I went by my new to me bike this morning and there was puddle of gas under the bike. Fuel is coming out of a hose attached to the right side carb (right side carb is #4 I assume). I think this hose is a bowl vent hose. I don't know too much about these Mikuni carbs. Assuming that it is a vent hose, the other three carbs don't have this hose. Does that mean the right side carb vents all 4 bowls???
I assume incorrect float height or failed o-ring or debris or something is causing one of the bowls to overflow.
It also means that I have fuel flowing through the petcock, which isn't right. It looks like an OEM petcock, as far as I can tell, with a lever to switch from ON to PRI (prime???). I pulled the tank off and fuel flows from petcock regardless of which position lever is in. I assume I need replace/rebuild the petcock as well.
Appreciate your thoughts and guidance.
Let me guess....Store your bike with E10 fuel?
That hose is a choke vent hose and you should have 3 more, one on each carb....in addition you should have 2 larger fuel bowl vent hoses, one between carbs #1 and 2 and the 2nd between carbs #3 and 4 (6 hoses total)
Your float level is fine, you have gunk holding the float needle off the needle seat.
Yes, that petcock should shut off the fuel (ON position) when there is no engine vacuum. Time for a rebuilding kit from RPM: http://www.rpmracingca.com/proddetail.asp?prod=M%2FC%3AVacuumPetcockRebuildKit
Before you do the petcock work... try running a tank of fuel with Seafoam thru your bike....see if that cleans things out.
Don't leave E10 fuel in your bike.
Check your gas tank for rust and sediment.
Don't be tempted to install a fuel filter. Our gravity flow FJ's don't like the restriction in flow.
Clean and seal your tank...if needed.
Cheers
Pat
Not sure who but another user in the group removed the mesh screen on his side panels and installed a manual fuel valve right on the inside of the panel so he can sneak his fingers in to manually turn fuel on/off, might be worth considering...leaking fuel in your home/garage is scary stuff especially if you just pulled in from a ride and bike is hot. Probably one of the more economical options.
Quote from: 86FJNJ on August 25, 2025, 01:24:36 PM.....leaking fuel in your home/garage is scary stuff especially if you just pulled in from a ride and bike is hot.....
Absolutely true ^^^ especially if you have a water heater in your garage. When those gasoline vapors reach the level of the pilot light....:bomb: things will get real interesting.
The beauty of the 84/85 petcocks is the Prime (P) setting, where gas flows all the time. The later 86/87 petcocks don't have this P setting.
With the petcock set on Prime you can install a normally closed 12v fuel solenoid. The solenoid will open when the key is turned on and closed when the key is off....
I would rather have a fuel pump and a real honest to God fuel filter. My carbs will be happier.
Our tanks aren't getting any younger.
Also never, ever (over)fill the tank after a long ride and park it.
Quote from: Pat Conlon on August 25, 2025, 01:00:35 PMLet me guess....Store your bike with E10 fuel?
That hose is a choke vent hose and you should have 3 more, one on each carb....in addition you should have 2 larger fuel bowl vent hoses, one between carbs #1 and 2 and the 2nd between carbs #3 and 4 (6 hoses total)
Your float level is fine, you have gunk holding the float needle off the needle seat.
Yes, that petcock should shut off the fuel (ON position) when there is no engine vacuum. Time for a rebuilding kit from RPM: http://www.rpmracingca.com/proddetail.asp?prod=M%2FC%3AVacuumPetcockRebuildKit
Before you do the petcock work... try running a tank of fuel with Seafoam thru your bike....see if that cleans things out.
Don't leave E10 fuel in your bike.
Check your gas tank for rust and sediment.
Don't be tempted to install a fuel filter. Our gravity flow FJ's don't like the restriction in flow.
Clean and seal your tank...if needed.
Cheers
Pat
Thanks Pat. I just pick up this bike a week ago or so. It's quite possible that the previous owner left old E10 fuel in the tank. I removed the tanka and drained the fuel. I'll rebuild the petcock so that I can stop the free flow of fuel that is leaking everywhere, that's my first priority. After that, I can try sea foam or break into the carbs to clean them out. Yes one of previous owners installed a fuel filter between the tank and the carbs. Because of that bulky fuel filter, I can't route the fuel line from the tank to the carbs as per the drawing on the air cleaner assembly. And one of the fuel hoses is kinked. Should I just rempve the kicked hose and replace it without the fuel filter?
Regarding the carb vent hoses, the choke vent hose is missing on carbs 1, 2, and 3 and the fuel bowl vent hoses are also missing. How much of an issue are these missing vent hoses?
Paul, you need the carb hoses.
