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

[bmcdonou]

Hi all. Couldn't find anything on this issue when searching.

Took the bike (75 550 in a 73 500 frame) out for a longer ride than I normally get to.....and we ran into a series issues I've never experienced with the bike before. One in particular, which I think could be the root cause of the others, was that the gas tank was holding pressure as it got hot...

When I finally got home and parked it, I noticed a hissing coming for the tank lid. When I open it, it burped open with excess pressure. I had just replaced the rubber seal from what I assume was the original. I never really thought about how the tank lid works, but I see that the underside plate has a little hole that I assume is there to allow venting. Perhaps the new rubber sealed it completely disallowing it to vent? Let me know any thoughts on what could be happening here.

Here is the full story - series of issues and my theory around the gas pressure:
 - Went for a longer ride than normal, about 3-4 hours in total with a mix of expressway, easy cruising, stop-and-go traffic
 - The ride up was fine, when we stopped I noticed the smell of gas and a leaking carb. I assumed it was a stuck float, whacked it, and hoped that would resolve (btw, don't typically have this issue as i service the carb every winter)
 - The ride back hit traffic and the bike felt hot. There was a point of some smoke, but assumed it was oil from the little leak I am well aware of form the valve cover getting to the pipes. At that point, I also noticed I still had a little gas leak which made me nervous
 - We got on the express way and about after 10min, I started losing power and eventually stalled.
 - I thought perhaps I had lost fuel from the leak but found out that I had lost a qt of oil. I also noticed at this time that the bowl was leaking from the seal, or perhaps higher. I have overflow lines that run out the back as is stock, and that was leaking too. #2 carb.
 - We filled her with oil and limped her home. That is when I noticed the gas tank pressure
 - Did a proper check and found oil caked to the bottom of the pan (not a qts worth tho....that is still a puzzle as I assume a qt would make a proper mess. Did not see smoke from form the pipes.)
 - I changed the oil the next day it was full of gas
 - Pressure washed engine and took for a warm-up ride. No oil on bottom, only the known value cover areas (about a teaspoon or less every 30 min of riding - so small that I've never had to top her off or anything)

So here is my theory:
 - Pressure in tank pushed gas into the carbs, overtaking the float valve, and into the head (at least on #2)
 - The gas in the oil decrease the viscosity, causing it to leak more severely (perhaps some burning off), generally perform less, resulting in overheat and eventually oil pressure loss, and then failure
 - something to do with the new lid seal or the install prohibited proper venting (assume it is designed to allow for venting)

LMK your thoughts a take on this?

Next up is a compression and leakdown test to see what damage I've done.
 
   

[scottly]

 - something to do with the new lid seal or the install prohibited proper venting (assume it is designed to allow for venting)

The gas cap is supposed to be vented.

[HondaMan]

It only takes about 2 ounces of gasoline to turn 3 quarts of 20w50 oil into 10w or less viscosity. That could have made the engine quite unhappy and have high friction: this could make it stall at low or idle speeds, easily. Oil with gas in it makes a terrible lubricant, despite them both coming from the same substrate.

I've also seen this situation accidentally IMPROVE a CL350 twin that had been driven very slowly for years, in high gears at in-town speeds (lugging it) when one of his carbs got a stuck float one Spring and put almost a gallon of gas into the oil. He rode it to the shop to complain about the gas smell: when I pulled the dipstick it was full to the top of the hole with gas! I drained it, fixed the carbs, installed new oil and ran it for 2 days' commute work-to-home and back, then changed the oil again. After that it would wheelie going into 2nd gear at redline, was probably the single 'hottest' CL350 I ever rode!

[seanbarney41]

I think your diagnosis of what happened is mostly on the money.  However, plugged tank vent is more usually associated with negative pressure, i.e. as the fuel in the tank is used atmospheric pressure cannot fill the vacuum fast enough causing fuel starvation.  The tank is trying to implode, not explode.  I can imagine that heat from the engine could cause the opposite but I have never encountered this problem.  I am thinking you likely have some simultaneous carb issues.  Be sure to check your carbs for correct float valve operation, casting cracks, plugged overflow drains, and plugged float bowl vents.

