Gas on the ground....Carb question.

[GS750GUY]

GS750GUY here again.

This just suddenly started happening the other day:

I was getting my 78 GS750EC ready for a ride the other day and when I dropped the bike down from its center stand and began pushing the bike out of the garage I noticed on the floor of the garage the bike left two perfectly parallel lines of fuel on the garage floor about three feet long that was created as I pushed the bike across the floor and out to the driveway.

When I fired up the engine, it immediately started right up as usual but did not immediately rev to about 2500 to 3000 rpm as it usually does. The engine seemed to run smoothly but very slowly over a period of 30 to 60 seconds did it finally rev up to about the 3000 rpm mark so I could back off the choke and drop it back down to the 2500 mark for a little warm up period.

After the warm up period, the bike idles, runs, and performs beautifully.

This has happened the last three times I have ridden the bike and I'm just wondering what is causing the bike to suddenly start dumping two short paralell lines of fuel onto the floor as I push the bike out of the garage and if this is possibly related to what I am experiencing with my initial engine start up idle speed.

Thanks again for your GS wisdom.

GS750GUY

 

[67fire]

Was your petcock in the PRI position?

Eric

 

[tkent02]

When the float bowls get too full it goes out the overflow tubes.
Most likely they are getting too full because of a petcock which no longer shuts off the fuel completely. If it does not go out on the floor until you move the bike it is not leaking really badly, if it was it would drip all the time when it is parked. These bowls being too full explains the odd running for the first minute or so until some fuel burns off, while the fuel level is too high it is getting more fuel than it was when everything was working normally.

Test the petcock for proper function, if it's not working take it apart and clean it out, you will likely end up buying a new one, which will last another thirty years or so...

Just saw the post above, yes check if it was in PRI.

 

[GS750GUY]

When the float bowls get too full it goes out the overflow tubes.
Most likely they are getting too full because of a petcock which no longer shuts off the fuel completely. If it does not go out on the floor until you move the bike it is not leaking really badly, if it was it would drip all the time when it is parked. These bowls being too full explains the odd running for the first minute or so until some fuel burns off, while the fuel level is too high it is getting more fuel than it was when everything was working normally.

Test the petcock for proper function, if it's not working take it apart and clean it out, you will likely end up buying a new one, which will last another thirty years or so...

Just saw the post above, yes check if it was in PRI.

Thanks guys for your response. No the petcock was not set on prime. How would I test the petcock for proper function? When I restored the bike I did rebuild the petcock and installed a new rubber internal gasket.
GS750Guy.

 

[doctorgonzo]

Thanks guys for your response. No the petcock was not set on prime. How would I test the petcock for proper function? When I restored the bike I did rebuild the petcock and installed a new rubber internal gasket.
GS750Guy.

With petcock in the "On" position if gas comes out of the fuel line without the bike running the petcock is bad (if it's a vacuum operated type), so a potentially messy test is just pull off the fuel line with the bike shut off.

Opposite problem (with vacuum type) would be fuel NOT coming out the fuel line in the "On" position when you suck on the vacuum hose.

 

[bonanzadave]

Keep an eye on your oil level. If gas leaks past your petcock it can end up in your crankcase.

 

[GS750GUY]

With petcock in the "On" position if gas comes out of the fuel line without the bike running the petcock is bad (if it's a vacuum operated type), so a potentially messy test is just pull off the fuel line with the bike shut off.

Opposite problem (with vacuum type) would be fuel NOT coming out the fuel line in the "On" position when you suck on the vacuum hose.

Mine is the vacuum type.
So if I'm thinking correctly.....my problem cannot be the opposite problem that you mention above because my engine does run. If this were the problem the carbs would be getting no fuel to the carbs at all and the engine would not run. Is that right?

If it is the first problem you mention above where the petcock is bad and leaking with the petcock in the ON position will I find fuel continuously dripping under the bike basically until the tank drains out of gas?

GS750Guy

 

[tkent02]

Keep an eye on your oil level. If gas leaks past your petcock it can end up in your crankcase.

No, these are VM carbs, with overflow tubes. Fuel should just **** out on the ground below the bike.
The EPA mandated lack of overflow tubes came with the CV carbs.

If it is the first problem you mention above where the petcock is bad and leaking with the petcock in the ON position will I find fuel continuously dripping under the bike basically until the tank drains out of gas?

GS750Guy

Yes, it can put a whole tank on the ground.

 

[Jagir]

Similar problem on a Honda twin that I was tinkering on, but it seems to FLOW right out of the float bowl overflow spout (only on one side) about a second after I turn the petcock on.

Engine seems to run fine, but I haven't run it much or for long because of all the gas on the engine case and ground.

No solution yet, will tinker more and keep my eyes on this thread.

(bike was on centre stand, not side stand)

 

[GS750GUY]

Here is a thought......could this possibly be the cause???:

I read somewhere that if the bike sits for a couple of weeks or so between rides you should move the petcock to its prime position for 30 seconds or so and then back to the ON position in order to prime the carbs before starting the engine.

