GS450E: Gas pouring from airbox vent, no gas tank on it!

[Guest]

I let my bike sit for about a week and it began giving me starting/idle problems. I stripped and dipped the carbs, added new o-rings and new boots (intake and airbox) not too long ago and it started, idled and rode fine. I was getting ready to do the valves (bike's only got 3k miles so they shouldn't be too bad) and synch the carbs when I started to have issues. After a week of sitting, it wouldn't start on choke. I would get it to start with a little throttle (I know it's bad) and then it would race to 3.5k for a while and then die. Subsequent attempts to start it would be unsuccessful. I would try to adjust the idle but it wouldn't do anything. Neither would adjusting the mixture screws.

I let it sit for a few days and decided that something was up with my idle circuit and decided to pull the bowls and see what was up, but I wanted to run them dry first. I removed the gas tank, plugged the vacuum nipple on the carb, put a box fan in front of the bike (I knew the RPMs would be high) and started it. It idled at 3.5k like before and then it started leaking TONS of gas onto the floor out of the airbox vent. There was a 3 foot wide puddle on the floor in a matter of seconds before I realized what was happening, so I cut the engine. My bike doesn't have an oil window so I couldn't easily tell if there was gas in the crankcase, but the gas had to come from somewhere. It was way more than two bowls could hold. Any ideas?

Here's the recap:

-1981 GS450E, running fine but sat for a week.
-Mikuni BS34 carbs, stripped and dipped, new o-rings, new boots, no vacuum leaks.
-After sitting a week, bike would start, race up to 3.5k RMPs, then die.
-Pulled tank to run bowls dry
-Tons of fuel pouring from the airbox

Help!

[cowboyup3371]

Sitting for a week causes the gas in the bowls to evaporate. Use the prime function on the petcock for about 30 seconds or so then start it. Just make sure to turn the petcock back to on

[hjfisk]

I would guess you need to change the oil as the gas probably over filled your crankcase, then you need to replace your leaky[bad] fuel petcock as it must be leaking by or through the vacuum line , then I would reset the floats , and check the needle seats as they had to leak by also. The reason the bike starts so hard is it is full of gas in the lower end.I would not try and run it until you do this as if the lower end is that full of fuel you will ruin the bearings in that motor, I believe the 450 is a insert bearing motor, and they don't like gasoline.

[hardline_42]

Sitting for a week causes the gas in the bowls to evaporate. Use the prime function on the petcock for about 30 seconds or so then start it. Just make sure to turn the petcock back to on

No prime on the '81 GS450 petcock. Just "ON" and "RESERVE." I did loosen the brass screw for a bit the first time I tried to start it to make sure there was fuel in the bowls. When I removed the tank today, there was fuel in the line as well.

[hardline_42]

I would guess you need to change the oil as the gas probably over filled your crankcase, then you need to replace your leaky[bad] fuel petcock as it must be leaking by or through the vacuum line , then I would reset the floats , and check the needle seats as they had to leak by also. The reason the bike starts so hard is it is full of gas in the lower end.I would not try and run it until you do this as if the lower end is that full of fuel you will ruin the bearings in that motor, I believe the 450 is a insert bearing motor, and they don't like gasoline.

I'll definitely change the oil, first thing tomorrow. I didn't think to check the petcock (this all happened with the tank OFF the bike). Would the petcock be leaking with the bike off (ie, no vacuum)? Because it doesn't leak at all sitting on the shelf, even though it's 3/4 full.

When I rebuilt the carbs, I did check the needles and seats and I didn't see any grooves on the needle. I also set the float height per the manual. I might just order a new set of needles/seats and replace them just to be safe.

[Gorminrider]

well, if the tank is off the bike, you can test the petcock really easily and fairly safely by attaching a hose to the vacuum port on the petcock and sucking on it..carefully at first! of course. But I bet you have done this already....
Next, the obvious thing to do is bench-test your carbs with a fuel supply and see if they overflow. (I will always do this...now,after trying the the hard way-struggling with put in-take out,put back,take out again with my own carbs)

OR You can test the float level with carbs on the (level) bike by attaching a CLEAR hose to the bowl drain and lifting the other end of the hose above the bowls. This will show you fuel level in the bowls. It should not be above the bowl gaskets, I recall. It's doesn't fix thingst but at least shows you if your fears are true. You can ride awhile on the level like this I guess, if the hoses are longish, just in case you suspect a moving bike is different than a bike relaxing in the shop