i was wondering if i was the only one with a mikuni vm22mm that has slat spot off idle. i have a vm20 with same thing? i have vm20mm on a honda 70 with 52mm bore and 52mm stroker, mini monster cam, xl70 head, 22mm intake, manual clutch. i am at 425 feet altitude. i have in pilot jet is 22.5, main jet is 140, slide is a 2.0, and needle in 2 from top ring. needle is stock and i have five other needles in bigger in size and smaller size. and i have changed all of these needles, the only thing that helps any is the smallest one and it is still there. the vm22mm is on a honda 70 with 52mm bore and 51 stroker,mini monstercam,xl70 head,24mm intake,manual clutch, the pilot is 30,main140

I would say it is a pilot jet problem. Could also be a float issue also. 1/8-1/4 throttle is the pilot 1/4-3/4 is the needle and slide and 3/4 to full is the main. If you are running a stock head and valves those may be to big of carbs, not sure. Do you have a 18 or 20mm carb to try on it. What are you running for a airfilter? I have had tuning issues with the cotton gauze ones before and have since switched to a uni two stage filter on all my bikes. I hope this helps in some way. Tres might have a better idea.

i have tried 20mm kehin and i can't get enough flow out of them and i say this because you can run wide open and then let off to half throttle and the bike doesn't change engine speed at all and don't have power. i have been running k&n filters. on all bikes and these are the only one with trouble. i also have two brother pipes on both of these. and mechanicl advancers on both out of the xl70s

Ok, Here is my answer on this. I doubt you will like it, and it may ( my opinion) be worth exactly what you payed for it.

but here goes:

After spending an ungodly amount of time trying to jet Esco's 2 bikes that had the 22mm mikuni's
I finally pitched them both and replaced them with Sheng Wey Carbs.

Both bikes instantly ran better and responded to proper jetting techniques and are running well to this day, as evidenced by the great run that Osoto's daughter made on the blue bike at the regatta.

Here is my theory on why the mikuni is not a good fit for these bikes.

This is a pic of both carbs, the mikuni is on the left:

Click to view attachment

They are both 22mm carbs


Here is a shot looking down the slide chamber
Click to view attachment


Here is a pic of the slides side by side, once again the mikuni is on the left
Click to view attachment

The mikuni slide
Click to view attachment

The Sheng Wey slide
Click to view attachment

As you can see, even though both carbs have the same flow size , 22mm

The mikuni slide is 6mm larger than the Sheng Wey

I believe that the mikuni has too big of a chamber and does not keep the air fuel mixture tight enough for this engine.

So Thats my thoughts on the subject

take it for what its worth

What Tres said is right on, I didnt know about the huge slide in a mikuni. That huge slide chamber is causing a low pressure area and now introducing enough fuel. Looks like you need the sheng wey carb. Or you could try a stock xr80 or xr100 carb. I know that bbr sell a xr80 style carb to go with there minibike kits. Post some pics of your bikes.

YES YOU ARE RIGHT THAT IS PROBLEM BUT NO BODY COULD JUST TELL ME HOW TO FIX UNTILL FOUND IT IN A OLD MIKUNI BOOK FATHER HAD AND I ALSO FOUND IN A OLD TRI-Z BOOK. ok! here it goes. this weekend my brother and i got to to lath cnc machine and we made new slides and we didn't make a cut out on the front side of the slide every thing else the same and guess what they worked!!!NO FLAT SPOT!!!! it was to rich so we made just a little radius and it was just right. so that tells me one thing that mikuni knows how to fix. it just high dollor fix for new slide from them but we made our own. now as for the number in the slides you have to email mikuni and they will make you one or do just as i did make your own!!not that hard. just time. will try to get pics of everything for show and tell thanks for the input i was thinking the same need more flow off idle. jets. so don't get ride of your mikuni just need slide for it and it will work!!!!!!!!!!!!!!!!!!!!!!!!