Mikuni VM34 roundslide carbs - questions for experts

[MB-Ian]

Hi everyone, my bike has been upgraded to the Mikuni VM34 roundslide carbs and ran pretty well - still does - but lately has an annoying habit of dying while idling away at the lights awaiting take off. I can tell the idle of the engine changes in character a few cycles before its about to stall/die and if I give it a blip on the throttle it can be recovered and will run for another 30-40 seconds before the cycle repeats. If it does stall all i need do is press the starter button and of she goes again.

Looking at the right-side carb I can clearly see two brass screws, one spring loaded one more-or-less below the choke, and one non-spring loaded one near the air cleaner.

Looking at the left-side carb, I can see the non-spring loaded brass screw, but the spring loaded one is missing completely?

1. what do these two screws do? What do they adjust?

2. is the LH side one supposed to be missing something?

My bike came with these carbs fitting but no intructions/details on how to tune/adjust them, and to be honest they are pretty well set up at the moment, with nice idle and good response all the way through the rev range...

Thanks in advance,

I.

 

[Jack]

one is for the slide adjustment and the other is for air adjustment to the Pilot jet. Up the pilot jet to prevent the motor from coughing or dying but make sure the carbs are in sync first.

 

[yamaman]

Hi Ian, welcome to the board. I hope sunny Melbourne is treating you well? some awesome weather your way just lately!

Lots of VM info HERE for you to digest! Good luck

 

[5twins]

The VM34s come in both right and left handed versions. The difference is the location of that spring loaded idle speed adjustment screw. It appears your set-up was done using 2 right handed carbs. The idle speed adjuster should be present on your left carb but will be in between the 2 carbs and difficult to access.

 

[MB-Ian]

Thanks for all the links to info everyone, looks like I'm getting somewhere. Printed me Mikuni manual and ready to put on the reading glasses

My bike has a 650central kit on it, and rides without problem at all rev ranges, no misfires or smoke/heavy exhaust so I reckon its just getting the idle speed/mix right.

Hi Yamaman, i see you're a fellow countryman way out west... Melbourne is... very Melbourne like today, was hoping to clock up some kays today. On my Ls so no one is afraid to talk to the middle-aged bloke on the old bike, and just as many surprised the XS is learner approved.

Cheers,

Ian.

 

[MB-Ian]

Ok so I've been fiddling with the idle speed screws and got a reasonable tick-over of 1400 rpm using the dead cyl method (removing a barb and running on 1 cyl, setting that for 1000 rpm, then repeating the other side resulted in a 1400rpm idle speed), however from time to time you can hear the "pft" misfire and the revs drop momentarily, but since the idle speed is now sufficiently high the engine will rotate to the next ignition cycle and pick up again - I'll video and post a youtube clip soon to show.

YESTERDAY was a warmish day - 24C - and I got caught in stop-start traffic and the misfire came back with a vengeance, with such regularity I practically had to blip the throttle continuously as the engine would only idle for a bit before the dreaded "pft" sound and revs drop. I also noted if I ease the throttle on instead of turning it abruptly, it was possible to stall the engine with the same "pft" sound. I'm starting to think that sound is a lean out, a pop thru the intake of one of the carbs. TODAY was an even warmer day - 28C - and I didn't even want to chance this problem re-occuring so i drove the car.

Now so far I've been messing around with the idle speed screws only, and not the air screws - they feel very loose, btw - and it seems like maybe at idle the engine is getting a little too much air from one of the carbs eventually leaning out and then "pft". I was going to try to adjust those, by putting a vac gauge on the barb and adjust for highest vacuum reading, with the opposite cylinder dead, and then repeat the process for the other side - is this right?

I'm also presuming the air screws admit more air the further they are screwed out, ie. turn them in to richen the mix and turn them out to lean it?

I.

 

[TeeCat]

Hello, Ian, and welcome!

I'm not a carb guy, but might be able to both learn and contribute a bit in your thread because I'm just in the midst of learning about VMs.

I'm also presuming the air screws admit more air the further they are screwed out, ie. turn them in to richen the mix and turn them out to lean it?

Yes, sir. This is the opposite of what I was used to with my old BS38s, and I got tripped up by it initially. But yes, you'll turn in to richen if the 34s are like the 36s.

You'll find lots of friendly expertise here. I'm quite friendly, typically, though not an expert. Keep us posted!

TC

 

[grizld1]

Ian, always start carb work by checking everything else--set valve lash, service and gap points, set ignition timing, etc. There are two reasons to do this. One: many times those preparatory adjustments will eliminate the problem you were cussing your carbies for, and Two: since you can only tune carbs by engine response, you need to deal with everything else that affects the engine first.

 

[smokenjoep01]

Lots of VM info HERE for you to digest! Good luck

Great info for me as well. Thanks for that link. Should have a sticky here some place. very helpful info.

 

[gold1]

Ok guys,A quick question.
I am fitting vm34 on my xs650 1979 special.Mikes 2into1.pamcoe ignition.
I am trying to find out if I need to change the float needle ,its a 3.3 Needle valve.

 

[MB-Ian]

Its my understanding if you buy the kits from an XS650 specialist (mikesxs, 650central, etc.) then they are supposed to be set up to work pretty well straight from the box.

Welcome, btw, I see you're new here. Where ya from?

I.

 

[gold1]

Auckland , New Zealand.
I already have my carbs but need advise on float valves?
any help greatly needed.
regards geo