MORE IDEAS ON PETROL VAPORISATION

Regarding the letter from in last month's newsletter, I have too have had trouble over the years with petrol vaporisation on my Chummy. I could never understand how, with my straight gravity feed pipe with no upward bends that it could be possible for vapour to lodge anywhere to stop the petrol feed to the carb, any vapour bubbles would surely rise back up to the tank or be pushed down to the carb to escape out of the vent in the top of the carb.

I have just put my Chummy back on the road after eight years hibernation, so I thought I would investigate the problem prior to putting the car back on the road. I started by making up the gadget described by Rob van Beaumont from Holland in the Association magazine 1996C. I used it as described and was amazed to find that the level was just on the very end of the tube. Could this be the reason why I always had to flood the carb to start it, hot or cold?

I have now managed to achieve the correct level, but the float is right up to the top of the carb. I have also had to put an extra fibre washer under the valve in the bottom of the float chamber. This now leaves no future adjustments. I now put forward my hypothesis that the problem thought to be varporisation, is infact due to modern petrol having a different density than that of petrol of years ago. The float is higher in the petrol thus dropping the level too far below the jet and starving the engine of fuel. The hotter the engine gets, the less dense the petrol is and the more the fuel level drops. I have a 1928 Zenith carburetter book on the 22FZ and it says that the carb is supplied adjusted for spirits of densities between 700 and 750. I will be writing to a petrol company to find out if the densities of modern fuels is different than older fuels.

With the correct level now achieved the Chummy runs much better and also starts easier when hot and cold. However, the carb would never have been designed to run with the float right at the limit of its adjustment. If anyone has any ideas on this please respond in the next newsletter or e-mail me on ianmoorcraft@talk21.com. I will try to get a response from an oil company before the next newsletter.