Advice on carburettors nc35

[SevenThreeSeven]

One of my motorcycles is an '05 Kawasaki EX-250 and it doesn't have much horsepower to work with (about 27-28). The EX-250 is notorious for its 4500 - 5500 RPM "flat spot", which is very noticeable due to the bike's low horsepower.

the EX-250's "flat spot" is due to...

... the CDI.

The EX-250's CDI uses a very simple advance curve: 2-dimensions, X and Y, with no compensation for load. It's pre-historic, rudimentary.

At about 4,000 or 4,200 rpm, depending on the year-model of the EX-250, the CDI has driven the ignition timing to full advance (38 degrees BTDC for the older engines or 42 degrees BTDC for the newer engines).

When you roll-on full throttle at about 4,000 rpm you suddenly make the air/fuel ratio much richer. In a modern car or motorcycle engine the ECU knows when this happens (from the information provided by the Throttle Position Sensor and the Manifold Absolute Pressure Sensor) and it retards the ignition a bit because richer air/fuel ratios burn faster than leaner mixtures.

Because the EX-250 CDI has no way to sense or compensate for this sudden rich (faster burning) mixture the ignition advance stays way up there at 38 or 42 degrees and you end up with too much of the burn occurring before the cylinder reaches TDC (it doesn't go as far as inducing "knock" but it definitely causes the "moment of maximum cylinder pressure" to be significantly misplaced).

With this type of "dumb" ignition control the most compromised moment of the engine's operating envelope is the RPM that occurs right when the CDI has finished advancing the ignition timing.

So, instead of instantly making a lot more power (as you're anticipating when you roll the throttle open wide) you end up making only a little bit more. And as the RPMs rise the engine slowly lifts itself out of the situation (back to a point where the ignition advance makes more sense).

Does your NC35 have a Throttle Position Sensor on its carbs? I know that some of the more advanced '80s and '90s carbed bikes had a Throttle Position Sensor to help the bike determine "load" in a rudimentary way (Throttle Position Sensor + RPMs computation to determine when to reduce spark advance).

[richyrd5]

I stand corrected.0.5 shims i meant!!

[racingt]

Yep, that's what I used. 112,s and 0.5mm shims. Just fitted Tyga system, runs even better, so perhaps it was running a bit rich. I can understand people wanting them to run perfectly, but for me 95% is good enough; haven't got much time to go removing carbs a few times. Checking the float heights was probably just as important, and that was free.

[LennonRVF400]

Thanks for all of the advice lads cheers

[LennonRVF400]

Sound lads