trouble starting. All of a sudden. (81 GS750L)

[sipman]

Hi,

my bike hates me. It's a GS750L from 1981. I have it up for sale and a prospective buyer came to look at it.
If the bike hasn't been started in a week+, it takes quite a bit of choking to start it.
Today, my buddy started it in the morning without any incident. But this afternoon, in front of the prospective buyer, the bike started immediately and then proceeded to immediately die. Then it would not restart. I tried, and tried. It wouldn't start at all. I'm sure the bike was flooded, but I've never had a flooding issue before. The battery drained completely in all my trying. I'll be really surprised if this prospective buyer ever comes back to look at the bike!
Any ideas why the bike would suddenly not start?

[Nessism]

Bikes love people that maintain them.

How are your valve clearances?

[almarconi]

More than likely the bike know you are trying to sell it and simply does not want to be sold.

[hikermikem]

Any ideas why the bike would suddenly not start?

I would be embarassed to admit how many times my hard starting bike was simply out of gas...

[cyclefvr2]

is the petcock diaphram good,and working? they need to be turned over just to fill the bowls with fuel, even on prime
my hose popped off,and it did that, or if its dry rotted

[sipman]

I've been maintaining it. It has gas. Valve clearances are OK. Petcock was just rebuilt about a month ago. I don't know what else could do it. Hoses are new having been replaced this summer when I replaced all the O-rings on the carb after dipping/cleaning.

I went back to it and used some starting fluid and got it started once again. I had tried on, prime, and res to no avail. I have a clear fuel filter so I can usually see fuel in there. I wonder if the petcock had been stuck closed? Is prime guaranteed to open the petcock? After the bike started on the ether, I wonder if it had enough vacuum to open the petcock and refill the floats?

[DPage]

I've read that sometimes a little vacuum is needed even when the petcock is on prime, so that may be your problem. But when you say the petcock has been rebuilt in the last month and you have a visible fuel filter (which is sometimes used to collect fuel tank rust upstream), I'd suspect some kind of fuel delivery problem. If you were keeping this bike, I'd say just gut it up and spend $60 to replace the petcock with a new one, plus check the condition of your tank and get rid of the external filter. Since you're selling it, make very sure that rebuilt petcock works. I haven't heard much good about the quality of those rebuild kits.

[49er]

I've read that sometimes a little vacuum is needed even when the petcock is on prime, so that may be your problem. But when you say the petcock has been rebuilt in the last month and you have a visible fuel filter (which is sometimes used to collect fuel tank rust upstream), I'd suspect some kind of fuel delivery problem. If you were keeping this bike, I'd say just gut it up and spend $60 to replace the petcock with a new one, plus check the condition of your tank and get rid of the external filter. Since you're selling it, make very sure that rebuilt petcock works. I haven't heard much good about the quality of those rebuild kits.

There is a problem with rebuilding the petcocks, but it's not in the kits as such.
I re-built mine on the 850 with a K&L kit, only to have it leak within the first month of service. The problem lies with the wear on the beveled seating surface where the o ring sits, when the engine vacuum is not holding the valve open. I remedied it by substituting the replacement o ring with one with a slightly larger diameter.
This allows the o ring to seat on a new part of the bevel area, which provides perfect sealing once more.

As said earlier, get rid of that inline filter. The tank filter and individual carb float valve filters should be adequate. Many inline after market filters cause fuel starvation. And no, you don't need any vacuum assistance for fuel to flow in the prime position. That's the very purpose of having a prime position.