Puzzled !

[tomo]

This one has me stumped, so just thought I would ask if anyone has had a similar problem or can suggest anything. Took my 78 GS750 out for a run the other day, as usual after sitting for a while I needed to blow a couple of times into the carb breather pipes before she would fire up. Bike ran well, so as it was a hot and sunny day (rare in the uk !) enjoyed a couple of hours exploring country lanes. Got home and put her back in the garage which is attached to the house. Later that evening, went to the kitchen and smelt petrol. Looked in the garage and there was a small pool under the bike. Mopped it up and tried to see where the petrol was coming from. First thought was the tap on prime, but it was`nt. Next maybe a stuck float but no sign of petrol on the carb bodies. One of the carb drain tubes looked wet at the bottom but not higher up. Checked under the airbox, no petrol there. No sign of any leakage from the tank. Next day, small pool of petrol under the bike again !. This has got me puzzled, as I can not for the life of me find the source !.

[Rijko]

as usual after sitting for a while I needed to blow a couple of times into the carb breather pipes before she would fire up.

that is strange.
Since you mention "as usual after sitting for a while", i get the feeling this means you already had carb issues for some time but managed to work around them this way.

Could be you have 1 or more sticky float needles, that get stuck in the closed position.
And you release that by blowing into the vents.

And since this is persistent ... check/clean the float needles and seats at least.

[gtem]

Based on needing to use ether(?) to get it started and then the leakage, frankly it sounds like the carbs are gumming up (clogged enrichment pickup, sticky float valves). I forget whether the earlier GSes used a vacuum petcock, but over time vacuum petcocks tend to let fuel by into the carbs even with the engine off, and then in turn the float valves are your last "line of defense" to keep fuel from overflowing the bowls, and the constant dribble from the petcocks will only accelerate the gumming up process as fuel evaporates from the bowls and more then dribbles in. Vicious cycle.

[tomo]

Thanks guys. I think I do need to pull the carbs and give them a thorough service, but the breather pipe trick is something I was told about many years ago for when a bike is hard to start after a while of sitting unused, I guess it just sort of primes the carbs. Works every time though. I have found even after a carb clean/re-build, some bikes can be a sod to get going. I did wonder if on this occasion a float had stuck, but there was no indication, no fuel on the bowls. I have recently had running problems though, brief intermittent power loss and no.1 pot not always catching on start up, but I was suspecting that to be electrical, maybe now it could be carbs. Looks like time to pull those carbs !.

[salty_monk]

Is it on the sidestand often? I had this twice. Once was a failing petcock, fuel was coming from the overflows. This happened pretty consistently.

One was pinholes (not visible until I stripped the paint) in the back corner of the tank (the bit that would be the highest corner when on the side stand and thus dry when the tank is close to empty so would rust faster). This one would come & go depending on whether I left it on side stand, centre stand & how much petrol was actually in the tank when I left it (fuller tank would leak even on side stand but it would also run under the tank & drip on the engine).

In both cases it wasn't a large amount, much easier to smell than to see really.

[tomo]

Is it on the sidestand often? I had this twice. Once was a failing petcock, fuel was coming from the overflows. This happened pretty consistently.

One was pinholes (not visible until I stripped the paint) in the back corner of the tank (the bit that would be the highest corner when on the side stand and thus dry when the tank is close to empty so would rust faster). This one would come & go depending on whether I left it on side stand, centre stand & how much petrol was actually in the tank when I left it (fuller tank would leak even on side stand but it would also run under the tank & drip on the engine).

In both cases it wasn't a large amount, much easier to smell than to see really.

Thanks Salty-monk, sorry for delay in saying that, been a few things keeping me off the bike and forum for a while !. I will check the tank and the petcock. I suspect I may have other issues with the latter so have posted another query in the fuel/carbs section.