Overflow/Petcock issues with my 1978 GS750 (First Post)

[alkswheetinjur]

Hello GS Resources,



I have been lurking this forum since I first saw the add for my 1978 GS750 posted on craigslist last spring. Since then with the a lot help of you guys I have been able to turn out a pretty neat little cafe project. Always something new to learn here. Almost every issue I came across somebody else had already experienced and posted about. I have had my bike up and running strong for a few months now, never having any issues other than little things. (leaky tach drive and drain plug) I ride mostly on weekends and I am very please with what this 38 year old machine can do.

Now for my problem(s?). I was out cruising around last weekend when I had to stop for fuel. While pumping my gas I saw a puddle form under the bike. I realized it was fuel leaking out of the overflow of carb #4 (not just a trickle but a pretty steady stream) . I knocked on the bowl a few times to free up the float and hammered down back to my place a few miles away. Every stop sign and intersection where I had the chance to look down I saw the puddle reappear. I somehow made it back to my garage with some fuel left in my tank and without turning into a fireball.
Engine off, I opened up the drain plug on carb #4 so I could drop the bowl and see what was jamming the float up. The gas never stopped flowing from the drain plug so I pulled the fuel line for the petcock and saw that it was never losing flow (steady stream in all positions). After draining all the fuel from my tank I was able to pull the bowl on carb #4 and clean my floats and needle and get everything moving smooth. Got it back together added a little fuel, fired it up and now carb #1 is overflowing.


I am assuming my petcock is shot but would this cause the carbs to overflow or just a coincidence? Everything seemed to be in excellent order up until stopping at that gas pump. Also not sure if related but the petcock also seems to have some what of a yellowish oxidation. I believe the carbs are Mikuni VM26SS with K&N pods and the petcock is the vacuum three position (RES,ON,PRI)

Thanks in advance, sorry for any ignorance.

 

[chuck hahn]

Recheck the float hts across the board. When the bowls are off, hold the carbs upright and lift the floats and observe that the float needles rise and fall without fail and there isnt any binding when you lift the floats from fully at the bottom..follow me???

EDIT..yes it sounds like the petcock is junked. As for the drain never stopping the flow..do the float rechecks for jamming especially as the floats hang all the way down with the bowls off and holding the carbs upright. slowly lift the floats and carefully feel if there is ANY resistance to your lifting them.

If there is, then youll see a stop tab on the hinge of the floats that hits one of the float posts. This tab hits against the float post to prevent them from dropping too far..which causes the float needles to bind against the hinge instead of working up and down smoothly. Bend the tab toward the post and recheck float movement till they are no longer binding on the float needles.

 

[chuck hahn]

service manual here.

http://zeus.mtsac.edu/~cliff/storage/gs/GS750_Service_Manual_early8valve.pdf

 

[alkswheetinjur]

Thanks for the manual. Do you think there is any correlation between the broken petcock (constant flow) and the floats needing adjustment or does that not make any sense?

 

[chuck hahn]

my impression is its a little of each. Granted Im not there looking at them, but the floats should stop the flow even if the petcock goes away. Now , the drain may still drip just a little if the floats are a bit wek and seeping. But , as I took what you wrote, they are steadily running out the drains and the flow doesnt slow down any....was I reading that right???

 

[chuck hahn]

Probaby a needle jamming against the float hinge is what I would suspect right off. Thats why i said check the stop that rests against the float post and feel the float isnt getting hung up as you work it from fully dropped down and up to close off the needles.

 

[alkswheetinjur]

The initial leak from carb #4 would pour out pretty steady until I knocked the bowl. I tore it down and got the float and needle moving smooth with no hang ups. Then Carb #1 started leaking after I got everything back together. Carb #1 would trickle a little then stop on its own regardless if the bike was running or not. Haven't torn it back apart yet thinking there may be more to the issue. Might be as simple as some debris breaking free when I fueled up. I'll check it out tomorrow if I get the chance. Thanks for the advice.