Hullo all,

I will try and keep this short and straightforward. I have a 160cc 4 stroke single cylinder motorcycle. I have installed a big bore kit, a high-lift camshaft, a performance CDI, free-flow exhaust. I have also increased the carburettor size from 26mm Mikuni BS26 to larger 29mm BS29s along with a K7N high flow air filter. The bike has also lost a substantial amount of weight. However, I am sure if I can get the right state of tune, I can extract better wheel bhp. I haven't had the opportunity to test it on a dyno but before I bear the expense of checking it on the Dyno, I want to ensure it is in the best possible tune. Can anyone suggest the best possible testing methods taking in to consideration the 8-map CDI, adjustments to the carb jet size, etc. The mechaicals are done, now I want to optimise combustion and flow. Thanks!


  • What does CDI stand for?
    – Zaid
    Commented Jul 19, 2017 at 20:25
  • Capacitive Discharge Ignition
    – user30065
    Commented Jul 23, 2017 at 13:16
  • Tune (timing, jets) is always adjusted to the fuel you plan to use and the compression ratio of the engine. The higher octane fuel and compression ratio the more max timing you can set and and slightly leaner the fuel mixture you jet for. Its complicated. Remember idle jet affects the total fuel mix the carb delivers. You want just enough idle jet for a good idle quality, then choose your main jet for the rest of the fuel ratio. Dyno is the best way to tune an engine correctly. If the exhaust pipes glow red when free running at 2500 rpm, the main jet is too small.
    – Moab
    Commented Jul 25, 2017 at 0:40
  • You can do it without a dyno but takes more time and experience.
    – Moab
    Commented Jul 25, 2017 at 0:42

1 Answer 1


It is possible to check a few things with relative ease before taking it to the dyno:

  • ensure air-tightness of the intake path

    Confirm there are no paths for intake air to escape through. Crank the cylinder to top-dead center (TDC) and introduce pressurized air into the intake. It should be able to sustain pressure for a few minutes if it is air-tight.

  • measure air-fuel ratio (AFR)

    The idea behind this is to do some basic carb tuning across the rev range to control fuel delivery such that the resultant air-fuel mixture is neither too rich nor too lean.

    AFR data can be supplied by a gas emissions analyser or a wideband O2 sensor installed in the exhaust. If you opt for the latter you will need to drill a hole and weld an O2 bung into the exhaust.

Other comments

It looks like CDI is a digital ignition control system that is proprietary to Bajaj. I don't see that you would gain much performance benefit from flashing new maps unless the existing maps are pulling out ignition timing to avoid pre-detonation.

  • Hey thanks, I have the O2 Sensor bung hole installed. Question is how do I interpret the readings? It ranges widely between 11 - 18 on the AFR meter?
    – user30065
    Commented Jul 23, 2017 at 13:18
  • This is petrol, right? Under what conditions are you seeing AFR 18? At idle?
    – Zaid
    Commented Jul 23, 2017 at 17:20

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