I have an 81 CB750 Custom that has been in storage for nearly a decade. I went through and replaced all that needed replacing, all the rubber basically, electricals, brakes and so forth, cleaned the carbs and now have an extremely annoying issue.
All four carbs will randomly leak from the overflow, but not with any regularity. The only one that reliably leaks is #2, the rest just sometimes. I believe the issue is dirty gas due to how much debris I could not get out of the tank. I cleaned it as best I could with the budget I had, nothing heh, using acetone and particulates, drained and flushed it out numerous times, but there was still more rust than I would have liked in the gas.
The overflow tubes are all intact, no cracks or bad seals, the drainplugs are all intact as well as the floats and float valves with the rubber tips. I can exchange any part with any other carb, in whole or in groups, and the leak does not follow the part. I can tap a bunch on them and they eventually will stop leaking... mostly... but after riding a few miles they start leaking again. I can shut off the gas at the tank and they stop dripping, and if I remove the float bowls without draining them, they will be full of gas.
This is particularly a hassle due to having a 900 mile ride coming up and I don't want to be trailing gas everywhere since that can really affect the mileage or just simply set it on fire thanks to the aftermarket pipes getting dripped on.
If anyone has any ideas of what to check for I would greatly appreciate it since this is the last issue keeping me from riding. New floats, seats, needles, gaskets, tested for and found no cracks in the tube, tube seat, bowls or needle seat, bike runs properly without any issues.
I believe it is just dirty gas, since #2 basically gets all the gas first, and any debris is most likely to collect there more than anywhere else.
Have you got a filter fitted in the fuel line? If not, I would fit one. If the leak sometimes stops when you tap the carbs, I would suggest the float valve seats and needle tips need proper cleaning and polishing. Also, if the needles are old, the rubber tips harden and don't seal properly. Replace the needles. I assume your float heights are non-adjustable? Make sure the floats have full movement without touching or binding on anything.
I just picked one up this week, but other work prevents me from installing it yet. Yes, it has the non adjustable plastic floats, they aren't touching anything and if I manually move them with the bowl off, they feel like they move freely and stop the gas perfectly without having to compress the needle spring. I will be draining, filtering and refilling the gas tank and carbs a few times in the next day or so.
I was worried that the floats themselves may have developed a crack and not moved properly, but they are intact, filling the bowl when it is off and moving it up to the bottom of the carb and the floats all move with it. I have had to clear debris off the tips a few times already and with how dirty the gas gets in the bowl is what leads me to think that it is just a case of dirty gas. I just wanted to cover all the possibilities before I wasted money on parts I don't need.