Big Bore FAQ

I get quite a few emails from people who’ve done a big bore kit and then run into problems. Most of the issues are repeated so I’m going to try and list the problems along with the solutions here. I’ll just copy and paste from the emails to save time. It’ll probably be a bit chaotic till i’ve figured out the layout/format…but here goes:

Big bore starting problems

Big bore doesn’t run right, no power

  • how are you adjusting the fuelling? If “i don’t know” then that’ll probably be why. You’re going to need a fuel controller, and a suitable fuel map.

Big bore keeps overheating

  • how are you adjusting the fuelling? If “i don’t know” then that’ll probably be why. You’ve increased the cylinder size so it’s going to need more fuel. You’re going to need a fuel controller, and a suitable fuel map.

Big bore no top end power

If it runs ok low + mid but there’s no top end:

  • It could be your injector is running out of available timing/duty cycle – i.e. it’s open all the time and still doesn’t flow enough fuel. You may want to try a higher flow injector like 160cc/min. You will need some sort of method of adjustiung the fuelling in order for this to work (e.g. PCV, PCFC or whatever) or it’ll run like garbage.
  • You may also be running out of available valve timing – bigger cylinder to fill takes more time so you could benefit from a cam with longer duration (usually look for a “stage 2”. I quite like this one).
  • It could also just be the changed physics of the engine. By putting a bigbore kit on you changed the balance factor of the piston & cylinder assembly so now the changes are it is “happy” at different RPMs. This is something I’ve touched on in recent videos

It could well be a combination of all 3 of the above.

Can I modify the stock ECU to give the correct fuelling for the big-bore kit?

  • No. Apparently it’s possible but the number of people who know how to do that and are willing to share that (so far) is effectively 0. So unless you have a friendly yamaha engineer who’s willing to help you out, or a friendly hardware hacker, it’s unlikely to happen. If you know different and have info to back it up then please do let me know. Being able to hack the original ecu would be a very elegant solution but is currently beyond my skills.
If you’ve got a big-bore issue you need help with email info@northcust.co.uk or use the contact form

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