fuel leaking ?

[Wayne Meuir]

Got a problem with my 76 GT750 and need advice.

Bike had been setting up all winter without being ridden. Gas had gone bad, but there was not very much in it, so I filled it up with fresh gas and added about 2 ounces of Sea Foam, cranked it up and rode it about 7-8 miles and parked it. It ran OK but not perfect. Went out a few days later to crank it again, it rotated about one revolution and locked up and I smell raw gas. Pulled the plugs and kicked it over manually and squirted a bunch of raw fuel out the left plug hole. I tapped on the left float bowl with a screwdriver becasue I thought the float might be stuck. Put the plugs back in and it started but ran ragged for a few minutes and then smoothed out.

Couple of day later, I see a pool of gas on the floor under it, pop the fuel cap, and sure enough, a lot of the gas was gone, so I assumed the stock vacuum petcock was leaking so I pulled the fuel line off and put it in a bucket, filled the tank back up with gas. That was two days ago and it has not leaked a drop.

My assumption here is that the vacuum petcock is bad and that the needle and seat on the left carb is not seating properly. When I turn the petcock onto reserve, it pours fuel into the bucket.

Question is: Why does the petcock not leak into the bucket if it is really bad, and how did the gas get out of the tank if the petcock is not bad?

I don't mind replacing the petcock if it is bad, but that is a $110 mistake if nothing is wrong with it and I don't know how else to check it.

I am going to pull the carbs and clean them out and do the air jet mod anyway, so the carbs are not an issue.

Wayne

[Suzukidave]

Wayne , i would go right to the O ring on the vacuum side of the petcock , its letting gas get thru when the engine isnt running .

[Wayne Meuir]

Dave, that is what I thought as well, but why will it not leak into the bucket? I simply pulled the fuel line off and installed a longer fuel line and put it into a bucket on the floor. Has not leaked a drop.

I have not had time to take the petcock apart, and I am going out of town for about 10 days, so it's going to have to wait until I get back.

Wayne

[Craig380]

The vac petcock on my 380 did this a few years ago - after several years of me never thinking about it, it suddenly decided NOT to shut off properly and pissed fuel all over my garage floor.

I drained the tank and pulled the petcock apart, but the diaphragm and o-ring seemed fine. It's never done it since. I reckon a bit of crud from inside the tank had stopped the valve shutting off properly.

As I only use my bike at weekends, when I'm parking it up I tend to pull the main fuel line off the petcock just to check it's not passing fuel (if I remember to, that is )

[Wayne Meuir]

When I get back from vacation, I will pull it apart and see what I find. I may put an in line manual one on it for insurance.

Wayne

[Madbuffalo]

I may put an in line manual one on it for insurance.

That's exactly what I did a couple years back. The petcock had failed during winter storage so I had a crankcase full of fuel in the spring which I suspect also had something to do with the hardenned crank seals I replaced recently. The petcock and float valve have since been fixed but I just feel better having that manual shut-off too.

[Wayne Meuir]

OK, I finally got around to the Buffalo today and have a couple more questions:

1. I did the air jet mod. Do I need to put loc-tite on the new air jet or just screw it in. Right now, they are just screwed in, so I can take them out and loc-tite them if need be. Not sure if they might vibrate out or not.

2. The leaking petcock was a NOS petcock in 2007. I disassembled it today and I can find nothing that I can be sure is causing it to leak. The little 4 hole rubber baffle appears to be in good condition, there are no burrs on anything that I can see or feel, the lower bowl was clean and the seal appears to be OK, the screen in the bottom was clean, and the rubber seal that is on the outside of the movable lever does not have any visible damage. Some one said it might be the O-Ring, but I have not found an O-Ring, so help me understand what you guys are talking about. The seal on the outside of the lever sorta looks like and O-Ring until you take it out, then it almost looks like a lip seal.

I have an in-line manual cut-off ordered, but I would like to fix the stock vacuum petcock anyway if I can figure out what's wrong with it.

Wayne

[RING_DING]

I would put Loctite on those air jets ( I did). Imagine the mess if they did happen to vibrate loose!

[Suzukidave]

Wayne , when you pull apart the diaphragm side of the petcock you will find the O ring that can get a split in it or just hard from years of being in gas.... and when it does it will not stop the gas when the engine isnt running.

[Wayne Meuir]

Thanks, guys!

I will loc-tite the air jets. I thought it might be a good idea, just was not sure.

I have not taken the diaphgram side of the petcock apart. I was afraid I would screw up the diaphgram and I don't think it is available anywhere. I will do that in the morning.

Wayne

[Wayne Meuir]

Well, I pulled the diaphgram side off and changed the o-ring that fit on the nose of the diaphgram and put it all back together. Still have the same problem. Diaphgram appeared to be in good shape and I don't have any external leaks that I can see so far, but gas is still leaking from the fuel line outlet.

I got the o-ring at a hardware store and it was exactly the same OD, but appeared to be a little thicker cross section. I assumed it might have been because the old one had been compressed for more than 5 years. The old o-ring looked ok but was hard and brittle.


When I put the nose of the diaphtram with the new o-ring on it back into the body of the petcock, I was expecting it to fit fairly tight, but I really did not feal any resistance when I pushed it back in there.

What else is likely to cause this?

Wayne

[Suzukidave]

As far as i know there's only 2 ways the fuel gets thru the petcock .. past that O ring or thru the prime circuit . http://www.oldjapanesebikes.com/mraxl_G ... /sr71.html" onclick="window.open(this.href);return false;

[Wayne Meuir]

Dave, This is getting stranger by the minute. The new o-ring leaked like a seive for about 10 minutes and I left the shop for about an hour, went back and it was not leaking, so I just assumed that the gas had swelled up the o-ring, which is probably not good for a long term fix since that means the fuel is attacking the rubber.

I found another properly sized o-ring that was in an Athena gasket kit that I had left over from some earlier unknown project, but becasue it is an automotive kit, I assume it is fuel resistant. I replaced the one I put in there this morning with this one and it leaked again for a couple of minutes and so far has not leaked another drop. I am hoping the fuel that leaked out was just residual fuel that got trapped in the bowl when I took it apart.

I am going to watch it for a while before I put the tank back on the bike, but hopefully it is fixed.

I have another question for you though. Do you know if the petcock from a late 70's through early 80's GS (750. 850, & 1000 are all the same, I think) will work on a Buffalo? They look darn close to the same to me. The base where it mounts to the tank is exactly the same dimension (21mm wide X 67mm long X 44mm hole spacing with 6mm bolt holes). The only thing that is different that I can see is the location of the vacuum port. Reason I ask is that the GS petcock is about half the price of the GT one.

Wayne

[Suzukidave]

Its very possible .. only thing i can think of .. doesnt the GS valves have just 1 outlet so you would have to put together a 3 way splitter for the fuel lines .. still not hard to do and they should have enough fuel flow .

[Wayne Meuir]

Dave, My Buffalo is a 76 and has the BS40 Carbs on it, so there is only one fuel line outlet on the petcock already. I think the 72/73 had individual lines, but I am not sure.

Still not leaking as of a few minutes ago! I sure as heck hope the raw gas that got in my crankcase did not screw up the crank seals. I put new seals and bearings in the bike when I built it in 2007 amnd I don't want to ahve to do it again.

We are in the middle of one helluva rainstorm right now. It has rained 3 inches in the last hour and a half.

Wayne