I’ve recently restored a 1939 KB 24 Silver Star (350cc OHV).
Fully rebuilt engine, it fired up within a few kicks!
But……….no oil was returning to tank!
Diagnostic steps taken so far;
Ensure oil was feeding to the crank case.
Remove return line and ensure oil was ejecting when kicking over.
Change from 40w monograde (freezing Derbyshire workshop) to thinner multigrade. Now a small amount of oil is returning, but not feeding up to the rockers.
Remove the oil pump, non return ball valve and oil lines/fittings.
Blow through oilways to ensure not blocked (I did all this when I rebuilt the engine, but had to make sure).
According to the 1939 parts book, the same oil pump is used throughout the whole model range. The pump I have has no non return valve in the short pick up pipe.
My next step is to free up a pump I have from an M20 engine in my ‘stores’ and compare and contrast how much oil each pump moves, when operated with a hand drill.
Can anyone add anything constructive, in case I’ve missed something obvious, please?
I have photos of the pump base/pick up pipe, if needed.
It is very unusual for these pumps not to pump at all...Has the pump been worked on at all up to this point?...Pumping against a 'blind' gallery these pumps can achieve 60 psi before they start to 'back feed' past the gears..With a standard spring the anti drain ball lifts of its seat at approx 7psi so there shouldn't be an issue there...Clearance between the faces of the gears and the pump plates/body should only be a couple of thou...and of course most pumps have some gear tooth wear...I'd be inclined to fit the M20 pump and see what result you get, it would at least help to determine where the problem lies....There are also two different tooth forms on the pump gears and they aren't interchangeable...Something else to consider if the pump has been worked on previously...Finally, check that the oiltank breather pipe is clear....Ian
Hi Ian, I was hoping you would reply with technical know how, thanks.
Initially I thought the non return valve was to prevent back feeding, but then I remembered they only tend to be in the suction side of Impeller pumps (non positive displacement etc).
Thinking diagnostically it can only be two things…………not enough is getting into the ‘sump’ to be returned, which could be oilway or pump related. Or the suction side of the pump is lacking………am I thinking correctly here?
I eased the Mazak body of the pump, when I rebuilt the engine. It’s not been taxed since 1959 and the chaps father worked for Kings Of Oxford and took the bike in as a part ex. It may have been robbed for parts, I can’t be sure. But I suspect it’s its original oil pump. The piston was 20 thou oversize and it’s bore perfectly serviceable, so I don’t think the bike was out of service due to a knackered water pump…….
The sprung loaded ball is 'downstream' from the pump and is there to prevent oil draining through the pump into the crankcase when stationary...Pump pressure lifts it off its seat...In the base plate of the oil pump early ones didn't have a ball in the pick up tube...This was added later to stop oil drainng past the gears, through the pump and into the crankcases, thus by passing the sprung loaded ball which is in the oil feed drillngs...The base plate ball stayed on its seat with gravity only...It was retained by a wire clip..
Feed to the crankcases can be limited by a blocked oil tank vent pipe leading to a vacuum in the tank (a check on that is just to leave the cap open with the engine running), a blocked oil pipe, a defective pump (excessive wear), drive shaft failure or an incorrect spring fitted to the anti drain valve...Return from the crankcase, as you don't have a ball fitted in the pick up tube, comes down to pump wear...I'm assuming the pump to crankcase gasket is good...Ian
Ian!! It’s the pump. I tried the one I have from a 2nd hand M20 engine, after easing it so it span nice and freely.
It ‘spuzzes’ out way more oil than the other one when run at the same speed! The original one doesnt look worn inside, in fact the end plates look less worn that the ‘M20’ one. So go figure.
Kind of embarrassing, should have tested the pump before refitting it to the rebuilt engine.
Thanks for your technical input, it all helped to Wade through the thought soup!
Cheers,
Tim.
