Gas cap not sealing?

I didn't feel that my gas cap was closing properly so I shimed it with a washer and now it looks like its good, but when I toped up last night I stood her up and the gas splashed out from under the cap like it wasn't even closed!! my concern is that if gas can get out that easy can water get in as well?? I didn't really overfill the tank and i've never seen this before but I didn't have my tank bag on, and I have gotten heavy smells of gas before after a fill but didn't notice gas because of the bag. should a gas cap seal fairly tight?
Shaun

 

yes it should. There is a rubber seal under the lip - it is easy to change - Chacal carries them. I had gas leaking after fill sometimes - replaced the seal and all is well.

 

thanks bill it seemed funny to me that it leaked so easily
Shaun

 

looking at the fuel left in my tank I can't help but wonder... how much fuel could evaporate through a improperly sealed cap over the course of an eight hour day parked in the sun?? it seems as tho I have a lot less than expected, any idea's or is this just my brain playing tricks on me?
Shaun

 

I have a problem with gas leaking from around the cap. It is the rubber seal. I ordered one from chacal but it didn't fit the cap on my maxim. It did fit the one on my seca so I switched the caps. I haven't got around to pm chacal about it yet.

 

Probably quite a bit, actually. Make sure it's not your petcock and carbs filling the crankcase with gas.

Bill made a good point, and I want to drive it home a bit. These bikes are OLD. The rubber bits DRY OUT. The gas cap seal takes a heck of a beating, once it's old and dry and hard it really doesn't work any more. (I completely killed mine by leaving it on for the acid flush.) The gas cap seal is an interesting composite metal/rubber piece, first cousin to your valve cover bolt donuts.

REPLACE IT. Easy to do, and it gives you a reason to disassemble/clean/lube the fuel cap latch mechanism (don't lose the springs!) in the process. You'll get improved fuel economy (honest) and no more dribbles. I got mine from you-know-who.

 

sweet thanks fitz I'm sure it's the next piece of the puzzle that is my maxim!! I picked up a petcock rebuild kit today locally (sorry Len) because the diaphram is leaking fuel into the vaccum tube (disconecting the vaccum tube and running on pri took me from 15 to 35mpg!!) not where I want to be but better except I rode to work today and now my tank is close to empty and I've only riden about 50 km!! but the bike was parked in the sun all day and it was about 25 degrees today (70F.ish)
Shaun

 

rob, kinda funny, my new seal doesn't really fit either. maybe it is an '80 Maxim thing, like the stanping for the tank is just slightly deeper. I have to smack it hard for both lugs to seat in the filler hole. Otherwise, more leaking. I never thought about Chacal, I just figured I didn't install it right.

 

Well since I swapped the cap from my seca onto my maxim, I have to carry two keys with me. I always afraid I'm going to lose it.

 

There are two different styles of gas cap, that look the same; one you can simply push shut and one requires the use of the key to close. There's a Yamaha dealer bulletin out there somewhere (I'll try to find it) that tells the dealer about instructing the customers on the "new" correct way to close the gas cap when the change occurred.

Also, it is important to get it aligned just right when installing. It's a matter of getting the screw good and snug but not tight, and closing the cap a few times, then tightening the screw, then check. Repeat if necessary.

 

Thanks for the tip Fitz I'll try that when I rebuild it
Shaun

 

Chacal told me that some of the early caps had a plastic internal body. The seals that he carries fit the metal internal bodies. My maxim has a plastic one so that is why the seal I bought from him didn't fit.