[Oddjob]

Is the engine vented to the atmosphere like the 500 or vented to the air filter like the 550? Failed rings or a holed piston allow the compression of a cylinder to pressurise the crankcase, this can cause the oil to be pumped out of the engine via the easiest route, which is normally the venting system.

The petrol cap has a small needle pinprick hole in the domed part of the fitting, I'm presuming it's a latch type 500 fitting? behind the dome is a loose washer the same OD size as the domed part, fitting a new washer on this part can be a little tricky, parts never seem to want to line up to allow the rubber to engage in it's slot. The inner part of the cap does detach from the chromed outer cover BTW, it's only held in place by 4 sprung prongs, it's fairly easy to remove the inner part from the outer and doing so makes replacing that rubber seal SO much easier. Just a matter of using a screwdriver to disengage the sprung prongs and it pops out. Got a couple dismantled as we speak so I can get the outer part rechromed. Check the pinhole hasn't been blocked by rust particles, which moved as you were fitting the new seal.

I can post pics of the parts if that helps.

[bmcdonou]

Is the engine vented to the atmosphere like the 500 or vented to the air filter like the 550?

Current setup is the 500 orientation, for both airbox and venting (i.e. to atmosphere). I will double check that all are free and open.

I removed the inner to replace the rubber seal. Does it go around both the inner and the attached 'washer of the same diameter? I forgot to pay attention when I remove the old one as I did it before realizing I could remove the entire inner element. I DID NOT put the new rubber around both the inner and the washer as I assumed the washer was there to prohibit the rubber from sealing the top cap portion, and thus allow for venting.

Is the cap the only area of venting on the tank? I also just before that took apart the petcock, cleaned the passages etc. I noticed then that the 'straw' was splitting down the middle, but assumed that wouldn't hurt operation.

However, plugged tank vent is more usually associated with negative pressure... I am thinking you likely have some simultaneous carb issues.  Be sure to check your carbs for correct float valve operation, casting cracks, plugged overflow drains, and plugged float bowl vents.

This makes sense, but definitely felt like positive pressure. I.e. Cap pushed open, vs being sucked closed. I was certainly having some carb/overflow issues, but not before this ride. Another odd symptom was that when I turned off the petcock, gas was still dripping out. At the time I couldn't tell from where (meaning, it was dripping from the bottom of the bowl, but originating from somewhere else). This was in addition tot he overflow hose our the back. At home, with a closer look, it almost seemed like it may have been coming from the intake boot, as if the entire carb was flooded with gas.

During my short rides, all is okay now. But I feel like I need to take her on a real ride to get her up to temp to see if I can recreate the tank pressure issue. Given I live in a large city, the only place I can think to do that would be the expressway....for which I now have some fear to do so. Maybe I just take it one exit at a time and do a loop until it happens again or I feel it has resolved.

[newday777]

You are in Chicago right? Winter fuel mix is happening, different formulation now, which boils the gas in bikes if it gets hot out(happens in the spring too). When the gas boils it will even whistle while the cap is shut pushing fuel out into the carbs and if high enough into the cylinders, then into the crankcase, besides out on the floor.

[bmcdonou]

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.

[Don R]

 You can remove the vent mechanism from the gas cap, it's just wedged in there. An o ring pick under one of the 4 flat springs will loosen it enough to get it started coming out. Then you can make sure all of the holes are opened up.
  My race gas almost always whooshes when I open the 5 gallon can. If it doesn't, I know it wasn't sealed good enough and they never sit over an engine or get shaken up.

[scottly]

I can just rock my bike with the front brake on sloshing the gas around, and generate some pressure in the tank, hissing from the cap vent.

[HondaMan]

Is the cap the only area of venting on the tank? I also just before that took apart the petcock, cleaned the passages etc. I noticed then that the 'straw' was splitting down the middle, but assumed that wouldn't hurt operation.

That 'straw' in the petcock is the normal fuel-feed pipe. When the level of fuel falls below it in the tank, then you must switch to RESERVE to keep running.

When they split, there is no RESERVE left! You need a new petcock, or at least you need to solder that pipe's crack shut again.

