Topic: Gas tank pressurizing....causing other issues?

[bmcdonou]

Not sure on the level of caution.  I guess better safe than blown up.

It really comes down to the force needed to create a spark between a steel screw and washer - and if that can be created when opening/closing the cap, or vibrations while riding. Flint is special bc it is extremely tough, sharp, and coarse; it chips off little shards of iron in the steal that then react to the oxygen. Not sure if the threads of a screw would replicate that or not. Also, I believe stainless (if using as the screw) is way less likely to spark bc it has less carbon in it - doesn't solve for the other bits. You could probably do a test and rub the screw on the outside of the disks and see if you can get a spark. That said, It wont hurt anything if you can find a brass or copper screw. can also grind off the unneeded threads, or coat in something.

I'll probably start poking around for solutions on Friday when I have more time.

There are also replicas available at 4into1, CommonMotorCollective, and some ebay/amazon sellers. Unfortunately its the entire outer and inner cap assembly, and looks like they've modified the design to be a screw-in vs the four prongs. For whatever reason, I try not to discard the original OEMs elements if I can avoid it, and prefer to fix what I got vs buy new - and saves $50. https://4into1.com/reproduction-gas-tank-cap-latch-set-honda-cb-cl200-350-360-450-sl350k-cb400f-500-550k-750k/

[Oddjob]

Sorry again for the delay but some news.

I tried to see if rubbing any of the discs against the threads of a normal JIS screw produced any sparks and I didn't get any, that's not saying it's not possible, just that I didn't see any in about 20 attempts.

Parts have started to arrive and I have tentatively assembled the cap again, just to see how it went. Still awaiting some 5mm nyloc nuts so I used a 5mm flanged stainless steel nut instead.

First off let me explain a couple of things, the discs have a central hole just smaller than 5mm so a standard 5mm screw will not fit unless you enlarge the hole slightly. The end boss where the head of the rivet sits is larger, somewhere around 6.5mm. The rivet has a small sort of captive dowel, can't think of the correct name for it right now but it looks like this. It's short because I had to cut the pressed part off.


I didn't realise what this was for until I went to assemble the cap, it acts as a stop so the rivet doesn't clamp too hard, when the discs reach the larger step it stops the rivet being crushed anymore. In order to stop the 5mm screw from floating around this larger hole and possibly causing some misalignment problems I had ordered some top hat washers, to try and avoid any spark problems, I had as you can see from the pic, decided on nylon ones. The tophat part is exactly the same size as the rivet, taking into account the thicker flange on the bottom. So when I screwed everything together the tophat section stopped me overtightening everything. It's a little stiffer in action that an untouched one (I have another to test against) but this could be because when the cap is pressed shut and the spring compresses the head of the standard rivet tends to travel up the boss rather than the bottom part sticking out. The new nylon tophat washer is still a little stiff in moving through the boss and it needed to be enlarged to 7mm to get the new nylon tophat washer to pass through. As the cap gets compressed again and again in use I'd imagine it will feel exactly the same.

All together again. New sealing gaskets on order.



Still waiting on some brass screws to replace the steel one I used in the pic, can't take the chance of any sparks. When they arrive I'll advise on the ideal length to use, I'm thinking maybe 12 or 14mm.

Hope this helps.

[HondaMan]

A cool study on the cap.

I used to offer "Gas Cap Hinge pin kits" (for donations to these forums) of an (American-made) 18-8 stainless screw, washers and acorn nut for these bikes for a long time - until suddenly it became impossible to get an 18-8 #4 screw in 1-1/8" length, for some reason. I still have a bunch of the acorn nuts (and I think, washers). Now the screw has to be cut down from the Chinko 18-8 1-1/4" screws we can get easily. When I've tried to cut down the 1-1/4" versions, invariably the length comes out wrong, or the nut won't start of the exposed thread - so I quit doing them over that.
The 303 stainless screws will work, being 100% sparkproof, so I'm told. Cheap Chinese 18-8 stainless screws are said to have too much ferrous steel in them now, and that's all I can find.

[HondaMan]

Oh, very interesting. Yes, I am in Chicago. Anything I can do to avoid this from happening; additives or something? When I opened the tank, I definitely felt the heat.

