carb overflow mystery

[wompin]

I have been attempting to diagnose a carb overflow issue I encountered upon initial start up after a complete motor rebuild.

While the carbs are on the bike, at least two of the bowls are overflowing.

I took the petcock and carb bank of the bike, and ran tests to check for leaks or overflow. I fabricated a temporary tank bottle and hose to mimic the gas tank. I filled it with water, and check for function of the vacuum petcock on both reserve and on settings. The petcock doesnt leak and does let fuel through with vacuum as it should.

I used the same bottle/hose rig hooked up to the fuel line to the carb bank to simulate a constant source of gas as would exist with a leaking petcock (even though mine doesnt leak when tested off the bike). All four of the float needles appear to be seating correctly and preventing bowl overflow during the test. The needles and needle seats all look flawless and clean as well. Float heights are set to 25mm

The problem is, the last two times I have reinstalled the petcock and carbs to run the bike, I have bowls overflowing. Is there some variable present when the carbs are on the bike, being choked or throttled while attempt to start the motor, that would cause overflow when there is no overflow in isolated (off bike) testing?

I know I need to set pilot air and fuel screws. But I have started with fuel screw 1 turn out and air screw 2 turns out. I have pods and cut off drags. Advice?

Also, how far up should I set the throttle stop scerw on my vm26ss round slide carbs? Does the slide need to be slightly open aka slightly throttled to start up? FYI the starting issues I have may be due to 30 degree F temps.

I am stupified here, please help with any or all issues. Thanks

 

[Guest]

Only way I can see it flooding is this, needles aren't working or the petcock is messed up. I would pick the petcock. Check and make sure it isn't set on prime. If it is, it has a continuous flow of gas to the bowls. In most cases it will over come the needles causing the over flow. which two bowls are over flowing? Would be helpful to know what bike you are working on as well.

Do you have your carbs at the same angle they would be on the bike when you tested them?

 

[MisterCinders]

Have you checked the floats with the bowls in place and the carbs off the bike? I had an overflow problem when the floats were snagging on the bowl gasket, and that issue would not show up until I tested the carbs with the bowls and gaskets in place.

Also are the washers for the needle seats working properly? Even if the needles seat well, if fuel drips past that washer, it will flood the bowls.

Finally, check your oil for gas and change it and the filter before starting the bike again if there is any chance of fuel entering the crankcase.

 

[SqDancerLynn1]

Water will react different than gas ! What carbs ? The needles need to be changed or floats set too high, or float valve O rings are bad (80 up BS carbs)

 

[wompin]

Only way I can see it flooding is this, needles aren't working or the petcock is messed up. I would pick the petcock. Check and make sure it isn't set on prime. If it is, it has a continuous flow of gas to the bowls. In most cases it will over come the needles causing the over flow. which two bowls are over flowing? Would be helpful to know what bike you are working on as well.

Do you have your carbs at the same angle they would be on the bike when you tested them?

The bike is a 79 gs1000L. Carbs are vm26ss. the petcock has no prime setting, only on and reserve. The petcock has been tested and is functioning. I agree a constant flow of gas will likely get past even properly seating needles. There is no constant flow, though (at least not when testing off the bike)

During the test, just to be thorough, I tilted the carbs at every angle. No leaks at any angle.

The carbs that have leaked, although at different times, are #1 and #4.

 

[wompin]

Water will react different than gas ! What carbs ? The needles need to be changed or floats set too high, or float valve O rings are bad (80 up BS carbs)

The needles and float valves appear flawless. I have cleaned and inspected both thoroughly. I will recheck the o-rings. Finally something to check that I havent checked twice. Thanks!

Obviously water has different qualities than gas. Are you saying the different densities (leading to different bouyancy) are throwing off the float heights during my test vs in situ on the bike? Please elaborate on "react differently".

Carbs are vm26ss

 

[wompin]

Have you checked the floats with the bowls in place and the carbs off the bike? I had an overflow problem when the floats were snagging on the bowl gasket, and that issue would not show up until I tested the carbs with the bowls and gaskets in place.

