Badly designed or failed fuel systems are actually one of the leading causes of engine outs and forced landings. Causes range from clogged filters and pinched lines to rotten primer bulbs and dried out pumps. Yet the cold facts are the simplest of oversights can put an end to fun weekends and expensive aircraft . . .
Full Article by Mike Stratman, below :
https://www.cps-parts.com/cps/pdf/Part41.pdfSome extracts:
Impulse pumps function (as their name implies) off a pulse created in the crankcase of the engine. As the piston travels up and down, the area below the piston is subject to alternating pressure and vacuum, much the same as the combustion chamber itself. This pulse is used to flex a diaphragm inside the fuel pump. One way flapper valves route the fuel thru the pump in a one way fashion.
A lot of people will run the dual round pump even on a single carb engine by routing the lines back together after the pump. If you inspect the internals of the pump you will find a common discharge cavity in the round pump. You will also find a much more sophisticated style valve than the flappers in the single or rectangular pump. The thought of hang your hide on a dangling piece of gas-ket material is a little unsettling. But for as simple as these pumps are they do a surprisingly good job.
Because these pumps work off the crankcase impulse there are several considerations. First the line from the crankcase to the pump must be no longer than 12” and must be fairly rigid so as not to absorb the pulse. A one piece or solid line is preferable. Hoses using several lay-ers can separate internally collapsing the passage and are impossible to detect from external inspection. You must also consider that because they function off the crankcase pulse, they are subject to performance based on engine rpm. A constantly changing variable in the equation of fuel delivery.
Pulse Pump Weep Holes: Recently a brouhaha has raged over the existence of the weep hole in fuel pumps. If you look very carefully you should find a microscopic hole (.017” or about 1/64” diameter) either in the indent on the rectangular pump or on impulse boss on the round dual pump.
Rotax has issued a service bul-letin claiming that all aircraft pumps must have this hole. This hole is there in case the operator installs the pump in such a way to allow fuel from the crankcase pulse line to fill the pump. This hole will allow the pump to purge the fuel rather than become inhibited with a blockage. The pump must be mounted so that this hole is at a low point. This will allow any build up to by purged quickly. Theoreti-cally the fuel should never reach the pump if the pulse line is installed correctly. The line should run up hill at some point before reaching the pump, kind of like a toilet trap.
I have heard unsubstantiated reports of dramatic changes in pump performance with the hole covered or not. The weep hole obviously compromises the pulse cavity of the pump with a “leak”, although very small, leading me to think there may be some truth to this rumor. It would cer-tainly make for some interesting tests if someone wanted to take the time. Regardless, in order to keep current with the bulletin from Rotax, we are carrying only the weep hole flavor.
You should check when purchasing a new pump to see if they have the weep hole type.
Rebuild kits are available for most brands of impulse pumps containing all diaphragms and gaskets, allowing you to zero time most any pump. This should be done an-nually to avoid dried out and/or cracked diaphragms or gaskets.
+++++++++
Logged
