This is almost my first post in this forum (apart from my introduction), so I might as well starting off with contributing something to the forum that could be useful to others:
I had troubles with my Honda CB250G (I know its not a SOHC4, and neither is It very common outside of Europe, but the gas cap is the same flip-up design used on most SOHC4-K models).
whenever I rode it for some time, it died on red lights. It runs great when cold, but it gets worse as it gets warm, more so, if the fuel level is low. First I suspected an intake leak, or bad coils, or whatever else you can blame when your bike just doesn’t run right.
But then it dawned on me when I opened the gas cap for a refill and a lot of pressure escaped with a very well audible ‘Pffffftt’: my gas cap vent was clogged! The tank started to pressurize, causing an overly rich mixture.
After some Google-Fu I was able to pry out the spring-loaded gas cap assembly from the hinged chrome cover. Like a lot of people recommended, I blew some compressed air through the vent hole. But when I did, something inside the gas cap assembly went ’pop’ and got cocked in a way that the different shims and discs of the assembly no longer laid flush on top of each other… I googled some more and saw that the very early diecast gas caps were held together with a screw. I figured I could take my assembly apart and replace the rivet in my assembly with a bolt and nut. Since I had not much to lose, I went at it with the Dremel.
As soon as the rivet head was gone, the spring-loaded assembly came apart (I had clamped it down, but the ‘disassembly’ was still rather instantaneous…), and to my surprise, I found another rubber gasket inside. Interesting! Why would Honda go all the way and install a non-serviceable rubber gasket inside the gas cap? Some of the threads I browsed claimed the gas cap to be a one-way air valve. That’s not true, you can blow and suck air through the venthole. Also, I quickly modeled all the parts in CAD, to look at a cross section.
It looks to me that the internal seal only allows ventilation through the provided holes, both ways. It stops air from whistling trough between the various discs. But if Honda added additional holes to the parts, why did they add another gasket to seal off ‘natural’ ventilation? Is it to prevent fuel from flowing out (too quickly) if the bike is laying on its side, like a very restrictive baffle?
It’s a very complicated design for something that could be achieved with a big cork. But I think Honda had its reason to produce a 10-parts-assembly gas cap.
Whatever its purpose might be, the rubber and the steel parts of the assembly held lots of crud on the inside, and no soaking or ultrasonic cleaner would have reached it. I cleaned everything by hand, coated all the pieces with a thin film of grease (like: really, really thin) and assembled the gas cap with an M5 JIS screw and nut. After popping it back into the chrome cover, it closed and opened just as nice as before, but now it also ventilates, and stops my tank from force-feeding my carbs.
Popping out the gas cap assembly from the chrome cover is a neat trick to change the gas cap seal with ease, and to make sure the seal is wrapped around all the discs! Mounting it inside the chrome cover is a PITA, especially with stubby fingers.
Would be interesting to see if the old K0 die-cast screw type gas caps are built just as complicated. For anybody interested, here’s some pictures of the innards of a Honda gas cap:
