1973 GT750 – Oil Pump

The build continues !

Recall when I cracked open the cases on the engine I wanted to use, I found a bit of a mess as it appears the oil had leaked out and been replaced with water over the years, and things were rusted solid. Other than the cases, I was unable to salvage much of the internals at all.

I basically had must of what I needed to replace all of the internals, but I did not have a good gear shift selector shaft and so had to do a bit of searching around to get one. The gear selector shaft on the GT750 was changed sometime in 1974 to have a longer rod on the left engine side. At the same time, the gear shift selector arm design was also changed, and the new ones had much less angle than the ones used in 1972, 1973 and much of 1974. Although I had the correct, old style gear selector lever, the gear selector shafts I had available to me had either corrosion or wear right where the oil seal would ride.  After a bit of head scratching, I was very lucky (and thankful !) to have one offered to me for the cost of the postage by Barry, who is a fellow member on the Sundial Moto Sports Board, which is a great resource if you are dealing with these old 2 strokes.

I previously highlighted a few of the other updates I made during the engine rebuild here and at last I had got to the point where I was ready to button up the cases, as can be seen to the right.

I had four GT750 injector oil pumps available to me – one was definitely suspect as on the bike I’d pulled it from, the oil injector tank was bone dry and missing its cap, which likely meant that water had been able to get into the pump itself. Although it turned, I didn’t really trust it, and on the three other pumps I didn’t have any history. A really good write-up on the Suzuki injector oil pumps is available on Richard’s site here.

Having a working oil injector pump is a must – unlike other 2 strokes, the crankshaft design means you will likely burn out your main bearings if you try to run solely on pre-mix with out having the crankshaft re-designed. This sort of conversion is possible – Bill Bune’s shop will do this for folks who need to run pre-mix for vintage racing applications for example, and I’m sure there are others. In my case though, it was going to be injector oil so I had to confirm I had a working injector oil pump ! To do this, I took a piece of flat steel stock, and drilled and taps mounting holes so I could bolt on the oil distribution loom and oil pump in a similar way to how it was mounted on the engine. The pump driven shaft protrudes though the bottom. I had a scrap oil pump drive shaft from one of my parts engines, and I drilled and tapped a hole in the bottom of it to take an 8mm bolt that I could drive with a 1/4 inch variable speed drill. The reason for doing this is that as the oil pump shaft is turned, it actually travels up and down about 4mm which meant I couldn’t just connect the drill chuck directly onto the shaft – using the original drive shaft allows the pump shaft to both be turned and also allows it to move up and down

After testing the four pumps I had available, I was able to confirm that two worked fine, and that two were scrap so it was time well spent now rather than later after I’d seized up the new engine due to lack of oil ! In the photo below, you can see test rig mounted on the bench and the drive shaft on the bench beside one of the oil pumps. With the engine in the frame I can finally move onto other things !!