I am NOT saying to go out and hop on your new bike (or any freshly rebuilt motor), and just thoughtlessly nail the throttle ... some caution and common sense are in order here ... or you could possibly end up seizing it.
Explanation
Here's the low down ...
When you break in a new motor, you are essentially trying to "wear-in" or "mate" metal to metal surfaces within the motor. Think of it in this way . . . It's similar (in principle) to lapping a 4-cycle engine's "valve seat" for better sealing ... you are wearing (by friction from the lapping compound) the two surfaces together so that they both match each other. Only after these two surfaces have worn together do you get the perfect valve seal.
You're trying to do that very thing with the piston/cylinder & rings, but without the aid of the lapping compound ... the friction comes from the honing "cross hatches". This wearing together of the rings to cylinder wall is best achieved early on while the cross hatches are still sharp and able to "cut" and wear the two surfaces together. Just try lapping in a valve without the friction from the grinding compound ... it would take forever, and you never would archive as good of results.
When you work your motor against a load (like under acceleration), the piston rings are being forced out against the cylinder walls with tremendous pressure ... the more gas you give it, the harder the rings push out. This is the best time and way to mate/wear the two surfaces together ... while the motor is fresh!
Now ... think of breaking in a motor while idling it along ... babying the motor ... as if you might break something? The combustion that is being generated is pretty weak and the ring pressure against the cylinder walls is minimal ... so ... after about 400 or 500 miles of this pampering, you have only succeeded in rounding off the sharp honing edges on the cylinder wall, and still have not mated the two surfaces together for a good seal.
At this point, you have missed your opportunity and the engine's cylinder probably has blow-by, less compression, less power!! Have you ever seen a black & brown stained piston and/or glazed over cylinder walls? That is baked on oil ... caused from the hot gases that are blowing by the rings.
Tests have shown time and again that the power losses from breaking a engine in gently (as per many owners manuals), are in the neighborhood of 5 to 7 percent. By the time your engine finally wears together, it could be worn out!!
Break-in Procedure
I usually start out recommending that you go to a slightly bigger (richer) main jet & sometimes, even a richer jet needle position ... until you get through these initial stages.
Also, if you are mixing your oil & gas, put a little more oil in ... if you are using the auto lube system, also mix a little oil into your gas .... It's just better to have slightly richer jetting & oil to gas ratios ... rather then risking possible seizure from overheating caused by a lean mixture, or from a lack of lubrication.
If at any time during the break-in you feel that the engine might be overheating, bogging down, or not accelerating as quickly as it just was beforehand, (with the same amount of throttle), then shut it off and let it cool down ... check your spark plug for a proper reading. Always be prepared to pull in the clutch and kill the motor if it starts to bog or pull down during the break-in period ... it could be the onset of piston seizure.
Any seizure that might occur, would probably mean one (or more) of the following:
- You are too lean on your jetting ... and/or the gas to oil mix ratio
- The rebuild has too tight a ring gap or piston clearance
- Inadequate warm-up ... immediately followed by pushing the new engine unreasonably hard.
- Not watching how the spark plug is burning as you proceed through the process ... in other words, ignoring it's state of tune... probably NOT because of the stress graduated break-in outlined below.
Example:
Here's an example of how you might do it ...
NOTE: (analyze the spark plug for any carb or ignition problems during each step below ... making tuning adjustments as necessary)
1. Start the bike up ... occasionally blipping the throttle until the motor is warmed up (say about 4 or 5 minutes), then shut it off and let it cool down completely.
2. Warm up engine, and then go out and do two or three separate five (5) minute sessions of some brisk accelerating & decelerating .... that is: 5 or 6 sec on the throttle - - - then 5 or 6 seconds off ... be sure you are in a high enough gear to keep the bike pulling without reaching red-line.
Use about 50-60 percent throttle during this particular exercise. Shut it off and let the bike cool down before repeating these individual sessions.
NOTE: Best to do this in a secluded area with little or no traffic issues.
So, when you apply throttle, you are seating the rings ... when you let off the gas, you unload the pressure on the rings and are actually drawing up more cooling mixture onto the cylinder walls .... both on and off throttle play an equally important role in the break-in.
3. Warm up engine and then repeat step 2 ... but this time, only do (4 minute sessions) at about 75% throttle and in shorter on/off intervals of just 4 or 5 seconds duration. Again, be sure to let the bike cool down between these sessions.
4. Ok, if your spark plug looks good, you are probably ready to give it some short bursts of full throttle: Warm up the bike and then do a few three (3) minute session like before, but this time you will give it full throttle with shorter on/off intervals of just 3 to 4 seconds .... taking it up to red line should be ok at this point.
After this, the bike should just be driven reasonably hard whenever ridden ... until you have logged ... say ... 100 to 150 miles on it.
If you follow this kind of a stress graduated procedure, the motor break-in will be nearly complete ... time will take care of any unfinished business.
5. Try to avoid steady cruising speeds for the first couple hundred miles or so.
Related notes
A. Safety 1st ... Don't try to do this in heavy traffic ... you will just end up ticking off other motorists, and may very well cause an accident from drivers that are not paying attention to your erratic driving.
B. If breaking in a four cycle, you should change the oil NOW. Why so early you ask ... better to get rid of all the metallic flecks that are floating around in your transmission oil before they have a chance to start lodging themselves in bearing surfaces and other non desirable places.
CAUTION: AVOID USING "FULL" SYNTHETIC 2-STROKE OIL UNTIL YOU'VE BROKEN THE MOTOR IN ... you may not get the desired results if you do ... 4-cycles = Use either regular 10/30 or 10/40 motor oil for at least the 1st 500 miles or so.
C. When doing these break-in accelerations, you should start out at a high enough rpm that you are not lugging the motor, but low enough that you do not exceed red line during any given interval.
D. One of the reasons for choosing higher gears (like 3rd, 4th, & 5th), is so that you are taking advantage of your speed (the air movement) to help carry away some of the excessive heat your new motor is producing. Let your rpm's dictate which gear is most appropriate for each stage listed above (see: related note C. directly above).
E. After the initial break-in (several hundred miles)... re-torque the head, install new spark plug, warm up the engine, & then do a plug chop to determine any final carb tuning adjustments that may be needed.
F. The break-in process does not have to be too complicated, this just happens to be one way of doing it. Any similar method can be used so long as you are progressively increasing towards a full load on your bike as reasonably soon as possible.