I tested the carbs as a complete unit as they would be with on the bike. I also checked float height with the bowls off.

Can you clarify what you mean by testing the carbs with the bowls and gaskets in place. Do you mean simply fully assembled carbs or actually mounted on the motor? Thanks

 

[Steve]

I fabricated a temporary tank bottle and hose to mimic the gas tank.

How high is the bottle above the carbs when you do this test? You want to have the fuel level in the bottle about the same height above the carbs as it would be on the bike.

IFloat heights are set to 25mm.

Any particular reason you set them there? I believe the specs call for 24mm.

I know I need to set pilot air and fuel screws. But I have started with fuel screw 1 turn out and air screw 2 turns out. I have pods and cut off drags. Advice?

Reasonable enough starting point for your setup.

Also, how far up should I set the throttle stop scerw on my vm26ss round slide carbs? Does the slide need to be slightly open aka slightly throttled to start up? FYI the starting issues I have may be due to 30 degree F temps.

Can't give you any real specifics here, but loosen the idle screw until you see clearance, then turn it in until the screw touches the linkage and give it about one more turn of the screw.


.

 

[wompin]

How high is the bottle above the carbs when you do this test? You want to have the fuel level in the bottle about the same height above the carbs as it would be on the bike...

...Any particular reason you set them there? I believe the specs call for 24mm.

Test Bottle height:
The bottle is more or less one foot above the carbs. If anything, this height is greater than that of the mounted tank. In theory, that would mean even more hydrostatic pressure, thus more chance of needle seat failure. Even with the additional height, there is no overflow.

I did notice something odd last night when I repeated the test, though. After I did the test, I drained the bowls, but only carbs 1 and 4 had fluid. The others were empty.

Talk about gremlins, come on here!

Float Height:
The source I used (haynes and suzuki manual) calls for floats set to 23-25mm. I suppose I am more comfortable with 25mm since that would theoretically seat the needle sooner, and provide further insurance against overflow.

 

[demonspeedracing]

Have you checked the vent on your gas tank?
Just moving the fuel tank will cause it to create pressure within the tank. Enough to overpower needles and seats.

Also there is more head pressure with a full or even a half full gas tank then with a small remote fuel tank.

Timothy

 

[MisterCinders]

I tested the carbs as a complete unit as they would be with on the bike. I also checked float height with the bowls off.

Can you clarify what you mean by testing the carbs with the bowls and gaskets in place. Do you mean simply fully assembled carbs or actually mounted on the motor? Thanks

You should try two tests on carbs that overflow. Both happen off the bike.

First test the needles with the rack assembled and the bowls off. Gently lift the floats to make sure the fuel stops flowing when the needles seat. If fuel dribbles out past the washer or the seated needle valve, well, you know what to do.

Then put the gaskets in place and see if the floats still move without snagging. If so, do the next test after you close up the carbs by putting the bowls on. Check the float height with the bowl drain tube (Z1 has this for about $7) and see where the fuel level sits in comparison to the bowl edge.

 

[chuck hahn]

23 to 25 MM is the service manual spec as well. So I always go in between at 24 and havent ever had a problem. Fuel obviously isnt getting shut off..so if the float ht is at 25 then that wouldnt be it. Back track and check each float needle..theres a little spring loaded tit on them..are they all free and working right? Are there gaskets on all the float seats themselves?? Next, put the carbs as they would be on the bike and work the floats and observe the float needles arent sticking. Next..install the gaskets without the bowls on and move he floats past the gasket edges and be sure none are hitting the gasket material. Lastly..you check both floats in each bowl to be sure they are both at the same hgt. If all checks out ok, then i dont see any reason for them to not shut off the fuel flow properly.

 

[Steve]

The source I used (haynes and suzuki manual) calls for floats set to 23-25mm. I suppose I am more comfortable with 25mm since that would theoretically seat the needle sooner, and provide further insurance against overflow.

