Mikuni VM34's Tuning??!! Help!!

[Dxs650]

The choke enrichens the mixture, so if it runs with choke on but dies with choke off then obviously it's running to lean. Your plug colour will tell you that. Screwing the mixture screw in leans the mixture, out enrichens it. Screw the mixture screw out until it runs OK with the choke off, After warming up. From memory I think the bench setting is 1.5 turns out, over two and you need a bigger pilot jet.

I run VMs on my 74. It has 42 mm pipes, pods, longer manifolds (which make no difference) and pretty open DIY mufflers. Can't remember the jetting or needle position, but I did have to change them.

To start try the idle screw 1.5 out and needle clip in the centre groove to start, then read your plugs.

READING THE PLUG COLOUR IS KEY.

You also need to sync the carbs to get it revving cleanly. Either buy a set of vacuum gauges or make yourself a manometer to sync. A couple of coke bottles will suffice rather than going to the extreme I did. You need something with a reasonable volume of water, stay away from a looped tube, it'll only confuse and annoy you if you have no experience. Here's a video showing how to sync using the manometer shown.

With VMs, when synced and jetted correctly, you CAN snap the throttle open for a clean response, they will work better than BS carbs, that's why people run them.

Stock BS34 manifolds will fit VM34s, they should have spigots fitted for vacuum petcocks, if not you can fit them, or swap to manual petcocks.

That’s kind of the problem I’m having. Can’t snap the throttle and it feels like it’s idling a bit high. I have #25 pilot jets in there now I Saad thinking of trying out #20. Because I can’t get the bike to idle any lower and I have the idle screws all the way out . What number pilot nets do you have in ???

 

[toglhot]

Oops, I made a boo boo in my original post, it should have said screwing the air screw 'out' gives a leaner mixture, not a richer one. So screwing the air screw in gives a richer mixture.
I think my pilot jets are 25. But the air screw is right on the cusp. So, screw the air screw in for a richer mixture, until it stumbles, then screw it out until it stumbles, then pick a point half way.
You may have to go up a size in the pilot.
May I suggest you tinker with your carb settings and timing settings, so, you know how settings affect running. The best teacher in life is not a book, it is mistakes. If you never make mistakes, you will never learn!

Come on people, you didn't pick up my mistake - bad forum etiquette.

 

[Dxs650]

Oops, I made a boo boo in my original post, it should have said screwing the air screw 'out' gives a leaner mixture, not a richer one. So screwing the air screw in gives a richer mixture.
I think my pilot jets are 25. But the air screw is right on the cusp. So, screw the air screw in for a richer mixture, until it stumbles, then screw it out until it stumbles, then pick a point half way.
You may have to go up a size in the pilot.
May I suggest you tinker with your carb settings and timing settings, so, you know how settings affect running. The best teacher in life is not a book, it is mistakes. If you never make mistakes, you will never learn!

Come on people, you didn't pick up my mistake - bad forum etiquette.

I have 25s in there now and the idle screw is all the way. So I have nothing left to lower the idle That’s why I was considering putting in 20s so that it gives me more adjustability with the idle screws

 

[Kevin Werner]

Come on people, you didn't pick up my mistake - bad forum etetiquette.

Tog, buddy, friend, pal... My son and I were on a 12 hour road trip from the "Smokies Rally" and I couldn't formulate a suitably scathing reply...
I briefly considered you were talking BS and not VM

 

[Kevin Werner]

Come on people, you didn't pick up my mistake - bad forum etiquette.

TOG, my irreverent reply notwithstanding, I have been wrestling with my VMs and it seems I am going assbackwards. I am currently at 190 main and 20 pilots when I came across this thread. I have some barking going through ~4500rpm so I will go to baseline 180/25 as it seems I have been leaning one and richening the other . Dumbassery.