Subject: Fuel Flow 101
Author: Keith Ruff
Date: Apr 11, 2003, 7:55 AM
Post ID: 1712586677
I need an education on how the fuel flow works from tank to carb. On
all of the BMW's I had owned in the past, a fuel line from each petcock
connected to a crossover, which then had fuel lines running from the
crossover to each carb. This is the exact same set up on my 71 Ambo. I
used to also run in line filters on my Beemers but have not done so on
the Ambo. When I opened the petcocks on the Beemers, fuel would flow
through the lines, down to the carbs, and fill all the way up back up to
the petcock. There was fuel throughout the entire line, either when
running or not running. The Ambo has got me confused. When I open the
petcocks, nothing flows. I can see fuel in the lower lines just below
the crossover, but the fuel will not fill the remainder of the lines. I
just checked the petcocks, removed the hose from one side and put a test
fuel hose on it and ran it to a container. Opened the petcock and fuel
was a flowin'. Closed the petcock, let the line drain, then put my
thumb on the end of the hose. Opened the petcock and NO FLOW. So it
appears any air in the line stops the flow. I then tested the right
side petcock while the left side was disconnected so air could escape
and it flowed perfect, but gas would not flow when I put my thumb over
the open end of the left side fuel hose. Can I deduce that the petcocks
are not functioning correctly? Should the petcocks allow air pockets in
the lines to escape up into the tank, or is that the job of the carbs?
I squirted some carb cleaner into the carb vent holes but that did not
change the situation, still no flow out of the petcocks when the lines
were reconnected. Does this sytem not want fuel all the way up the
lines to the petcocks, but rather just enough in the lower lines to feed
the carbs? Does it let air escape through the carb vent holes when the
carbs need fuel, thus letting fuel flow out of the petcocks? I am
confused at the operation. I am not sure if I am worried about nothing
or if I have a fuel flow problem. Please educate me.
Thanx!
Keith Ruff
71 Police Ambassador - NJ