Hello! Rebuilding a very modified Sea Doo HX

[Peterbertine]

I bought a 1996 HX with many modifications a couple months ago. It ran great for several weeks but finally seized up, I was running 91 non-ethanol premixed with Amsoil 2 stroke synthetic at 40:1 (should have been 30:1 I’ve since been lead to believe)

Pulled the engine got it turning freely, rebuilding the 44 mm carbs and am checking to see if they are set up for the Factory Pipe (can’t imagine they’re not).

I and am now trying to figure out the rev limiter/solenoid control that has been hard wired into the computer.

I’m not expecting 100% reliability, but I’d definitely like to cool the engine down. I don’t mind losing 5 MPH off the top end, and I’m happy if it takes a bit longer from idle to get there.

To do so, I thought I’d lower the RPM limit to the mid 6000’s and if possible get more water flowing in the Factory Pipe to cool down the ski. Ultimately I’d like to retard the timing to get it cooler but I have no actual idea what’s been done to the internals of the engine so all I’m doing before a full engine rebuild is to try to tune the carbs and exhaust system.

But check out the revlimiter/solenoid control for the Factory Ski! Can’t find anything like it out there. Could it be a one off device programmed by someone with advanced skills? The guy who sold it to me said it came that way and he had no idea how to program it or what the symbols on the readout are.

Should I leave it as is? Should I take it out and start with a new rev limiter and solinoid control. Or take it out entirely and have water constantly flowing into the factory pipe?

The big question is: “Has anyone seen an Intellitronix circuit board like this with the 2 buttons and 4 digit readout with 4 wires used in this application?

Thanks

 

[mikidymac]

Are you sure that isn't just a tach?

It sure doesn't look like any rev limiter or water injection controller.

If it seized you aren't going to just get it running again without a rebuild.

Is the head stock?

 

[Peterbertine]

Hi. Thanks for responding.

I haven’t opened up the engine, I figure the head isn’t stock given all that was done with the carbs and factory pipe.

It wasn’t a “bad seize” (if there is such a thing), I immediately filled the cylinders with ATF and as soon as I got the engine out of the ski and had the carbs off I poured in more ATF and acetone. I waited a day and the flywheel turned with the help of a small wrench and immediately turned free and easy by hand… so, nothing appears to have broken, worst is I rebuild the top end, I hope?

I was just going to put engine back in the ski after rebuilding the carbs and do a compression test to see how bad the damage is. Do you think it’s better to do a pressure test on the engine as is?

The black box could be just a tach, but the wires coming out of it are not normal (red, black, green and white) tachometer colors, see photos. One wire goes to the solenoid for the factory pipe. I’m inclined to remove the box entirely and run the pipe without the water injection. But I’ll wait till the engine is running to do more tests with the box still attached.

Main question is: since the engine did seize, should I rebuild it before doing anything else?

 

[mikidymac]

The engine seized, the damage has been done, doesn't matter if your "broke it free".
If the tach is attached to the water solenoid then it is just for water injection.
Post a picture of the head.

Two things on that Factory Pipe.
1. You have to measure the outlet diameter. If it is 38mm you are good to go. If it is 34mm the pipe can't be used, no matter what you do it will burn the rear poston, period.
2. You can't run the stock 720 38mm carbs, they will never supply enough fuel and air to support that pipe no matter what you do. Easiest fix is a set of stock 40mm carbs from the Seadoo 800 and they will just bolt on.

 

[nibroc]

pic of the head will do

 

[Peterbertine]

The engine seized, the damage has been done, doesn't matter if your "broke it free".
If the tach is attached to the water solenoid then it is just for water injection.
Post a picture of the head.

Two things on that Factory Pipe.
1. You have to measure the outlet diameter. If it is 38mm you are good to go. If it is 34mm the pipe can't be used, no matter what you do it will burn the rear poston, period.
2. You can't run the stock 720 38mm carbs, they will never supply enough fuel and air to support that pipe no matter what you do. Easiest fix is a set of stock 40mm carbs from the Seadoo 800 and they will just bolt on.

I measured the diameter of the pipe
and it’s 38mm.
It’s got the 44mm carbs. They’ve been disassembled and one is really deteriorated, had to have been running lean.

I’ll take the top end apart this week and see what the damage is. Given that it seized, should I worry about the bearings and the rest of the lower end?

Here’s some photos.

 

[Peterbertine]

pic of the head will do

 

[mikidymac]

Head is stock but could have been milled.