True, they will seat sooner, but that also lowers the fuel height in the bowl, making EVERY circuit in the carb (except starting) run just a little bit leaner.

One silly millimeter doesn't sound like much, but it will definitely make a difference. It might run just lean enough that you might want to go one size larger on the jets, and in the process of changing the jets, you will re-check the float height, possibly adjust it a bit, then wonder why you are now running so RICH.

Yeah, while the actual range is 23-25mm, I believe it's actually written as "24.0mm +/- 1.0mm". Because of that, I prefer to go to the middle, as much as possible.

.

 

[wompin]

Good point Steve. I did already jet up three sizes on the main and one on the pilot, plus one groove up on the needle, just as preliminary compensation for the pods and open pipes.

Wouldnt the circuits take up the same amount of fuel regardless of the gas level as long as their intake openings are submerged? Maybe at 25mm they are on the brink of no longer being submerged.

Anyway we probably beat this point to death, Ill shoot for 24mm just for good karma.

Chuck, Mister Cinders, demon:

Ill be running all of your suggested tests tonight. Thanks for the info. Hopefully I can see something new. Demon, what am I looking for when I check the gas cap vent? clogged vent?

 

[Big T]

One other possibility is that the overflow tubes are partially broken at the base, allowing the fuel to flow out

Try swapping the bowls around and see what happens

 

[chuck hahn]

Yes...clogged vent. Take the 2 screws out of the brown kidney bean looking part..being sure of how the BBs go back in. Then youll see a vent hole. Blow into it and see that air passes thru the vent. The vent channel igtself is a molded in tench in the grey base metal and is covered by the chrome cap cover. The outside air flows thru the channel and into the tank. Also , if you take off the black cover, youll see that there is an oring at the base of the lock cylinder that keeps gas from splashing up and out via the key hole..youll need to be sure the oring is good. While the cover is of, give the lock cylinder a good blowing out with some air and relubed the tumblers with a shot of penetrating oil.

 

[chuck hahn]

Or temporarily cap off the overflow nipples on the bottom of the bowls.

 

[barnbiketom]

TRUST ME
make sure the individual CARB VENT lines are not blocked or hooked wrong ( to each other)

if in doubt just reach in with needle nose pliers and pull them oof the carbs then retest

 

[Steve]

Wouldnt the circuits take up the same amount of fuel regardless of the gas level as long as their intake openings are submerged? Maybe at 25mm they are on the brink of no longer being submerged.

Oh, they will still be submerged. The problem is that fuel is drawn up through the jets by the air that is passing over the top. The air over the top is accelerated by the venturi, and the faster-moving air has less pressure. This lower-pressure air will suck the gas up the jets. Now, if the level of the gas is lower (due to setting the float height at 25mm vs 24mm), it will have to suck harder to raise the gas the extra mm. Since it doesn't suck any harder, it siimply draws up less gas, therefore, a leaner mixture.

I was looking for the reason you went with bigger jetting and saw that you have pods and cut-off drag pipes.

Not sure that three main jet sizes will be enough for that combination, and a larger pilot might not have been necessary, but the part that confuses me is "plus one groove up on the needle". Did you move the clip down (toward the point) or up (toward the blunt end) of the needle? If you moved the clip UP one groove, like it sounds in your post, you actually LOWERED the needle, which will lean out your mid-range, you should have lowered the clip, which will raise the needle.

.

 

[wompin]

Ya what I meant was I raised the needle one groove, so theres more flow on midrange.

If three sizes up for the main isnt enough, do you have any suggestion for a proper jet size?

I had seen from a few sources that a good rule of thumb is 2 jet sizes up for pods and two for open pipes. Then one pilot size up for each 3 main jet size increase. I plan to baffle the drag pipes, and when I can afford it, put on a 4 into 1 with a nice turnout muffler. So I made a judgement call and went 3 sized up for main and one for pilot, plus the needle moved up one setting. I figure I can also back out the fuel screw more if needed.