[geoffbaker]

Hi everybody,

I bought my 2000 a month ago and just finished rebuilding the head. It's going very well, and the beast starts up on the first kick (with the choke down).

My only problem is the idle. It just doesnt seem to be able to idle, I have to give it a little throttle. It's almost there ... after a couple of minutes it will lope along but eventually will cough and die.

I set the main jet to the fourth setting (one below the richest) as at the middle setting it was not idling at all.

The spark plug was pretty black when I reset the timing today, but it's only got 4 miles since the rebuild and I'm expecting it to burn a little oil for a while...

Im at 3500 feet here in Tucson and its already in the 80's so this isn't a cold start issue.

Thanks in advance!

[baird4444]

What pilot jet are you running??
Have adjusted the air screw??
         - Mike

[dogbone]

The key was the needle setting made a difference, it shouldn't .
Try to adjust your float level.

[geoffbaker]

Ive adjusted the air screw and that helped some. But it just won't hold a low idle until its been running for quite a while.

dogbone, adjusting the float is a matter of bending the arms of the float, right?
Can you tell me what this will do?

thx

[dogbone]

If the float level is too high, the gas will bleed thru the emulsion tube (needle jet),and run down the intake runner. Maybe bleed isn't the correct term, sorta sucked by the intake vacuum pulse.  This should be done by an experienced mechanic, but yes,  the tang gets bent. A genuine mikuni manual or rebuild kit will have the correct float height adjustment spec's.
This may only be a partial part of the problem, it could still have too rich of an idle circuit, read up on mikuni on the internet. DB

[geoffbaker]

thanks DB, I'll keep working on it.

gb

[Kevin Mahoney]

1. The RE's are cold blooded, but after 4-5 minutes this should be a non-issue
2. I would look very carefully at the rubber hose between the carb and the head. They are a wear item and crack, especially under the clamps. This sound like a classic vacuum leak

[luoma]

You might be running too rich at idle to keep it running without sooting up. If you can start a cold-blooded bike that easily, could be because you are burning too much gas.

[Byway]

Geoff,

Mike asked which pilot jet you're running.  I would suspect the pilot jet first off since the jet needle shouldn't come into play at idle. 
Kevin mentioned checking for air leaks at the intake rubber.  This is always an excellent idea. 
I just checked my float height while i was swapping out pilot jets.  Download a copy of the Mikuni VM Manual at http://www.mikuni.com/fs-carburetor.html (click on the "VM Round Slide Tuning Manual" link.  This is an overall good reference with the exception of the float height listing for the VM28 being 15-17mm.  I found this to be virtually impossible.  I believe the proper float height is 22-24mm, as listed for some of the other models.  This is the range mine is in and it's just fine.  With the carb. turned upside down and the float resting on the float needle by it's own weight, measure between the raised rib (on the surface on which the float bowl mounts) and the very top of the float.  If adjustment is required remove the float, gently (and slightly) bend the tang that contacts the float needle (use needle nose pliers).  Reinstall and remeasure.
Hope this helps!

Al

[geoffbaker]

Im running the standard pilot which is #25.

Rubber is new and perfect as far as I can tell.

It is running rich because the plug is black. I can set the main jet back to the center setting.

I'll check the float tomorrow. Thanks for all the suggestions!

[luoma]

Check the air screw setting. I believe it should be between three quarters and one and a quarter turns out. I set mine at one turn. Also, an easier way to adjust overall mix is to move the needle one notch. My carb is a flat slide, so this might be different, but i think they are similar.

[geoffbaker]

I disassembled the carb and sure enough, the float was off - way off. Something like 30mm from float to lip!
I reset it to 24, put everything back together and now she starts up better than ever.
I reset the main jet to the center circlip (factory setting) and I think that helped too.

But she STILL won't hold a low idle, she coughs and dies.

SCRATCH THAT... I went in and readjusted both air and main screw, and now she is holding a fine, steady idle!!!!!

Thanks for all the help!

[Byway]

Good that you're getting all the kinks ironed out before the good weather gets here!   

Al