Petcock Repair Woes - Rebuild Kid didn't work or I screwed up

[buickyouth]

1978 Yamaha XS750SE

Problem: One of the petcocks was leaky, about two drips per second.

Attempted Solution: Rebuild petcocks.

Application: Drained tank, took petcocks off tank, disassembled (noting of what goes where and how it all fits together), reassembled with the new pieces from the rebuild kit, attached petcocks to tank, reattached fuel lines, filled with gas. All seemed right. No leaks. This was two weeks ago.

What happened next: I was getting a lot of front brake drag and decided to rebuild the front master cylinder. I rebuilt the MC last night and reinstalled it on the bike this afternoon. After bleeding the brakes I figured I should take it around the block and check the brakes.

No brake drag. But the bike was having a hard time idling at intersections and it ultimately died and I couldn't get it to start again (neither push button or kick start).

Then I saw the gas dripping on the ground.

All the hoses were on securely and it looked like the petcock / gas tank gasket was where it was coming from. But when I got home and took off one of the hoses to drain the tank it just kept pouring out. This was the case with both petcocks.

I tried all three positions (ON, RESERVE and PRIME). None of them would stop the gas from flowing. Prime was a fuller stream. But RES/ON didn't stop the flow.

I've pulled the petcocks off the tank again and inspected my handiwork. Everything seems like it was in the right place (to the best of my knowledge), so what gives?
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If anyone that could give me some insight as to what I should be double/tripple checking or where I can troubleshoot, i'd really appreciate it.

Looking forward to your take on this.

Thanks,
Ian

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[buickyouth]

Also, I got the petcock repair kits from Partsnmore, #20-0021

http://www.partsnmore.com/parts/yamaha/xs750sf/?filters[fitting]=custom

[petejw]

im not familiar with the 750's,
but if u have gas leaking its more than likely
leaking from the needle and seat of the carbies,
which would also explain the ruff idle/ engine stall.
tho the petcocks shouldnt leak, but if they do
it still shouldnt affect the performance of the bike in anyway, they're
a precautionary thing in case the needle and seat leaks.
if theyre similar to the 1100's check for fuel in the oil, sometimes they can leak forward
thru an open valve.

[buickyouth]

So this morning, I put the petcocks back together and while they're still leaky, it's only out of where the fuel is supposed to come out. All external leaking spots from yesterday are now sealed up nicely.

I put the tank back on the bike, put a little gas in and let it run for a bit to check for leaks. Dry all around. I drained the battery pretty bad trying to start it with the push button. But the kick starter did the trick right away, didn't even have to open the choke at all.

After letting it run for around 10 minutes with no leakage, I decided to take it for another test ride. Once again it seemed like the bike was having a harder and harder time at idle and after about a half mile of round the block riding, it stalled out and refused to start again.

The petcocks and everything around it remained dry.

However, there was some fluid coming out of the bottom of the air box (this time I got a few photos). I'm not sure if it's something happening from the crankcase breather hose or if it's carbs overflowing because of the petcocks still leaking a bit in the inside. Or if it might be a sign of a bigger problem that needs my attention.

Take a gander below gents and let me know what you think my next step should be.

Once again, thanks for all the guidance and insight.

Ian
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[jetmechmarty]

Rebuilding Yamaha Vacuum Operated Fuel Valves (Petcocks.)


The above link will show you how to properly rebuild the petcock. Once you read through it, you'll know if you did it right.

[petejw]

lol refer back to reply 3.

[rpgoerlich]

Needs more spring pressure on that petcock diaphragm. Check the length of the diaphragm push rod, as I've had one that was just a tad shorter and wouldn't seal until I stretched the spring a bit.

I'd check that starter as it should've started with it and not just the kick start, pulling the voltage down. Probably needs the commutator cleaned and you can check the brushes while in there.

Battery is suspect also. If the voltage drops below 11volts or so when cranking and it wont start.

Both of the above could be reasons why it would only start with the kick start.

[jjz28]

If you hac gas coming out of the airbox, you probably have gas in the crankcase. Hopefully, you changed the oil before you tried starting it again. On my 1100, i just flipped the rubber over and bent that little washer that goes on the outside a little more so it would have more pressure and it worked for me. Make sure to change that oil if it has any gas in it or you will have very serious problems if you dont already. Your float valves and seats are not working right either or it would stop the fuel from getting into the airbox.

[DGXSER]

First question, is the fluid gas or oil? Looks like it could be oil, but could also be gas mixing with oil residue from the airbox.

If it is oil only, then you may have overfilled the crankcase with oil. Or you may have gotten a bunch of gas in your oil that now has the crankcase over filled.

If it is gas, then your petcocks are not the main problem. The petcocks just shut off fuel from getting to the carbs, they do not control fuel level in the carbs. Inside the carbs is the float valve, that controls fuel level in the carbs. I like to compare it to your kitchen sink, there are valves under the sink to stop the water from flowing to the faucet for when you need to work on it, like the petcocks, and then there is the faucet, like the lfoat valve. If water keeps running when you turn the faucet off, you do not blame the valves underneath, it is in the faucet.

Running or not, the fuel level in the carbs should not change, and if gas is running out the airbox, at least one is over filling. To see which one, open the airbox and see which inlet to the carbs is wet with fuel.

[buickyouth]

I'm pretty sure it was the float valves.

I had a new set of them. So, I pulled the carbs again and opened them up. The old ones definitely had a lot less action on them than the new. Made the swap, cleared out it's underside and put them back together.

Once I reinstalled the carbs on the bike, I started it put some gas in the tank and wondered, "is there gas in the oil?" I checked the oil level window and it looked overfilled. Opened up the fill plug for the oil and I was sure that there was gas mixed in.

Drained the oil and replaced it with new. Fired it up and let it warm up a bit.

Thinking back on my last two outings that ended with me looking like I'm just taking my 600lb dog out for a walk, here's what I was remembering:

1. After only a short time of running, the bike would have a hard time at idle with my hand off the throttle.

2. The leaking started happening around 0.5miles and stalling out shortly thereafter.

So, I kept myself limited to my immediate block and did some laps. I'd go once around, back it onto a sheet of cardboard and set it on it's stand for a minute looking leaks and then saddled up for another lap.

After a few, I still saw nothing and the idle remained strong on it's own. I'm planning on testing it for a longer duration (not distance from home) this evening after work. I'll report back with my findings.
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I also checked out the petcock rebuild post from the XJ site and all I can say is, wow. Such a great, informative and helpful post. I for sure didn't do that good of a job. But now that I know all that I need to do and how to do it, I'm looking forward to opening the petcocks up again and doing it right.

Appreciating all your words of wisdom,
Ian

[sparks]

Think I would have also changed the oil filter.. There still would be contaminated oil there..

[buickyouth]

Great point sparks. I let it drip for a while, but felt that no matter what, I'd still be left with some contaminated oil in there. I figured on doing another oil change as soon as I get another filter in.

This is probably an item I should have one or a couple extras of lying around.

[buickyouth]

Also, Regarding my longer test ride last night after work, I'm happy to say that the bike is consistently idling smoothly at idle and didn't leak a single drop.