Gas cap

[SonnyT]

Has anyone replaced the original gas cap? My original locking cap has leaked from day one. If the tank is even half full and I have to apply the brakes slightly aggressively gas leaks and runs down the side of the tank. Any ideas?

[The Prophet of Doom]

They are available on ebay and heaps of bike shops.  Same as RZ350.
Having said that I got a replacement seal from a local seal supplier for about $5 just by taking the old perished one is as a pattern.

[injuhneer]

I'd like to buy a new RZ350 fuel tank cap with an RZ350 attached to it.   

[SonnyT]

Getting lazy in my old age I start looking at replacing much too soon. Decided to take the cap apart and check it out. Turns out there is a paper gasket that had shriveled up which left the two screws loose. Made a new paper gasket, cleaned everything and reversed the rubber gasket. All is good now. Thanks for the response.

[fret not]

Good answer!   

[SonnyT]

My gas cap repair fizzled and started leaking again. Disassembled the cap, removed the paper gasket, and made a new one out of a bicycle inner tube, and replaced the o-ring around the tumbler assembly. Seals great now, BUT, now it seems that vapor pressure builds up and when I shut the bike down the pressure gets by the petcock (On position), fills at least one carb then runs out its overflow. If I loosen the cap and release the pressure it doesn't leak, and if the petcock is left in Prime with no pressure in tank there is no overflow, so I'm assuming both needles and seats must be okay.
So, is the cap supposed to let air in but not out? Did I seal the cap too well?
Help!

[Walt_M.]

The vent hole is in the tank filler neck. It's very small and easily blocked. You didn't notice while your cap was leaking.

[SonnyT]

Thanks for your quick response. I removed the cap and can't find it. Had the tank sealed several years ago so it may be sealed. Is it in front, rear or side? Also don"t find a tube or hole that it vents to on under side.

[Walt_M.]

I can't remember for certain but I think it's in the front. And it's very small, the tank sealer would easily block it.

[SonnyT]

Any idea where it vents to, so when I do find it in the neck I can blow air back through?

[Walt_M.]

I'm not 100% but I think it's just to atmosphere. I can't find it in the service manual and I'm not convenient to the bike to look. I looked at my '84 RZ350 and the cap is vented so no help. As a suggestion, drain your tank, stuff a rag into the filler opening and sand around the filler neck until you find it. You'll need a very fine wire to clear it, a sewing needle is too big.

[SonnyT]

Couldn't find a vent hole inside the tank or an exterior vent hole. Disassembled the cap and found what possibly looks like vent holes. But it looks like it needs the original gasket inside to operate properly, and mine is long gone. Looks like the rubber gasket I made is blocking the venting channel.
Ordered a new cap on ebay.
Thanks Walt. Is it possible your tank is an 83 and has different vent than my 82?

[SonnyT]

Got it! Figured out how the cap vents. Paper gasket is not special, just flat solid piece. Reassembled cap (correctly) and luckily found a rubber sealing ring to replace the old, cracking one. No leaks and no pressure build-up.

[fret not]

I am thinking that inner tube rubber is not a good material to use exposed to gasoline.  An inspection of a list of sealing materials at an industrial supplier site can really help select a good material.  I often use McMaster-Carr, though MSC, and others are very good.  Just checking for materials that are resistant to certain chemicals (acids, alkali, fuels, solvents, etc.) is very educational.  They usually have helpful information and lots of choices.  This can help find the type of material, then you need to find that material in a useful form, and they often have a selection to choose from.