Sea-Doo 951 Top End

This is my very ambitious project to rebuild the motor of my 135HP Sea-Doo 1998 GTX Limited. If you've looked around at other pages on this site you might notice that I dabble in a lot of stuff, but I'm not really an expert at any of them. Well wish me luck rebuilding this tightly tuned, high performance, and specialized engine.


Two years ago my ski started running really bad on a multi-day trip to the lake. The first sign of any trouble was one morning when I got up early to ride. It was very calm and I must have run it for about 30 minutes wide open. While at full speed the motor suddenly bogged real hard and died. I cranked it and it took off again. It did crank a little slow like it was real tight so I was thinking it might have seized. After that it would run at mid RPM but bog and die at higher RPM. Idle was very rough. I looked it all over, pulled the RAVE valves to clean them thinking one might be stuck. That's when I noticed that the PTO piston was missing a small chunk right at the RAVE / exhaust port. Now there is some history here. I purchased the Sea-Doo from a girl that popped a hole in the hull and sank it with only 20 hours. I noticed all along that it had very noticeable scars on the PTO piston when viewed from the RAVE opening. I guessed water ingestion either when she sank it or at some other time. But otherwise it ran fine (65 MPH) in between major problems. I'm certain that one of the major problems, probably the air leak, caused the final destruction of the piston. This is a problem history:
  • Jet pump oil filler cross threaded causing a small oil leak - Had to replace the jet pump bearing cover
  • Oil pump cable holder fell off thankfully wide open - This is when I purchased the Shop Manual and quit going to the dealer for maintenance. They cross threaded the jet pump oil filler and should have been tightening the nut on the oil pump according to the Shop Manual. The nut and washers fell into the bottom of the ski and out the drain holes. My son had the unit at the lake and kept running it even though it smoked like crazy. I ended up purchasing a used pump off Ebay and took the nut and washers to reassemble my pump.
  • Very annoying loose ground at the starter - The starter bolts kept coming loose. Those bolts were the electrical ground connection between the battery negative cable and the engine block. The MPEM was grounded to the block at a different location. This meant that the MPEM kept loosing power intermittently. Very frustrating to locate. I ended up breaking off one of the starter bolts trying to tighten them. Since I can't get an easy-out on that broken bolt without removing the engine, I opted to just run a new bridging ground from the starter grounding bolt to the MPEM grounding bolt. The starter still only has one bolt that comes loose occasionally but I don't have a problem with grounding anymore.
  • "big" air leak in the fuel pump pulse line - The ski slowly developed, what I now know were, classic signs of an air leak like running away on the ramp.... running away on the hose.... I never knew that a 2-stroke could diesel on it's own oil!! I never knew that the PTO carb could pull so much air that it formed ice on the outside!! I think that 5 minutes took 10 years off my life. After a little googling I learned what caused the run away and most important that stopping it requires full throttle and choke to flood it with fuel. The ski started having intermittent problems running at the lake. It would run fine sometimes and then bog down to barely idle speed at full throttle. Put it on the trailer; take it off the trailer; it is fine again. I initially thought that a clogged fuel system was to blame. Rebuilt the carbs, replaced the fuel lines, found that the 0-rings in the fuel selector had disintegrated. Ran it on the hose where it seemed fine but it still bogged down at the lake. I put it on the trailer and left it for a summer. The next spring I was about to break down the engine enough to perform an engine leak test. I pulled the carbs then the fuel pump pulse line. I had never changed the fuel pump pulse line because it seemed really hard to reach under the engine. It turns out that that line had deteriorated from the inside out until there was about a 1/4" hole in it. I replaced the line and reinstalled the carbs. The ski ran like new again. Well I did notice the top speed fell to about 60 MPH but after running lean for who knows how long and running away on the hose for 5 minutes until the PTO carb had ice on it, I thought I was really lucky.
  • Gas in the oil tank - Kids.... well it was a big holiday weekend so rather than removing the tank to drain it we simply siphoned all we could out of the tank and refilled it with injection oil. But did we really get all the gas out?

The Plan

The plan is to purchase a matched cylinder and piston set from SBT (i.e. top end rebuild). I know that there is a good chance that some of the piston debris has worked its way into the crank case. However I'm willing to take that chance because I don't think that I can manage an entire engine swap; there are just too many settings, adjustments, and specialized skills that I don't have. If the crank bearings go later, I'll just part out the ski since it is now 10+ years old.

I plan to replace the reed valves and rebuild the carbs while I'm at it. I also want to put back the original A1 or B1 rated fuel hose. I have A2 rated hose now and I've noticed the increased smell of fumes in the hull (no there is not a leak).

