The timing and ignition are set, there are no leaks around the intake boots, and the carb is fresh. The new NGK plug is wet when removed after a few kicks. It still has a runaway idle however.

It will only start with the idle screw nearly all the way in. When it cranks over it runs hard no matter how much I turn the screw out for about 20 seconds before it starts going wide open and I have to kill it. 

The carb is slightly overflowing fuel from the overflow nipple. I checked the float height and set to 23.5mm from the surface to the middle of the arm, as the manual states. 

It's still overflowing. Could the engine be sucking in too much fuel? This would mean that it only runs with a lot of air being pushed in (idle screw all the way in is the only way it starts).

has anyone used a different float height? I cleaned and double checked the overflow tube for cracks and there are none. 

Thanks!

  

For it to be running away i'd first look to see the slide is touching right down 'Clang' with speed screw backed out of the way & lower cable clearance. Slide doesn't have to be up far to give you a runaway fright but a long way from wide open--not far off shut down. 1 or 2mm of daylight under the slide would have it climbing in revs.
Spilling fuel out has to be fixed of course but won't be causing it to rev. In fact if anything it would help to slow it down by being over rich. Be careful & have a plan for runaway situations. Even run it minus the filter hose so you can put your hand over the carb mouth to stop it. Not like i did on my Fergy diesel tractor & nearly lost my glove down the spout haha.

I pulled the carb again this morning just to quadruple check..

The slide 100% slaps all the way down when the throttle is released and the idle screw is turned all the way out.

You should probably check the dry side crank seal.

Just another randon thought. If the choke jet in the bowl is blocked then opening the choke only lets in more air exactly like opening the throttle. So close the choke if not already closed & try again. If it's overflowing fuel it'll be plenty rich without [a working] choke.

Good thoughts, I'll keep at it. I'm also in the process of replacing the LH crank seal in another thread:

yamaha-enduros.com/index.php/forum/1972-...nstall-lh-crank-seal

This type of problem has been discussed here many many times.  So there is tons of info. 

I must have typed this 100 times at least...  on the float level, are the arms parallel to the float bowl gasket surface just as the float valve closes?  If not parallel or very, very close to parallel you are using the wrong level spec. 

Have you replaced the float needle and seat?  No amount of adjustment is going to fix a bad float valve. 

One thing I'm not clear on is when you say "idle screw", EXACTLY which screw are you talking about?  There is a throttle slide stop screw and an idle air screw and some members here don't seem to understand the difference so I have to ask.

Where do you have the idle air screw set?

If you are talking about needing to have the throttle slide stop screw all the way in to start my best guess is your "fresh carb" has a plugged pilot jet?

But you also need to find out why it's overflowing...  a minor drip or two probably isn't causing an issue but if it dribbles constantly when you turn petcock on then figure it out and fix.  Extra fuel is likely to make it flood and idle slower...  and the only way it can "runaway" is by getting extra air, not too much fuel...  just always best to fix the basics or you end up chasing your tail.  Even if you get it to idle and stop runaway...  then if you fix the overflow you might have to start over on the idle problem. 

 

MarkT,

Good to know about the parallel test. I'll test when I pull the carb again.

The needle and seat and new OEM.

When I say 'idle screw' i'm referring to what you call the "throttle slide stop screw".

The air screw is set to what it said in the manual which is 1.5 out.

The pilot jet is 100% clean and clear.

I have a pressure tester on order and will run a pressure test on the engine within the next few days. I put a new aftermarket LH crank seal on it, so maybe that's still leaking and sucking air in.