Jawa oil pumps....

I am very fond of my Jawa model 634 350cc twin, it is identical to the bike I had when I was 18, but without the tradesman's box sidecar I had in those days (I always was a bit "anti-fashion"). Originally the bikes were fitted with a 2 stroke oil pump. These are something of a disaster as they always seem to pump too much oil into the engine and are so stiff to operate they usually break the throttle cable every few months. I remember on at least one occasion when I was a teenager I had to get home with a pair of mole grips clamped on to the broken cable end to operate the throttle. On my present machine, a previous owner has done the right thing and disconnected the pump, oil now being mixed in with the petrol at a ratio of approximately 30:1. However, the oil pump remained, still driven by the end of the crankshaft and just pumping oil round through a loop of plastic pipe back to the pump inlet.

Recently oil has been escaping from the oil pump housing, so on Friday evening after work I set to. I removed the oil pump and the gasket where it bolts on to the engine appeared to have failed. I don't have another but I decided I would stop the oil pump from driving altogether so it can never fail out on the road. The oil pump has a quill drive and I attempted to cut it with a hacksaw so that the oil pump could not turn with the crankshaft. Two blunt blades later I gave up, the metal is surprisingly good quality and has been well hardened. Instead I stripped the oil pump down to its constituent parts and removed the drive shaft altogether, fixing the pump body back on to the engine with gasket sealer.

Later today I intend to go out for a ride to see if this modification has been successful. I do now have a worry that the pump body will fill up with gear oil and leak. If this does happen, I have a spare pump which I can fit and be back to where I started so I cannot really lose out. What I really need is a replacement primary drive cover for the engine with no oil pump fitted, as used on earlier and some later models of this bike. I have yet to find one though.

More later, with pictures if I can work out how to post them!