Gas on the ground....Carb question.

[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.

[Jagir]

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.

I replaced my petcock with a manual one and left it on one night (oops). The next day the carbs were flooded - as I figured they would be. At that time, I did not realize that a little fuel had trickled down into the crankcase. It added too much fluid, so some came out through the overflow which was stupidly placed in the centre of the the bike (in front of the back wheel).
The bike started acting real funny; really high revs, engine 'slipping' then, the rear wheel slipping on gassy oil overflow!

I parked it right then and there and then came back later to change the oil and wipe the tire down and move the overflow tube to the side. Then i rode home slowly and remembered to shut the petcock off and park on the centre stand.

[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.

Thanks tkent02! I will double check my overflow hoses and make sure they are routed and flowing properly.
GS750Guy.

[GS750GUY]

[QUOTE=Jagir;1120627]I replaced my petcock with a manual one


Thanks Jagir!

For the purpose of simplicity (and hopefully for the sake of added reliability), I have thought about replacing the vacuum operated petcock with a manual one.

When installing a manual petcock do you just plug off the vacuum line?
You have already mentioned one of the downsides to a manual petcock when you forget to shut it off.

Does anyone out there have any other experience with other downsides to manual petcocks?

GS750GUY

[tkent02]

[QUOTE=GS750GUY;1121073]

I replaced my petcock with a manual one

You have already mentioned one of the downsides to a manual petcock when you forget to shut it off.

Does anyone out there have any other experience with other downsides to manual petcocks?

GS750GUY

Just that if you forget to turn it back on, and if you crank it wide open to get back on a highway or something, it will lean out and die right about when it should be getting all the fuel it needs.
You could ask Chef for more information about running lean.

[GS750GUY]

Just a thought....regarding the fuel in the crankcase issue:

Is it ever possible that oil that has been in the crankcase for a couple thousand miles to take on a very very slight hint of fuel odor just simply because the oil has been exposed to engine vapors over a 2000 mile period of time?

The reason I ask is because I stuck a long flat bladed screw driver blade down into the bottom of the crankcase in order to bring up an oil sample and found that if I concentrated on my sniffer hard enough I could sense a very very slight fuel odor.

I checked the routing of my overflow hoses and they all seem to be fine and none were plugged or kinked.

GS750GUY

[Skateguy50]

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

Not sure if anyone adressed this post but if you are priming a bike that doesnt need to be primed it will overflow the carbs and do exactly what you described. The only time I ever used prime was after cleaning the carbs and installing them dry. Maybe after a winter but never during the riding season.

[Steve]

Not sure if anyone adressed this post but if you are priming a bike that doesnt need to be primed it will overflow the carbs and do exactly what you described.

Close, but not quite correct.

If your carbs have overflowed, it means that you have TWO failures.

First, the petcock is leaking when it shouldn't be, second, the float valves are not holding it back. It has been mentioned that the float valves are not designed to hold back fuel flow. I can't fully agree with that. How else do they regulate fuel level during engine idle, when you have very low usage? They should be able to stop fuel flow, but they shouldn't have to, if the petcock is working properly.

If your petcock works, but your float valves leak, you may never notice. If your petcock leaks, but your float valves are good, you may never notice. Only when they BOTH fail do you see the puddles on the ground. Unless, that is, you simply have the float level too high and simply moving the bike off the centerstand sloshes fuel into the overflow tubes.

.

[tkent02]

Close, but not quite correct.

If your carbs have overflowed, it means that you have TWO failures.

First, the petcock is leaking when it shouldn't be, second, the float valves are not holding it back. It has been mentioned that the float valves are not designed to hold back fuel flow. I can't fully agree with that. How else do they regulate fuel level during engine idle, when you have very low usage? They should be able to stop fuel flow, but they shouldn't have to, if the petcock is working properly.

If your petcock works, but your float valves leak, you may never notice. If your petcock leaks, but your float valves are good, you may never notice. Only when they BOTH fail do you see the puddles on the ground. Unless, that is, you simply have the float level too high and simply moving the bike off the centerstand sloshes fuel into the overflow tubes.

.

I'll disagree with your disagreement, Steve.