[bmcdonou]

Took apart the cap again last night. Shot air into the vent hole but couldn't really tell if anything was happening - mostly blowing back at me. Did again with carb cleaner, and did see some liquid coming out of the puck at the 'sandwich' seam.

Can someone help me with the mechanics of how this vent works? I assumed there would be another corresponding hole on the other side, but I did not see one. If it is venting through the seam of the two halves of the 'puck', that would be covered by the rubber when installed, and that wouldn't allow for venting. Air did not seem to be venting where the spring attaches on the back plate or anything.

Then you can make sure all of the holes are opened up

You say 'all the holes' vs 'hole'....where are the others I am missing (which is what I am/was looking for per above)?

I can just rock my bike with the front brake on sloshing the gas around, and generate some pressure in the tank, hissing from the cap vent.

Sounds like perhaps this is fairly common, and I've just never experienced it because I've always had a crummy seal on the cap. It would seem to me that you'd want to have some sort of venting function to eliminate both negative and positive pressure situations. I don't believe petcock does so perhaps it happens at the carbs?

If this is common/expected, then perhaps my hypothesis that the pressure overtook the floats (i.e. tank pressure > pressure from float buoyancy) is garbage, and I just had a bad float/clogged overflow that allowed gas to pour in?

I also checked compression and all seem good there: (120-130 psi) cold and dry.
I guess I could take off the carb bowls and inspect the floats, overflow passage. Can I get to #2 and #3 while on the bike?
Beyond that, I guess I'll just try to test. If I continue to have the pressure issue overtaking the floats, I may make some holes on the side of the rubber seal to allow for venting through the puck seam.

Appreciate any thoughts / guidance.

[scottly]

I've just never experienced it because I've always had a crummy seal on the cap. It would seem to me that you'd want to have some sort of venting function to eliminate both negative and positive pressure situations. I don't believe petcock does so perhaps it happens at the carbs?

The vent for the gas tank is in the cap!

[scottly]

http://forums.sohc4.net/index.php/topic,193663.msg2261802.html#msg2261802

[bmcdonou]

Thank you Scottly! I think I was focused on the 'tank pressure' vs 'cap' keywords when I was searching before posting, so I did not run across that. Always hate to make people 'google' things for me.

I see hondaman's solution of penetrating oil and gas soak for a few days vs disassembly, so I will try that next.

[Oddjob]

Also answers the question about where the seal locates, around all the discs is the answer.

Personally I'd be tempted to remove the rivet like he did and check the condition of the rubber gasket inside, maybe that's collapsed and is blocking the vent hole somehow. As he says, just use a small 5mm screw and nyloc nut in place of the rivet.

I'll be doing this with one of my cap internals tomorrow so I can see how it looks inside, I'll then replate the discs etc with a coating of zinc/nickel to stop it rusting in the future.

[scottly]

Thank you Scottly! I think I was focused on the 'tank pressure' vs 'cap' keywords when I was searching before posting, so I did not run across that. Always hate to make people 'google' things for me.

The link I posted may not be the same cap as yours. Could you please post pics of both the top and bottom of your cap?

[Oddjob]

Took one of mine apart tonight, I was wrong btw, what I thought was a pinprick hole in the domed part was in fact just a dent, the vent is the triangular section. The seal underneath was in good condition, however as the author of that other thread said there was quite a bit of corrosion on the internal discs. All cleaned up now ready to be replated. One thing the other thread didn’t mention was that the holes for the rivet are actually 2 different sizes, less than 5mm where it’s peened over but around 6mm just under the head. The rivet is actually shouldered. A small top hat washer like the headlight bowl uses may be just the ticket I’m thinking.

[bmcdonou]

Mine looks like the one in the original thread. I certainly did not have the rubber around all discs; only the puck - so that was wrong. See pics attached.

I still can't visualize how this venting system functionally works. Seems to me if gas enters the triangle hole, it will be stuck inside the puck as the rubber is fitted around and seals it in. But I guess there is enough slack there to allow for venting?

Interestingly, as I just opened the tank to remove the venting device to take pics, it burped open. Its not warm in my garage. I guess its from swooshing it around last night (~20 hours ago) taking the tank off/on to do the compression tests. So fair to say I have little-to-no venting ability at the moment.

