1995 Sea-Doo HX engine trouble when warm

[Enorats]

I recently bought a 1995 Sea-Doo HX. It's got aftermarket handlebars, FPP pipes, and an aftermarket flame arrestor in place of the stock air filter. The guy I bought it from only ever used non-ethanol fuel in it (actually mixed 91 and 110 in a 4:1 ratio, no idea why as the manual doesn't call for anything that high). I don't have access to 110 easily, so I put 91 non-ethanol in it (the manual only calls for 87).

The ski ran perfectly fine my first few times out, though it seemed to be rather hard to start after the first ride of the day. No hesitation that first time, but any time after that would require playing with the choke and throttle to get it to fire up. It also tended to be stuck at an idle, refusing to go above that for up to a minute or so after starting regardless as to the throttle. No idea why it's doing that, but once it started running normally again it would run fine until I fell off or shut it down again. After reading about the fuel lines in these things I decided to take one off and see what it looked like inside. Replaced the original grey tempo line with some regular black automotive line, then cut open the old one and saw it was pristine inside. The non-ethanol fuel must have prevented the decay typically seen.

The next time I took it out it stopped starting entirely, with the solenoid just going "click" when the button was pressed. Ended up putting a new battery in it, as I'd thought it was bad. Turned out that wasn't the problem though. Replaced the solenoid as well, and that didn't fix it. Put new plugs in it (gapped to the proper amount), because why not? Ended up being the starter itself, which was filled with black dust and had basically no brushes left. Replaced the starter, and it ran again.

Now we get to the present issue - I took it out again this last weekend and it ran great for around an hour. Still had difficulty starting, but otherwise ran normal once it was up and moving. Then I was cruising along, at maybe a third throttle (and had been leisurely doing so for a good ten minutes) and all of a sudden it slowed to an idle and then stopped in just a couple seconds. Completely shut down, and just went "click" every time I pushed the button. When I got home, it still wouldn't start. Ended up taking the spark plugs out and.. bam! Cranks over fine. Put the plugs back in, and nothing. Just click. Take one plug out (either one, makes no difference) and it'll run fine on one cylinder. After it cooled down, it runs fine with both plugs in again.

In this time I'd noticed that I'd burned little (or maybe even no) oil, despite burning through around an entire tank of gas. I'm not sure if I should have noticed any drop in the oil in that time or not, but I figured I'd go ahead and disable the oil pump. I pulled the carbs off, then the intake manifold, and pulled the nylon gear out. Left the pump on, because why not? It's basically just a block off plate without the gear driving it. All the oil lines are still connected, so the rotary valve is still getting oil. Opened the carbs up and found the filters and needles to be perfectly clean, nothing noticeably wrong with them. Put it all back together, and took it out to the lake to test. Just slowly moved around at a quarter throttle or so for maybe 10 minutes.. and bam, shut down on me and just goes "click".

Took it home, pulled the plugs.. cranks over fine. No fluid on the plugs, nothing coming out of the holes. Put one in, and it runs on one. Both in, nothing. Engine is uncomfortably warm to touch, but not burning. Same temperature on all sides. Put the thing away for the afternoon. Came back out a few hours later, pushed the button.. instantly starts with zero throttle or choke. No hesitation at all.

I'm at a loss. What could possibly be causing the engine to basically seize up, but only when hot.. and unseize and work just fine until it gets hot?

 

[mikidymac]

Just so we are clear, you bought a HX that has a lot of aftermarket pipes including the Infamous Factory pipe, the owner ran race gas and you think it should run on the manual stated 87 octane?

So it is set up to basically be a fast modified ski and isn't going to be like a stock ski.

Those pipes were known to fry pistons especially if you have the early one. You are better off installing the larger carbs from a 787.

You need to make sure you are premixing the oil if it doesn't have the oil injection which it probably doesn't judging from the aftermarket pipes.

At this point the first thing I would do is check compression and post some pictures of the engine and parts.

 

[Enorats]

Yeah, I added an appropriate amount of oil to the tank, then topped it off (was half full) with premixed fuel to mix it up. Added premixed fuel to the carbs before starting it up too. It had the stock oil injection system when I bought it (and for the first couple times I ran it), but I wanted to eliminate that as a possible source of my problems (and I'm not sure I really trust the system anyway).

