petcock question

[JRM83]

Hello all. I just picked up my very first bike. A gorgeous 81 xs400 special II. Now I've been getting acquainted With pre fuel injection technology. I've been stumped on the petcock. I understand it's operation in ON RES and PRI settings. EXCEPT I don't understand how it can fill the float bowls at full throttle in ON. It seem as soon as the throttle opens all the way the ball valve in the petcock will close. This will cause the float bowls to quickly drain. And since float level is so important for the proper operation how does it compensate. Especially when driving up a steep hill. Or on a top speed run. Or theoretically when "towing".

 

[ThePsuedoMonkey]

Welcome to the forum! Any pics of this gorgeous ride?

The vacuum that actuates the petcock is created by the venturi on the carb boots. As long as there is airflow through the boot, this vacuum will keep the petcock open. Besides this, they are exactly the same as a manual petcock: gravity pulls the fuel into the float bowls, which close a check ball in the carb itself when the float level is high enough. After enough fuel is used the float level settles and the check ball is released, and fuel again flows into the float bowl. I hope this helps, and you might be surprised at how far the bike can go on just the fuel in the bowls!

 

[HoughMade]

Exactly. It takes very little vacuum to keep it open and as long as high velocity air is passing an opening like that in the carb boot for the vacuum line to the petcock, enough vacuum will be generated. It's either on or off, it's not load dependent.