True a good needle valve will usually stop the flow, not always. It is not it's job, that is the function of the petcock. If the needle valve was intended to stop fuel flow 100% they would have never used a petcock, it would have been an unnecessary expense. Notice how quickly petcocks went away when fuel injection came in.

It does not need to seal 100% to control the level of fuel while it is flowing, and the fuel never stops flowing while the engine is running, no matter how slow the idle. The needle only needs to squeeze off the flow to whatever it takes to idle the engine, a ml or two per minute or something like that. Compare the design of the needles with that of a toilet's float valve, which IS designed to completely shut off the flow. The water pressure in the pipe helps to close the valve, once it starts to close it slams shut.
These needle valves are the opposite, pressure in the line pushes the valve open, only the tiny float applies any pressure to close the valve, a very poor design for a shut off. It is a perfect design for controlling the rate of flow.

[KEITH KRAUSE]

Not trying to argue but VM carbs can allow fuel to enter the cylinder and then to the crankcase. I've seen it a few times. Without touching anything I've slipped the carbs out and saw/felt a puddle at the bottom of the manifold. I will add that each time I saw this the bike had been on the sidestand.
When level on the centerstand, the overflow tube SHOULD collect any excess fuel and allow it to drain out the line. What exactly happens when the bike is leaning on the sidestand I won't bother to wonder but it will allow fuel to enter the cylinder if for whatever reason the fuel overfills the floatbowl.
Most of the time the float needle valve will stop the flow if the float pushes up against it enough. If every part is operating as new there will rarely be any leaking. But on our old bikes there are lots of things that can go wrong. A bit of dirt on the needle tip. Varnish from sitting too long. A small groove in the tip area. A burr on the side. A float not working smoothly. A new replacement float that isn't up to Mikuni standards...any combo of the above. Lots of things.
I've seen it and worked on it. My own bike...with a new Pingel fuel valve that I knew was working right, I once in awhile would see a small leakage of fuel. This only happened if the bike was pushed out and placed on the sidestand a minute or so. I'd come out after closing the garage door and see it. Never see it actually dripping, just a small puddle (several drops). I decided to check one day even though I knew the carbs have been serviced correctly. Maybe a bit of dirt or a burr? I checked carefully and found nothing. Float level correct. No dirt. Floats moving smoothly, etc. I even went as far as to replace the float valve with a brand new Mikuni valve. Still would leak once in awhile. I decided it wasn't worth going crazy over.
Just my experience. I'm sure others have had it happen too.
But I just wanted to say it is possible for these carbs to leak into the crankcase because I've seen it myself.
From what I'm reading it doesn't sound like the petcock is leaking fuel into the case but I can only go by his description. You generally can smell the fuel in there without much question. You can also test the petcock overnight if you want. The stock '78 petcock can fail and allow fuel to run in the VACUUM line and go straight to the #3 cylinder/crank too. Testing can be hit or miss sometimes for unknown reasons. Maybe at the first stages of failing the petcock will sometimes work right but the next day/week it won't.
I had a suspect petcock many years ago and learned a little about testing but decided a Pingel was my best choice.

[Jagir]

[QUOTE=GS750GUY;1121073]

I replaced my petcock with a manual one


Thanks Jagir!

For the purpose of simplicity (and hopefully for the sake of added reliability), I have thought about replacing the vacuum operated petcock with a manual one.

When installing a manual petcock do you just plug off the vacuum line?
You have already mentioned one of the downsides to a manual petcock when you forget to shut it off.

Does anyone out there have any other experience with other downsides to manual petcocks?

GS750GUY

Yeah, the job was actually done by someone else as a favor to me and it was, but the 'plug' in the vacuum line wasn't very good. In fact, it was a screw in the end of the line and it did not plug the line properly.

Once I (rather - the GS Forum) realized the issue (running like crap -all air leaky, about 5 minutes after I took off), I got rid of the screw and ran the hose back to one of the cylindrical protuberances on the side of the carbs and zapstrapped it on tight.

Also, ditto on the "forget to turn it on" mention, but at least you always know what that issue is, let's just hope that you are not in traffic when you are 'reminded'.