Purchase List

Parker Yamaha - SeaDoo Fuel Line 6mm - $3.45/ft - 11ft - ~1/4"
Parker Yamaha - SeaDoo Fuel Line 8mm - $4.99/ft - 14ft - ~5/16"
NOTE: Don't buy fuel line from Parker Yamaha. Neither parts above are "actually" metric even though the pictures and description indicate they are metric. Parker Yamaha overcharges for the same 1/4 and 5/16 fuel line you can purchase other places. Worse, the 15 ft of 5/16 came as 3 x 5 ft sections. My longest 5/16 run is 6 ft. Because I didn't have time to mail order more, I went to West Marine and overpaid for a full 14 ft.

Short Block Technologies
SBT Cylinder Exchange,Sea-Doo 951 Silver Code: 62-109
SBT Rebuild Kit for Mikuni SBN-I Carbs, Sea-Doo Code:35-180
SBT 951 Reeds, Sea-Doo Code: 45-109-10K
SBT Sea-Doo 951 Exhaust Wrench Code: 80-109
SBT Intake Gasket Kit, Sea-Doo 951 Code: 52-108
SBT Break-in Oil

NOTE: I didn't like the exhaust manifold gaskets from SBT. The didn't fit and required some work with an exacto knife.

Granger - Loctite 5920 Copper Gasket Adhesive
Granger - Loctite 534 High-Tack Gasket Sealer

Loctite 242 Blue
Loctite 271 Red


SBT Sea-Doo Forum
PWC Today Sea-Doo Forum

SBT Forum: Tips for Cylinder replacement
SBT short block replacement instructions

SBT Forum: Listing of Stock Carb Settings
SBT Forum: Carb Tuning w/ plug pictures

New Boat Builders: FUEL SYSTEMS - Good article describing difference between A1, A2, B1, and B2 fuel line. Personal water craft should have type A1 (fire rated / low permeability) although the OEM Sea-Doo lines are B1 (low permeability)
USGC Boating: Fuel Systems / Equipment Standards - Cross reference between USCG and SAE specifications
USCG 183.542 - USCG Rules governing use of fuel lines

West Marine A1 Fuel Hose - not metric. ($2.99 / $3.19)
1/4" is 15 thousandths bigger than 6mm; 5/16" is 2 thousandths less than 8mm
Parker Yamaha: SeaDoo 8mm Fuel Line
Parker Yamaha: SeaDoo 6mm Fuel Line

Notes from various SBT and PWC Today threads

  • Exhaust Gaskets: Loctite® 5920 Copper Gasket Adhesive
  • All paper Gaskets: Loctite® 2 Gasket Sealer / Loctite® 534 High-Tack Gasket Sealer
  • Alternative Opinion: Blue loc-tite on all the bolt threads and 3M 1211 on the base gaskets. Use silicone high temp sealer on the exhaust gaskets. (Three Bond 1211)

  • Loctite 518 replacement: Autozone - Permatex 51813

I ended up using Permatex High-Tack on the base, head, and exhaust manifold gaskets. I used Permatex copper on the exhaust and cone gaskets.

  • Acetone for cleaning
  • Coat cylinder walls with oil
  • Double nut the studs to remove them. You don't need to completely remove the cylinder studs. Just pull them up until they are un-threaded from the crank case
  • Measure squish before tear down so you can select the right gasket thickness
  • Don't split the pipe; leave it in one piece


I purchased the top end kit, new fuel hose, gasket adhesives, and gasket remover. Of course it arrived while I was out of town. The first weekend my brother and I pulled the motor apart. First the exhaust. I couldn't get the pipe out of the ski without splitting the cone. I'll need to figure out exactly what I need to put those pieces back together. The carbs and intake came next since I wanted to change out the reed valves while I'm at it.

Removing the head and cylinders was a bit of effort. I read on a forum post to double nut the cylinder studs to unscrew them before pulling the cylinders. That was great advice and worked well. Getting the wrist pins out was a challenge. The circlips came off easy enough; very anti-climatic once I put on my safety goggles (what a dork). I could not figure out how to get the wrist pins out. One suggested method was to "tap" them out with a socket. The PTO pin was really hard to remove and I didn't want to damage the connecting rod bearings at the crank by tapping sideways on the connecting rod. The solution was some 1" gas pipe, a bit of 1/4" all thread rod, and a couple of washers and nuts. I managed to press the pins right out, nothing to it, all for $5 at Home Depot.

