Mikuni VM Carb Help?

S

sierra_ta

New Member
I have a new set of Mikuni VM30 carbs that I placed on a cb360. Once they were installed the bike fired right up and idled well, however, it will only throttle to about 5k rpms. I took off the carbs to look at them and noticed that the needle jet rises with the needle as the throttle pulls. I would assume that this might be causing the problem because the needle isn't fully raising from the jet. With this said, I can't figure out how to keep it down.

Does this seem right and does anybody know a fix? Could something else be causing this if its normal? Thanks.
 
I cant visualise what youre talking about. The jet is literally lifting out of the bottom of the carb? Take them apart and make sure everything is screwed in tight.

Otherwise, theyre probably jetted wrong for you bike. Actually, most likely they are either way.
 
I don't know, it seems to have the washer on it (keeping it from falling down further) put I may be missing something else. I'm going to have to take them apart this weekend and get a better look.
 
It can't 'fall down' as there is a flange at top, must be assembled wrong or your description doesn't fit what I'm thinking.
Can you get a pic?

PJ
 
Well the summer got extremely busy and I haven't even been able to look at this since my last post in June. With that said, I finally got a chance to take a look again. I am pretty sure I know what the problem is, just not how to fix it. The needle jets seem to not be held in place and will stick to the needle/raise up when the throttle is pulled. Below is a picture.

3e1b10ee.jpg


Both carbs are doing this. It runs great until the jets start rising (about 2 minutes and once it must vibrate enough). I must be missing something on the jets but do not know. Any ideas?
 
As I said at the beginning, the brass washer is missing between main jet and needle jet.
Needle jet fits in from top (slide side) the washer is about 8~9mm dia then main jet locks it in position.
Washer fits into recess at bottom of needle jet tube on carb body
There is als a slot on side of needle jet that has to be aligned with pin on jet tube
 
The MAIN JET which is threaded into the bottom of the NEEDLE JET has a big cup/ring (#35 in the diagram from the manual) between it and the needle jet / carb housing to sandwich it all together. The main jet holds the needle jet down when assembled correctly.

Download this Mikuni manual:

http://www.mikuni.com/pdf/vmmanual.pdf
 
you only need the cup if your motocrossing, it keeps fuel around main jet on bumpy tracks. all you need is brass washer.
I'll measure size, you can probably find some at Home Depot
Or, you can just ignore everything I said a few months ago until you get an answer you like ::)
The FLANGE is at top and part of emulsion tube/needle jet, WASHER is at bottom
 
I didn't know the cup was there for motocrossing. I'd better start dirt-jumping my XS ;)

If you can't find a brass washer the right size search out your local Mikuni distributor and buy the bits you need.

PJ - do the carbs always come with the cups? I've got three pairs of VM34's and all of them have cups, including the ones I've bought new.
 
Hi Tim, all the MX ones come with cups (that's how you know the 'correct jetting' is for MX bike, 190 main, 45 pilot ::))
Street versions rarely come with the cup although I'm sure there are a couple of exceptions?
 
You must log in or register to reply here.

DTT Light or Dark

  • DTT Light

    • Spread the Love

    Back
    Top Bottom