Hey Guys,

Not sure if its been mentioned before but ive done a lil research reading up previous posts and found out fitting a MC21 flywheel on a MC28 (stock card) provides advancement to firing timing, but i also understand that its not recommended to use the MC21 flywheel on a HRC Card PGM-IV... as the HRC card already does the job of advancing the firing timings. The question thats been circulating my thoughts... im using a MC28 running HRC-030 and i got it up and running with the MC21 lightened flywheel (30% lightened) the throttle response no doubt proved snappier but at the same time the rev limit barely peaked at 13,000 RPM.. can anyone advice me what are the cons of using a MC21 lightened flywheel on a HRC PGM IV?

Thanks mates!

13,000rpm? Are you crazy?

This is StephenRC45's dyno chart, you will note his bike makes peak power at about 11,700 rpm.
Even an RS250 (on 38mm carbs) only makes (claimed) peak power at 12,250rpm.

Back to the question, the HRC 030 card and the mc21 flywheel togeather give more advanced than HRC thought the bike should have.

The cons of running too much advance?
Well, if the air/fuel mix is ideal it'll possibly denonate and seize.

Really you want to run the least advance you can that still gives the maximium power the engine can. The only place you can do this is on the dyno with a few woodruff keys. £20's worth of bits from Tyga and 3 or 4 runs on a dyno will tell you all you need to know;

Do a power run, fit a 2 degree offset key so the flywheel is further round clockwise than it "should" be, do another power run. That will tell you if the '28 flywheel timing is a better for your engine. If it is you can try going back even further with a 4 degree key and see what that does.
And so on.
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Thanks for the reply Fonty,

i've done afew laps on track this morning and did plug chops with the mc21 flywheel, by the way im using a lightened mc 21 flywheel (30% lighter) its running really smoth and with a really fast throttle response but this time the engine peaked at 12,000 RPM on the tachometer~

Did some research on ignition tuning on the web and I've sent in a spare MC28 flywheel for lightening (25% lighter) and to test it out later next track session.. as for Dyno runs.. i guess thats not really a matter to me and they can be kinda tedious in Singapore... (Finding a good dyno tuner garage for 2stroke.. kinda practically impossible here).

then again i'd like to thank you again for your reply !

In that case switch to the '28 flywheel when you get it back and compare laptimes, feelings.
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i've tried the MC28 flywheel (25% lightened) and the power delivery slacks slighty compared to the MC21 i was using before, but top end definitely showed what the 28's flywheel is capable of.. peaking its rev at 13,000rpm at final gear throttle maxed out =)

both flywheels are awesome but i would still personally feel that mc21 has much mucchh better engine response! inertia power!

More advance will give a stronger midrange, but at the expense of the top end as it will not rev. Fontyyy's correct in saying you should play with woodruff keys.

We found (on the dyno) once you got the mixture close to spot on too much advance meant power dropped off very sharply past peak, we found 500+ revs overrev and the best part of a horse retarding the ignition 5 degrees (was 2 degrees advanced, is now 3 degrees retarded).
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Ignition timing and jetting can be played with to get the characteristics you need for a particular track.

On a short and twisty track I would run more advance as this builds the midrange hit, but will not rev. But on long tracks i would retard the timing to give over rev and generally keep the combustion chamber cooler.

There is no perfect timing or jetting or pipe or cylinder spec, that will be optimized for all tracks/conditions. You will have to play to find out what works.

Getting the max hp on the dyno is not always the way to go. Sometimes a pony less can give you a bike that's easier to ride and will run at full power to the end of a race, without over heating or whatever.

Matt@TYGA wrote:
Getting the max hp on the dyno is not always the way to go. Sometimes a pony less can give you a bike that's easier to ride and will run at full power to the end of a race, without over heating or whatever.

I second that. My SP motor's made 68hp on the dyno, but that was just stupidly peaky. In "normal" trim it makes 66hp, and is a world apart from pretty much anything shy of an F-III. It'll even out-drag the best of them from a standing start in the first 3 gears.
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NSR-WORLD.COM

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