Performance CDI for China Gas/ Grubee/ Happy Times Engine48cc-80cc
A performance CDI optimizes the ignition timing for best engine performance, less heat, and less vibration. Many specifics determine the ideal ignition timing, including altitude, engine porting, carburetion, compression ratio, throttle habits. The "timing" is not one number though. It is a graphable curve since the correct timing changes with the changing RPM. (see graph below)
Copy Cat Competitors
my own CDI unit by copying the design of the KDX200 CDI and modifying
it for the Grubee engine. (I'm an electronics technician.) That, in
combination with a high voltage ignition coil, I improved the spark
strength and now have an ignition system that advances and then retards
the timing as revs are increased. (see video)
Here is a digital timing curve from a PLV CDI, just as an example to show that an advance/retard curve is mostly standard for 2 strokes:
Recently I became aware of the GP version of the RS125 with a modified intake system of a reed valve in place of a reed valve. Unusually it had a very advanced ignition curve. Why I think it's that way is it needs to be more advanced because without the extra turbulence created by the reed valve the fuel/air mixture needs more time to fully burn. So this would apply to piston port only intakes such as what the stock Grubee engine has. So for stock Grubee engines the only downfall for the stock CDI is it's a bit weak and the timing doesn't retard past 3500 RPM. Anyway now my performance CDI allows a good amount of timing advance for piston port only intakes, as well as selectable amounts of retard for engines with reed valve. Max timing now is adjustable from 20.5 BTDC to 24 BTDC. (see graphs) In case you're wondering about how late in the RPM range the RSA125 starts to retard, that is normal for very high reving engines. Max RPM for a stock Grubee is only 6000.
Anyone after better performance usually ups the compression of the engine also. But unfortunately if you don't also increase the ignition coil voltage you could be causing more erratic running because higher compression makes it harder to consistently ignite the fuel/air mixture unless you also increase voltage available to the spark plug. Here is a graph showing the relationship. (10 bar pressure is equal to 145psi at sea level.) A 25% increase in pressure requires a 15% increase in spark voltage.
How to Determine Timing: Remove the magneto cover, take the spark plug out, stick in a screwdriver to feel piston position, rotate the magnet till you find the pistons highest position, mark with a felt tip marker one of the corners of the magnet and the stator frame where the magnet corner is. That will be your TDC (top dead center) reference point. The ignition point (seen with an automotive timing light while running) on the stator frame will be a few millimeters counter-clockwise of the TDC mark which indicates the advancement of ignition before TDC. Advancing the ignition (at the CDI) more will make the magnet mark to be even more counter-clockwise than before. Use this formula to figure out how many degrees difference there is between the new position and the reference mark. Degrees=(distance between marks x 360)/(3.14 x 36.2 magnet diameter)which equates to distance between marks x 3.17
So if the marks distance is 3mm then the degree change is 9.5 degrees.
Best plug to use for standard engine: NGK BPR7HIX iridium extended tip spark plug (not for use in engines modified for higher compression because it will hit the piston. Install a spark plug washer or use a B7HS.) This and the Denso IWF22 iridium spark plug should be gapped at .032" (.8mm) when used with my CDI. Both have a reach of 21mm from washer to end of ground electrode. Best plug to use if the exhaust port has been raised and the intake port lowered for high RPM use (over 9000 RPM): NGK BPR8HIX
From "Devices to improve the performance of a conventional
two-stroke spark ignition engine"
Click here to read more about my CDI and how to buy it.