Subject: RE: Why does the vented gas cap not work?
Author: Keith Ruff
Date: Feb 15, 2005, 7:28 AM
Post ID: 1718394734
Hey Greg,
When I had "Patience" the 71 Ambo, I ran clear fuel lines to monitor
flow. What I found out was that the gas stopped flowing when I had
vapor lock, so the carbs would run dry. I cleaned the vent on the cap
AND also squirted some carb cleaner in the vent holes on the carbs.
When I squirted the carb vent holes, it was like a giant pressure relief
and the gas shot right down the fuels lines and filled up both sides.
The vent lock, I believe, is starving the engine from lack of fuel
flowing out of the tank through the fuel lines.
Keith Ruff
70 Cafe Racer Ambo
Manahawkin, NJ
Greg Bender wrote:
OK, I'm sitting at my desk with two identical gas caps in front of me. They are both from Dennis Kirk and are of the vented nature. Each has a nifty one way check valve that allows air in, but does not allow gas or air out. It is easy to test just by sucking and blowing to see how it works. So, what's the big deal? Well, too much pressure inside the tank will cause the engine to faulter until the pressure is released. The engine begins to act like it is starved of fuel and I can hear the valves start pinging more. This has happened to me many times while riding and the cure is always to loosen the gas cap and allow the excess pressure to escape. I am certain that the cause is too much pressure building up inside the tank because I've stopped (more than once) and put my fingers under the cap while unscrewing it...I can definitely feel air escaping. This effect is also in line with the design of the gas cap, to let air in, but neither gas nor air back out. I believe that temperature changes (cold gas being warmed by the engine and ambient temperature change increases) cause the gas (and the air inside the tank) to expand more quickly than the vacuum caused by the removal of gas is able to equilize the pressure. Now, I'm going to drill some holes in one (as per John Ulrich's advice). But, my question is this: Why does too much pressure inside the gas tank cause the engine to faulter in the first place? Is it forcing something to close off inside the carbs? Regards, Greg Bender 1971 Ambassador 2000 Quota http://www.thisoldtractor.com/gtbender |