Mikuni Carburetor probs on an '85 FJ600 - Motorcycle-Journal Forums

SatanMonkFJ600 · 08-03-2008, 10:07 PM

Hello everyone. I have a question about my bike. I have a 1985 Yamaha FJ600, I bought it off some people who had stored it 2 years with the same fuel in it. I pulled the carbs and cleaned them, checking all my gaskets, needles, seats, floats etc etc.
Put it all back together and I the bike flooded out. After some messing around with no luck, pulled the carbs again this time going over them with a fine tooth comb. (btw when I clean my carbs I do NOT use wire or anything harmful, only cleaner and low psi air) put them back in and the bike was still flooding out, now out the air box drain.
I found that the fuel cap was creating a lot of pressure in the tank so I took it off and it seemed to solve the problem. So I dismantled the fuel cap to check the vent hole, unplugged it and re-assembled it. Rode it for about 1 hr, and noticed that my cold idle was about 1200 rpm but after the hr of riding it was now about 3400 rpm. (I figured I must be getting too much fuel again) I shut it off and left it for 2 days.
Now I just went to start it today and fuel came GUSHING out the air box hose. (Now I know it's not the fuel cap) Shut it down and tore it all down. I noticed before that my tank, even with the valve on “On” or “res” still leaked fuel, with no vacuum hose on. So this time I pulled it apart and cleaned it, put it back on and it still leaked. (oh well)
Tore down the carbs again and spent about 2 hrs inspecting each needle jet and float. Everything seems to be perfect, even moving the float up and down there is 0 hesitation on the needle. Cleaned up the bike put the carbs back in hooked up all the hoses, put limited fuel in the tank, and when I went to start it, again fuel came gushing out the air box.
I'm looking into getting a fuel petcock rebuild kit, (because the flange that controls the vacuum valve IS kinda chewed and crappy and bothers me) but I know that this sounds like a float / needle problem to me. (BTW I'm going to try a small test tomorrow to see if I can find out what one(s) are in fact flooding out.)
So any ideas that may help?

Thanks in advance.

P.S. Sorry about the long story, but I thought I'd answer all obvious questions now.

Aussie Steve · 08-03-2008, 10:15 PM

Had a similar problem on my B12 # four carb suddenly began to flood. I ended up bending the float tab up a fraction to increase pressure on the float needle. This was only supposed to be a temp fix but I am still riding six months later. I inspected the needle when I had the carb down and could see no wear. You do need to identify which carb/s is/are the culprit/s.

As far as I am concerned even if the fuel valve is faulty the floats should shut off the flow.....so check out the float height too.

Good luck

SatanMonkFJ600 · 08-03-2008, 11:02 PM

Thank you, I was thinking about this.. sort of. I know I can do a float fuel level test, thought that I would try bending (very little) the float tab if one is over filling

Aussie Steve · 08-04-2008, 04:32 AM

Yes try that but remember that a 1mm rise or lowering of float level is attainable by a tiny bit of bending at the tab and that kind of change can affect high speed running.

Good luck.

SatanMonkFJ600 · 08-04-2008, 12:50 PM

Ok, well I did my test to check my floats and needles by putting the carbs on my garage floor connected to the gas tank above and turning it to prime (pri) and waited to see which carb started leaking.. None. So I scratched my head and gently starting moving the needle assembly up and down found that 3 of the 4 needles had good vacume and 1 did not. So I dissmantled the top of it to find my diaphram was bent back, so I stuck it proper, re-assembled the top and finger tested it again with good results. (not believing that this could be a cause, but you never know) re-assembled the carbs back into the bike and started. Started good, ran for about 20 seconds then the fuel started out the air box again. Now I'm REALLY stumped. ANY ideas at this point would be very helpful (besides selling the bike)

Thanks in advance.. again.

SatanMonkFJ600 · 08-04-2008, 05:17 PM

OK, found some of my problem? Maybe? I pulled back the air box and started the bike, seems the fuel was dumping out of my crank case. So I dumped out the oil / gas. Replaced it with fresh oil / filter. Started just fine, then started "jumping" when I let out on the clutch in first gear. Sigh.. I might just have to break down and take it to my local stiffy in the butt Yamaha dealer.

