fuel return line/fuel starvation/air bleeding

[Diesel Graham]

I just thought I would pass on some experience incase others have similar issues on their engines:

Air bleeding:
the easiest way to bleed air from the system is to fit a rubber bulb pump between the tank and filter. After changing my filter, I bled air from the entire system with 5 pumps on the rubber bulb. After hitting reserve on the motorway I no longer need to pull over, remove the bleed screw and blow into the tank, I just reach down, squeeze the bulb and switch the tap onto reserve. If you dont have one already get one!

Fuel return lines:
my understanding is that the fuel return lines should be above the level of fuel in the tank, neat as it may be to have one combined under the fuel tap you do risk un-filtered fuel running back down that line and reaching the fuel pump (see next paragraph)

Fuel starvation:
when I bought my bike, the seller told me "the only trouble you will ever get will be in the fuel filter" (he was right)
The original oiwner had fitted a car filter.... better filtration results in a greater restriction to the flow, I found this to be a problem as I reached half a tank and the engine stopped. No more fuel would pass the filter until a greater head of fuel was added (or the filter removed).
I fitted the original mico-bosch filter and all was well until some dirty fuel started to clog it. Despite a healthy head of fuel (10 litres or so) fuel would not reach the engine reliably. A quick squeeze on my rubber bulb had the engine running again but after 100 yards it died again.
This got me thinking... why is this high pressure pump not able to pull fuel from the tank even though it can be pumped at low pressure using the hand pump?
Well big thanks to MOUSE for his suggestion that air is being pulled in through the fuel return line:
Any filter restricts the flow, if it becomes easier to pull air through the return line than it is to draw fuel through the filter then air will enter the filter bowl and the engine stalls. Now for those of you with return lines below fuel level, in this situation you will probably be drawing unfiltered fuel through the return line and into your pump without even knowing it!

I have fitted a one way valve into the fuel return line and even with dirty fuel and a partially clogged filter, the engine runs well and no more air has entered the system.

[Diesel Dave]

Hi Graham,

Certainly true that virtually all the running problems you ever get with a diesel are fuel feed related.

Essentially I use 3 filters in the system, there's the filter pipe on the tap that sticks up into the tank, then I have a glass bowl type with a magnet in the bottom and then a large aftermarket inline jobbie set vertically with an old bicycle valve epoxied to the top so I can bleed the air out.

Replace filters annually though, regardless of mileage, and if you leave e bike unused for any length of time then make sure the tank is full.

[Diesel Graham]

"make sure the tank is full"

what is the theory behind that?

[Diesel Dave]

It stops the tank breathing vapor laden air in and out with daily temperature variations.

Eventually droplets form and sink to the bottom of the tank where they build up causing havoc, rusticles and black gooey scum where the border meets the fuel.

[Diesel Graham]

that makes sense and would also explain why one of my 2 spare tanks has gone rusty inside, the only one that had the cap on.

[Diesel Dave]

If you look at the stationary engine tanks the lowest point is usually a plastic bowl with the filter in it - protects the metal tank.

[coachgeo]

It stops the tank breathing vapor laden air in and out with daily temperature variations. ....

Should read.... "water" vapor laden...

also the water vapor invites algae

[coachgeo]

If you look at the stationary engine tanks the lowest point is usually a plastic bowl with the filter in it - protects the metal tank.

ahhh.... very wise consideration to keep in mind when designing a diesel bike