The only real modification to the ski is the pipes, the rest shouldn't really affect anything. Why would the pipes affect the octane rating the engine needs? I'm not super mechanically inclined, but I grew up riding a 1993 Yamaha racing ski that had aftermarket pipes on it, which were taken from a ski that won a world championship. That ski didn't require any special fuel, so I wouldn't have thought this one would either. The owner never said it needed that fuel, just that he used it because he liked to "go fast".

The engine compression was ~135 in each hole. Used a 30 dollar tool from Napa to do the test, and I've no idea how reliable it is. From what I've read that may be a little on the low side, but not so low that it'd be an issue.

 

[Enorats]

Ended up taking the top of the engine off to see what the pistons actually looked like, and from the top at least they didn't appear to be damaged at all. I'm not really comfortable taking the engine itself apart any farther than that on my own, soI thought I'd check other areas.

My dad suggested the issue with it seeming to seize up randomly (but otherwise run perfectly fine aside from the difficulty starting) might be due to an issue in the jet pump. I pulled it off, and found that the wear ring definitely has a couple gouges in it, so I'll be replacing that since it's off. Doubt it was causing my issue though.

I do have a question though. The rubber bit on the end of the impeller seems to be half missing. The other half isn't on the drive shift either. I'm having trouble figuring out which part this is exactly from the parts diagram. Is this part number 271000434, the impeller ring? I don't see anything that looks like part 271000422, the impeller boot. Is my boot missing entirely?

What's pictured here is exactly what I found when I removed the pump.

 

[Mraw]

When it dies does it feel like the key being pulled?

 

[mikidymac]

The parts you need for the pump are...
271000516
Impeller Boot
MSRP: $8.39
Dealer Price: $8.39

271000517
Ring
MSRP: $8.39
Dealer Price: $8.39

293200024
Neoprene Seal
MSRP: $10.99
Dealer Price: $10.99

I would also change the pump oil while it is out.

 

[Enorats]

When it dies does it feel like the key being pulled?

The two times it's died on me out on the lake it was running perfectly normally, and had been for at least a few minutes uninterrupted. The first was when I was just out cruising at half throttle or so for a good ten minutes. The second was when I was slowly moving around at a quarter throttle or so near the launch for a good ten minutes or so testing to see if it'd happen again.

Each time it dropped down to an idle and then died within a couple seconds. When I'd try to start it again it'd just go "click" loudly, which I assume is the solenoid. The engine would not crank over at all though, and would refuse to do so until at least one spark plug was removed, or a few hours had passed and it was cool again.

When the lanyard is removed it doesn't do anything at all. No response from the starter button.

It's got a new battery, new starter, new solenoid, and the starter wires looked to be in fairly good condition. The carb filters were completely clean, and the needles didn't seem damaged or worn either. I disabled the oil pump by removing its gear and premixed the fuel to eliminate that as a possible cause too. I'm honestly at a bit of a loss as to what could be causing the hard starting issue, let alone the way it seems to seize up while running.

 

[Enorats]

The parts you need for the pump are...
271000516
Impeller Boot
MSRP: $8.39
Dealer Price: $8.39

271000517
Ring
MSRP: $8.39
Dealer Price: $8.39

293200024
Neoprene Seal
MSRP: $10.99
Dealer Price: $10.99

I would also change the pump oil while it is out.

Thanks! Ended up ordering exactly those last night, so it's good to know I've got the right parts coming. Had to drain the oil since I'm taking the impeller off to replace the wear ring as well, so new oil and a wear ring are also on the way.

I swear, by the time I'm done with this thing it's going to be 90% new parts. At least I'm learning what everything does and how to fix it when it breaks.

 

[Enorats]

This is what things looked like with the top off the engine. The water droplets in the rear cylinder I'm hoping came from towing it back to shore after the first time it broke down. I didn't realize they needed to be towed super slowly, and we came back in at around 10-15 mph, which was likely why we found water in the cylinders after the first breakdown. It was hydrolocked, and after getting the water out it fired up easily.. only for us to then discover that it was still dying after running for awhile.

