🔧 Saving a Sea-Doo: My Hands-On Journey Fixing a Stubborn 787

If you’ve ever owned an older jet boat — or any 2-stroke personal watercraft — you know that half the fun is fixing it before you can enjoy it. I’ve been elbow-deep in my 1999 Sea-Doo Speedster SK for weeks now, tracking down one problem that kept leading to another. Here’s my story so far — maybe you’ll see yourself in it too.

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🚤 The Boat: Twin Rotax 787s and a Lot of History

My Speedster SK runs twin Rotax 787 (a.k.a. Rotax 800) engines — classic, simple, powerful, but sometimes fussy. When everything’s dialed in, it screams across the water like nothing else. But mine hadn’t been dialed in for a while.

It all started with a hard-starting starboard engine that would crank, cough, and sometimes pop — but never really run right. The port engine? Perfect. The starboard one? Not so much.

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🔋 First Suspect: The Starter and Spark

I dropped in a fresh starter, thinking a strong spin would fix it. But then the starboard engine only gave the odd fire — sometimes backfiring through the carb — and then nothing.

Checking spark showed it was weak. Could it be the coil? The plug caps? Maybe even the rectifier or the MPEM — the brain box for these old Sea-Doos? I read stories about people swapping coils and suddenly everything worked, so I added that to the list.

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🧰 The Carburetor Rabbit Hole

Then I turned to the carbs — the Mikuni Super BN. These are legendary for simplicity but demand perfection. I rebuilt both carbs, bench-tested the pop-off pressure (shooting for the sweet spot around 35 PSI), swapped out the spark plugs (bye-bye fancy iridiums, back to factory NGKs), and reinstalled everything.

The engine wanted to go, but still backfired and even blew an exhaust hose clean off — something it had clearly done before, judging by the beat-up hose clamps. That pointed to serious flooding or timing issues — backfires like that don’t happen by accident.

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⚙️ Chasing the Real Cause

I started second-guessing everything. Was it the flywheel key shearing, throwing off timing? Was the coil or MPEM to blame? Should I swap ignition channels to see if the MPEM’s one side was cooked? Maybe… but before tearing into the flywheel, I wanted to be sure it wasn’t still a fuel problem.

I realized the carbs are the heart of the 2-stroke — if they flood the crankcase, you’ll get all the same symptoms: fouled plugs, backfires, random hose explosions. So I swapped carbs between the good engine and the problem child. And guess what?

After confirming that the rotary valve was timed right and swapping the carbs…

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🎉 Breakthrough: The Carb is the Culprit

The moment I bolted the port (good) carb onto the starboard engine, it fired up beautifully and ran sweet. Problem found. Not ignition, not timing — just that one stubborn carb feeding too much fuel and flooding everything out.

So now, instead of chasing phantom electrical gremlins, I’m zeroed in on getting that “bad” carb right: rechecking the needle & seat, setting the pop-off perfectly, and dialing in the tiny lever arm that makes or breaks these carbs.

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🔑 What I’ve Learned

• Sometimes the simplest part is the culprit. In a 2-stroke, it’s almost always fuel or spark — and it’s usually fuel.
• Backfires are more than just loud — they tell you exactly what’s wrong. If you’re blowing exhaust hoses off, don’t ignore it.
• The right parts matter. Generic needle & seat kits can cause endless grief. Genuine Mikuni only.
• There’s no substitute for hands-on testing. Forums are gold — but swapping parts around is how you know.

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🚤 What’s Next

Next up, I’ll rebuild that “bad” carb one more time, double-check the lever arm height, verify pop-off, and get both engines running in sync. After that? Back on the water where this Speedster belongs.

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⚓️ Final Thought

Fixing old boats isn’t just about wrenching — it’s about chasing clues, learning more every step, and not giving up when the answer isn’t obvious. If you’re deep in your own 2-stroke adventure: keep at it, trust your gut, and listen to the engine — it always tells you what it needs.

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See you on the water — where the only backfires should be from the people left behind!