96-00 Sea-Doo Sportster 720 LT Twin Engine - Rotax engine problem

[del_scotland]

Hi, I just bought a Sportster with twin engines (See pics).

After 15 mins; on the water the drivers side engine cut out, then started and cut out a few mins; later and would not restart in the water. A few days later it restarted on the trailer thou.

I checked passenger side engine compression and both cylinders are sitting at 138psi.

Drivers side engine shows rear cylinder at 60psi and front cylinder at 115psi.

I removed the head on the drivers side to check the condition of the piston tops and cylinders and not scored or lipped. However, I see the front cylinder piston has markings on it 010-817-05 & 0.50, the rear cylinder piston has no numbers/ just the exhaust side arrow. It appears to me that there has been a 0.5mm oversized piston placed within the front cylinder and the original piston cylinder left in the rear?

I understand from the both cylinder psi readings of the drivers side engine (60 & 115psi) that both piston rings will probably need replaced with a top end gasket set at least.

I have included a lot of pics for further info.

Can anyone answer the following...

Going by the top piston markings, is one piston original size and one 0.5mm larger?
Is it ok to run an engine with these different looking size pistons?
When I buy new piston rings will i just buy the standard 84mm rings for both the existing pistons?

Any other help at this time would be much appreciated

Thanks for your time and help!

 

[del_scotland]

More pics

 

[burtshaver2021]

Your long past new rings, even when they an engine gets down to 120 pounds they are usually not in spec and this engine has much worse compression. You’re going to need to bore and go to the next size piston.

 

[burtshaver2021]

Also going to need all new gaskets, check the fuel system, pressure test the engine before disassembly and after rebuild, looks like it ran lean on the one cylinder

 

[del_scotland]

Your long past new rings, even when they an engine gets down to 120 pounds they are usually not in spec and this engine has much worse compression. You’re going to need to bore and go to the next size piston.

So as i suspected would you agree that someone has done a single cylinder rebore and put a bigger 0.5mm piston in it?

Will i need to go bigger than 0.5mm on both cylinders now do you think?

 

[del_scotland]

Also going to need all new gaskets, check the fuel system, pressure test the engine before disassembly and after rebuild, looks like it ran lean on the one cylinder

I'm thinking of changing over to a oil/ fuel straight mix in tank at 50/1 and doing away with the oil injection side... do you think this is best?

 

[burtshaver2021]

So as i suspected would you agree that someone has done a single cylinder rebore and put a bigger 0.5mm piston in it?

Will i need to go bigger than 0.5mm on both cylinders now do you think?

Someone has put a .5mm larger piston in, yes. You will almost certainly need to go bigger, have someone measure the bores for out of round and taper and they will be able to tell you if you need .75mm larger than stock or if you will need to go 1.00mm larger.

 

[burtshaver2021]

I'm thinking of changing over to a oil/ fuel straight mix in tank at 50/1 and doing away with the oil injection side... do you think this is best?

I’ve only ever heard one person say there oil pump failed and I’m not even sure I believe them. This is after seeing 1000’s of posts so I would say you’re fine sticking with your oil pump. There is a test in the manual you can perform to test the oil pump but myself, as long as it’s pumping oil, I’ve never been concerned with them, in seadoo’s anyways. I would clean out the oil tank, lines and filter and change the oil to an API-TC rated low ash oil if you’re not already using that and change the small oil lines if they haven’t been changed recently. Not sure if you have BRP dealership close? I always order the small oil lines from them