Trouble starting 79 GS750L

[Markur]

IMG_6692.jpg
I am trying to help a friend whose husband died less than 2 years ago sell (get running) this bike that he owned. Only has 15K miles out and it is in pristine condition. Apparently it started 3 months ago and had new gas at that time. I heard it running in a video she recorded and sounded fine. I was interested in purchasing for $1000 except for this issue. It cranks over and sounds like it wants to start so battery is good. My thoughts are perhaps it’s the spark plugs need replacing or the fuel pump has gone out. either that or the carbs need cleaning with new gaskets, etc. one thing that is apparent, is the lack of gas showing up in this fuel filter when I try to start it. I’m thinking it might be the petcock lever/pump that has gone out and it’s not allowing fuel to flow? Any thoughts and an easy way to test this theory?

[Brendan W]

The external filter makes me think the petcock is not original which would have had an integral filter in the tank.
There shouldn't be a fuel pump. Those bikes are gravity fed.
If the external filter is a pressurised fuel system type it won't flow enough for a gravity fed system.
If the petcock is the original type it is vacuum operated with a PRI prime bypass.
Set it to PRI and see if gas flows.
The other positions are ON and RES. If the level is low it will only run on PRI or RES ( assuming the vacuum is okay)
Check that the vacuum hose ( the smaller one) is connected at both ends and not kinked or blocked.
Fifteen months could be long enough to allow water to drop out of the fuel collecting in the float bowls.
It might be worth draining the float bowls into a glass jar to see what's happening there.
Open the fuel filler as well just in case it has a blocked vent preventing fuel running.

[rphillips]

If it was running fine 3 mo. ago, there can't, or shouldn't, be a carb. problem. For sure, no fuel in clear filter, no fuel to carbs. I never liked ad on filters on gravity flow bikes, from what I've seen they cause more problems than they fix... Haven't seen it but running good 3 mo. ago?/ $1K... I'd say go for it, especially if it's the champagne color.

[Markur]

The external filter makes me think the petcock is not original which would have had an integral filter in the tank.
There shouldn't be a fuel pump. Those bikes are gravity fed.
If the external filter is a pressurised fuel system type it won't flow enough for a gravity fed system.
If the petcock is the original type it is vacuum operated with a PRI prime bypass.
Set it to PRI and see if gas flows.
The other positions are ON and RES. If the level is low it will only run on PRI or RES ( assuming the vacuum is okay)
Check that the vacuum hose ( the smaller one) is connected at both ends and not kinked or blocked.
Fifteen months could be long enough to allow water to drop out of the fuel collecting in the float bowls.
It might be worth draining the float bowls into a glass jar to see what's happening there.
Open the fuel filler as well just in case it has a blocked vent preventing fuel running.

Thanks Brendan for your info. I will look into the vacuum hoses and such.