D,
Thanks for measuring the crank. This is perhaps a good time to have a bit of discussion on flywheel-effect, as T250K suggested above (or on another thread).
It certainly deserves consideration independant from the power production of the engine, but there is some overlap.
On those dim'ns, compared to a typical Jap bike (except YZa/b which have hollow flywheels!) the Maico crank is 13% heavier, & 11% larger dia - which is just as significant. Why?
Flywheels are storage devices (the mechanical equivalent of a battery) that operate via rotational momentum. Momentum stores energy which can either be absorbed or given up for short periods (just like a battery does electrically).
Now linear momentum is just the mass times the velocity. In the case of flywheels & angular momentum, that velocity is proportional to the dia of the flywheel as well as rpm. And its the extra dia where the Maico carries the extra weight so its a two-fold multiplyer effect.
So, as a rough approximation, the flywheel-effect of the maico is 1.11x1.13 (= 1.25) that of a typical jap 250 at the same rpm. ie. 25% greater, & from memory thats about how it feels too. Its considerable.
It may seem like 0.5 - 1kg extra flywheel weight is insignificant compared to a 100kg bike + 80kg rider, but once you get past optimum, its the-straw-that-broke-the-camels-back scenario. An extra 0.5-1kg spinning at 120mm dia @ 8000rpm is 0.5-1Kg moving at 50m/sec, which is a considerable force to be overcome.
But what effect do flywheels have in practice? Frankly I laugh every time I hear/read: "put a flywheel weight on to give it some torque" or "lighten the flywheel to give it some power" as if the flywheel can produce torque/horspower itself. Torque & horsepower (which are directly proprtional to ea other via a mathematical formula) are produced only by how much fuel-air mixture you combust, at what rpm, & how efficiently you harness the energy released.
If you do dyno runs w the same spec engine but w diff flywheels weights, stabalizing the rpm after you've raised it for ea reading, the diff in torque/power will be all-but negligible & probably not measurable.
However.... lets not forget the storage/momentum effect. When you are lugging an engine the flywheels (if sufficient weight, like the Maico) can give-up energy for a short period, & that is why they're so popular on trials bikes. Conversely when you accelerate the engine up its rev-range the flywheels absorb energy that something else must supply, ie the engine. (in much the same way you have put energy into a child's spinning-top/gyrscope to get it up to speed).
In that situation you have to first overcome the momentum-effect which holds it back. Plus you are raising the flywheels to a higher energy state (higher rpm) which requires energy input (just like the battery/spinning-top). That's the situation where too-heavy flywheels can & do hold back acceleration.
The test for this would not be a dyno run, but a roll-on acceleration comparo (w other effects neutralised)
It seems to me its made worse on the maico by it not having a very fat midrange to overcome that extra flywheel-effect when accelarating up the rev-range. On the other hand, the YZ(a/b)s also lose out cos of too-little flywheel effect, losing any potential gains in useless wheel-spin.
Obviously, if yr riding style is to use the breadth of the powerband coming out of corners rolling the power on smoothly from lower in the rpm range (as mine is) you are going to be held back more by too-heavy flywheels than if yr style is to keep the rpm up in the corners & row the gear lever. (Unless its a slippery track surface.)
On the other hand, heavier flywheels can be a distinct advantage for the average rider in the drag-race to the 1st corner, in that the momentum-effect tends to keep the engine spinning in its higher-rev/higher-power range rather than falling below it.
So it seems to me there is an optimum flywheel weight for every engine/track-condition/riding-style combination. Both too-heavy & too-light can affect acceleration to some degree. They also affect ease/difficulty of riding. To my mind/style its the spanish who got it in the sweet-spot best. Yam went too far one way, & Maico too far the other.
T250K also asked about the flywheel-effect of clutch weight & rear-wheel weight. It seems to me these are minimal, probably not even noticeable to the average rider, mainly on account of the significant reduction in rotational velocity due to drastic gearing reductions.
Having said that, I recall one incident when Marty Smith's mechanic replaced the rear tire on his RC125. He came back in complaining that the bike felt as slow as a tractor. They found that the new tire was 2-3lbs heavier & Marty felt it. OF course the effect would be multiplied by higher unsprung weight giving less traction as well.
But yr perfectly right, DL in saying its a lot of work reducing flywheel wt, & I wouldn't consider it either unless the bike needed a new rod/big-end. And then I would be skimming it off the OD of the crank where possible, where it has most effect.
You are also perfectly right in saying it does nothing about the intake limitations of the std Maico. Same about the pipe/stinger, & the transfer ports/timing. Those need to take MUCH higher priority.
Apologies for another lengthy post, but thinking things thro from 1st principles explains/resolves most problems.