Sorry to miss this earlier: yes, there is something that helps. You can add oil (almost any kind) to the gas in the tank, about 2 ounces or so per fillup. I first started this during the years of MTBE in the gas to help neutralize its corrosive effects, later discovering that it helps a LOT here at high altitude when the Winter gas is still in the tank and we get an 80 degree day in January (so I go riding). The rest of the year it helps to mitigate the other "stuff" they add to gas to [reduce emissions/prevent freezeup/meet some new DOT standard/etc.), like butane, ethane, alcohol...

[bmcdonou]

Very helpful. I soaked mine in PB for about a week, and that worked for a few rides, but we seem to be back to clogged. Yesterday was 80 and so of course had to take it out, and noticed gas leaking out of the #2 carb and tank pressurizing again. Put oil in the gas for a night event and did a longer joy ride and that seemed to help, but was also mindful to open the tank lid at stops to normalize pressure.

When you get all of your final parts all sorted, I'd love a shopping list. I'll hold off from taking apart until then and just manage around it until then.

Sorry to miss this earlier: yes, there is something that helps. You can add oil (almost any kind) to the gas in the tank, about 2 ounces or so per fillup. I first started this during the years of MTBE in the gas to help neutralize its corrosive effects, later discovering that it helps a LOT here at high altitude when the Winter gas is still in the tank and we get an 80 degree day in January (so I go riding). The rest of the year it helps to mitigate the other "stuff" they add to gas to [reduce emissions/prevent freezeup/meet some new DOT standard/etc.), like butane, ethane, alcohol...

Would have never thought of this.....what does that make it, 250:1 ratio? I put a cap into my half tank and will see how it goes. Certainly will add this to my bag of tricks. Thanks! I am also running seafoam at the moment; ever since the initial issue my idle has been off. Assume particles got pushed through the filter screen and clogged the idle jet. Was trying seafoam to see if I could cheat my way until the end of the riding season and not have to service the carbs before then. The idle has improved, but its not 100%. Rainy week ahead so may take then off and service.

[Rayzerman]

I have found one of the best cleaners is the purple Simple Green, either full strength or diluted somewhat with water.  I dropped my tank cap into it and left it overnight, no disassembly except to remove the 52 year old gasket. Needed a new one for sure.  Rinsed with water and gave it a blow with compressed air.... all is very apparent when it's working correctly..

[Oddjob]

Just waiting for the final parts to arrive, brass screws are coming from China it appears ☹️☹️

[Oddjob]

Sorry, done this mod for a few days now but keep forgetting to post the results.

Here goes.





I got 2 sizes of brass screws, a 12mm and a 16mm, I think 14mm would have been better TBH but maybe that size wasn't available. The 12mm fitted but the threads didn't reach the nylon bush in the nyloc nut so I deemed that too short, the 16mm does and protrudes out, I'll trim that when I go to fit the inner part into the chrome outer but at the moment it still needs to be worked on, all the metal parts need zinc/nickel plating so might as well leave it until it needs it.

It compresses ok, seems to work just fine, put a new rubber seal on it just to see how it looked. Managed to find a source of them fairly cheaply so bought 10 of them.

If anyone wants a 16mm brass screw, a 5mm nyloc nut and the plastic top hat bush, as well as a genuine Honda seal I can do a package at $10 inc P&P from the UK. Limited number though.

[PeWe]

I heard a strange bubbling sound in all carbs when parked in the garage after a ride. CB750 K2
This due to an aftermarket fuel cap rubber seal.

It had a too wide lip covering the vent hole on the inside.
I think the gasket had to be flipped right, one side was wider.

I checked a new Honda gasket in a bag now, both sides seems to have equal lip width. If different, wider  against the tank I suppose.

That reminds me to verify a new aftermarket gas cap I have. If vent is ok.
Cap for CB750 K0-K6 that most likely fit other  bikes too.

I had no sudden stop as another tank caused when the gas cap was too rusted with vent hole blocked.

Fuel starve after ca 15 minute ride. This must depend on how full tank was probably  far from full so air inside could flex.

[Oddjob]

Same seal on the CB500/550K as the 750K2-7 I believe, I have a few if your after one.