Petcock/Float relationship

Real quick question about why we need the petcock to be off while parked. Shouldn't the float needles prevent excess fuel from entering the bowls? Doesn't the float close the inlet once the proper level is reached??? I'm asking because my petcock drooled fuel at will when i removed the line, even when it was in the on and reserve position, as if it is bad. However, I never experienced any of the issues like gas in the crank case, smell of fuel, etc.

Total Newb to carbs and motorcycles so feel free to enlighten me.

 

canicula, welcome to the site. Have you rebuilt your fuel vlave? (petcock).
The only time you should have fuel coming from it would be when it is set on prime. So, with that in mind you have a leaking fuel valve probably due to a week spring on the diaphram on the vac side of the valve. I had the same problem after I rebuilt mine. I had to put a small washer behind the spring to increase tension of the spring on the diaphram to make it stop leaking. Give that a try & see what you have. It might take 2 washers as mine did.
Good luck,
Gregg

 

Sounds like your float valves are working fine, but you should rebuild your petcock, get a kit from Chacal.

 

it's a safety/idiot-proof thing
idiot-proof so people don't forget to turn the gas on or off
safety because without it the floats are the only thing holding back five gallons of gas from the crankcase and the parking lot

 

Gregg- Thanks, I've been lurking around here for a bit with a post here and there. My petcock is really tough to turn as it is but I'm in the process of a complete teardown/rebuild of my carbs for peace of mind/reliability sake.

wizard- yeah, gonna order the parts from chacal....his catalog is mesmerizing... =)

polock- that you confirmed what I suspected, thanks.

 

on a side note, anyone notice that the forum edits Petcock to Pet(edited) when it shows up in the last ten post scroll....Thought that was humorous...kinda made my evening :lol:

 

Polock hit the nail on the head. The only thing to add is that even when your floats are working PERFECTLY they can occasionally have an "issue." This is why we have a fuel valve, whether it be vacuum or manual.
(An occasional stuck float is NOT something limited to '80s Yamahas, believe me.)

 

I think Yamaha knew that the carbs have issues from time to time and the vacuum operated fuel petcock was the way to go. The owner/ rider would always forget to shut the fuel off when the bike is parked. The gas filling the crankcase and spilling on the ground is not a pretty sight. If you have looked inside these carbs that have sat for awhile you can't beleive what dryed gas can do.

 

There is a more common reason for your problem than a vacuum valve malfunction.

Behind the selector handle there is a rubber washer with five holes in it. That washer seals between the valve body and the passage in the selector that controls where the fuel flows (PRI, RES, ON). When that washer wears out fuel leaks around it and ends up bypassing the vacuum valve.

Given that the handle is hard to turn I'll place money on that washer being bad.

When I needed one for my XJ650J Rosenau had them in stock. I think it was $8. Price has probably gone up since then.....