The last few times I rode my GS500F (2004), the engine lost power on full throttle, which was very noticeable when going uphill. At first I thought that it was a dirty fuel filter, or muck in the fuel lines, clogging the needle and seats in the carbs.

Today, I decided to tackle the problem and took off the tank. Upon inspection, the aftermarket fuel filter turned out to be spotlessly clean. Then I turned my attention to the frame petcock. I disassembled the front handle and it looked fine and clean inside, so I assembled that part again.

Then I loosened the 5 screws holding the diaphragm cover on and carefully removed the cover and diaphragm. There was some dirt and corrosion around the "piston" and rubber O-ring. I cleaned it off by gentle scraping and blowing and finished by putting half an earbud in the Dremel and gently buffing the "port" where the O-ring seals and the 3 "cylinder" protrusions with a little Brasso on the earbud.

After assembly, a quick blow test confirmed that the O-ring seals when it is supposed to, and opens when there is vacuum from the engine - easily replicated by creating vacuum with a syringe attached to the vacuum tube.

I took the bike out for a test ride and the problem has gone away. I hope this helps someone else with a similar problem.

I suppose I should crack open my vacuum petcock and see if I can fix it like this without a rebuild kit. Of course my problem was the other way around, fuel leaking into the vacuum line. For my antique bike, rebuild kits are not really available.

If you're careful, I don't think that you'll cause any more harm than there already is. Maybe you'll even notice what the problem is. A torn diaphragm maybe.

yeah a local old-bike guru suggested making a diaphragm out of a piece of inner tube. Could work. Can't hurt. It's junk at the moment.

SK, do you have anyy pictures of opening the petcock?

I tried once and stopped quite rapidly as it seemed all the fasteners were frozen.

Sorry, man. I knew that I should have taken pictures, but I didn't.

Tell you what... I'll soon be replacing the fuel lines with fancy green transparent tubing from Italy, then I'll open the petcock again and take pictures. Give me a few days.

The screws came out easily for me. Maybe try some penetrating oil on yours. Be careful with the diaphragm when you take the cover off. Lift the cover just a little and use a small flat screwdriver to loosen the diaphragm all around, otherwise you might tear it.

As promised, some pics.

The 5 bolts on the diaphragm cover should loosen fairly easily - mine did. If stuck, let a little penetrating oil seep in for an hour or so and try again. Use a good fitting star screwdriver.


Carefully separate the diaphragm from the petcock body. I ran a small screwdriver all around before pulling the cover and diaphragm off the main body.


Clean buildup and corrosion as well as you can. I scraped and wiped and this is the result. Not bad after 18 years.
Pay attention to the 3 "pillars" around the hole in the center. They form a cylinder for the "piston" to move up and down. I used a Dremel to spin half an earbud dabbed with Brasso to get the cylinder surfaces nice and shiny.


Another view on a cylinder pillar. See how shiny it is after some brief polishing with the earbud.
Also note that the rim of the hole is pitted badly. It doesn't seem to make a difference as the rubber O-ring still seems to seal well enough.


I ran the spinning earbud all around the piston and O-ring. It's much cleaner than it first was.
Also, carefully inspect the diaphragm for tears. Mine is still in good shape.


The 2-year old black fuel hose is already cracked at the ends. I decided to upgrade to a superior product.

nice thanks!!!

Thanks for your post.

The 2-year old black fuel hose is already cracked at the ends. I decided to upgrade to a superior product.

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How do you find the Ariete fuel line? I had one on my bike when I bought it (the reserve). Compared to the standard on the main, it was hard and starting to crack and difficult to route. Maybe it was just old, but I replaced it with standard. How are they when new?

The continental rubber lines are tough as nails, but they are too small. They really will never ever die if you fit them to the roght size fitting.
The other clear/tinted tygon etc etc type lines dont like sun exposure, so put some type of opaque sleeve over the parts exposed to sun and they'll last a lot longer. Ofcourse they also sell for about $1 a foot, so just replace them if you have a stash.
Cool.
Buddha.

Quote from: dropbear on February 12, 2022, 07:05:04 PM

Thanks for your post.

How do you find the Ariete fuel line? I had one on my bike when I bought it (the reserve). Compared to the standard on the main, it was hard and starting to crack and difficult to route. Maybe it was just old, but I replaced it with standard. How are they when new?

New Ariete is quite soft and easy to work with. Time will tell how it holds up. I haven't used it before.

Quote from: The Buddha on February 15, 2022, 08:10:34 PM
The continental rubber lines are tough as nails, but they are too small. They really will never ever die if you fit them to the roght size fitting.
The other clear/tinted tygon etc etc type lines dont like sun exposure, so put some type of opaque sleeve over the parts exposed to sun and they'll last a lot longer. Ofcourse they also sell for about $1 a foot, so just replace them if you have a stash.
Cool.
Buddha.

You're quite right. I bought the Continental fuel lines too small. Should have been at least 7mm - if they even make that size.

As to your comment about sun exposure, on the GS we don't have that problem because all the fuel lines are hidden.

Quote from: SK Racing on February 15, 2022, 10:10:37 PM

As to your comment about sun exposure, on the GS we don't have that problem because all the fuel lines are hidden.

Not quite. The 3" of reserve line and the 2" of the main line that are open to sun will craze and disintegrate while the rest of the line hold up great. How do you think I figured out they dont like the sun ?
Cool.
Buddha.

Quote from: The Buddha on February 16, 2022, 10:22:20 AM
Not quite. The 3" of reserve line and the 2" of the main line that are open to sun will craze and disintegrate while the rest of the line hold up great. How do you think I figured out they dont like the sun ?

You're quite right about that for the stock bike. I forgot that I've rotated the petcock 90° clockwise on my build. The fuel lines now exit forwards and out of sight - not downwards anymore like on the stock GS, so they are completely hidden from the sun.




And on a stock bike, some fuel line is exposed to the sun.

Another view of the petcock on my bike, showing how little of the fuel lines is visible. No sun will ever reach there, so I guess I'm fine with the transparent hoses.