Carbs are Super BN Racing carbs. Get rid of that automotive fuel filter.

It is a shame Matt Braley isn't here anymore, he would get you going.

 

[Peterbertine]

Head is stock but could have been milled.

Carbs are Super BN Racing carbs. Get rid of that automotive fuel filter.

It is a shame Matt Braley isn't here anymore, he would get you going.

Thanks for your help. I’m sure Matt knew his stuff, if he’s past away I’m sorry.

I hastily bought this ski to entice my son to leave Brooklyn and spend weekends with me in the Hudson Valley, we are right near the Hudson River by Woodstock.

It ran great and he helped me work on it till it froze up, I suspect because I was only mixing 40:1. Or because the high temp silicone hump hose on the Factory Pipe split and was leaking and over heated the motor last time we were out. Could also have been the decaying carb that ran a piston lean.

I’ve since bought 2 cheap stock Wave Ventures a 700 1996 and a 760 1997 to keep him occupied and we’ll return the HX to how it ran when I got it over time, it’s really fun.

The gas filter is gone, I have a marine one without the paper membrane in it that will go in when the engine goes back in.

The motor is out of the boat. I figure I’ll rebuild the top end this winter if necessary. Unless you think it’s better to put it back in the ski and do a compression test.

 

[mikidymac]

Premix should be 32:1 with good oil.
Fuel filter should only be the OEM fuel seperator, do not use Marine ones for boats.
Yes, pull it apart and do all gaskets and seals if nothing else.
Rebuild the carbs with Genuine Mikuni parts only.
Pipe hose would choke it out from no air, would not rein the engine.
Install an electric bilge pump, the HX can take on a lot of water and the aftermarket air cleaners will not keep water out of the engine.

 

[Peterbertine]

Premix should be 32:1 with good oil.
Fuel filter should only be the OEM fuel seperator, do not use Marine ones for boats.
Yes, pull it apart and do all gaskets and seals if nothing else.
Rebuild the carbs with Genuine Mikuni parts only.
Pipe hose would choke it out from no air, would not rein the engine.
Install an electric bilge pump, the HX can take on a lot of water and the aftermarket air cleaners will not keep water out of the engine.

Thanks for your feedback, it’s keeping me inspired.

I’m using Amsoil for premix, wilI make it 32:1 now.

I have 2 genuine Mikuni carb rebuild kits.

And a big YES to putting in a little Rule automatic bilge pump.

Below is a photo of the fuel separator that I understood was OE

And here’s a couple photos of rebuild kits for the top end. I’ll take it apart and see what the pistons look like before buying anything.

I’m moving the engine inside today, so I should have it apart by the weekend.

 

[mikidymac]

You will need the housing for the water separator/filter. THe picture is only the element and o-ring. The filter with the barbs is the oil injection filter which you aren't using due to premix.
Also note that you still have to supply oil to the rotary valve gear on your engine which the premix does not do.

Let's see the pistons before you order any. For your application I would only use OEM or Pro-X pistons and have the machine work done by Group K. You are putting way more stress on this engine so I wouldn't use WSM or SBT pistons.

 

[The Wayniac]

You're not going to be able to make any use of that pipe running tach speeds in the mid 6k's. It takes a skilled builder to set up a 720 in an HX with Factory Pipe on pump gas.
That's a home brew water control. you want water control, it makes the pipe hit hard and doesn't affect reliability in any way as the water is turned off by 6k.
Ditch those Buckshot carbs, they're too big to be easily tuned and make the boat less safe on pump fuel. There's nothing they can do on that setup that a set of stock 38s Or a set of 40i can do while running on pump fuel.
Where about are you located?
Wayne

 

[The Wayniac]

Also, 40:1 is the maximum ratio you want to run, 32:1 you can run lean due to the excess mixture.
You can not run 91 octane on an Factory Piped 720, you must run minimum 93, forget the ethanol it won't hurt, in fact ethanol has higher anti-knock stability than octane does. Just don't let it sit unused in the tank for extended periods, and when you do, stabilize it.
I had youre in the Hudson Valley, our performance shop, WetWerX, is located in Schenectady. I recommend you reach out to us, there are very few people who have as much experience with HX, 720s and heavy racing modifications as we do. I raced Sport Mod for a very long time before we moved over to Sport GP which I race now on a lightweight 1050 HX.

Shoot us a message, 518-209-2664
Wayne