If I move the petcock to the prime position when in fact the carbs DO NOT NEED TO BE PRIMED, will this action dump excess fuel into the bowls causing some of the excess to dump out onto the floor through the overflow hoses when I drop the bike off its center stand??? Is this a possibility??

For the heck of it, I placed a clean piece of cardboard under the bike and left it overnight and find there are no signs of any fuel at all leaking onto the cardboard. The only time I see fuel drop to the floor is when I drop the bike off its center stand. I then only see two parallel three foot streaks of fuel on the floor as I push the bike out of the garage and then see no fuel drips at all as the bike sits in the driveway prior to me riding off.

GS750GUY

 

[Larry D]

The petcock works like this.

In the "Prime" position fuel will flow with the engine off.
In the "On" position fuel will flow only if the engine is running.
In the "Res" position it works just like the "on" position. Should be used when the tank is almost out of fuel.

If the petcock is failing and fuel is flowing when in the "On" theorically, the needle valve seats should prevent fuel from flooding the engine. So, I would say your rebuilt petcock is a bust OR the needle valves aren't doing the job. Did you rebuild the carbs with new o-rings ? If so, the needle valve seats should be good.

Take off the oil fill cap on the clutch cover and see if you can smell fuel.....that's funny..."see" if you can "smell" fuel.....anyway, if you get a strong smell of fuel, DO NOT run the engine. Fix the problem, drain the oil, change the filter, fill it with new oil, run the engine for 100 miles or so and change it all again. You do not want fuel in the oil.

 

[doctorgonzo]

In the "Res" position it works just like the "on" position. Should be used when the tank is almost out of fuel.

To clarify for those who have never been inside the tank. A plastic tube sticks upwards into the tank from the petcock. It has two openings. the one closer to the bottom of the tank is closed in the "on" position, this stops you from getting that last inch or so of fuel. In the "Pri" position this hole opens. I think it's a holdover from the days before fuel gauges when you needed a waring you only had about a gallon left.

 

[tkent02]

The petcock works like this.

In the "Prime" position fuel will flow with the engine off.
In the "On" position fuel will flow only if the engine is running.
In the "Res" position it works just like the "on" position. Should be used when the tank is almost out of fuel.

If the petcock is failing and fuel is flowing when in the "On" theorically, the needle valve seats should prevent fuel from flooding the engine. So, I would say your rebuilt petcock is a bust OR the needle valves aren't doing the job. Did you rebuild the carbs with new o-rings ? If so, the needle valve seats should be good.

Take off the oil fill cap on the clutch cover and see if you can smell fuel.....that's funny..."see" if you can "smell" fuel.....anyway, if you get a strong smell of fuel, DO NOT run the engine. Fix the problem, drain the oil, change the filter, fill it with new oil, run the engine for 100 miles or so and change it all again. You do not want fuel in the oil.

Again, these are not CV carbs, the fuel should not go into the crankcase, it has float bowl overflow tubes to prevent this. This could only happen if the overflow tubes were routed up over the carbs, or if they were completely plugged.

Depending on which petcock you have, the rebuild kits may or may not work. On some the parts don't fit at all, they go in but cannot shut off the fuel, they are designed incorrectly.
The only way to know if it works is to test it. The petcock should completely shut off the flow of fuel from the larger fuel port when there is no vacuum applied to the vacuum port.

If fuel is flowing when the engine is off, the petcock is at fault. Although the needle valves in the float bowls may hold this fuel back, it is not their job, and they are not designed to do this. They are only designed to keep the fuel at a constant level while the engine is running, not to stop the flow completely.

Similar problem on a Honda twin that I was tinkering on, but it seems to FLOW right out of the float bowl overflow spout (only on one side) about a second after I turn the petcock on.

Engine seems to run fine, but I haven't run it much or for long because of all the gas on the engine case and ground.

No solution yet, will tinker more and keep my eyes on this thread.

(bike was on centre stand, not side stand)

In this case the needle valve is not controlling the fuel in the bowl to a constant level, it is flowing more fuel that the engine can burn. Something to do with the needle valve is stuck open. Sometimes the float sticks to the bottom of the empty bowl, so try tapping on the float bowl while the petcock is turned on to see if the overflow stops, if not you will have to go in and find out why. Could be a chunk of something stuck in there, could be some part of the valve is bent somehow. Take the float bowl off and see what is wrong.

 

[Jagir]

In this case the needle valve is not controlling the fuel in the bowl to a constant level, it is flowing more fuel that the engine can burn. Something to do with the needle valve is stuck open. Sometimes the float sticks to the bottom of the empty bowl, so try tapping on the float bowl while the petcock is turned on to see if the overflow stops, if not you will have to go in and find out why. Could be a chunk of something stuck in there, could be some part of the valve is bent somehow. Take the float bowl off and see what is wrong.

Thanks for providing direction. I hope to get at it soon and I'll let y'all know what I figure out.

