Thread: PROgress on the Pro 785 Rebuild
10-27-2016, 09:45 AM #1
PROgress on the Pro 785 Rebuild
Well the time has come that my 1998 Pro 785 needed a complete rebuild. I’ve been working on this project for about 2 months now and have been taking my time to ensure I’m doing things right and that I am sourcing high quality replacement parts where needed. Some background on why I’m working on the ski. A few years back, I did a complete rebuild with new WSM 750 pistons, rebuilt crank with new rods and genuine Mikuni carburetor rebuild kits. It seemed to run well until one day I was trying to start the engine and heard a bit of a clank. Come to find out, there must have been some trash in the carb that caused the needle and seat to leak a lot of fuel into the crankcase causing a hydrolock when I tried to start. Compression was still good and it still ran okay. I didn’t really think anything more of it…
Fast forward to last summer. In the search for better RPMs, I put in Polarisracer166’s recommended jetting for stock Pro 785s. This included 122.5 low speed jets and 135/130/135 high speed jets. This really woke the boat up but it still just didn’t pull the tach like it should have and it still hesitated badly or even sometimes died on the low end when I punched the throttle. After running this jetting for a couple of weekends, I ended up burning down the PTO cylinder….holed right in the middle. First thought of course is the jetting is off but the CEN and MAG cylinders showed perfect wash. I replaced the piston, cleaned out the carbs and on I went. By the time this happened, the season was coming to an end so I winterized the boat and decided I would figure out what happened the next season.
Fast forward again to this last season. Thinking the jetting was off from the season before, I opened up the high speed screws and left it that way for a while. Again, the boat wasn’t running well and wasn’t pulling the tach. I was out on the lake in a back cove practicing some high speed turns and then suddenly the boat died. Go figure, the PTO burned down again….time for a rebuild. I pulled the pipes, engine, gas tank, oil tank and all electronics leaving absolutely nothing inside the hull other than the steering and trim cable.
10-27-2016, 09:48 AM #2
Trying to chase the cause for the constant burndown of the PTO cylinder, I made a set of aluminum block off plates and used some rubber gasket material to leak test the engine to check for any air leaks. I first tried using some Plexiglas but that was an ultimate fail. I ended up buying some quarter-inch aluminum plate off eBay and cut them to fit the exhaust port on the cylinders and on top of the intake manifold. After pressuring the engine up to 8 psi, the gauge showed only about a pound of pressure drop after 15 minutes. This told me that there was in fact not an air leak at the PTO crank seal like I thought there was and I needed to look elsewhere.
I honestly can’t think of why I thought that the crank would not have been out of phase considering the previous hydrolock from a couple of years ago. I checked…and rechecked…and rechecked again the crank by the now known very inaccurate “redneck” way of putting one cylinder at TDC or BDC and ensuring the other two cylinders were at the same height. It always seemed to check out…or so I thought. Wanting to get a little more precise, I used a degree wheel and bought a Harbor Freight dial indicator. I quickly realized that the CEN was 2 degrees advanced and the PTO was 3 or 4 degrees advanced. This was a red flag to me that the very reason why that cylinder just couldn’t ever survive when I ran the boat hard. I should mention here that I could cruise around at part throttle all day and not have any issues but once I started to run the boat hard, the piston would hole up in a matter of 10 or 15 minutes.
Realizing that this is for sure not something I can do myself, I tore down the engine and packaged up the crank and sent it to Crank Works in Tempe, Arizona. After a couple of days, I got a call back confirming that the CEN was 2 degrees out of phase and the PTO was actually 6 degrees out of phase! I suppose those Harbor Freight tools are not the most accurate things around but they were just accurate enough to tell me there was a problem. In addition, 2 of the rods had a good amount of pitting at the wrist pin side and some bearings were suspect. I ordered 3 new SBT Pro 785 rods and sourced OEM main bearings on eBay. I will have the crank welded and pinned to keep it from spinning out of phase again.
Sticking with using as many OEM things as possible, I located 3 OEM crank seals on eBay for a relatively decent price. They are definitely more expensive than say Winderosa or WSM, but the peace of mind is worth the extra few bucks I spent that I’m using the best quality parts.
The bottom crankcase bolts were also looking pretty corroded so I bought all new stainless steel bolts through an online metric fastener site. FYI, it is nearly IMPOSSIBLE to find the 4 bigger M10x1.25 bolts in stainless steel that the crankcase halves use on the PTO and MAG ends. I ended up paying around $35 for a set of ARP bolts, but again it is worth the few extra dollars to replace the stock steel bolts with stainless steel. This means that every external bolt to the engine, from the crankcase bolts to the starter bolts, will be stainless steel.
I also found a set of stainless steel M10x1.25 acorn nuts for the cylinder bases as the OEM chrome plated nuts always seem to rust up and look horrible. This might end up being a small problem as they require a 17 mm wrench, which doesn’t allow for enough clearance to turn the wrench to tighten up the acorn nuts on the exhaust side of the engine. I may have to “modify” my combo wrench by grinding it down on the outside to get it a little bit smaller to allow enough clearance to turn the wrench between the cylinders. I will be using a Motion Pro torque wrench adapter to allow me to torque these nuts to the correct value using my soon-to-be modified 17 mm wrench.
