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Blocked fuel cap vents

Tom Gaynor

Well Known and Active Forum Website User
VOC Member
I'd always thought this might be a hoary motorcyclist myth, until, you've guessed, it happened to me. Many people are unaware that Scottish section camping weekends have to be earned, usually by cresting Slocht Summit (1300 feet) 10 miles south of Inverness, in pissing rain. If the rider proves to have the right stuff, he is rewarded with a splendid, dry, weekend, in stunning scenery. And his kit is dry by Dingwall. No, really.
Battering up Slocht on Friday, in pissing rain, see above, the twin abruptly stopped battering, and went into "pathetic plonk" mode. Rice puddings retained their skin. Opening the throttles had no effect, nor did switching to the 150 yard reserve capacity. About a mile on, it stopped. So I took the tank bag off, and checked the fuel. Plenty fuel. Replaced cap, and tank bag (and the sticky mat underneath it) and pressed starter. Engine restored to health, no further problem. Sun came out, sky cleared, and on I went to Achmelvich Bay. If you have Google Earth, look it up. It was like that all weekend.
I reckon I had created enough vacuum in the tank to overcome suction from the carbs, which would explain why opening the throttles made zero difference to revs.

Discuss.


Tom
 

rapcom

Well Known and Active Forum Website User
VOC Member
Not a myth, Tom, it has happened to me too, twice, both times caused by tank bag covering the cap. I reckon it takes about 35 miles from a refuelling stop before the bike goes on to one pot, then stops. The symptoms are so like running out of fuel that you instinctively look in the tank, which of course cures the problem. I'm still wondering how to fit a decent breather to a standard Vincent tank cap.....
 

BigEd

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VOC Member
VOC Forum Website Moderator
A few years ago my brother was riding a vintage Sunbeam in the Banbury run and I was following him around the course. After a few miles The Sunbeam lost power and stopped. He suspected the magneto. (Classic hot magneto syndrome; it works until hot and then fails plus it is easy to blame the magneto as it can't defend itself unless you are holding the plug lead as you crank the engine.:eek:) After a few minutes fiddling about and checking for a spark the engine started and ran OK ....... for a few more miles.
This occurred several times until eventually we realised that the petrol filler cap vent hole was being sealed by the route card in a plastic wallet that was taped to the top of the petrol tank. A few seconds work making a strategic hole through the card and wallet saw the Sunbeam running perfectly for the rest of the day. Each time we stopped the carburettor must have slowly filled with petrol again.
When using a tank bag on my Rapide I have a short piece of wood that sits on top of and across the petrol and oil filler caps leaving the two breather holes exposed. Another piece of wood with two concave sides )_( is screwed underneath the first piece of wood. This locates between the two caps and keeps things in place. Cheap and cheerful, a bit like myself. ;-)

I reckon I had created enough vacuum in the tank to overcome suction from the carbs, which would explain why opening the throttles made zero difference to revs.

Discuss.


Tom
 
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peterg

Well Known and Active Forum Website User
Non-VOC Member
Plugged: relationship of air to fuel volume

Howdy Chaps,

If you've truly got a restricted/plugged fuel cap it will resort to misbehaving much sooner with the cap back in place the more fuel you have in that tank as liquid is not readily compressible or in this case expandable as plain atmosphere is.

I'd just gotten my basket case Shadow back on the road, few test miles with partial fuel load, ran great. Met one morning to go on a 200 miler with a local classic bike club and filled it to the base of the neck and made sure that cap was on tight. Within 5 miles it was running on one pot under load. Pulled to the roadside thinking a jet plugged and commenced to remove the jet block, to the accompanyment of a faint treblish whistle which subsided during this delicate operation. Five or so miles later, same scenario, this time I disturbed the cap and there was a mild rush of air: engine heat, once stopped on this chilly day had heated the small air cavity, expanding it, thus the whistle. Correspondingly, drawing a vacuum when chilled, once underway.

In a nut shell, I had to crack the cap less frequently the more fuel had burned off so subsequently only filled the tank half way each time to get home.
 
