I can get mine to run like that.

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.

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.

That's strange,possibly that jet is marked wrong?

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.