Now for the long cleaning job. I'm sure that I'm taking way too much time cleaning all the gasket surfaces and bolts but I just am not sure exactly how clean they should be. Gasket remover works well, but slowly for the paper gaskets after removing the bulk of the material with a razor blade. Nothing seems to get the old locktite out of the bolt threads except running them through a nut with some carb cleaner. Even then there is a little deep in the threads.

I posted pictures of the pistons to the PWC Today SeaDoo forums to get some advice as to exactly how the piston failed. The general consensus was water ingestion. The problem is that I saw absolutely nothing to indicate a water leak. The other possibility was lubrication failure but only on the PTO piston? The mag piston had a "little" scar on the side opposite the intake/exhaust ports. Might have also been oil. Ok, dammit; I'll quit using non-SeaDoo oil. I had been using a Pennzoil PWC full synthetic that is API-TC rated. Somehow I doubt that was the problem, but I'm a little superstitious. It might have been the "gas in the oil incident above" but I would have expected the problem to show up about 5 - 10 running hours earlier.

I learned something new; piston wash. Apparently another great way to read the fuel mixture is by examining the amount of carbon that is washed off the piston by the fuel entering the combustion chamber. This is probably only good for reading the low speed circuit. Checking the high speed circuit requires killing the engine and pulling the plugs immediately after a WOT; I doubt that I can look into the plug holes in the bright daylight on the lake.
Here's a link that explains it:
How to Read Pistonwash

Both of mine were indicating lean because the piston wash was non-existent on the PTO piston and barely visible on the mag piston. I'll be extra careful setting the mixture this time.

While reassembling, I decided to check the RAVE clearance since I have 0.5mm over sized pistons. I cleaned and reassembled the RAVE valves then bolted them on the new cylinders. I pushed one of the new rings down in each cylinder with the piston. Humm.. I don't have feeler gauges. Something else to purchase. As soon as I verify the clearance it will be time to reassemble the cylinders.

Another question.... Do I need to have the heads opened up since SBT sent me 0.5mm over sized pistons?

With feeler gauge in hand I determined that the rave guillotines are too close to the piston ring. I used the drimmel to rework the guillotines. Getting the cylinders back on the ski was a trick. I used two hose clamps to compress the rings. My son held the cylinders while I slid the two hose clamps down and worked the pistons into the cylinders. The bottom of the cylinders have a taper that helps quite a bit.

Once I had the cylinder reassembled, I went to work on the fuel and oil systems. Pulled the oil tank completely out and drained all the Pennzoil. I replaced the fuel lines, cleaned the fuel filter, and siphoned all the fuel out of the gas tank. The fuel selector valve had already been replaced two years earlier. Damn parts companies. I ordered SeaDoo fuel lines from Parker Yamaha so I could get the 8mm and 6mm size. Parker Yamaha shipped me 1/4" and 5/16" hose. Worse the 5/16" is about 3/4 OD; way to big around to make the bends needed. On top of that I ordered 15' of hose by the foot. They shipped me 3 x 5' sections. The longest run is 6'. Dip shits. I'll be sending it back. Time to make an emergency run to West Marine to over pay for fuel line.

Time to install the pipe and cone. I covered the exhaust gasket and cone ring with Permatex Copper. With the manual in hand I started installing bolts w/ loctite 242 using the sequence and torque listed in the manual. I realized about half way through that the carbs were in the way. I dropped them leaving everything connected. Installing the exhaust is complex but not hard. The Clymer manual has a great picture that clearly illustrates the torquing sequence. Highly recommended.

To Do: Post a few pictures.....
Contributors to this page: michael .
Page last modified on Monday 24 of August, 2009 07:21:58 CDT by michael.

Attached files

ID Name Comment Uploaded Size Downloads
27 cylinder_mag-1.JPG MAG Cylinder michael Tue 04 of Aug, 2009 07:39 CDT 3.64 Mb 3871
26 Cylinder_PTO-3.JPG PTO Cylinder michael Tue 04 of Aug, 2009 07:39 CDT 2.44 Mb 3704
25 Cylinder_PTO-2.JPG PTO Cylinder michael Tue 04 of Aug, 2009 07:39 CDT 2.35 Mb 3581
24 Cylinder_PTO-1.JPG PTO Cylinder michael Tue 04 of Aug, 2009 07:38 CDT 2.53 Mb 3613
23 98 GTS Ltd Cooling 2.pdf michael Sat 01 of Aug, 2009 10:21 CDT 45.14 Kb 4706
22 98 GTX Ltd Cooling 1.pdf michael Sat 01 of Aug, 2009 10:21 CDT 38.47 Kb 3626