Its about time to get the old seadoo running again for the summer.. last summer though I had trouble tuning the carbs "dual mikuni 38i's on a stock 720 seadoo/rotax 2-stroke" I truely hate carbs.. especially these "high/low adjustments, high/low speed jets, and pop-off pressure for the diaphrams" my idea is to retrofit a 1 bbl tbi w/ 747 ecm onto a single bbl intake for this motor. before I go grab a 1 bbl tbi, i'd like to throw up some potential problems and hopefully get some ideas and solutions here. first off, ignition... since it is a 2 cyl 2 stroke with a single coil, the coil fires ever 180 deg crank rotation. since this motor uses a modern CDI ignition theoretically I could connect the stator output directly to a GM ign module? now to keep the rpms correct to the ecm I would set the "cyl select" to 4 since on a 4-cyl 4-stroke it would also fire every 180 degrees crank rotation. now an O2 would be out of the question I think so I'd have to run this purely off of VE adjustments. so EGT might be the only way to get it tuned correctly? is this even a feasible project to attempt?

 

I am sorry but I will not be able to let this thread live. This one is pushing the rules as far as 3rd gen TBI is concered.

 

I agree that it should be locked, but I still cant resist replying

Yes, you could potentially use tbi on that motor. The only thing thats dependant on the firing rate is the rpms. There is a constant for the # of cylinders that you can change to match the motor. The injector will fire each time the ign. fires and the fueling is based on the volume of the cylinder and the volumetric efficiency, among other things. Delphi also has very small flowrate injectors available. A tbi off of a saturn or something with the small style injector is what youd want.

The real problem is that the ecm will only be useful up to around 6000 rpms. If this motor turns faster then that, your SOL as far as using an ecm goes. Also, the stock code in an ecm is only adiquate at best and is only designed to fuel up to around 3200 rpms. It can go over that, but if the motors normal operating range is above this, some other ecm, maybe a 730 or something, might be better. IIRC, the factory also used those ecms in some tbi setups and the code should be better in those.

If you want to persue this further, ask each question in more detail on the ECM/DIY-prom boards, and carefully word it so you dont give away the fact that youll be using it on a Seadoo. www.diy-efi.org is also a very good site.