Build an XR 750 engine for recreation or competition on flat tracks.
Harley-Davidson XR 750 motorcycles remain one of the most popular bikes in racing history. The twin-cylinder, 748 cc engines develop white-knuckle torque that continually proved successful on oval dirt tracks. Mechanics favored the factory engines because the bottom end is relatively bulletproof and seldom needed rebuilding. This reduced racing costs by limiting a build-out to the top end. Building a Harley-Davidson XR 750 dirt track engine requires precision machine work, a full complement of engine tools and mechanical expertise.
Disassembly and Machine Work
1. Drain the engine oil. Follow the procedures in the XR 750 workshop manual and remove the engine from the motorcycle. Place the engine on a workbench and remove both carburetors. Discard the intake gaskets. Remove and discard both spark plugs.
2. Remove the front and rear rocker covers and discard the gaskets. Remove the cylinder heads with the valves intact and discard the head gaskets. Lift the push rods out of each cylinder.
3. Pry the front and rear cylinders off the crankcase and discard the crankcase gasket. Remove the wrist pin clips from both pistons. Tap the wrist pins out and take the pistons off each connecting rod. Discard the pistons, wrist pins and clips.
4. Loosen and remove the clutch cover bolts at the primary case and remove the cover. Save the cover and bolts. Remove and discard the clutch pressure plate, springs, clutch discs and the clutch basket.
5. Turn the engine around. Remove and save the timing cover bolts and the cover. Disconnect and discard the stator and ignition points.
6. Send the twin cylinders and cylinder heads to a racing shop. Have the cylinders honed. Have the heads ported and polished. Have the shop install racing valves, valve guides, valve springs and keepers. Ask the shop to provide the new pistons, rings, wrist pins and wrist-pin clips.
7. Attach the new pistons to the connecting rods using the new wrist pins and clips. Put the new rings on each piston. Place the new cylinder gaskets from the racing kit atop the crankcase.
8. Install the pistons into the cylinder sleeves with a ring compressor tool. Torque the cylinders to the specified foot-pounds given in the workshop manual. Drop new push rods into the lifter-ports on each cylinder.
9. Place the head gaskets from the racing kit atop each cylinder. Install the newly serviced cylinder heads atop each cylinder and torque the head bolts in a crisscross pattern. Place the new rocker-cover gaskets atop each head. Fit the covers on the heads and torque the bolts in a crisscross pattern.
10. Follow the procedures in the workshop manual and install the racing ignition components. Install the racing clutch components. Reattach the saved timing cover and clutch cover using the original bolts.
11. Position new intake gaskets at the front and rear intake ports on the corresponding cylinder heads. Attach a 40 mm Keihin racing carburetor to each intake port and tighten the nuts to the specified torque. Install new spark plugs at each head. Install the engine in the motorcycle. Fill the crankcase with oil.