10-27-2016, 09:50 AM #3
Pipes and Exhaust:
After being subjected to some salt water and just typical wear and tear over the course of 18 years, the pipes, collector and water box definitely needed some attention as most of the OEM powder coating was flaking off badly. I decided to have them blasted and powder coated the same stock hammer tone color as from Factory Pipe. Prior to having them done, I pressure tested all the pipes to 10 psi and all passed with flying colors. I took the pipes to a local powder coating shop to do the work. Well go figure, after blasting the pipes, one had a quarter inch hole by the water inlet fitting and one had a cracked mounting bracket. They fixed both those issues for $20 for a total of $220 for all 3 pipes, the collector and my Hot Seat water box in the OEM hammer tone powder coat.
While I did this previously this year, I wanted to also mention that I used McMaster Carr for sourcing new rubber pipe / hull mounts. They work perfectly and look a lot beefier than the OEM mounts. The rubber part is a bit short so I ended up using some stainless steel nuts to make up the height of the center section.
For reference, here is the part number:
5823K21 Natural Rubber Sandwich Mount W/304 SS Stud Cylindrical, Male/Male, 3/8"-16 Thread, 3/4" Height.
See this thread for some pictures of when I installed the mounts earlier this year: http://www.greenhulk.net/forums/showthread.php?t=251032
10-27-2016, 09:55 AM #4
Just like with the pipes, these carburetors needed to be blasted and recoated as they were very rough looking and had the paint peeling off. My brother has a blast cabinet so we blasted them ourselves and coated them with Rustoleum Appliance Epoxy. They definitely came out awesome looking. I bought 3 genuine Mikuni rebuild kits and also replaced all oil injection and primer lines with good Tygon tubing. I routed the oil injection lines to a single connection point on the front end of the carb rack and will use barbed fittings to allow me to remove the carburetors easily without having to destroy the clamps. All fuel hoses were replaced with good quality reinforced rubber fuel line from Gates and I used Oetiker clamps on every fuel and oil connection. I used to have that clear Fast Flow tubing from eBay but I have decided to stay away from that as it got very brittle over the last couple of years I presume form either poor quality control or the fact there is ethanol in the fuel.
I also figured since they were 18 years old, I replaced the needle and seats with genuine Mikuni 2.0 needle and seats. All 3 carbs pop-off at 18 psi with the 65 gram springs provided with the Mikuni rebuild kits.
I also elected to again go with the 122.5 pilot/low speed jets and 135/130/135 main/high speed jets. I think with the crank fixed with correct index, this jetting will prove itself to be the best for my needs of buoy racing.
In the quest for building a limited class Pro 785 buoy racer, I sourced a Skat-Trak adjustable exit nozzle, Hot Seat manual trim and Hot Seat auto drop nozzle. Being these items are several years old, they both needed some work to bring them back up to operating condition.
The manual trim needed a complete new cable as the old one was corroded inside and didn’t allow it to pull smoothly. After scrounging the internet for a replacement, I tried to have a custom cable made. This proved to be both difficult to source and expensive when I did find a source. I happened to stumble across a Skat-Trak cable that looks like it will fit. This cable goes on a Yamaha manual trim system. The only modification you have to do to make it work is extend the threaded rod end by about 3 inches where it connects to the trim system as it is a bit too short. This can be done by purchasing some brass or stainless steel all-thread and a small threaded coupler. This cable has worked perfectly since I installed it.
Here is where I bought that cable at:
My through-hull bearing / driveshaft seal definitely needed some attention as both seals are missing the garter spring. Luckily the internal brass metal bearings are still in great shape. A quick search of Greenhulk showed that an 8702 seal would work as a drop-in replacement. I sourced new SKF seals off eBay and they definitely do work and are of good quality. Be sure to use a seal puller tool to get those old seals out without any fuss.
With all the electronics and wires pulled out, I went ahead and got rid of all the crappy plastic tubing on the wires that has become brittle and cracked. I am using some good nylon braided wire sleeving with heat shrink tubing on the ends to keep it from fraying. The wires will be routed up and along the side of the hull and not be run along the bottom like stock. I did this on my 1010 Pro 785 and it looks neat and professional.
Since I will be using the manual trim only, I bought a new start/stop/bilge switch off eBay for a great price. This one is without the trim buttons but still retains the ability to manually turn on the bilge pump. I will keep this until I can find an Octane start switch so I can put it over next to the grip on the throttle side in case I jump the start and need to quickly start back up.
More to come…
10-27-2016, 09:58 AM #5
Couple more miscellaneous pictures...
10-27-2016, 10:45 AM #6
- Join Date
- Jul 2015
- Addison, IL
So nice!!! Love it!
10-27-2016, 10:49 AM #7
Thanks, man! Hopefully this is the last rebuild I have to do for several years.
10-27-2016, 11:08 AM #8
- Join Date
- Jul 2015
- Addison, IL
10-27-2016, 11:28 AM #9
I'm thinking of going to 166's jetting ,I still can't pull over 7780 but it's so reliable amazingly enough
thanks for finally posting the progress lol I needed something to read
10-28-2016, 11:44 AM #10
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