Last pic is just of the old rubber....just to see how bad it was. Fair to say....plenty of venting at that point in time.

[scottly]

what does the top of the cap look like? For instance, notice the small vent hole in this K7 750 cap.

[Oddjob]

I’ll post some pics up tomorrow so you can visualise how the vent works. The seal around the first internal disc is held away from the outer disc with the vent hole. This allows air to pass around it, the next disc has a hole near the dished centre to allow the air to pass through. The next disc is the top one of the trio you can see in your pics, the hole in its centre is a lot wider than the spring which passes through it, so air can pass through that. Finally there is the star plate, slightly dished and with a removable collar in the centre where the rivet sits. This then as you know locates in the chromed cap but is held away from a seal due to its shape, so air can pass to atmosphere. Can petrol escape if the bikes on its side, yes, but the way it would need to travel means it would drip rather than pour out.

I’ve tried hold my lips to the vent hole with the rubber seal installed and you can breathe through just that small opening so it clearly works. Air can pass either way, either in or out.

[bmcdonou]

This is how I thought it would work, but I missed the 'hole towards the dish centre' on the back/middle plate the corresponds with the front vent hole - if that's what you're referring to. I could not locate this hole on mine with a flashlight and in studying the pics in the link shared, could not see it there either. So I started to think it only vented through the outer seam, which made no sense given it's covered by the rubber seal. Would love some pics of yours when you get them!

I started soaking in penetration oil and will check every day or so. If by weekend it's not passing air, I'll drill out, clean, and replace with nut and bolt.

what does the top of the cap look like?

It's the standard chrome variety you find in the early-mid 70s variety. See attached pic.

[Oddjob]

Hang fire on drilling out the rivet, which by the way I just ground the top off. Less chance of accidentally drilling through anything inside that way I reckoned.

The problem is possibly a catastrophic fire/explosion caused by replacing the rivet with a 5mm screw. I'll advise more later and post some pics up.

I've been looking at all the parts of the cap and just spotted the disc with the seal around it also has a vent hole, so air can go through the outer vent hole, passes in a void in the seal disc and then through the hole there (slightly hidden by the seal, which is why I didn't spot it earlier), then through the hole in the next disc, this then means it's above the level of the outer seal sealing the cap to the tank, this can then vent to atmosphere etc. The outer seal BTW 100% goes around the outside of ALL the discs, if you don't get it around the uppermost disc it can move and it jams up the spring movement by sliding into the spring coils, it needs to be locked in place by the rubber seal.

I may have a way to avoid the fire/explosion risk but still looking around for suitable parts.

[bmcdonou]

Any updates, Oddjob? Mine has been soaking for several days but still not venting. So I think its time to take apart. I did realize that I have a spare in an old tank I acquired. Way more rusty than my current one - also not venting. I may start with that one just to have a practice run...then rebuild both.

[Oddjob]

Sorry, still awaiting parts to be delivered. My main concern is sparks, the original rivet is smooth all the way down, when the cap is pressed down this compresses the internal spring, the internal discs only just clear the rivet and the rivet is smaller than the 5mm screw you plan to fit to replace it. Of course you'll need to drill the holes slightly bigger in order to fit the new 5mm screw. However all (or should I say everyone I've checked up to date) screws less than 30mm are threaded full length, this means the threads form a serrated edge for the discs to rub against as the spring compresses, it's like the old fashion flint lighters, a flint rubs against a serrated surface and causes a spark. So I'm worried that it could potentially cause an accidental spark just as shut the cap on a tank full of petrol, that's to be avoided at all costs.

I tried to find 5mm screws partially threaded but not much luck, unless I can find some much longer and cut them down. Brass evidently doesn't cause sparks according to what I've read, so I've ordered some 5mm brass rivets and will be attempting to rivet it back shut. It that doesn't work out maybe some brass 5mm screws will work. Rivets arrived today, just need to experiment on how to peen them over to the correct length. Bear with me a few days and hopefully I'll have an update for you.

Am I being too cautious here?

[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.