1978 GS550e Carb Bowls Overflowing

[satoricycles]

I am having a hard time getting my carb bowls to stop overflowing. I have tried new needles, seats and floats and the bowls keep overflowing. I have also adjusted the floats up to 28mm with no luck. When I blow air through the fuel line with the carbs upside down, I get no air blow by at all. But when I put fuel in, a carb or two will always start to overflow at the bowl gasket. So, I try to adjust the ones leaking, refill with gas and a different one will start leaking. There doesn’t seem to be any rhyme or reason to which one is going to leak, but it seems like one will always overflow. I find it hard to believe that I have eight faulty sets up needles and seats (old set and new set leak). Does any one have any advice or anything else I should look into?

[Brendan W]

Gut feeling there is dirt in the fuel or the fuel delivery pressure is too high.
Are the carbs on or off the bike when you test them?

[Rich82GS750TZ]

Is your vacuum tube from carb 3 (assuming VM) connected to the smaller nipple on the back side of the petcock? On the ON and RES petcock positions, the carbs should only pull fuel when engine is running. Set on PRI, the petcock should flow continuously.

[satoricycles]

I have been testing with the carbs off the bike mainly. I have been just dribbling the fuel into the line, so the pressure should only be gravity related. Off of Rich's picture, I have the vent tubes on the float bowl vents, but nothing on the vacuum or overflow. Do I need to plug the vacuum possibly since I am not connected to the tank?

[allojohn]

One thing to check is the float valve needle dropping out of the valve and jamming the float. There is a tang on the float next to the bowl post to adjust and limit movement of the float and prevents the needle from dropping out. HTH.
And no, you do not need to plug the vacuum line.

[satoricycles]

The needles seem to move fine when I hold the carb up and move the float with my hand. But, I will try to adjust them to not drop as far and see if that helps.

[Nessism]

The vacuum line can be left open when testing the carbs off the bike.

First thing I'd do is make sure the floats are not installed up side down. Yes obvious, but I've bought carbs off ebay that came that way before.

Next is to test fuel level in the bowls like shown in the factory service manual. A piece of clear tubing can be ground on the end into sort of a pencil point shape and then threaded into the drain hole threads.

One other thing to do is make sure the overflow tubes in the float bowl are not cracked. That's been know to happen. Also, make sure you have quality gaskets. Some of the cheap junky "carb kits" come with really poor gaskets. First things first though, measure fuel level.

[satoricycles]

I did do a double check on the floats to make sure the are on correctly. I am not getting fuel out of the overflow on he bottom of the bowls, so I think the tube are fine. The gas is coming out of the bowl gasket, so I am sure I need better gaskets, but I still think the fuel should shut off before it gets to the top of the bowl anyhow. I will try checking the fuel levels tonight to see if the float height needs more adjustment.

[satoricycles]

Update, I checked the fuel levels and all were at the ridge of the bowl, which seems to be in spec (4mm down from the carb body). I put the drain plugs back in and it starts leaking out the bowl gaskets again.... I did change the fuel source as well in case it was dirty gas, but still leaks. Any other suggestions before I buy another needle and seat kit?

[allojohn]

Did you buy Mikuni parts? If you bought a K&L kit, that may be the issue... K&L gear is considered very poor here.

FYI, Genuine Mikuni parts from JetRus. Looks like there may two options for your bike.

[roeme]

The bowl gaskets should not leak, at all. I've had the needle valve obstructed in my VM's, but the gaskets didn't leak – fuel got spit out of the overflow tube, like it should in that case.

Get good bowl gaskets first, you'll need them anyway, and it's way cheaper than a needle valve set. You can even cut out the gaskets by yourself from good (paper) gasket sheet, but the correct thickness eludes me right now.

Make sure the floats don't scrape on the gaskets. Rubber-type gaskets may shrink when exposed to fuel - not a problem in 90% of the use cases, but might become one when taking off the bowls repeatedly, or years after installation.

On the bench, while the carb bowls are off, you can manually close the needle valve. If they still leak, they're bad, and you surely need new ones. If they don't, they might be good – "might", because your finger has more force behind it than the floats.