As a rule, we try to focus our How-Tos on repairs we think the weekend mechanic can aspire to, so we’ll kick this one off with a qualifying statement: Rebuilding a Yamaha Autolube oil pump requires more than average care and attention.
Our factory Yamaha manual specifically says of the pump, “Make no attempt to disassemble it,” yet it can be rebuilt. Pump failure is typically due to bad seals or, as we found with our 1974 Yamaha DT125, sludged or stuck parts. In our case the spring for the pump shaft had rusted in the compressed position. A previous owner bypassed the pump in favor of premixing the fuel/oil, likely because the pump couldn’t stroke with a stuck spring.
As the photo above shows, the Autolube may be a little pump, but it has a surprisingly large number of parts: We counted 39 pieces total, including gaskets. Some of those parts — like the springs and pawls for the pump sprag — are really tiny, and for that reason we strongly suggest taking the pump apart inside a clear plastic bag so parts don’t go flying. If you lose any of the small hard parts, you’ll be looking for a replacement pump. We also strongly suggest taking photos during disassembly to aid assembly.
Yet even with those warnings in mind, a rebuild is doable and in our case netted a perfectly functioning pump. It’s also affordable: HVCcycle.net’s comprehensive $37.50 kit (see inset photo above) covers all Yamaha oil pumps of this type and includes every seal and gasket you’ll need, plus new pump body screws and new Allen head body mounting bolts, a nice touch. A manual is handy, and bleeding air from the pump before operation is critical.
1. The oil pump is under the right front cover. Remove the cover, noting the oil throttle cable that pulls on the pump. Rotate the pulley to slacken the cable. Disconnect the cable from the pulley, then disconnect the oil feed lines. Remove the two Phillips head screws securing the pump.
2. With the pump removed from the engine, remove the small cotter pin securing the manual oil starter plate, followed by the plate itself. The plate might be stuck to the shaft. If it is, spray it with penetrant and carefully work it off to avoid accidentally breaking the plate.
3. Next, turn the pump over and loosen the 9mm nut securing the cable pulley to the oil pump plunger shaft. Hold the pulley against the pump body, then remove the nut, wave washer, adjustment plate and any shims under the adjustment plate. Keep the shims safe; they’re crucial to pump calibration.
4. Remove roll pin that keys the pulley, followed by the pulley and its spring, noting the tabs on the spring to locate it in the oil pump body and to the pulley.
5. Remove the four Phillips head screws securing the oil pump plunger cover to the oil pump body.
6. Separate the oil pump plunger from the body. In hindsight, we would have been wise to do this working inside a clear plastic bag to avoid losing small parts. We were lucky and didn’t.
7. Removing the large flat washer exposes the worm drive gear. Note the small pins keying it to the oil pump plunger cam. They’re spring-loaded and can fly off if removing the gear. Instead, working inside a plastic bag, remove the plunger cam complete with the gear and pins, holding it together and pushing it up through the pump body.
8. The plunger cam coming out with the gear and pins removed. Note the two washers on the stem. A flat washer goes against the pump body and a wave washer against the bottom of the plunger cam.
9. The plunger cam has a small oil seal inside its shaft for the pump plunger. Using a small hooked tool, reach in and remove the seal.
10. Next, remove the pump plunger from the plunger cover. Note the guide pin on the plunger shaft; the plunger cam pushes against this to stroke the pump. Also note the spring, which was rusted in a compressed state; this pump wasn’t stroking at all.
11. Using a screwdriver, gently pry the oil seal from the pump cover (shown) and from the pump body. They’re both a fairly gentle press fit and come out quite easily.
12. With the pump stripped, clean all the parts thoroughly. Following degreasing and rinsing, we gave everything a 15-minute soak in an ultrasonic bath.
13. With the parts clean it’s time to start putting the pump back together. Here we’ve just installed the new plunger cam seal (in green), using the old seal as a guide to help push the new seal into place.
14. Despite expectations, the ultrasonic bath didn’t release the seized pump plunger spring so we carefully worked each coil apart.
15. Next, install the new seals in the pump cover and the pump body. The seal for the pump body should be installed flat or rubber side out, the seal for the cover should be installed flat or rubber side in.
16. Next, install the pump plunger with its spring in the pump cover, with the threaded shaft to the outside of the cover and the cam guide pin positioned under the seal.
17. Making sure the shaft washers are properly located, install the plunger cam complete with the worm gear, pins and springs, followed by the large flat washer. Install the new cover gasket and lightly grease the pump plunger shaft before installing the cover.
18. With the cover secured with new Phillips screws, key the pulley spring to the body. Follow with the pump pulley, turning it counterclockwise under spring pressure until the hole for the roll pin lines up, then install the pin.
19. Next, install the shims removed earlier, followed by the pump adjusting plate, wave washer and securing nut.
20. With new body to engine gasket in place, install the oil pump to engine using new mounting screws.
21. Reconnect oil hoses and the throttle cable. Check for pump stroke by turning the manual starter plate. Adjust cable preload so the adjuster plate aligns with the arrow on the pulley, then bleed the system per the manual. MC