Think of it this way.... When the bike is running, the engine vacuum opens the petcock and the carbs are full of fuel, now if one of those other 3 carbs starts free flowing fuel, or you have a stuck float, do you want that fuel dropping down on the top of your hot engine....or would you rather have the fuel run through a hose and exit at the bottom of the bike?
Tip: I put number tags on the ends of small choke vent hoses corresponding to the carb# and L and R on the larger vent hoses. That way, if I see gas dribbling I know what carb is misbehaving.
Good on draining the tank, now wash it out (see any rust?) and clean the petcock inlet screen.
Remove the fuel filter. Follow the factory route for the fuel line, it's counterintuitive until you visualize how low the petcock outlet is positioned.
If you don't, as your engine heats up the fuel line softens and a pinch will occur.
Ask me how I know this...
So, 1) put all the carb hoses on, 2) remove the aftermarket fuel filter, 3) follow proper fuel line route 4) clean out tank 5) rebuild petcock, 6) run a tank or two of Seafoam.
If after a tank or two of Seafoam, if you still get gas dribbling out (onto the ground and not the top of your engine and now because your petcock is fixed, it only dribbles out while the bike is running) it will then be time to take your carbs off for cleaning and fresh o rings on the float needle seats.
Report back at that time....
Cheers
Pat
Quote from: 86FJNJ on August 25, 2025, 01:24:36 PMNot sure who but another user in the group removed the mesh screen on his side panels and installed a manual fuel valve right on the inside of the panel so he can sneak his fingers in to manually turn fuel on/off, might be worth considering...leaking fuel in your home/garage is scary stuff especially if you just pulled in from a ride and bike is hot. Probably one of the more economical options.
Yeah I did that. If you fold the wire mesh down on the top slot on the left side panel. You only notice if I tell you it's missing. And you can fold it back if you change out your petcock. I had RPM install their pingel petcock when I was out there for the Cali rally. Going to install one on my 92. Just cuz the stock one is so hard to turn off n on when removing the tank.
The first trick to try with a suspected stuck float is with the bike on the center stand use the handle of a large screw driver and give the offending carb a few good whacks....It's saved me once or twice from chasing my tail. :crazy:
Yup, did that already, but didn't work for me.
I forgot to mention...check your crankcase oil level, do a sniff test, to be sure fuel didn't get past your piston rings and corrupt your oil.
Quote from: Pat Conlon on August 25, 2025, 10:15:37 PMPaul, you need the carb hoses.
Think of it this way.... When the bike is running, the engine vacuum opens the petcock and the carbs are full of fuel, now if one of those other 3 carbs starts free flowing fuel, or you have a stuck float, do you want that fuel dropping down on the top of your hot engine....or would you rather have the fuel run through a hose and exit at the bottom of the bike?
Tip: I put number tags on the ends of small choke vent hoses corresponding to the carb# and L and R on the larger vent hoses. That way, if I see gas dribbling I know what carb is misbehaving.
Good on draining the tank, now wash it out (see any rust?) and clean the petcock inlet screen.
Remove the fuel filter. Follow the factory route for the fuel line, it's counterintuitive until you visualize how low the petcock outlet is positioned.
If you don't, as your engine heats up the fuel line softens and a pinch will occur.
Ask me how I know this...
So, 1) put all the carb hoses on, 2) remove the aftermarket fuel filter, 3) follow proper fuel line route 4) clean out tank 5) rebuild petcock, 6) run a tank or two of Seafoam.
If after a tank or two of Seafoam, if you still get gas dribbling out (onto the ground and not the top of your engine and now because your petcock is fixed, it only dribbles out while the bike is running) it will then be time to take your carbs off for cleaning and fresh o rings on the float needle seats.
Report back at that time....
Cheers
Pat
Thanks again Pat. That's sounds like a good plan. And I'll install the missing hoses on the carbs - that makes sense. Do you know the size of the two larger float bowl vent hoses?
And I'll check the oil level and smell. Only thing I'm struggling with right now is finding a petcock rebuild kit. RPM is out of stock and I need it shipped to Canada.
We ship to Canada and we got 5 kits in this morning.
.......and Robert for the score! :good:
I think I used 5/16 for the 2 float bowl vents. Run them along with your choke vents down the back side of your engine, not too long mind you.... just long enough to get the fuel off the engine.
One time in band camp, I ran my hoses too long and the end of one of my float vent hoses melted shut from the tip touching the exhaust collector...... I had a hard time figuring out why all of a sudden my bike was surging at freeway speeds. Those float bowls gotta breathe, they get cranky when they can't....
Yep I had to drain my oil in the parking lot of a hotel. On a ride around the lakes up through the U.P. and skip breakfast. And catch up to the group. Easy enough all Harleys. Luckily the grounds guy for the hotel was was there to take the oil and new drain pan. The rapping on the float bowls worked that time. But the crankcase was already full.
