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Guide To Four Stroke Motorcycle Engines, Part 10

Updated on December 31, 2009

Buttoning Up

Reinstall the carburetors / fuel injectors, points, plates, covers, and accessories that should be in place before the engine is put back into the frame. Leave yourself access to the valve adjustment and the points, however, and set them as well as you can before you start the engine. Poor timing and bad valve adjustment can ruin the "breaking in" of a rebuilt engine.

Sure-Fire Valve Adjustment

If adjusting valves is still a mystery to you, remember one thing: ''The crankshaft rotates twice while the cam rotates once." It follows, then, that if you rotate the engine with a box end wrench on the bolt head end of the crank until a valve is all the way down, the same cam follower will be at the center of its base circle with one more revolution of the crank. When the cam follower is on the base circle in the proper place to adjust valves - it doesn't matter if the valve is an exhaust or intake valve as you will have the valve in the proper place on the cam for adjustment.

Check the valve adjustment settings with a feeler gauge. Set them as close to factory specifications as you can, but don't wear out the adjuster nuts doing it. Re-time the engine statically, carefully following the directions given in the ignition chapter in this book, then you should be ready to reinstall the engine.

Remounting the Engine

Make sure the engine cases, mounting plates, bolts, and nuts are clean before you remount the engine. Dirty mounting areas are bound to loosen up after a few hours of riding, and no one wants their engine to fall out, especially after top-end overhaul!

Remember, when you reconnect the wiring, carburetors, exhaust system, and fuel system that these are all delicately tuned sections of the motorcycle. Keep everything clean. Take time to reinstall all nuts, bolts, brackets, clips, gaskets, and retainers properly. Sure, you're eager to tryout your newly rebuilt top end, but don't hurry the final stages of this job and risk damaging what you've just improved.


This same principle of careful restraint holds true also for the break-in procedure. Disregard the stupid old tale: "Break 'em in fast, they run fast. Break 'em in slow, they run slow."

Any engine that's flogged right after it is rebuilt is destined for a short life and poor performance. Since no ring manufacturer has control of the exact finish and crosshatch angle of a cylinder, the new rings require a certain period of time to seat against the cylinder wall. If this break-in period is a controlled, leisurely one, then the rings can seat without too much wear either to the wall or the rings.

This careful approach insures good seating and keeps the final ring end gap smaller. A violent break-in period will make the rings and cylinder wear too fast causing excessive end gap.

Having given the bike a good static tune-up, start the engine on a good set of plugs and fresh oil. Listen for any distinct metallic noises that weren't there before. If you hear strange noises, turn off the engine and correct the problem.

Again with the engine warming up, check the timing with a strobe light and adjust the timing if necessary. After the engine has warmed up, check the carburetion adjustments for cable synchronization, idle mixture, and idle speed.

By now the machine should be ready to take for a break-in ride, assuming, of course, you've adjusted the chain, control cables, and other items on your "pre-flight" checklist. Be careful as you dab the shift lever into first gear. Often a clutch that hasn't been run for a while is sticky and will stall the engine. Instead, coast the bike along a bit in neutral and dab for first while you're rolling. Pull in the clutch and rev the engine a bit to free the sticky plates.

Remember three important points during your initial break-in ride:

  1. Don't overheat the engine.
  2. Don't lug the engine.
  3. Don't over-rev the engine.

After an hour or two of gentle riding, change the oil, reset the valves, and double-check the timing. Look for seeping gaskets, fuel leaks, loose bolts, etc. After the engine cools down, re-torque the cylinder head and all other nuts and bolts you worked on.

Changing the oil a couple of times in the next thousand miles or so will carry away any microscopic particles that sluff off during the break-in period and will help insure the success of your four stroke top-end overhaul.

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      5 years ago

      It's really great that people are sharing this innarmotiof.


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