I am working on a 2 cylinder, 250 cc motorcycle which I am increasing the displacement to about 350 cc but I have no way of remapping the ECU. Is it possible to get a satisfactory result by changing the injectors to a larger size and use a adjustable fuel pressure regulator to fine tune the fuel supply?

  • What is the make/model/year of the vehicle. It may just turn out that everything automatically adjusts to the amount of air flowing by a MAF. Welcome to the site. Cheers! Commented Jul 15, 2016 at 13:39
  • Have you resolved your issue? If not, please provide the make/model/year of the bike. Cheers! Commented Oct 13, 2016 at 0:46

1 Answer 1


A few assumptions first:

  • It is an open loop alpha-n type of injection system (just has a TPS, no MAF or MAP sensors, no O2 sensor, no catalyst)

If the above is true, you could probably make it run using that method, but there will be a trade-off, the difference in fuel requirements at one condition (say idle) between the 250 and 350, will be different to the difference at another condition (say high rpm, WOT).

i.e. for your method to work well the volumetric efficiency of the new engine must the the same as the old engine, at all operating conditions. It won't be.

That said, it might be close enough that you can get it to run acceptably: If you tune so that the leanest operating condition on the new engine is stoichiometric you will avoid lean misfires and overheating, then you can only hope that it doesn't happen to be too rich to run properly in other conditions.

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