Stalling when engine speed falls from a high rpm...

SClay · Full Member Joined: May 30, 2008 Posts: 93

It goes like this:

Starts fine, idles fine, but if you hold the revs at a steady amount at a decent clip, and then just chop the throttle all together, it will drop and then stall.

But if you are casually blipping the throttle, it will return to idle no problem.

I can't ride it(snow/cold), this is just preliminary testing. It's a VM36-34, something, not totally sure. I suppose it's unimportant, as the problem is not the carb itself, but in the mechanics of it, or between it and the engine.

Air leak? Jetting? What do you guys think? It's too cold out there for me to troubleshoot for days.

Steve

yamaman

There is a great tuning manual for VM's here, might help:

www.mikuni.com/fs-manuals.html

Shannon60

it sound like its lean on the pilot circuit. open the mixture screw about 1/2 to 3/4 turn.

Tomterrific

Is the idle set at 1200rpm? I'm used to my Bonneville with heavier flywheels and a lower idle rpm but my XS needed the idle rpm set much higher. Very Japanese of Yamaha.

Tom Graham

xsjohn

Tom has a very good point about flywheel weight on these........another good reason to watch compression ratios......can make them run rattier the higher you go ....that's why I settled on 8-1 as a good comprimise for what I was looking for...................

xsjohn

SClay · Full Member Joined: May 30, 2008 Posts: 93

Idle is around 1200, I have no tachometer, but I have my mag-tach set up on it at the moment for tuning and that's about where she sits at. I will check out the pilot circuit, I was thinking that is probably the cause, but why not get some opinions as well right? It's just so bloody cold outside it makes it quite difficult...

Steve

SClay · Full Member Joined: May 30, 2008 Posts: 93

Oh and yamaman, thanks for that link, not sure why I always missed that on the Mikuni site but I never did see that before. Awesome bit of literature!

Steve

SClay · Full Member Joined: May 30, 2008 Posts: 93

Hey guys figured I would report back. My inclination is that I am running a bit lean, but my plugs seem to tell a bit different of a story, or so it seems....

I took some pictures to show you what I'm looking at:

Right cylinder:

Left cylinder:

To me, at least the left one, looks a bit on the rich side. Which seems strange. However, I also took a horrible picture of the intake port after I ran it(around the block, various RPM, etc.), while the picture is in itself blurry, there is a residue in the ports, looks to me like excess gasoline, which would mean I'm a bit on the rich side, it is in both ports, in case you wondered. Bad picture, but it gives you an idea.

Now, this wasn't there before I ran it, it was clean.

So what do you guys think? One of my manifolds just toasted itself, so I can't do any more diagnostic testing, but figured I would put this information out there for the XS650 gurus to mill over. Thoughts?

Steve

Shannon60

What do you mean your manifold toasted itself?

SClay · Full Member Joined: May 30, 2008 Posts: 93

There was a tear in it that I repaired with some ever so elegant JB weld, held fine, until I re-installed everything, I guess the combination of the parts being malleable and warm, caused it to just tear apart. I'll have to see about ordering new ones.

Steve

yamaman

Hopefuly the new manifold will cure everything!.........The VM's have a larger outside diameter than the stockers, can be awkward to fit!
MM might have custom jobbies made for it?????? Or know of a good replacement. I wonder if LC manifolds fit?

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