The Ford Barn - View Single Post - Update on Vapor Lock and Changes made with Gas Cap/Sig difference in the 2 caps

James Rogers · 06-01-2010, 05:34 AM

[QUOTE=Mike V. Florida;19367]I want to take a little closer look at why a bad vent could cause vaporlock.

An unvented cap/tank won't cause "vaporlock" (wives tale). What happens with an unvented tank/cap is, when you remove fuel/liquid from a container it must be replaced by an equil amount of something else such as another liquid or solid or gas (air). In the Model A fuel system the fuel is replaced by air unless the tank/cap is unvented. In that case the fuel flowing to the carb is not replaced and causes the tank to create a vacuum which will eventually overcome the effect of gravity and starve the carb with stopping the fuel. The only way to overcome this is to add a vent to the cap/tank or open the cap. I have this problem on my cabbie and have to leave the cap loose to defeat the vent. I have tried several repo caps and all the gaskets seem to eventually rotate with removing and installing the cap at fuel time till the vent is covered. When this happens I only get a couple of miles and the car begins to starve for gas and eventually quits. If I let it sit for a while it will start and drive a couple more miles and quit again. This distance is dependent on the amount of gas in the tank, more gas less miles less gas more miles before it quits.

I still don't believe in vaporlock in a gravity fuel system.

06-01-2010, 05:34 AM	#19
James Rogers Senior Member Join Date: May 2010 Location: Asheville,NC Posts: 3,104	Re: Update on Vapor Lock and Changes made with Gas Cap/Sig difference in the 2 caps [QUOTE=Mike V. Florida;19367]I want to take a little closer look at why a bad vent could cause vaporlock. An unvented cap/tank won't cause "vaporlock" (wives tale). What happens with an unvented tank/cap is, when you remove fuel/liquid from a container it must be replaced by an equil amount of something else such as another liquid or solid or gas (air). In the Model A fuel system the fuel is replaced by air unless the tank/cap is unvented. In that case the fuel flowing to the carb is not replaced and causes the tank to create a vacuum which will eventually overcome the effect of gravity and starve the carb with stopping the fuel. The only way to overcome this is to add a vent to the cap/tank or open the cap. I have this problem on my cabbie and have to leave the cap loose to defeat the vent. I have tried several repo caps and all the gaskets seem to eventually rotate with removing and installing the cap at fuel time till the vent is covered. When this happens I only get a couple of miles and the car begins to starve for gas and eventually quits. If I let it sit for a while it will start and drive a couple more miles and quit again. This distance is dependent on the amount of gas in the tank, more gas less miles less gas more miles before it quits. I still don't believe in vaporlock in a gravity fuel system.