Hi all,
I have a hose on my carb that I don't understand. It comes out of the carb behind the main fuel supply line. It exits the carb/oilpump chamber through the side of the large air intake hose that goes from the air filter down to the carb chamber. This mystery hose then goes between the engine and frame. It seems the hose was then connected to nothing. Is it supposed to be connected to nothing? When I took it out it was plugged so I cleared it with compressed air. That end of the hose was very dirty and seems to have been hanging out there for a long time. Normally I would guess this hose is supposed to be connected to a crankcase ventilator but I don't see one of those on schematics. I don't feel like taking out the engine just to see if there is something in there to connect the hose to. Does anyone know what this hose is, what it does, and if it is supposed to be attached to anything?

Thanks for the help!
Dave

Is this the hose? Over flow.

Hi - lots of hoses to figure out, I got a months head start.
the one you refer to is a vent (the other skinny hose is an overflow that connects to a very cool standpipe inside the bowl

My 69 I am bringing back to life had no hose on the vent, just the hose barb. Do you even need hoses on them?

Not if you don't mind the occasional fire in your crotch.
There may occasionally be some fuel that burbles out in the event of an overflow situation with a stuck inlet valve. In that case, there may be some raw gas "vent" out of the barb and onto the hot engine. Better to have it routed onto the road racing by underneath...

Thanks for the info! I can't read your carb diagram but it looks better than mine. I use the parts fiche at www.yamahapartshouse.com/oemparts/a/yam/...9bc7842ad/carburetor. My diagram puts the fuel lines at the back of the picture.

It's good to know my hose is doing the right thing. Unfortunately, I don't think it's long enough to hang over the back of the engine, it probably just dumps the overflow fuel on the top of the motor. I'll get a longer hose to make certain it passes over the motor and hangs down the back.

Thanks again!

Interesting since the carb is inside the side case since it is a rotary valve motor, I thought if it leaked gravity would carry it out the drain in the case. Wouldn't the vent hose drain to the bottom of the case?

I thought the one from the base of the bowl IE bowl vent/overflow pipe had to vent to the outside by the hose being fed out through a hole in the case directly under it. Might be a little gauze filter in a fitting next to it under the case. Hole is the right size for the hose but the go brittle with age so easily broken if bent.

After writing that i went looking & here's a G5T cover which is the 80cc but thought they'd be the same. Reasoning is that with the carb trapped in the cover for rotary valve inlet the vacuum on open throttle can stop the float dropping & stop fuel getting to the bowl--so they vent it to fresh air. Not sure how true that is but we always vented then out there. I'd have thought with the gauze covered hole it'd stop any vacuum happening.

Early models.... circa 1964... relied on the air filter and steel gauze drain. This system sort of self-compensates for a dirty air filter... when air filter is restricted the pressure drops in the float bowl which effectively leans out the jetting.

(If I remember correctly, the much later RD 400 vented the carb bowls into the air box and I'm sure many other bikes did too)

Sometime later Yamaha added an external float bowl vent hose to the rotary valve models... too high to be an "overflow"... restricted air filter on that type will run much richer.

Thanks for all your help everyone! Your knowledge and expertise are much apreciated! All your suggestions got me on track.