Leaky Carbs

[ribbert]

Noel : Is it possable for a stuck open needle and seat to flow raw fuel into a intake manifold ? Seems it would just flow out the over flow drain but if it was plugged ?

George

I've never bothered to determine this. Mine leaks intermittently and it just runs out the vent hose, sometimes in considerable quantities.
Sometimes it only goes 5 kms and leaks again, sometimes it goes 5000 kms between episodes. Sometimes it starts hours after I have parked it. This has the potential to outdo the demise of Dan's bike for spectacle, it lives in a well sealed, unventilated garage under the house which has a gas furnace in it!

I know what causes it and I'm not too fussed about it at the moment. A rap on the offending bowl stops it, but it also means it is leaking past the fuel pump, which I know to be the case.

To answer your question, I don't know. The main jet is lower than the vent but I have never seen any evidence of it on mine, it just runs out the vent pipe.

If DavidR was talking to us he would know or most likely someone else here will know the path it travels for sure.

If there are no answers forthcoming I will do a bench test or have a look to satisfy my own curiosity. ( I have wondered about this from time to time)

Noel

[movenon]

Noel : Is it possable for a stuck open needle and seat to flow raw fuel into a intake manifold ? Seems it would just flow out the over flow drain but if it was plugged ?

George

I've never bothered to determine this. Mine leaks intermittently and it just runs out the vent hose, sometimes in considerable quantities.
Sometimes it only goes 5 km and leaks again, sometimes it goes 5000 km between episodes. Sometimes it starts hours after I have parked it. This has the potential to outdo the demise of Dan's bike for spectacle, it lives in a well sealed, unventilated garage under the house which has a gas furnace in it!

I know what causes it and I'm not too fussed about it at the moment. A rap on the offending bowl stops it, but it also means it is leaking past the fuel pump, which I know to be the case.

To answer your question, I don't know. The main jet is lower than the vent but I have never seen any evidence of it on mine, it just runs out the vent pipe.

If DavidR was talking to us he would know or most likely someone else here will know the path it travels for sure.

If there are no answers forthcoming I will do a bench test or have a look to satisfy my own curiosity. ( I have wondered about this from time to time)

Noel

Indeed leaking fuel can and should be of great concern ! I am familiar with the garage layout you are describing. An unvented condition can be dangerous. Gas fumes and carbon monoxide issues from gas furnaces and auto's...

When we shut off the bike, the line going into the carb and presumably the carb bowl will be pressurized for a very short time and I could envision a slight dribble out the overflow tube or tubes. Or if shut off during colder weather and the a with the temp warming up perhaps again a small amount of fluid could expand/vent out.

I have never had a FJ fuel pump apart, perhaps the pump shutting fuel off isn't as solid an operation as we think. With the fuel tank above it's under small amount of pressure. We don't give it much though in auto's usually because the tank is lower and can't gravity feed. Inside the FJ pump there would have to be a one way check valve.. Perhaps someone who has torn into the actual pump area can give some input ?

The other day after a short trip on the freeway (120miles / 193KM) my bike (1990 model) vented a very small amount of fuel from what I believe to be the fuel tank vent. Which was odd. It was a very small amount, maybe 5-10 drops. I will follow on in the next few days with another test run. I am not to concerned yet but I like to have an understanding of why it happened.
George

[Steve_in_Florida]

This is a discussion from last year that *MIGHT* pertain to the situation:

http://www.fjowners.com/index.php?topic=7056.0

It seems that under certain conditions a "siphon" may be established that can be broken by pulling the choke knob open.

Don't know if it is relevant in this case, but wanted to mention it.

Steve

[giantkiller]

Yes, it can indeed fill your crankcase full. I started my 86 and oil gushed out the crankcase breather with more volume, than if I had left the clutch slave off. I installed a manual petcock right after that.

Then on a 3 day ride. I forgot to shut it off. Got up in the morning too a 3' puddle under the bike and a full crankcase. Had to change the oil in the motel parking lot.

[ribbert]

Yes, it can indeed fill your crankcase full. I started my 86 and oil gushed out the crankcase breather with more volume, than if I had left the clutch slave off. I installed a manual petcock right after that.

Then on a 3 day ride. I forgot to shut it off. Got up in the morning too a 3' puddle under the bike and a full crankcase. Had to change the oil in the motel parking lot.

Well, there ya go. Who needs technical explanations with first hand experience like that!

[movenon]

Yes, it can indeed fill your crankcase full. I started my 86 and oil gushed out the crankcase breather with more volume, than if I had left the clutch slave off. I installed a manual petcock right after that.

Then on a 3 day ride. I forgot to shut it off. Got up in the morning too a 3' puddle under the bike and a full crankcase. Had to change the oil in the motel parking lot.

Well, there ya go. Who needs technical explanations with first hand experience like that!

OK it happens....   Was it the result of 2 failers or one ? Still have to confirm that the fuel pump truly is suppost to shut the fuel off, yes, no ? And there was an obvious needle seat failer. Hard to believe that Yamaha would have that large of a design flaw. But.....?

Was there a leading indicator of a failer ? Fuel leaking/dribbling from the vent tubes ? Running rich, miss firing ? Tells me that on the fuel pump model if you have a dribbling fuel leak from one or more of the carbs that it becomes a priority to fix it correctly. But I guess that statement applies to all models..

A manual fuel shut off might be nice for long term storage for added safety or if you develop said problem on the road.
I need more knowledge.... Things to ponder around the campfire.
George