Full Tank, Runs Great. 1/2 full not so good...

[StinkiestBink]

Hello Friends i have a nice one here. Lately i have been noticing that my 1981 GS650 will run differently depending on the amount of fuel in the tank. Mostly when i hit 65+ it will bog down and seems like it is running out of gas when the tank is 1/2 full or less. When i fill it up, it will run great 80-85 no problem. i am thinking that the added pressure of a full tank helps to drive the fuel down the line. Is that a petcock or vacum line issue?

Thanks.

[SPARKSS]

Sounds like an interesting problem for sure, considering that the fuel pickup should be coming from near the bottom of the tank. You might try the following to narrow it down: Drive it until it's running poor and then reach down and switch the petcock to the prime position and see if it improves or comes out of it (petcock issue). Secondly you might try stopping and removing the fuel cap briefly - was there any vacuum noticed when you did this (inability to vent).

Not much help here - just questions!

[StinkiestBink]

there wasnt any vacuum when i removed the cap to fuel up this morning, so it would seem to be venting fine. I have also noticed that when i set the petcock to prime, it barely trickles out. Should that be an unrestricted flow?

[Calvin Blackmore]

fule should be a slow steady stream form a propely wneted tank with the petcok on Prime

with cap off try it and schek the flow

disassemly of the petcock istn hard mine has gotten stuck int he past so i disassembled and lubed with motor oil

then cleaned nad it worked fine

[Steve]

Sounds like the problem is just before the petcock.

Drain the tank and remove the petcock. I would bet the screen is clogged. 8-[

With a full tank, there is enough pressure to push through the crud. You say it happens at 65+. What if you slow down with less than half a tank?


.

[StinkiestBink]

its fine a low speeds. i really havent tried just revving it out to see what RPM it is actually occurring at. Also, i do have an inline filter. Not sure if it is needed with the screen in the tank though. I will pull her apart this weekend.

[t3rmin]

its fine a low speeds. i really havent tried just revving it out to see what RPM it is actually occurring at. Also, i do have an inline filter. Not sure if it is needed with the screen in the tank though. I will pull her apart this weekend.

I'd never run without a filter on these old bikes. A little bit of tank rust will go a long way toward gumming up your carbs and a good in-line will catch a lot of that. But it may be time to change your in-line filter! Also as mentioned before you should pull off your petcock and inspect the screen and internal workings. If you only get a trickle on prime and you're sure the cap is venting properly then YOU DO HAVE A PROBLEM WITH YOUR PETCOCK.

[Steve]

I'd never run without a filter on these old bikes. A little bit of tank rust will go a long way toward gumming up your carbs and a good in-line will catch a lot of that.

A couple of issues come to mind here.
1) There is a screen on the petcock intake. That IS a filter. It should catch any tank rust.
2) An inline filter should be rated for gravity-fed system. Most automotive filters have thicker membranes because they are going into a fuel system that is fed by a pump that will force the gas through. Used on a gravity-fed system like ours, it may not flow enough. A good alternative is a filter for a lawn tractor.

.

[chef1366]

If you are going to use an inline filter make sure it's a gravity type like used on lawn mowers with 5/16 ends to flow the gas at the rate you need. If your tank is rust free lose the inline filter and as suggested earlier clean the petcock screen filter.
I personnally would just get a new petcock and forget about messing with such an old piece.
Steve you beat me to it.

[t3rmin]

The screen on the petcock is nothing compared to a real fuel filter. What's more the petcock screen is often torn or detached a bit. But even in perfect shape it can only be good for stopping the largest of particles.

I've had zero trouble with off-the-shelf "universal" type 5/16 fuel filters (is it Fram with the orange packaging?). If you have any doubt about the flow rate, hook one up to a funnel and pour some fuel through. It'll flow thick and steady just like out of the petcock. If you think these little motorcycle engines need more flow than that, you overestimate their fuel needs. Think of how long you can drive around on a tank of gas versus how long it takes to drain a tank on the bench through the petcock. Think of how long your bike will run on just the fuel in the bowls when you forget to hook the vacuum back to the petcock (whoops!). They just sip it. IMHO you'd have to really try hard to find a fuel filter that doesn't flow enough.

First thing I did after cleaning the carbs and before hooking the tank back up on my new 750 was install a filter. I'd rather ride than clean (and re-clean) carburetors! Cheap/easy insurance.

End of rant. ;-)

[nabrams]

While I'm all for in-line fuel filters, I think you should get a transparent one because they DO get clogged with gunk from a tank that's filthy. When it gets really bad, replace it. The transparent ones also let you see if the flow rate is sufficient.
I had an old Datsun 240Z that I used transparent fuel filters on just for this reason. Every few weeks or so the car would act like it was starving for fuel even though I had plenty (by the way, the problem was more pronounced at higher RPM, just as in your case - because of the higher flow rate needed).
As soon as I put the new filter in the problem would disappear (until that filter would clog several weeks later!!!!).
And yes, the petcock screen inside the tank DOES let in very fine silt from the tank.