As soon as I put gas in the tank it all drains out, presumably passing through the carbs and out of the air intake. bike's a 1980 yamaha maxim 650cc.

These models are typically very lean and respond well to a little richer slowspeed jet. However if the bike has sat up for longer than 3 months try draining the carb and tank a refilling with fresh gas. If this dosent work a proper carb cleaning may be in order. Also ckeck for an air leak at the intake manifold by spraying this area with carb cleaner while the bike is idling steady. If the engine rpm changes you have located a leak. repair as necessary.

Spray starter into the carb intake with no choke, if she runs for a few seconds then you know its fuel starvation
clean jets & airways & blow through with compressed air

Hi Stuart,
Carbies will always look clean inside the plunger assembly. fuel does not make it up into there in normal operation. They exists for vacuum operation of the constant velocity plungers to go up and down adjusting the mixture automatically for the engine with different throttle applied. If the bike runs with you spraying carb cleaner into the intake... then thats good... at least it runs for you :)
You need to check that you have the fuel petcock turned to prime initially to be sure that there is gas filling the carbies. Vacuum operated fuel petcocks can become nogo from sitting for extended periods. The bike uses a BS34 Mikuni carb. Parts are pretty good to find for them. They often have a drain screw on the bottom of the float bowl. You can check here to see if fuel is filling the carbies by backing this off a couple of turns, fuel should drain.
Most likely you will need to remove, strip and clean out the carbies thoroughly. They will have varnish from old fuel clogging jets and passageways in the carbs. Only way to do this is remove them and completely disassemble and clean them to be sure that they are good to go. Having a new gasket kit there to do this is often worthwhile. There is also a rubber O ring under the air/fuel idle mixture screw that perishes and will stop the bike from idling correctly.
You can have a read/download of an excellent guide written for the XS650 that uses the same carbies here. It will at least help you to learn what you are dealing with.
Have fun... well it aint much fun rebuilding carbs, but will be worth it to get the bike going again. Cheers
regards robotek

stored gasoline begins to deteriorate/lose octane/after only two weeks.drain fuel tank label and disconnect fuel lines at petcock,remove petcock from tank,clean fuel screen.re-assemble,check for supply to carbs. Honda's of that vintage have a seperate 'reserve' tank under the battery,which tends to collect all the 'crud',and must also be purged.trace your fuel lines to see if your unit is so equipped,

It seems the float chamber of the carbs are not getting enough fuel (gas).
Try to check this first by opening the carburator float chamber drain screws....thn select the petcock to 'prime' positon and see if theres fuel running out of those drain screws freely.If not thn it time to clean the carbs.
Not bad too to remove the spark plugs and clean them and regap.
Hope this helps!

The pilot jet controls the air/gas fuel mix at idle . If the jet gets plugged up not enough gas gets to the emgine at idle. You will need to remove the carbs and float bowls and clean them, paying special attention to the pilot valve. Count the number of turns to remove the pilot screw and replace the same number of turns then turn counter clockwise 1/2 turn.

A “very helpful” rating for this answer? Thanks!

so you have a hole in your tank somewhere. or leaky fuel **** gasket. firstly you need to find the leak. or is your gas just flowing right thru your carbs and out the overflow? shut off fuel to eliminate carbs. if it stops leaking then its probably gummed up carb floats. use Berrymans B12 fuel additive this will work into the carb and hopefully break down the gum. or strip and clean them by hand with a good spray carb cleaner. or remove air cleaner and spray directly into carb and let sit for a few days.
if it's a gas tank leak then you will have to get it lined, (liquid sealer) so long as the hole is not too big. if it is then drain and dry the tank, get it welded professionally or patch it with a filler or fiberglass and then get it sealed, i used my local radiator repair shop to het mine done.

Carbs gummed up. Need a serious cleaning or a full ruubers replacement/rebuild kit. Gas turns to gummy when left stting in the fuel bowls.

Use a couple of cans of Sea Foam.. best stuff in the world.