Fuel starvation? – 4 May 2017.

It’s taken me a while to get around to writing this post. Other stuff has been going on!

On a couple of occasions recently The Fire Bike has come to a halt without any prior warning. I’d be riding normally and the bike would appear to run out of fuel. It would be just as if I needed to switch over to “reserve”. However there was plenty of fuel in the tank. The first time it happened, I had refuelled and the bike faded about 40 miles later. I pulled over suspecting a vacuum in the tank and opened the filler cap but there was no sound of air rushing in. The bike then ran without any problem for the remaining 50 miles or so home. On the second occasion I had ridden the 20 miles to Aberystwyth seafront where there was a MAG gathering. After an hour or so I left to call at some shops nearby then set off for home. After about 5 or 6 miles the bike ran out of steam again. Again I opened the fuel filler but heard nothing. The bike restarted and I carried on the rest of the way home as if nothing had happened.

I hate it when things like this happen and the fault can’t be recreated in the workshop.

General opinion amongst fellow Guzzisti supported my initial thoughts that the problem was caused by fuel starvation with a second possibility that the ignition coil was beginning to break down when hot. The brief stop being long enough for it to recover. I ordered a replacement coil, just in case, but decided to check the fuel system from tank to carbs and the ignition wiring.

Before doing that, I thought I would just see if I could get the fuel supply to the carbs to fail. I made sure there was a decent amount of fuel in the tank and that the cap was done up tight. I then disconnected the fuel line from the running tap (as opposed to reserve) and added a short hose into my fuel can. I turned the fuel on and watched as it all ran out! I had hoped that the flow would reduce until it stopped or sucked air up the pipe proving that the filler cap was blocked. However it all ran out unimpeded.

The next step was remove the tank so I could make my checks but, first the seat needs to come off.


Now the tank itself can be removed.


  • Electrical connections in the battery compartment. Especially earths. All good.
  • Connections at the coil. These were fine.
  • Connections to ignition switch. All good.
  • I rattled the key in the ignition switch but couldn’t make it fail.
  • Fuel lines. All clear.
  • Fuel taps and filters. Fine.
  • Fuel filters at carbs. Clear.

I went back to the fuel tank which was sitting on my bench in the sun. When I opened the filler cap I quietly swore as I heard it exhale! If the expansion caused by the tank getting warm can’t be vented then neither can air get in to replace the fuel as it’s used up. So, after going around the houses, I was back at my first suspicion that the fuel cap was the problem. This is despite the fact that the bike has run without any problem for the last 2 years.

I had always suspected the cap because, way back, I had puzzled about how it was vented. There were two small holes outside the area covered by the rubber seal but no corresponding vent hole inside the seal that I could find. I drilled a 2mm hole in the inner layer of the fuel cap where I would have expected to find a vent hole. In this picture you can see my newly drilled hole and the outer two holes to the left and right of the rubber seal.


To test it out, I squirted some WD40 into the hole I had drilled and saw it come out of the other two. I then washed it out in petrol and refitted it.

Before I refitted the tank I straightened out the rear mounting flanges. They had been crushed a bit. I had bought a stainless steel rear tank mounting bolt a year or so back so this was fitted as well. The old one was pretty nasty.


I haven’t got a replacement front bolt yet. This is a stepped bolt which prevents it being over-tightened and could be something to make on the little old lathe I’ve bought. It’s just about set up now.


Anyway, Everything went back together and a few days later I managed to complete a 55 mile test ride without any sign of trouble. Time will tell if I’ve really solved the issue.

I never did fit the replacement for what is probably a 45-year old ignition coil. It arrived damaged and was returned but I haven’t bought another. I wouldn’t have fitted it anyway until I was sure it wasn’t a fuel issue.