1996 Seadoo HX 718cc Motor Still Runs Away

[Joe Snuffy]

Hello,
I have a HX 718.2cc it has dual carbs. When you rev the motor up and down and don't exceed 1/2 throttle everythng works. When you rev it up to 2/3 throttle it takes off and wont slow down this is while it is out of water which I only run about 15-20 seconds as a test. With the ski in the water it doesn't want to rev up it acts like its running on 1 carb I then choke it a couple of pulls and the motor will then rev up like it should until I let it idle about 15 seconds then it needs to be choked again a couple of times to get it to run correctly again but if you let it idle again for the 15 seconds it does it again. I tried idle it up a little and that helped the situation tremendously but still no cure I also adjusted the low and high sides of both carbs which didn't help much. I have an extra pair of carbs attached to their rotary valve plate and oil pump assembly. I decided to put them on as such to try and eliminate as much as I could. I bolted them on rotary valve cover and all to see what would happen. The motor is doing the exact same thing which kinda frustrated me but it does at least let me know its not the carbs.

Here is where I need help to figure out what the hecks going on with the motor I have found and/or thought of the following but need some imput. I did notice a scratch about an inch long and 1/32 deep on the rotary valve port closest to the magneto which could probablly be sanded out with emory cloth I thought maybee that might be the problem if need be I can have it machined right now I am just trying to figure this out I thought it might be pulling fuel to the other cylinder through that scratch on the rotary valve surface or losing fuel to that cylinder while trying to idle. I also thought it might be a head gasket leak when the head gets to a hot enough temp it seals both plugs are dry when I pull them after running it in the water. I thought it might be the fuel filter on the side of craft its clean the one inside the tank is also so are the ones in the carbs. Would the wrong rotary valve maybe? Does anyone have an idea or suggestion?
Thanks
Joe

[Bill O'Neal2]

Re: 1996 Seadoo HX 718cc Motor Still Runs Away

First off, you don't have a headgasket at all on the 718 engine.

A scratch in the RV surface needs to be filled, not sanded. The idea is to get it back to the orginal dimension, not remove more material.
What bothers me is that the RV surface has a scratch. It must have processed some metal through the motor in order to get a scratch that deep. You may have some serious damage to the pistons and the head from bearing debris if a bearing race has come apart in the motor. Remove your head. It has orings, not gaskets. Inspect the piston tops for damages.

Another thing that could cause your carbs to starve for fuel would be the fuel selector valve, a part that is known to get clogged up and restrict the fuel flowing to the carbs. Also, there are internal fuel filters in each carb that could be clogged up in both sets of carbs that you tried.

[Joe Snuffy]

Re: 1996 Seadoo HX 718cc Motor Still Runs Away

Bill,
I pulled the head the pistons look fine so does the head the rings on top of the piston seem to be ok also with just a small slit between them when they are compressed as they are right now. The small filters are ok also that are in the carbs. I bypassed the selector switch and still no luck when the ski throttle approaches 2/3s it takes off and revs way up and won't come down I tried choking each individual carb to see which one would bring it back down and it was the rear carb closest to the pto. I am at a loss for what the heck is going on. I have never seen anything like it. I am going to do a compression check on the motor tommorrow my gauge is at my buisness but the compression seems to be very high just doing a finger over the plug test and the motor starts very easily. I did notice that the plastic wheel that drives the oil pump on the carbs I took off today was also a bit chewed if that helps in the diagnosis process.
Thanks for the help
Joe

[Bill O'Neal1]

Re: 1996 Seadoo HX 718cc Motor Still Runs Away

Look very closely at the intake manifold for cracks. They can be hard to see.