Jackster,

Only for tapered bore engines (ABC/AAC/ABN/Plasma ceramic) is it imperative to do the break-in at full throttle, high RPM and a slightly rich mixture. This is because these engines must reach their correct operating temperature, so the cylinder sleeve will become parallel, due to differential heating and the break-in will proceed as it should.

For a cast iron ringed engine, that in its life will run at 10,000 RPM at full throttle, it doesn't matter how rich you make the mixture, it will not be able to sustain 4,000 RPM at full throttle.
It will have to produce less than 1/6th of its maximum HP, to do it and the mixture will be "too rich to fire".

If this is what the engine manufacturer says and you want to follow his instructions to the letter, you will have to partly close the throttle (to about 1/4 open) and richen the low speed mixture (by opening the idle needle), because the main needle has no direct control whatsoever, over the mixture at this throttle setting.

So you either disregard what he tells you about the throttle setting, or what he tells you about the RPM.
You cannot keep the cake and eat it at the same time.

Some car engine manufacturers tell you not to exceed 2/3 of the maximum RPM (If the red-line is 6,000, then it is 4,000 RPM...), or to sustain full throttle acceleration for more than a few seconds, for the first x hundred kilometers, or miles.

In either case, you must heat-cycle the cast iron ring, with several (10-20) very rich, short (20-60 seconds), 2/3 RPM runs and complete cooling in between them. Cast iron pistons need the exact same treatment, to anneal the piston and to stabilize its size.
After that initial treatment, 20-30 minutes of rich running will put an end to it.


I invite Thunder-Tiger's and OS' engineers; those who wrote these impossible-to-fulfill break-in instructions, to prove to me that if I do the break-in as I described in the previous paragraphs, the engine's lifespan will be compromised, or its power, or its reliability.

They will not bother to try, because they know it will not happen.

These break-in instructions are merely a warranty "trap-door", if they happen to receive too many warranty returns...