I've had an issue with my Mikuni carb for some time now where forgetting to shut off the fuel petcocks almost guarantees that when I return, fuel will be dripping from the carburetor overflow. The problem is intermittent. Sometimes it won't leak at all (rare), sometimes it takes a few hours, and sometimes it will begin within a few minutes of shutting down. Normally I turn the valves off after a ride and sometimes I forget, but lately I've been leaving them on intentionally trying to figure out what's going on.
I inspected the float needle and saw no problem. Blowing air by mouth into the fuel inlet with the float up also reveled no leakage. I moved the bike from shade to full sun, thinking maybe the gas tank was not venting properly, causing it to pressurize with the rising temperatures. It didn't leak.
Thinking perhaps I was missing something, I sent the carb to a reputable and well known Mikuni technician, who overhauled it. In the process he replaced both the float needle and seat.
When I got the carb reinstalled on the bike, I intentionally left the petcocks open. With the bike sitting in the garage, several hours went by with no leakage. Finally, after about five hours, I returned to find a small puddle of fuel under the bike. Damn! It doesn't necessarily take five hours either. This morning, after running the bike, fuel was dripping from the overflow within 10 or 15 minutes of shutting down.
Now I know you're supposed to shut off the fuel petcocks, but I'm beginning to wonder if this behavior is common with the Mikuni. Do any of you with Mikuni's leave your petcocks open? Do you get overflow?
The only thing I've changed with the fuel system on this bike is I've replaced the fuel lines and both petcocks. The old ones had very anemic fuel flow so I replaced them with a modern set that have a 90 degree outlet which allowed for much neater and shorter fuel lines. My fuel flow is most excellent. Could it be too good?
-John
I inspected the float needle and saw no problem. Blowing air by mouth into the fuel inlet with the float up also reveled no leakage. I moved the bike from shade to full sun, thinking maybe the gas tank was not venting properly, causing it to pressurize with the rising temperatures. It didn't leak.
Thinking perhaps I was missing something, I sent the carb to a reputable and well known Mikuni technician, who overhauled it. In the process he replaced both the float needle and seat.
When I got the carb reinstalled on the bike, I intentionally left the petcocks open. With the bike sitting in the garage, several hours went by with no leakage. Finally, after about five hours, I returned to find a small puddle of fuel under the bike. Damn! It doesn't necessarily take five hours either. This morning, after running the bike, fuel was dripping from the overflow within 10 or 15 minutes of shutting down.
Now I know you're supposed to shut off the fuel petcocks, but I'm beginning to wonder if this behavior is common with the Mikuni. Do any of you with Mikuni's leave your petcocks open? Do you get overflow?
The only thing I've changed with the fuel system on this bike is I've replaced the fuel lines and both petcocks. The old ones had very anemic fuel flow so I replaced them with a modern set that have a 90 degree outlet which allowed for much neater and shorter fuel lines. My fuel flow is most excellent. Could it be too good?
-John