Carb leaking - finally me as well

[wolfgangh]

Hi, I was often following threads of others about leaking carbs even after a rebuild. Amongst my 4 RD500, it never happened to me except for short leakages after a longer pause, which stopped within a minute or so when the needle got soaked in petrol and finally closed.
On my current rebuild - a 47X - I first only replaced the O-rings of the valve seats and the needles ( genuine MIKUNI parts), but I had random leakages. It was different carbs each time I opened the fuel tap. I then ordered 4 complete original YAMAHA sets of needle valves. Checked again float height of 24mm and fuel level with external tube method. At first attempt, all carbs were working OK, I was happy and turned around to other projects.
Yesterday, I started the bike again in the garage, and the left upper cylinder carb leaked like crazy, fuel even ran out of the carb mouth. Tapping with a screwdriver did not help. I turned off the fuel and went home. Today I tried again, this time the right upper cylinder carb leaked. Tapping did not help.

The bike was parked upright, not on the side stand.
I guess the only thing left as cause for this are the floats - either they get stuck when the bowl is empty, or they have leak and soak fuel, thus getting heavier and dont lift as much as they should.

Any other ideas for this?

[kpke]

I have no additional fix to offer. The Band-Aid I use is a secondary fuel shut off valve. But all this does is prevent the fuel from flowing when the bike sits. Not on the side stand like yours.

This summer right before leaving for the two-stroke extravaganza I went to start up my bike. It is typical once the petcock and secondary fuel valve are turned on that it will overflow a carb or two. Turn off the fuel, turn it on, turn it off, etc., and it will usually stop after a short bit. This time it didn't. So all four carburetors came off for float valve and seat cleaning. This worked. Put it back together, manually synchronize the carburetors, then vacuum synchronize the carburetors. This is not the first time. What a pain.

Good luck.

[wolfgangh]

Hi, I already did the fuel tap conversion for manual shut-off. Normally with my other RD´s it is as you described, it leaks for a few moments after a longer pause but then stops. This time is does not stop, and effects different carbs each time. I will check if the float bowls get stuck when the bowl is empty, and adjust the stops at the floats if needed. I though this only happens to others.

[rz500guytexas]

Post below

[rz500guytexas]

I have randomly had stuck float valves. I installed the ball valves Rick Lance made years ago due to the alcohol degrading the needles over time. Mine would still get stuck after travelling and sometimes after sitting for a while. Mr Rick suggested upon start up just crack the fuel petcock or shutoff. Reason was that the tank is so high up that the weight of the fuel slams the valves open causing them to stick. Try starting the bike with fuel trickling and after start the vibration of the engine will help keep them from sticking. It has worked most of the time.

Where I had all the issues originally was after transporting the bike in the trailer to the track. Every time I would try to get the bike started it would not run. What I was doing was loading the bike in the trailer and I would let it idle to get all the fuel out of the bowls. Pack up and leave. The problem was on transporting, the bike is solidly secured in the trailer so trailer bouncing would cause the float needles to bang in the seats and get stuck. Once stuck in full open they were a pain to get loose. Final solution has been leave the fuel in the carbs after shutting bike and tank off. Upon transport the fuel sloshes around keeping them loose. Then on arrival at track I try starting the bike, slightly open fuel petcock, no choke because the excess fuel has gone into the engine. If does not start a bit of choke, get it running, then slowly open petcock.

By the way did they send the correct needles? I know there are two sizes with the smaller one I believe used on carbs with a fuel pump. Go to the SUDCO web site and there float valves are on page 209.

[wolfgangh]

I just got an idea. I am currently running the engine with an external fuel bottle, which is positioned 50cm (or 1.5 feet or 20 inches) above the position of the original tank, This causes higher fuel pressure on the valves, and might be too much.
I will lower my bottle tonight and check again. I would assume that the fuel valves should be able to cope with this little extra pressure.

[rz500guytexas]

The aux tank is small so should not be that much weight of fuel. I use the aux tank on my TZR and has been ok. What about some lapping compound on a Q-tip and polish the seats. Maybe a burr or something. They are the brass ones or not?

[rd84]

Hi Wolfgang,

Have you checked your fuel tank for rust or other debris ?

Cheers
Paul