Bubbles from second shut off

[lilmilo]

After motor has warmed and been shut down there are bubbles going up fuel line. Turning the secondary shut off valve to the off position has no affect on them. there does not seem to be a leak at the valve. My old motor had a leak there and it never caused that. I did a heat gun test today and my carb was only 10 degrees cooler than my engine (110 degrees). I do not think my old carb ever got that hot. Could this be causin bubbles even if not boiling point? I have a Starfire GenII Phantom (chrome w/ black jug) that has a different ntake on it that seems to be aluminum. Could this be conducting heat to carb? Also upon shut down it diesels then backfires. hope this is enough info. any questions post them will do best to answer. TY

 

[professor]

My 2 cents- 110 degrees is not hot.
Carb is in line with the heat off the engine anyway.
The bubbles would no bother me if the engine ran good.
The running on is usually caused by lean mixture (as long as the spark plug is not a hot range plug not designed for the engine).
Possibly you have an air leak that leans everything at idle.
Some guys shut down the engine with the choke and avoid the stock kill switch.

 

[motorpsycho]

remember, the fuel system on these bike engines (gas cap, and carb bowl) are not sealed...they are vented.
air will get into the fuel line on occasion and get trapped in the fuel filter. once enough tiny air bubbles build up in the filter, the bubbles will rise up the gas line towards the gas tank (because air will always want to rise when it's in a liquid such as gasoline)
as stated, dieseling/ run on and backfiring on shut off is normally caused by a too lean conditon.

 

[lilmilo]

The bubbles are not coming from my filter they are coming from the nipple of the secondary shut off valve, which does not leak, while in the on AND off postions . They stop after motor cools so would seem to be heat related. The problem they cause is i can not start until cooled and bubbles stop. The measured temp was after only 3/4 of a mile or so. Can anyone tell me the temp of their carb on a fully warmed motor?

 

[motorpsycho]

no, what i am saying is that it is possible for air to be in the carb float bowl.
when the engine gets hot, this air will get hot and it will want to rise.
i thought you meant that the air bubbles were coming from the fuel filter.

 

[lilmilo]

Thank you professor and motorpsycho. Does anyone else have anything to add? I still cant bring myself to beieive that the boiling gas is ok. escpecially when i did not have this problem with my last kit. no one i know has had it, and there are no references to it anywhere in search. I am running rich in an effort to keep cool. with a NGKB6 plug. Should i maybe refinish my jug? as the factory black coating is quit thick. not only does it make it hard to start when warm, but doesnt exactly make me feel safe as it is a problem even when not completely warmed up. do not want to find out what would happen after a long ride.

 

[TYFOON]

Do you close the carb fuel valve (if you have one) when you get close to you destination? I have been. Figure less fuell in the bowl the better when sitting.

Just wondering if you were cuz I noticed small bubbles as well.

 

[lilmilo]

Shut it off before flipping kill toggle, but do not usually let run dry. but bubbles happen regardless of order of shutdown and amount of fuel in bowl.

 

[lilmilo]

So the backfire is back. redid gaskets w/ rtv. o ring at carb. clip on richest setting. I'm at about 5500 ft so if anything main jet should be to big? would think from everything ive read and the way my last motor ran that the clip would need to be much leaner.

 

[GearNut]

no, what i am saying is that it is possible for air to be in the carb float bowl.
when the engine gets hot, this air will get hot and it will want to rise.
i thought you meant that the air bubbles were coming from the fuel filter.

It would not be possible for a carburetor to work unless the float bowl was vented to the atmosphere. Any pressure developed in the float bowl chamber by boiling of fuel would be immediately vented out of the float bowl vent.
I can surmise that any fuel in contact with the carb above the float valve, hot enough to boil the fuel, could possibly create bubbles in the fuel line. I have never seen or heard of it before.