1975 Yamaha XS 650, wouldn't start....

So I pulled the Yamaha out of the barn from it's winter sleep. Pulled the carbs off, opened them up and cleaned them out, a yearly ritual. Noticed one float chamber a little depressed, no biggie. Put it all together and tried to fire it up. it poured gass out the overflow, like it usually does in the spring until everything shakes back in place. It kept popping but wouldn't stay running. So again I pulled off the carbs and tore them open and found this on the RH side........WTF>>>>>>BTW these floats were new 4 years ago when I rebuilt the carbs.







LH depression



So a quick order to MikesXS for new floats and more waiting.....

Like they collapsed in a freeze?

No, i think it was a backfire. It was fine when I cleaned the carb, then 5 minutes later of trying to start it this happened.

Brent

The collapsed floats reminded me of what might have happened to von Guericke's Magdeburg hemispheres if they hadn't been made of sterner stuff--perhaps the floats left the factory with a good vacuum level inside


Tim Kraakevik
kraakevik@voyager.net

That is.. A. Amazing.
B. Most unusual.
Explain your theory Brent.
Surely not enough pressure from backfire, down through the Needle jet to do such damage ?
But how else then.......
AC.

Got me stumped. One minute the floats look fine. The next complete destruction. The only thing I can think of it the chambers were under a vaccume some how and the right conditions set them off. The backfire wasn't really that big, just a little pop. I'm really tired of dicking with the carbs on this bike. It's a sign that it is tooo often when I can remove them in less than a minute.

Time to put some AMAL's on it!!!

bite your tongue :shock:

I've got a pair of 34 Miks to go straight on, all I need now is another XS.
What's the sale price ?

I have tried to make sense of the sequent of events that would cause this amazing event, and here’s my theory;

The metal floats are heat soldered, hence heated when assembled when the hot air inside one or both are caused to expand, so that when it is finally sealed on the last run of the hot soldering iron there becomes a partial vacuum inside the float. – In this case one more than the other? (This only goes to show that this theory may not be consistent.)
When the engine backfired, I am in two minds about this as to whether it created a vacuum or increased the air pressure, inside the float bowl, either way, the float with the most partial vacuum inside it collapsed due to the change in air pressure or vacuum alone. Hang on a minute, I hear you say, shouldn’t the float bowl be ventilated to atmosphere?
Yep, so it should, so this poses the question; was the overflow vent pipe blocked at the time :?:
If the answer is yes, then, mystery solved.
It is possible that the pipe could have been blocked with overflowed petrol that had not fully drained off and could have caused a partial hydraulic air lock.
What have I learned from this unusual frame of events?
1 set your engine up so that it will not backfire. (Easier said than done, I think!)
2 junk the metal floats and replace with the plastic ones.

there are brass and foam floats available. I hate foam ones, they don't last anymore. Until we get decent gas around here (snowballs chance) I'll be sticking with brass. I think this was an example of the perfect storm. I should have gone right out and purchased a powerball ticket.

The overflow wasn't blocked, but it did have fuel coming out of it.

New floats arrived last night. Installed them, no leaks no overflow and bike started first kick. No drama. Now to finish putting it together today and figure out how to get it home. Hard to do with 1 person short of the trailer or hitch carrier.

Have been thinking about this and what I think has happened is that there has been an explosion in the float bowl itself. Can't think of any other way to generate such sufficient pressure to collapse the floats. Can imagine there being just a little fuel in there - enough to make a combustible mix, but how would it get ignited? Flame front travelling out the intake manifold as a result of the backfire somehow propagating to the bowl? Dunno.