 

[GS750GUY]

Again, these are not CV carbs, the fuel should not go into the crankcase, it has float bowl overflow tubes to prevent this. This could only happen if the overflow tubes were routed up over the carbs, or if they were completely plugged.

Thanks tkent02!

Please enlighten me a little more regarding your statement above regarding my carbs not being the CV type on my 78 GS750EC and how fuel should not end up in the crankcase.

I checked and find my overflow tubes are routed properly below the carbs and shoot straight to the floor. And I find there is no fuel smell in the crankcase.


GS750GUY

 

[tlepac]

A Quick peek at Bass Cliff's site can tell you the outward differences ---and the inward if you are interested---between the CV carbs and the VM carbs. They don't look at all alike

 

[tkent02]

Please enlighten me a little more regarding your statement above regarding my carbs not being the CV type on my 78 GS750EC and how fuel should not end up in the crankcase.

Carburetors before 1979 on most models, were Mikuni VM series carbs, the throttle cable directly opens the throttle slides, there is no correction for changes in elevations, and there was an overflow tube which dumped fuel on the ground under the bike any time there was a failure of the needle valve causing too much fuel to be in the float bowls.

In 1980, they switched to Mikuni BS series carbs, These are Constant Velocity style carburetors. There is a throttle plate controlled by the cable, and a separate slide, which raises by a difference in air pressure between the air passing under the slide and the pressure in the airbox. The position of this slide controls the flow of fuel through the needle jets, controls the fuel mixture at partial throttle. This gave a correction for operating at different elevations, and better fuel efficiency at the cost of slightly slower throttle response. Suzuki put much larger CVs on than the size the VMs were previously, for instance the GS1000 went from 26mm VMs to 34mm BS carbs. Because of this, the overall power did not decrease, just the quickness the power came on. Another change is that the overflow tubes were removed, excess fuel can no longer dump on the ground. This looks like a great idea to the EPA, but for us bike owners not good at all as any overflowing now fuel goes up into the main carb venturi and spills into the engine through the intake port, in some cases gallons of fuel can go into and dilute the engine oil which can ruin the engine, or it can fill one cylinder completely hydro-locking it, when the starter in engaged it can bend a connecting rod or cause other damage.

I actually prefer BS carbs for my own bikes, but only because I live and ride with huge changes in elevation, they just work better here. They are much harder to tune for different exhausts or intake systems, and you do have to make absolutely sure the float system is working properly.

 

[GS750GUY]

Carburetors before 1979 on most models, were Mikuni VM series carbs, the throttle cable directly opens the throttle slides, there is no correction for changes in elevations, and there was an overflow tube which dumped fuel on the ground under the bike any time there was a failure of the needle valve causing too much fuel to be in the float bowls.

In 1980, they switched to Mikuni BS series carbs, These are Constant Velocity style carburetors. There is a throttle plate controlled by the cable, and a separate slide, which raises by a difference in air pressure between the air passing under the slide and the pressure in the airbox. The position of this slide controls the flow of fuel through the needle jets, controls the fuel mixture at partial throttle. This gave a correction for operating at different elevations, and better fuel efficiency at the cost of slightly slower throttle response. Suzuki put much larger CVs on than the size the VMs were previously, for instance the GS1000 went from 26mm VMs to 34mm BS carbs. Because of this, the overall power did not decrease, just the quickness the power came on. Another change is that the overflow tubes were removed, excess fuel can no longer dump on the ground. This looks like a great idea to the EPA, but for us bike owners not good at all as any overflowing now fuel goes up into the main carb venturi and spills into the engine through the intake port, in some cases gallons of fuel can go into and dilute the engine oil which can ruin the engine, or it can fill one cylinder completely hydro-locking it, when the starter in engaged it can bend a connecting rod or cause other damage.

I actually prefer BS carbs for my own bikes, but only because I live and ride with huge changes in elevation, they just work better here. They are much harder to tune for different exhausts or intake systems, and you do have to make absolutely sure the float system is working properly.

Thanks tkent02! What a thorough and well explained explanation. I appreciate you taking the time to be so clear.

It's good news to know that there is no way on my 78 with VM carbs that fuel could end up in the crankcase.

Thanks again!!

GS750GUY

 

[GS750GUY]

Thanks tkent02! What a thorough and well explained explanation. I appreciate you taking the time to be so clear.

It's good news to know that there is no way on my 78 with VM carbs that fuel could end up in the crankcase.

Thanks again!!

GS750GUY

As suggested, I went into the Bass Cliff site and looked at the difference between the VM and CV carbs and see as you said that there is no way with the VM carbs that fuel could get into the crankcase.
GS750GUY

 

[tkent02]

As suggested, I went into the Bass Cliff site and looked at the difference between the VM and CV carbs and see as you said that there is no way with the VM carbs that fuel could get into the crankcase.
GS750GUY

Don't be so sure, there is not supposed to be a way, this does not mean it can't happen.
If the overflows are plugged, or kinked, or routed upwards, fuel could still do this.