We tried running it on the hose for a few minutes a good half dozen times and never got any water coming out of the cylinders with the plugs out, so I doubt they're leaking.

When putting the top back on, do I need to torque the bolts in a particular pattern? Should the rubber seals be replaced, or are they reusable?

 

[mikidymac]

You are running lean and that is a recipe for disaster.

I would want to see if your pistons have any scuffing since your compression is lower than it should be.

As I said above, those Factory pipes are known to destroy 720 top ends without larger carbs.

 

[Enorats]

You are running lean and that is a recipe for disaster.

I would want to see if your pistons have any scuffing since your compression is lower than it should be.

As I said above, those Factory pipes are known to destroy 720 top ends without larger carbs.
View attachment 56158

So the carbs aren't giving the engine enough fuel because the aftermarket pipes are somehow making the engine use more fuel? Would using race gas mixed in to raise the octane rating somehow alleviate that issue? The original owner used 1 gallon of 110 for every 4 gallons of 91, but I don't have easy access to 110.

I haven't seen a good explanation for what exactly these pipes are supposed to do, or why they have the effects they do. Best explanation I've seen is that these pipes somehow make the pistons overheat and expand, which can then cause them to seize. Could this be my issue?

I do have the original stock pipes as well, and a set of gaskets for them. Would it be a simple matter to just put them back on, or would I then need to modify god only knows what else to make them work right again?

 

[mikidymac]

The pipe makes more power which requires more air and fuel. In a way think of it as kind of a supercharger allowing engine to draw in more air and fuel so you need to supply it with more air and fuel.

Increasing the octane will not help it get more air and fuel.

THe easiest fix is to go back to stock on everything if you just want a reliable good running ski.

Here is some good info from Group K...

The Objectives The key to this build-up is that we did not need to seek exceptionally high peak speeds, but we did want to get the maximum in strong acceleration. The big choice that we had to make was whether to use the stock pipe, or use the Factory Pipe Products race pipe. There is no doubt that the FPP pipe makes a huge increase in overall power, however much of that power is generated by a huge increase in peak rpms ( from oem6800 to FPP7600+). Normally it’s not a big deal to rev a 68mm stroked engine to 7600rpm. However the SD 720 engine has a very long heavy crankshaft whose bearings do not stand up very well (long term) to the loads of 7500+rpm operation. In addition, those same high rpms net a big increase in fuel consumption… not a good feature for a practice boat.​

With all this considered, we opted to configure this HX “Shredder” around the stock exhaust system. While the stock pipe does not allow high rpms, it does offer a very strong and linear acceleration curve. The objective would be to produce the biggest possible increases in torque and response along with modest increases in rpms.

The stock HX uses Mikuni 38mm SBN carburetors. While these carbs work well on a stock machine, they are a bit undersized to create the kind of acceleration we wanted for this project. We found ourselves stuck between the desire for good fuel range and the desire for bigger carbs. The ideal solution was to obtain a pair of stock carbs from a 782cc 96XP. These carbs are very commonly available on Ebay and at PWC salvage shops nationwide for under $200. The 96XP carbs are identical to the stock HX carbs in appearance and fitment. The only difference is that they use 40mm butterflies and throats instead of 38mm butterflies and throats. The 40mm carbs bolt up perfectly to the 718`cc HX manifold, and even have the same cable attachment points for the throttle and oil injection cables. The stock HX inlet manifold has inlet runners that are 39.7 – 40mm in size, so it does not offer any restriction for the 40mm carbs. Given that inlet tracts of the stock 718cc manifold were just barely big enough to accommodate our new 40mm carbs, we saw little advantage in removing the choke plates. It bears noting that leaving the choke plates in helps slightly to increase inlet signal (a problem area for the 718cc engines). This increased signal helps the engine to carburate better, particularly at lower rpms.

The performance result of adding the 40mm carbs was very noticeable. There was a clear increase in the authority of acceleration, as well as slightly increased peak rpms. The jetting of the 40s did need to be changed to accommodate the fuel demands of the 718cc HX, but once the jetting was done, the HX started, idled, and accelerated perfectly.

 

[POPPS]

Group K have so much experience,,,and therefore so much knowledge.