Hi, I need a little direction. My ski mechanic quit working on skis and I am hoping to fix a couple things myself. I have two 1996 Seadoos with both having the 720 engines. One ski seems to perform well. It starts right up and when I put it in the water it will run all day at 50 mph. My problem with this ski is that it smokes excessively. Its really bad at start up. It gets a little better as it warms up but is still worse than it should be. I have had the ski for 5 years and it did not do this when I bought it. I have also not made any changes to any adjustments. Is the smoke most likely due to the rings wearing out ? Can I just replace the rings ? Or is a top end rebuild mandatory ? The 2nd ski isn't running at the moment. The brass gear that helps turn the rotary valve striped out. As it did, it put a couple grooves the the crankcase surface where the rotary valve turns against it. There are 2 grooves both about 1/8 deep and an inch to an inch and a half long. The rest of the surface is in very good shape. Will the ski run ok with these grooves present ? Cost is a huge issue, as I need to sell the skis as well. I would like them to run decent but I am not going to claim the motors are rebuilt. Any advice appreciated and thanks in advance.......
Hi. I’m kid of new at this too, but before rebuilding, I would check the compression first, and using a mirror and flashlight, check the calibration of the injection pump. The two lines should line up with each other with throttle in idle position. I run all my skis just slightly under, that is, the line on the pump arm just slightly below the stationary line. The best way to tell if it’s too much or not enough is by visually inspecting the spark plugs after a long fast run at WOT.
Since Rotax uses a variable injection rate, at wide open throttle (WOT), the mixing ratio changes with engine speed, does not stay constant as if you premixed and didn’t use the injection pump, so after checking the plugs right after WOT, let it idle for 5 min or so, and check the plugs again. Mine are always black and wet looking after idle, but not near as bad since I adjusted the inj pump to slightly under, and after a good fast run, they are tan-brownish and dry.
If you’ve got 150 psi compression on both cylinders, I would not rebuild. If the rings are worn enough to make it smoke bad, you won’t have 150 on both, maybe only one cylinder, and if it ever overheated, like from no oil, a cooling water hose coming loose, debris deposited in the Water jacket, you will find a ring or two stuck in the piston groove, and scarring on the piston skirts. But if this is the case, you won’t get 150 on both, and yes, it’s possible that just one cylinder, piston & rings is bad.
Have you been using the same brand/type of injection oil for the last 5 years? 100% synthetic, or a blend?
Check your compression: both plugs out, plug wires grounded on the grounding posts provided, fuel off, and throttle wide open. When the gauge quits climbing, stop and record the pressure. The two should be very close. If one cylinder is considerably lower, you might pull the head, cylinders, and replace the rings, if the cylinders are not too badly scored.
You should replace all of the O-ring gaskets, and others too. The base gasket comes in a variety of thicknesses, so check the squish gap before tear down, and order that gasket accordingly.
Use never sieze on plug threads, cylinder bolts, head bolts after chasing with tap & die.
Clean the water jackets and o-ring grooves.
Watch for debris inside the water jacket, and head, remove with vacuum cleaner if necessary befor pulling cylinders.
And all of this can be done without pulling engine.