08-03-2008, 10:07 PM	#1 (permalink)
SatanMonkFJ600 Newbie Joined: Aug 2008 Bike: FJ600 Posts: 4	Mikuni Carburetor probs on an '85 FJ600 Hello everyone. I have a question about my bike. I have a 1985 Yamaha FJ600, I bought it off some people who had stored it 2 years with the same fuel in it. I pulled the carbs and cleaned them, checking all my gaskets, needles, seats, floats etc etc. Put it all back together and I the bike flooded out. After some messing around with no luck, pulled the carbs again this time going over them with a fine tooth comb. (btw when I clean my carbs I do NOT use wire or anything harmful, only cleaner and low psi air) put them back in and the bike was still flooding out, now out the air box drain. I found that the fuel cap was creating a lot of pressure in the tank so I took it off and it seemed to solve the problem. So I dismantled the fuel cap to check the vent hole, unplugged it and re-assembled it. Rode it for about 1 hr, and noticed that my cold idle was about 1200 rpm but after the hr of riding it was now about 3400 rpm. (I figured I must be getting too much fuel again) I shut it off and left it for 2 days. Now I just went to start it today and fuel came GUSHING out the air box hose. (Now I know it's not the fuel cap) Shut it down and tore it all down. I noticed before that my tank, even with the valve on “On” or “res” still leaked fuel, with no vacuum hose on. So this time I pulled it apart and cleaned it, put it back on and it still leaked. (oh well) Tore down the carbs again and spent about 2 hrs inspecting each needle jet and float. Everything seems to be perfect, even moving the float up and down there is 0 hesitation on the needle. Cleaned up the bike put the carbs back in hooked up all the hoses, put limited fuel in the tank, and when I went to start it, again fuel came gushing out the air box. I'm looking into getting a fuel petcock rebuild kit, (because the flange that controls the vacuum valve IS kinda chewed and crappy and bothers me) but I know that this sounds like a float / needle problem to me. (BTW I'm going to try a small test tomorrow to see if I can find out what one(s) are in fact flooding out.) So any ideas that may help? Thanks in advance. P.S. Sorry about the long story, but I thought I'd answer all obvious questions now.

08-03-2008, 10:15 PM	#2 (permalink)
Aussie Steve U.B's LoveChild Joined: Jan 2007 Bike: 02 1200 Bandit Location: Western Australia Posts: 1,115	Had a similar problem on my B12 # four carb suddenly began to flood. I ended up bending the float tab up a fraction to increase pressure on the float needle. This was only supposed to be a temp fix but I am still riding six months later. I inspected the needle when I had the carb down and could see no wear. You do need to identify which carb/s is/are the culprit/s. As far as I am concerned even if the fuel valve is faulty the floats should shut off the flow.....so check out the float height too. Good luck __________________ Just remember you ARE entitled to my opinion!!!!

08-04-2008, 04:32 AM	#4 (permalink)
Aussie Steve U.B's LoveChild Joined: Jan 2007 Bike: 02 1200 Bandit Location: Western Australia Posts: 1,115	Yes try that but remember that a 1mm rise or lowering of float level is attainable by a tiny bit of bending at the tab and that kind of change can affect high speed running. Good luck. __________________ Just remember you ARE entitled to my opinion!!!!

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08-03-2008, 11:02 PM	#3 (permalink)
SatanMonkFJ600 Newbie Joined: Aug 2008 Bike: FJ600 Posts: 4	Thank you, I was thinking about this.. sort of. I know I can do a float fuel level test, thought that I would try bending (very little) the float tab if one is over filling

08-04-2008, 12:50 PM	#5 (permalink)
SatanMonkFJ600 Newbie Joined: Aug 2008 Bike: FJ600 Posts: 4	Ok, well I did my test to check my floats and needles by putting the carbs on my garage floor connected to the gas tank above and turning it to prime (pri) and waited to see which carb started leaking.. None. So I scratched my head and gently starting moving the needle assembly up and down found that 3 of the 4 needles had good vacume and 1 did not. So I dissmantled the top of it to find my diaphram was bent back, so I stuck it proper, re-assembled the top and finger tested it again with good results. (not believing that this could be a cause, but you never know) re-assembled the carbs back into the bike and started. Started good, ran for about 20 seconds then the fuel started out the air box again. Now I'm REALLY stumped. ANY ideas at this point would be very helpful (besides selling the bike) Thanks in advance.. again.

08-04-2008, 05:17 PM	#6 (permalink)
SatanMonkFJ600 Newbie Joined: Aug 2008 Bike: FJ600 Posts: 4	OK, found some of my problem? Maybe? I pulled back the air box and started the bike, seems the fuel was dumping out of my crank case. So I dumped out the oil / gas. Replaced it with fresh oil / filter. Started just fine, then started "jumping" when I let out on the clutch in first gear. Sigh.. I might just have to break down and take it to my local stiffy in the butt Yamaha dealer.

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