Engine races to infinite rpms as soon as engine st

Well, first off we're talking about a Yam xj 650 here so don't get all huffy. You guys are usually real nice and knowledgable and my buddy rode two hours to hang out and has to get back to school Monday. The bike has unidentified cv carbs. I had four ideas let's see what you guys think. Air/vacuum leak-bike has had this problem before but has been fixed so I can't find any. Stuck ignition advancer-tried to find it but it's electronic ignition and has no visible advance mechanism or even means of adjusting the timing. Head gasket air leak-bike has over 100k and seeps a little oil in this area, may have gotten worse on this last ride. Would a head gasket even cause the engine to race like this? Can't find any thing wrong with the throttle linkage. Sorry to bother you guys with a Yami but the owner claims guys on the xj forum are neither helpful nor friendly. Also, check the cv diaphrams for leaks. One of the screws is stripped but the top still seems to be tight with the other three.

I am rebuilding a Yammy Radian,but am NOT an expert.Is throttle cable properly installed and lubricated? Is the choke mechanism also installed correctly and lubricated?These bikes do have elec ignition so advancer shouldnt be an issue.Are the springs on the throttle shaft ok and have they been lubricated?Petcock working properly?Does the bike race if the petcock is set on reserve and not PRI or ON? bj

seanbarney41, my money is on the air/vacuum leak, most likely a hole or bad seat in a diaphram.

I have a 440 twin with CV carbs that had this same symptom. Turned out to be a small tear in one of the diaphrams. fixed it with supper glue and a small piece of electrical tape.
If no hole found, it may be leaking between the diaphram seat and the carb body on the side of the stripped screw.

Super gluing the diaphram to the carb body might be a bit extream (permenant). Try some silicone sealer (bathtub silicone caulk). That would be removable.

Another option would be a smaller diameter screw (metric or standard thread does not matter) long enough to go thru the cap and carb body (if the bottom of the carb body hole is open to the outside of the carb) and a nut on the bottom to hold the cap down.

Good Luck
Jim

hocbj23-yeah we checked all that stuff except for putting the petcock on reserve. That petcock is kinda funny and stiff too.

Rainman-I'm leaning towards your cv top seal idea. We will try that tomorrow.

Thanks guys. If any body else has any more ideas please post 'em. I'm still worried about a head gasket air leak but don't know if that would even exhibit those symptoms. Anybody have any experience with this?

Timing advancement is done by adjusting the cdi pickup plate, and no I don’t belive a leaking head gasket will make a motor race well not the ones I had did….

Don’t want to trivialise anything….
you haven’t mentioned the main idle adjustment maybe it is screwed up too many turns doesn’t take many turns to get it to rev its guts out.
For it or any bike with carb related issues, to be revving its guts out in a consistent rev, ALL carbs must be doing the same thing one or two wont make it rev properly. I would start with what links all of them, i.e. the throttle linkage, choke linkage etc and go from there.

Like said, check the main idle adjustment. Drop the idle/slides all the way down till they hit bottom, then turn the idle up just a couple turns. If you look through the carbs, there should only be a small 1-2mm crack of air passable at idle.

Yeah guys, this is sumpin' that happened all of a sudden, out on the road, everything running great and properly adjusted before. Idle agjustment nob did absolutely nothing. Anyone else got any insight into the head gasket possibly sucking air?

Well, we're puttin' it back together. Got a screw and nut for the stripped hole in the cv lid, found some slightly suspicious cracks in carb holders(but they don't look like they go all the way through) and put silicon on em. Carb holders are thick rubber and still nice and soft. If the engine still races when we get it together, does anyone know a trick to find air leaks that doesn't involve running the engine and spraying wd40 etc., can't really leave it running very long at 8k rpm!

God dang, keep it simple stupid. I'm talking to myself of course. Got it all buttoned up and the first thing it did was rev to the moon. Turned the idle knob all the way out, that got it. Then I turned the cable adjustment all the way in at the handlebar, started the bike and re-adjusted the idle speed with the knob at the carbs. Ride around the block went well, hope the thing makes it back to Big Rapids:unsure: (about 150 miles) I still don't understand how that throttle cable screwed up in the middle of a ride.

So, Subby and Tek9ine you guys win the super duper smart guy award for the day

When he's riding, the throttle is not on the idle stop, so maybe it vibrated? Was there a spring on the idle speed screw, to keep it from turning from vibration?

I’m glad to here the issue was resolved and with the day I had at work your appreciation is greatly received………

Although loudhvx does have a point there must be a reason this happened during a ride as that spring mentioned has some serious tension. I have a hard time squashing it in my fingers… check to see why this happened, by the sounds of it if you don’t you may find yourself doing this exercise all over again!