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AFR tuning on a 2 stroke

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Hey Andre,

Was wondering if you have any experience tuning 2 stroke carburetted engines?

If so can you give me some insight on the considerations of AFRs for these engines in terms of best performance?

I have a pretty good understanding of AFR selection as it pertains to 4 stroke piston engines and I know there are many considerations and maximum power alone is not always the driving factor, but in a 2 stroke they seem to be much more sensitive to changes in AFR in terms of performance.

For instance running a 2 stroke rich to cool combustion charge would most likely see you sacrifice significant amounts of specific power.

I'd love to hear your thoughts.


Hey Chris, I'm really sorry but when it comes to 2 stroke tuning I can't offer any help. I've simply never had any experience with 2 stroke engines and have never had any reason to do my own research into their tuning. If nobody else on here can step in with their experience, I'd suggest looking for SAE papers on the subject. These papers can be an absolute gold mine and for a minimal investment you can learn a huge amount. I'll move this to the 'General' section as it may give the thread a little more visibility to others who are tuning 2 stroke engines.

When it comes to my 2 stroke I go off plug colour, I am tempted to throw my spare WB on but the plug check works fine for me.

HI Chris

I do not profess to be an expert on two stroke tuning, but I can give you my experience with a EFI two stroke project I have been involved with for quite a while. Most of what I have read gives a full throttle mixture of around 0.86 lambda, and that matches the engine I work with. This gives and EGT of ~650C on this engine. 250 GP engines ran around 650-680C, but I could not find anyone who had used an WBO on a GP race engine. EGT position effects this number of course.

The biggest development on 2T we found was in the snowmobile market, so maybe this is a good place to do some research

I also want to be pointed in the right direction with 2 stroke I have 2 stroke s3000 mercury outboard and being a 2 stroke want to know even combustion temperature which are good and safe. Being 2 stroke less time to cool pistion. Who has information on this


Without digging through old text books and track notes i couldn't really give firm parameters to work to but I can say AFR on a 2 stroke does indeed have a big effect on power. To tune this correctly we monitored EGT, knock with a det counter and frequently inspected the piston crown/spark plugs but didn't use an O2 sensor due to the high 'valve' overlap and oil content killing sensors very frequently. For GP125/250's this meant running as lean as possible often to the point of replacing the pistons after a qualifying session or race as an example with power numbers ranging from 80bhp for a test session to 120bhp for quali. The karting guys will have a world of knowledge out there as there are far more still in use today than motorcycles but from memory you will need to build up your own histograms for EGT vs. det vs. power and plot this again how often you're happy to change pistons. EGT will again depend on sensor location, piston porting, combustion and expansion chamber design and will be somewhat specific to your application. The other factors come down to time, budget and power requirement.

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