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vince998

Well Known and Active Forum Website User
VOC Member
Howdy Chaps,

If you've truly got a restricted/plugged fuel cap it will resort to misbehaving much sooner with the cap back in place the more fuel you have in that tank as liquid is not readily compressible or in this case expandable as plain atmosphere is.

I'd just gotten my basket case Shadow back on the road, few test miles with partial fuel load, ran great. Met one morning to go on a 200 miler with a local classic bike club and filled it to the base of the neck and made sure that cap was on tight. Within 5 miles it was running on one pot under load. Pulled to the roadside thinking a jet plugged and commenced to remove the jet block, to the accompanying of a faint treblish whistle which subsided during this delicate operation. Five or so miles later, same scenario, this time I disturbed the cap and there was a mild rush of air: engine heat, once stopped on this chilly day had heated the small air cavity, expanding it, thus the whistle. Correspondingly, drawing a vacuum when chilled, once underway.

In a nut shell, I had to crack the cap less frequently the more fuel had burned off so subsequently only filled the tank half way each time to get home.

I won´t bother repairing my leaky tank then!!
If i glue the vent hole up, it´ll suck air in through the pin hole as opposed to leaking fuel out :rolleyes:
 

peterg

Well Known and Active Forum Website User
Non-VOC Member
Howdy sir,

Funny you should mention that, I hold yard equipment in general disdain: the lawn mower has had a dirty gym sock for an air cleaner for 10 years and being a two stroke is now protesting that neglect with a loud rod knock. My weed whacker/hedge trimmer/edger leaks fuel like a sive around the two plastic fuel lines that go into the tank ungrommeted. I epoxed the gas cap vent shut, store it with no fuel in it, only filling it up right at the moment it's to be started, from which point it runs like a top breathing the tank around those two lines. Also helps that 2 strokes like to run in a vacuum, explains why they'll run upside down with a carb.
 

vince998

Well Known and Active Forum Website User
VOC Member
Howdy sir,

Funny you should mention that, I hold yard equipment in general disdain: the lawn mower has had a dirty gym sock for an air cleaner for 10 years and being a two stroke is now protesting that neglect with a loud rod knock. My weed whacker/hedge trimmer/edger leaks fuel like a sive around the two plastic fuel lines that go into the tank ungrommeted. I epoxed the gas cap vent shut, store it with no fuel in it, only filling it up right at the moment it's to be started, from which point it runs like a top breathing the tank around those two lines. Also helps that 2 strokes like to run in a vacuum, explains why they'll run upside down with a carb.

:D In England we call it bodging.
I actually took part in a bodging course during my military time (they gave it an official name "Battle Damage Repair"). I seem to remember having to make up a distributor cap for a petrol landrover using a bean tin and black nasty, and half tracking a main battle tank as a "get your arse out of the firing zone" repair
Never had so much fun with me pants on!!:D:D
 
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Trevor Rowlands

Well Known and Active Forum Website User
VOC Member
Years ago my old mate Wilf Owens had the same problem while we were touring Europe,he had soft sponge under his tank bag, when we got home I drilled a hole in the centre of a spare petrol cap and fitted a small right angle oil conection with a nut on the inside, I then put a length of screen washer pipe on to go under the tank bag, no problems since, a simple job,even a plasterer can do it.Trev.
 

Len Matthews

Well Known and Active Forum Website User
VOC Member
Not a myth, Tom, it has happened to me too, twice, both times caused by tank bag covering the cap. I reckon it takes about 35 miles from a refuelling stop before the bike goes on to one pot, then stops. The symptoms are so like running out of fuel that you instinctively look in the tank, which of course cures the problem. I'm still wondering how to fit a decent breather to a standard Vincent tank cap.....

Yep, me too! I lost one cylinder after five miles from home when setting off to IOM TT. Great, I thought, only another two hundred plus miles to do. What's happened? Ignition fault. loss of compression or what? No, just as Dick says foam base of tank bag had blocked the tank cap vent. Phew-what a relief!
 

Tom Gaynor

Well Known and Active Forum Website User
VOC Member
Now THAT'S what I call thinking out of the box. Genius! Cure a pinhole leak by converting it to a fuel tank breather!
 
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