Carb overflow

[AgentX]

Hey, all...

I have a Royal Enfield with a Mikuni TM32. I know the practical solution to my question already..."turn off the gas"...but wondered about the mechanics behind it.

If I leave the petcock on overnight, in the morning, the air filter will be full of gas. I don't understand why this is, or if it's indicative of a problem. The floats should hold the needle against the seat and keep anything from flowing, regardless of how long it sits, ya? In fact, as any leaks past the floats should push up even harder, I'd think. So is this just a matter of my needle and seat needing to be lapped together, or is it unavoidable and endemic to bikes in general?

 

[cxman]

no it wont hold

mikuni ships the carbs with a gravity flow needle and seat it will open in the 1-2 pound range that

will keep the bowl filled at high speed

that pressure can be exceeded by the gas in the tank heating up after you ride or in a warm place (direct sunlight one the tank)

and it will then push past the needle valve open

turn the gas off or put a vacuum shut off in the line or put a higher pressure non gravity needle and seat in

and risk not having enough fuel at high rpm probably not a big risk on your bike

lapping the needle and seat is not something i would do on a rubber tipped fnv

 

[pidjones]

Takes next to nothing on the seat, needle or sides to make one leak. Turn the petcock off - get the habit. And put a filter in-line to keep it clean.

 

[AgentX]

Thanks! I'm not nuts or nothing's broken, then. Got an inline filter in there since its rebirth.

I did once have an issue with a crumb of fuel line holding the needle open occasionally, but cleared that up.

I am in the habit of turning it off...I have to be...but every once in a while when I get home something's pressing me and I forget, or in this case, I started it up for another maintenance task, went off, did other things, and missed my normal dismount-shut-down routine.

 

[SONICJK]

I have noticed with my SR500 with a VM34 that after a ride when the tank is hot the expansion surpasses the ability of the tank to vent and some pressure builds. If the petcock is left on this will cause the floatbowl to overflow. cracking open the gascap will eliminate the issue in my setup.

That being said It shouldn't do it overnight on a cold bike I don't think.

 

[AgentX]

Funny you mention fuel starvation on the bike, though...this thing is a MONSTER of an Enfield ( )and will not fuel properly after a few seconds WOT. Still working it out...at this point assuming jetting is the choke point rather than petcock or float needle capacity.

 

[pidjones]

Funny you mention fuel starvation on the bike, though...this thing is a MONSTER of an Enfield ( )and will not fuel properly after a few seconds WOT. Still working it out...at this point assuming jetting is the choke point rather than petcock or float needle capacity.

That might also point to a venting restriction. Does this vary with fuel level? I doubt it is a jet issue.

 

[AgentX]

Never checked it against fuel level. Interesting idea.

There's no gauge so I never know how much is in the tank anyhow, lol.