1980 GS 750 not sucking fuel when hot

[Guest]

Hello all,
So I have an interesting problem that I have noticed lately on long rides. Bike starts up with no problem and I can usually ride her for a good half hour but after some time and when she really gets warm the bike stops sucking fuel and essentially starts running on fumes... I have come up with this conclusion because I have a nice size fuel filter that I installed and I can see the gas flowing through it... When bike is cold or has been sitting the fuel filter is full, I can start bike up and let it idle for 5 minutes and it stays full... I can ride it around town and if I stop I can see that its about half full and then after awhile I can see that the filter is empty and that when the bike starts running like **** and leans out and I have a hard time getting her started again.... I have also tested sucking on the vacuum line and that's not the problem... the problems seems to me like that when the bike is hot the carb no longer sucking the gas through the gas line almost something like a vacuum lock... Also it is not the gas cap it dies it both with the gas cap on and off... I cant see it being the piston and rings because I have 160-180 psi on each cylinder...

Any ideas?

[JEEPRUSTY]

Ditch the fuel filter and test

[ericeder141]

Did that once already and same problem... with the filter installed I can see the gas flowing nice and smoothly for basically the first 15 minutes then I can see the bike is sucking in less and less gas until eventually there is nothing in the line andbit in the bowls, just enough to keep it running.

[JEEPRUSTY]

Well for one thing the thing does not suck gas its a gravity fed system. If you bowls are going dry over time it may be some wonky vacuum thing witht he petcock.

[ericeder141]

I don't think its the petcock as I disconnected the fuel line while bike was running and it was dumping out gas... is there something that could cause the carb to build up pressure inside it and not allow the gas to flow through the gas line and into the carb bowl? I am thinking the more and more I read that its a vacuum issue somewhere between the carb bowl and the pistons....

[Steve]

As much as you might not like the idea, please re-consider your fuel filter. Where did you get it? What was it designed for?
Might seem like silly questions, but most fuel filters available at the auto parts stores are designed for ... cars, which have pressurized fuel systems. The filter elements tend to be rather restrictive because they are designed to filter out some really fine crud that might clog an injector. The few inches that gravity has a chance to pull on the fuel in our bikes simply does not have enough pressure to make it through the filter in sufficient quantity. Your best bet, if you really want to add the inconvenience of an external fuel filter, is to get one from Lowe's or Home Depot that is designed for a lawn tractor. They also have gravity-fed fuel systems.

There is nothing that will build up pressure inside a carb, especially if you have the vent ports open. Those are the Ts between carbs 1&2 and 3&4. There should be a hose attached to each one. The other end of the hose should end behind the airbox or under the seat, in relatively calm air.

Not sure where you came up with this "vacuum issue somewhere between the carb bowl and the pistons" idea, there is simply nothing there that makes sense. The engine is trying to run. The throttle butterfly is restricting it. THAT creates a vacuum, but only in the intake tube between the carb's throttle butterfly and the intake valve. Nothing makes it to the float bowl.

.

[cowboyup3371]

The filter that is already attached to your petcock inside the tank is all you need for your bike.

[Brendan W]

If I'm reading this right you have the same issue with the external fuel filter removed ?
How clean is the tank ?
If there is dirt in there it will gather on the petcock filter when running and can fall back when the flow stops.

[ericeder141]

Tank is actually very clean so I will try it without thefilter and see what happens. I am also going to replace the fuel line... maybe it collapsesonce the motor get hot and the hose softens up. Thanks for the responses guys iappreciate it. Hopefully i can get her running flawlessly again...

[JEEPRUSTY]

but you said you did try it without the filter. Or was that only at idle?

[ericeder141]

It was only at idle... this weekend im going to dig into it... its got to be something so simple im just overlooking it... ill start with no Filter and a new line... I believe its a 7mm fuel line... right?

[Steve]

You are correct, ... 7mm line. Might have to go to a VW dealer if you want to find some locally.

Just for reference, here is what you will find if you walk into the auto parts store: 1/4" line is 6.25mm, 5/16" line is close to 8mm.

.

[gustovh]

Clean the vent in your gas cap.

V

[JEEPRUSTY]

if he does both he will never know what the root cause was.
But then it could be combo of both.

[tkent02]

You are correct, ... 7mm line. Might have to go to a VW dealer if you want to find some locally.

Just for reference, here is what you will find if you walk into the auto parts store: 1/4" line is 6.25mm, 5/16" line is close to 8mm.

.

I think a foreign parts place should have it, if not any indy VW, Volvo, BMW, Mercedes or whatever shop will have it.