Oil pumps, and more specifically Suzuki oil injector pumps, have been occupying a good deal of my attention recently. For an in depth understanding of what they are and how they work, I suggest you take a look at the excellent overview of Suzuki GT750 injector pumps written by a friend of mine, Richard Nowson, which can be read at this link. The design of these for the Suzuki GT750 is quite robust, but they are also 40+ years old now, and like all things mechanical (and otherwise I suppose !) are starting to show their age. I needed a working pump for my current project (which is documented here), and so I dug into my ‘box of bits’ to see what I could come up with.
The oil injector pump on a two stroke is obviously a critical engine component and if the pump fails you risk having a solid block of scrap metal in fairly short order ! It is possible to run pre-mix, but due to the design of the crankshaft bearings, doing so is not recommended. Before trusting your engine to a pump with no known history, as a minimum it makes sense to bench test it and I wrote about one way to do that some years ago at this link. Since then, I have made a bench top tester from a scrap crankcase I had. The kickstart shaft on the Suzuki GT750 also drives the oil pump, so a friend of mine turned down the end of the shaft on his lathe so I could mount a pulley and then drive the assembly from a fractional horse electric motor. I’ve attached a video of it in operation below:
I really should fit a guard as the HS&E folks I used to work with would likely have a fit, but it does the job.
If you listen to the sound of the pump, you can hear the change in tone as the pump lever is pulled forward. Below is a photo (just ‘click’ to see a larger image) of the underside of the top cap showing the rod that is being rotated. The slot in the middle rides against a pin on the top of the rotating piston. As the shaft rotates, the slot allows the piston to increase its stroke and so pumps more oil. It is important to note that the pump always pumps at least some oil – backing off the pump arm adjustment will only decrease oil flow at higher throttle openings, so if your engine smokes a lot at idle then backing off the adjustment doesn’t actually do anything. Over time, what can happen is that the top of the rotating piston will wear, and/or the shaft will wear, and should this happen the pump stroke increases and more oil is pumped – a sort of built in fail safe design I suppose.
Note: when reassembling the cap, it is possible to reinstall the shaft 180 degrees out of phase so that the pump will run at full output volume at engine idle. Make sure that the slot in the shaft faces away from the bleed screw and toward the pin as shown in the photo. Also note that later style pumps do not have the pin.
Often when pulling the rotating piston out of the barrel, you forget to hold the pump upside down, and so all the springs and pins fall out of the rotating piston ! Early pumps have three pins/plungers and later ones have only two. Should this happen to you, don’t panic – the springs are all the same and the short plunger fits the bore having the draw hole nearest the driven end of the pump shaft, as shown in the photo to the left.
The third plunger in the early pumps actually is used to draw oil from the injector tank and was found to be unnecessary and removed. I don’t recommend mixing and matching pump parts as there are a number of design differences.
For example the rotating piston in the early type having the third plunger also has a restricted inlet at the top of the rotating piston. Richard goes into much more detail about the different pump designs in his write-up.
There is an oil seal in the underside of the pump body which is still available from Suzuki (part 09285-08002). The rest of the pump was not intended to be user repairable, but as is often the case necessity is the mother of invention. New top gaskets are available from several places – I buy mine from Reiner in Germany as the exchange rate for the Canadian dollar usually favours the Euro, but Ian Beardsley in the UK (check this link) also sells them and the quality is excellent. The same gasket is also used on pumps fitting the GT550 and the GT380.
One item that so far at least I have not been able to source is the small u-cup oil seal used on the actuator shaft. When this seal fails, injector oil starts to leak out the end of the shaft at the top of the oil pump. This seal is an odd size, and while I have managed to find some that are close, the fit isn’t ideal. I’d be happier if there was more contact area between the seal and the shaft bore. The replacements do seem to work, but I only have about 8 hours of run time on one and so am not yet confident of how long they will last. I’ve attached a photo here showing a replacement seal installed on the shaft – as you can see it is the correct ID and OD, but is really too narrow for the slot in the shaft, so I plan to keep looking for something closer to the right size.
The bottom line is that after a couple of days of sorting, testing, rebuilding I now have a couple of injector pumps I’m happy with ! I’ve put the test pump with the replacement oil seals to one side for the moment and will circle back to run it for a few more hours on the test rig before I actually try it on an engine.
The build is getting close to being complete – just fiddly things like the handlebar switch gear to be done, and then perhaps I can make some smoke in a week or two – maybe. 🙂