gas is leaking from the carb

[kjodell10]

My bike has developed a slow but persist leak of gas from the interior- mixing chamber?- of the left-hand carb, and into the air box. (I'm new so I may not have the nomenclature right.) I pulled the air box and it appears to be welling up from around the first brass bits- which I think are the jet needle and needle jet- although the origin of the leak could be further inside; I can't see to tell if that's so. Can anyone suggest where I should be looking? Any help will be greatly appreciated.

 

[gggGary]

It has a leaking float valve.
And a leaking petcock.
Why? is the $64,000 question.
Pic of the carbs? Reason can vary, depending on what carbs it has.

 

[XSLeo]

You don't mention the year or model of the bike. The carbs got major changes every two years with some changes every year. So knowing that can help us help you.
They useddifferent petcocks early and late. Early were manual, later vacuum operated.
Leaking such as this comes from the float valve being blocked open. Usually by dirt in the gas or crud build up from old gas.
Either thing can easily be fixed by a tear down and cleaning.
www.amckayltd.com/carbguide.pdf will tell how to do this. Just follow the stepsQuite often you don't even need any new parts. Care when you seperate the float bowl and the gasket is still usable. THe metal parts clean up wll with just spray can carb cleaner you can get most anywhere.
Leaky petcocks just make the leaky floatvalves more noticable when setting parked. Leaky flot valves can effect the way the bike uns. The float valves maintain a set level of fuel in the float bowls. If the valves leak the fuel level can vary enough to make the bike run rich.
Leo

 

[kjodell10]

It has a leaking float valve.

It has a leaking float valve.
And a leaking petcock.
Why? is the $64,000 question.
Pic of the carbs? Reason can vary, depending on what carbs it has.

Here's a photo of the carb- with aluminum foil drainage system...Thank you!

 

[kjodell10]

You don't mention the year or model of the bike. The carbs got major changes every two years with some changes every year. So knowing that can help us help you.
They useddifferent petcocks early and late. Early were manual, later vacuum operated.
Leaking such as this comes from the float valve being blocked open. Usually by dirt in the gas or crud build up from old gas.
Either thing can easily be fixed by a tear down and cleaning.
www.amckayltd.com/carbguide.pdf will tell how to do this. Just follow the stepsQuite often you don't even need any new parts. Care when you seperate the float bowl and the gasket is still usable. THe metal parts clean up wll with just spray can carb cleaner you can get most anywhere.
Leaky petcocks just make the leaky floatvalves more noticable when setting parked. Leaky flot valves can effect the way the bike uns. The float valves maintain a set level of fuel in the float bowls. If the valves leak the fuel level can vary enough to make the bike run rich.
Leo

It's a 1978 (650E). I've posted a photo. Thanks; I appreciate the help!

 

[gggGary]

BS38 carbs have brass tipped float valves so a new valve and seat are a periodic maintence item. While it's apart do a dip test to be sure the float has not started leaking. Petcocks can be rebuilt but buy only genuine Yamaha 4 hole rubber "valves", the ones in the kits seldom work well.
From here it appears you have a 1978 standard with the original carbs.

 

[kjodell10]

BS38 carbs have brass tipped float valves so a new valve and seat are a periodic maintence item. While it's apart do a dip test to be sure the float has not started leaking. Petcocks can be rebuilt but buy only genuine Yamaha 4 hole rubber "valves", the ones in the kits seldom work well.
From here it appears you have a 1978 standard with the original carbs.

Two questions: What's a dip test? And what's the best way to staunch the leak until I can get to both float valve and petcock? I could just keep collecting the run-off, I guess... Okay, three questions: Is there a best way to see if the petcock is also at fault? Thanks, Gary.

 

[kjodell10]

You don't mention the year or model of the bike. The carbs got major changes every two years with some changes every year. So knowing that can help us help you.
They useddifferent petcocks early and late. Early were manual, later vacuum operated.
Leaking such as this comes from the float valve being blocked open. Usually by dirt in the gas or crud build up from old gas.
Either thing can easily be fixed by a tear down and cleaning.
www.amckayltd.com/carbguide.pdf will tell how to do this. Just follow the stepsQuite often you don't even need any new parts. Care when you seperate the float bowl and the gasket is still usable. THe metal parts clean up wll with just spray can carb cleaner you can get most anywhere.
Leaky petcocks just make the leaky floatvalves more noticable when setting parked. Leaky flot valves can effect the way the bike uns. The float valves maintain a set level of fuel in the float bowls. If the valves leak the fuel level can vary enough to make the bike run rich.
Leo

I've bookmarked the carb pdf; thanks a million.

 

[XSLeo]

That is a vacuum petcock. You have it in the PRIime position. This bypasses the vacuum ON function and lets gas flow to carbs all thge time. You should leave it in the ON position. This lets fuel flow only when the bike is running. Vacuum from the engine opens the petcock. When not running, no vacuum, no fuel flow.
The PRIme position is used after the bike ha sat awhile nd you want to be sure the float bowls are full before you start the bike. Once running move to ON.

 

[kjodell10]

That is a vacuum petcock. You have it in the prime position. This bypasses the vacuum on function and lets gas flow to carbs all thge time. You should leave it in the ON position. This lets fuel flow only when the bike is running. Vacuum from the engine open the petcock. When not running, no vacuum, no fuel flow.
The PRIme position is used after the bike ha sat awhile nd you want to be sure the float bowls are full before you start the bike. Once running move to ON.

Whoa- even MORE stuff I don't know! Thank you, Leo. I've moved it to ON.

 

[gggGary]

Will be interesting to see if that "cures" your problem.

 

[kjodell10]

Will be interesting to see if that "cures" your problem.

With the petcock to ON, there doesn't appear to be any more leaking. But I'm assuming the float valve should still be attended to. I'm kind of looking forward to it- in an apprehensive way.

 

[gggGary]

With the petcock to ON, there doesn't appear to be any more leaking. But I'm assuming the float valve should still be attended to. I'm kind of looking forward to it- in an apprehensive way.

Well the old saying is float valves are intended to maintain fuel level in a RUNNING motor not act as shut off valves for PARKED motorcycles.

 

[kjodell10]

Well the old saying is float valves are intended to maintain fuel level in a RUNNING motor not act as shut off valves for PARKED motorcycles.

I shall take that to heart. Thanks, Gary.