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Fuel Tank Not Venting

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I have a 1972 Norton Interstate Combat Commando.  I am the bikes only owner. Been riding her for 48 years.

 

The problem:  Last week the motor started running on one cylinder for a few seconds and then the other quit.  All electrics were sound – good horn, bright lights, etc.; fuel tank full.  No oil in point cavity.  The feeling was as if it had run out of gas/petrol.

 

After waiting about 5 minutes, I kicked it over and it started on the first kick.  I drove it back a mile to my hangar located on a small airdrome in the country.  Rode it up and down the taxiway and it quit again.

 

I put it on the hoist and disconnected both fuel lines from the tank.  With both petcocks open very little fuel was coming from them.  So before draining the fuel to investigate the filters on the petcocks, I just opened the gas cap.  That produced good fuel flow from both ‘taps’.

 

So here is my question.  How does the Norton gas cap allow air into the tank to prevent a vacuum that prevents the fuel from reaching the carburetors? 

 

The problem is obviously in the cap’s vent system.  When it shuts the rubber seal on the metal retainer ring is snug up against the rim of the tank entry.  There is a spring pressure on this fuel retainer.  The only thing I can see is a very small hole on the metal retaining ring that, with the help of the rubber seal on its outer circumference, seals the fuel from splashing from the tank.  The only way I can see that air would then be ‘sucked’ into the tank through the small hole in metal part of that retaining disk.

 

I do have the gas cap with the locking mechanism that came with the steel tank I had to purchase to avoid the bad issues with methanol and fiberglass.

 

I cannot see any blockage in that small, what I believe to be the vent hole.

 

It runs perfectly with the tank gas cap open and secured only by the rear roll pin.  Such a simple problem that any dolt should be able to sort.

 

Your thoughts would be greatly appreciated.

 

John Morrissey

In the colonies

NOC 402598

 

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The small hole in the cap is in a sort of raised ring. There is another hole in the top of that ring but not in line with the bottom one so that air can get through but petrol doesn't, The air goes through that raised bit, or should. Probably blocked somewhere. Try an airline to blow through it.

Ian

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Had same problem on my Mk1a 850 after it just quit when steaming down a dual carriageway half a mile from home. What I should have done is just flip open the fuel tank cap which would have relieved the vacuum created by blocked breather hole and engine would have cut in again as I was winding down hugging the kerb - instead ground to a halt and pushed bike home !  Having diagnosed tank cap vent hole blocked gave it a thorough clean with carb cleaner removing copious amounts of white powdery crud , and have not experienced the problem since.

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Not saying this is the problem but on another make of bike I have someone had fitted a pattern part fuel cap and that had no vent. It looked the same but did not vent under the cap. The rubber seal effectively did just that - sealed the air out!

I tested the cap by turning it upside down away from the bike and poured a small amount of petrol into it. Nothing leaked out proving there was no route for fuel to get out or air to get in. The original cap has a tiny pin hole vent, off centre  of the cap, so I drilled one and cured the problem. After a long run you can hear air being sucked into the tank when the engine is switched off.

Some caps vent under the lid across the inner and out surface so it may not be obvious if they are vented or not.

Dennis

Dear James, Ian, et all.

Exactly!  Your replies spot on.  I removed the round plate with the offset holes (top and bottom)and noted an airway that would allow air in.  I disassembled the plate and soaked it in Hoppes #9 gun cleaner (same stuff I use to clean our fuel injection nozzles in my aircraft engine.  A few hours in the #9 cleaner and a high pressure blow through and all is well.  There was a spot of rust/debris firmly attached to the upper hole as well.

Thank you for your prompt and perfect replies.

 

John 

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I have a one owner 1972 Combat Interstate Commando with ~ 50,000 miles.  I have kept the cam chain properly adjusted and always keep good fresh oil (Vavoline 50 wt racing oil that has a touch of zinc).

What has been the reliability experience of the chain?  Not certain, but if it fails I would imagine that some valve head piston top clamoring might happen.  I might ad that I have placed a personal 6,000 rpm limit on the engine and have had no serious problem with its reliability.

John Morrissey

NOC 402598

 

Hi John, interesting reading your post that you now have a steel tank replacement for your fibreglass one. I have a '68 Fastback and Ethanol has eaten the tank so I am contemplating a steel or aluminium replacement and would be interested in where you bought yours and if it fits properly. 

Many Thanx.

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John:

Bad karma to change subjects in the middle of a posting.

A lot of people who were not interested in your first posts will likely miss your addendum.

Better to start a new posting.

Just saying

Mike

 

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