New fuel supply and carb vent hoses ordered, as well as petcock rebuild kit. I flushed fuel tank and checked for rust/debris - none found, looks really clean as far as I can see. I haven't checked my crankcase oil for fuel contamination yet but I will.
Waiting for parts to arrive and will report back.
Update
Parts finally arrived and I got some time to look into this again. Installed new choke vent lines on all carbs. Installed 2 new float bowl vent hoses as well. Replaced fuel line from petcock to carbs with new. Routed as per diagram and removed fuel filter that previous owner had added. Installed new diaphragm and spring in petcock. Petcock now holds closed unless vacuum is applied. Tank was flushed and is free of debris.
Put everything back together and started bike. Bike started up nicely but fuel started leaking badly out of choke vent for carb #1 and #4, maybe 2 or 3 too - it was hard to tell, fuel was gushing out, had to stop bike immediately. Fuel stopped leaking a couple minutes after I stopped the bike - so petcolk seems to be doing its job. So I guess my next step is to clean and rebuild the carbs.
It seems very coincidental that I would have a bad needle o-ring or stuck float on two (may more than two) carbs at the same time. I'm wondering if I missed something when I put it back together. One thing I wasn't sure about is the hose on bottom of the gas tank. I suspect this is a vent hose for the tank? Anyhow, I couldn't remember if it was connected to anything so I routed it down the back of the engine along with the carb vent hoses.
My money is on dry leaking o rings on the float needle seats.
Very common.
Update #2. Not as much progress as I would like.
Installed new float needles, needle seats and o-rings in carbs 1-3. For carb #4, couldn't get float pin out without breaking something, so that carb did not get a new needle and seat. Cleaned rest of carbs and found very little debris or varnish in there. Floats looked high but left as is. Put back together and tested on bench to see if carbs would overflow. Carbs #1 and 2 immediately overflowed. Took these apart and noticed the floats from the rebuild kit had a different orifice size. I don't think that's critical but I decided to put the old seats and needles back into carbs 1-3 with new o-rings. I also adjusted the float level lower on carbs 1-2. Put back together and bench tested and it did not leak. Installed carbs on bike with a temporary gas tank. Started bike up and it ran fine.
I then replaced the engine oil as it has been contaminated with some fuel. Tested again with a temporary gas tank and carb #3 overflowed immediately. Seemed to seal itself, so I started up the bike and again it ran fine, and carbs did not overflow. Ran bike out of fuel and drained carbs as I will be away for a while and bike will sit for several months.
When I'm back (in the New Year), I'll look into those float levels again and adjust lower as they are still higher than spec. And I'll find a new float and pin for carb #4 so that I can cut out the one that's stuck in there. Lastly, the gas tank petcock, that I replaced the diaphragm on, was holding for a while but started leaking again. I'll need to dig into this one again in the spring. All in all, quite frustrating.
Call it a day with the oem 40 year old petcock and buy a new one.
There's other things in life to worry about....those wonky vacuum petcocks ain't one of them.
Quote from: Pat Conlon on October 08, 2025, 02:41:33 PMCall it a day with the oem 40 year old petcock and buy a new one.
There's other things in life to worry about....those wonky vacuum petcocks ain't one of them.
True. But those petcock's aren't cheap. Are there any aftermarket ones that would fit that bike?
Yes, RPM sells a non vacuum Pingel petcock with an adaptor plate that fits our tanks but you need access in your left side cover to get your fingers in under the tank to move the fuel lever to the off and on position.
I have heard folks take the screen out of the side cover vent to do this.
I recall Dan aka: Giantkiller mentioning this.
(http://www.rpmracingca.com/prodimages/large/M%20C%20FJPetcock-1.jpg)
http://www.rpmracingca.com/proddetail.asp?prod=M%2FC%3AFJPetcock
$225.00 for the manual petcock vs. $190 for the oem Yamaha vacuum petcock, I know which I would choose.
Quote from: Pat Conlon on October 11, 2025, 06:59:58 PMYes, RPM sells a non vacuum Pingel petcock with an adaptor plate that fits our tanks but you need access in your left side cover to get your fingers in under the tank to move the fuel lever to the off and on position.
I have heard folks take the screen out of the side cover vent to do this.
I recall Dan aka: Giantkiller mentioning this.
(http://www.rpmracingca.com/prodimages/large/M%20C%20FJPetcock-1.jpg)
http://www.rpmracingca.com/proddetail.asp?prod=M%2FC%3AFJPetcock
$225.00 for the manual petcock vs. $190 for the oem Yamaha vacuum petcock, I know which I would choose.
Thanks Pat. I agree, that makes no sense.