Vapour Lock Gas Tank

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I have tried everything to get my 74 Commando with a roadster tank to stop vapour locking. I drilled out 3 additional holes in the ring under the cap; I cleaned it thoroughly...nothing seems to help. I have to ride with the cap open or open it periodically when the bike starts to stall. It is a standard steel original roadster tank with the standard cap. Can anyone give advice? Thanks!
 
Quebec Bob said:
I have tried everything to get my 74 Commando with a roadster tank to stop vapour locking. I drilled out 3 additional holes in the ring under the cap; I cleaned it thoroughly...nothing seems to help. I have to ride with the cap open or open it periodically when the bike starts to stall. It is a standard steel original roadster tank with the standard cap. Can anyone give advice? Thanks!


Same bike. I had the same issue after I put a new cap gasket on. It would go for a couple miles and then create a vacuum. I did not drill any additional holes. I sprayed some solvent in the original hole and used a wire to probe it and some compressed air to clear it. Come to think of it, I am not sure where it vents to after the hole is cleared.
 
drones76 said:
I sprayed some solvent in the original hole and used a wire to probe it and some compressed air to clear it. Come to think of it, I am not sure where it vents to after the hole is cleared.

The cap seal holder is made up of two sheet metal plates joined together. Each plate has a breather hole which is offset from the other but the holes are connected by the circular 'gallery' so the cap acts as a splash baffle, both holes and the gallery need to be clear for the tank to breathe properly.

Vapour Lock Gas Tank


Vapour Lock Gas Tank
 
Thanks LAB. Now I remember. My cap has a rivet instead of the screw that yours shows. I was concerned about disassembling it if the air did not do the trick.
 
Since the cap doesn't work properly anyway, you've got nothing to lose by drilling out the rivet, drilling and tapping the cap and replacing the rivet with a screw. This way you can remove the sealing disc and clear out the venting path.
 
I followed the same ideas and had the same concern about the rivet, but you are absolutley right: it's not working anyway so I'll drill out the rivet and follow it through. It's amazing how long I will let a situation go along without doing something about it over such a little fix as a rivet...but now that you have shown me the second, interior ring in the photo I really get it. Thanks for the excellent replies.
 
Quebec Bob said:
I followed the same ideas and had the same concern about the rivet, but you are absolutley right: it's not working anyway so I'll drill out the rivet and follow it through. It's amazing how long I will let a situation go along without doing something about it over such a little fix as a rivet...but now that you have shown me the second, interior ring in the photo I really get it. Thanks for the excellent replies.


How about taking some photos of your cap, when you disassemble it, so that we may see what you find.
 
Might just try pulling out the rubber ring and scrubbing the space clear of clogs then just work a new rubber ring back in. If that don't do it then its ancient practice to drill rivet and screw back.
 
hobot said:
Might just try pulling out the rubber ring and scrubbing the space clear of clogs then just work a new rubber ring back in.

Unfortunately the gallery isn't accessible when the sealing ring is removed.

What I did just notice when I pulled the seal away to check is that there appears to be a third plate (like a dished washer) between the other two.
 
Ok I've not had vent clogging to look deeper, mostly just leakage with full tank, so paying attention if vent clog strike me later too. Its mettle testing that such a tiny feature can be a show stopper and mystery to solve but that's why Commando's aren't for everyone, including me at times.
 
I pulled mine apart when I sent it in for chrome. It's easy to drill and tap a #6 or 8 screw into it, then it can always be dis-assembled easily.

Dave
69S
 
The rubber seal on my filler cap was perished and no matter how much I fiddled about I couldn't get the new one to stay put. So had to revert to drilling out the rivet and rebuilding it with a self Tapper, you would not have believed the amount of muck that was in between the plates!
 
I had the same problem with mine when I replaced the seal. The previous owner sealed the tank with the Kreme crap and he managed to seal up the vent in the cap. Wasn't aproblem until my new gasket actually sealed the cap. I just used a small screw driver and a knife to get into it to peel away the gunk. Then it was fine.
 
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