KZ305 gas cap leaking

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I recently bought my first motorcycle, a 1988 KZ305, and have finally gotten it to riding condition as it didn't run when I bought it. I did have to replace the gas cap since someone before me tried to force the gas cap open, so it couldn't be recovered. I bought a replacement on eBay, . It seems to fit properly.

What I've noticed is that when I fill the tank, and after riding a bit a small amount of gas leaks from the gas cap, and runs down the gas tank. Also I had a mishap (:facepalm:) where the bike fell over on its side and gas gushed out of the gas cap. I never intended, nor do I ever want to drop the bike again, but it was surprising how much gas leaked out. I imagine that is not suppose to happen and the seal should have been tighter.

One idea is buy another replacement gas cap, and take the rubber seal from the new one, and double up the rubber to make an even tighter seal. 

I have tried to more-or-less fill the tank, when adding gas. I've noticed on another forum post here, someone said they only fill the tank up 3/4 of the way. I don't think I want to take the bike on any long road trips, so that could work, but I also don't like the idea of leaking any gas.

Any recommendations?

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Dave

On some models the rubber gasket is listed as a separate part, but it isn't on the 88 305. Maybe you can use the gasket from an earlier model, such as the 81 305. Measure yours & report back:  ID. OD. thickness. also, any distinguishing features to the gasket, such as a raised rib or something.

Fuel cap gasket. #11009-1051. fits a number of models, including the earlier 305s.
www.partzilla.com/product/kawasaki/11009...4892236f2e30ab9e946a

The one I have is:

ID: 1 1/2"
OD: 2 1/4"
Thickness: 3/16"

From the partzilla page looks to be the same.

If someone tried to force (pry?) the previous cap open, it's possible the top of the tank was bent in such a way that now the new cap cannot seal. You can look for anything obvious - e.g. by holding a flashlight horizontally - but even a small bend could cause a leak.

The fuel cap gasket on my 81 KZ650-CSR is:  57mm OD. 40mm ID. 3mm thick.  (inch conversion:  2.25" OD x1.57" ID x 0.12" thick).  The gasket should be soft, not hard.

ps: my fuel cap is a very well made reproduction (Taiwan) & the gasket is very soft even though it's more than 5 yrs old. It's a very good replacement for the stock fuel cap.

Isn't that the same cap used on a lot of Kawasaki's in the early 1980's ?  Should be easy to get a tank seal if it is...

From the diagrams the 88 305 fuel cap has a replaceable lock cylinder, so it isn't exactly the same. but it may be pretty much the same otherwise. The gasket from the earlier models very well may fit.

When looking from the side, you should be able to see the gasket seal as you close the cap.
Make sure the metal cone the gasket is on has a spring loaded wobble that compensates for sealing angle as the cap is closed.
Put the end of a paper strip on the tank flange. When the cap is closed, you should not be able to easily pull out the paper.