New Engine Time...
After many hours and 3 seasons on the JM 718 motor (it's about 754cc now, not by choice) I have started to find a niche in riding preference. The JM is undeniably a solid arm ripper, but I've started to adopt a more adventure jet skiing (
ADVSKI - I trademarked it, jk) style of riding which involves 40-50 mile group rides packed with following high mountain/desert lake coast lines, finding remote coves, jumping boat wake, plenty of old skewl trickery and the occasional buoy session. I started to get curious about what a 760 would feel like... I also really like making stuff, almost to a masochist extent some time. Let me preface this build with this realization -
Jetmaniac could easily charge a lot more for his motors and it would still be an amazing deal - this is not easy stuff and there are so many opportunities to instantly turn a motor into garbage, potentially costing you hundreds if not thousands.
Over the last year and a half, I've compiled a hodgepodge of 760 parts and started the process of making a motor out of them this summer. After many hours on the water I had a very good idea of what type of power curve I wanted and I knew that being able to hold it WOT, and having a mid rpm pipe hit were high up there - but most importantly I wanted to keep the excellent throttle response I'm used to with a ported motor. I did a lot of research on the 760 builds on here, what people like and don't like about them, and started to compile a list of what I would do if I built a 760.
I knew that an excessively wide exhaust port would not allow me to have WOT fun. I studied exhaust port shapes in general and decided on trying the yamanube template combined with careful intake port matching, intake side sleeve trimming and then finished with media blasted transfer tunnels. I didn't go wider like a lot recommend given the 701 vs 760 bore diameter, etc. I wanted to keep this ski around the safe limit of exhaust port width for rec riding. I did all the porting / blasting and then had it bored 0.5mm over (84.5mm).
So I went down the rabbit hole of getting a 1/4 straight rotary tool and a 1/8" right angle rotary tool. I used a VEVOR 500W 1/4" and a no-name pneumatic right angle as the pneumatic right angles are way cheaper than any right angle tool head for the electric rotary machines. Both worked great. I got aluminum and double cut carbine burr sets both in 1/8" and 1/4" shank for all the work. aluminum = wider single cut style for hogging work, double cut = steel work (can do aluminum as well but clogs up way easier). Get a steel brush to clean your burrs every few minutes and spray them with WD40 to help with material adhesion.
Below is the Cylinder porting work. Note - I did not raise the exhaust port even a micron. I read enough about this porting mistake on 760s to know better. The top profile in the pic below is actually more rounded than it appears - it's just the angle and it's more of a squished octagon.
I filled the 760 intake manifold crossover after cutting a 'lock in' groove with a ball burr to keep the JB Weld from shifting. - simple enough.
I haagen daz cases out by finger porting (filled the intake side top case voids as well as the flywheel side wth JB weld - again using the lock in groove method, not pictured, as these are where you mill into for finger porting)
1) Initial grinding - basically have no idea what I'm doing here. Just jamming a big 1/4" aluminum burr into the edges trying to get a finger groove going.
2) Getting a little better - still not finished - but much bigger reliefs now and can see the JB weld so you know you've done some work.
3) After masking all seal and bearing areas with T-Rex tape, I media blasted most of the contact surfaces to get a relatively uniform finish. I used 80grit aluminum oxide on both cylinder transfer tunnels and the top case halves.
I didn't do lower epoxy feed ramps / stuffers like you see in a lot of case work.
Slapped it all back together. I'm using an older non-girdled RIVA head with 84mm 38cc domes. Might try and either remove the base gasket or find 35cc/36cc domes this winter - base gasket removal seems like a win/win and it's "Free". The Riva head got all new o-rings.
The carbs are the cursed OEM SBN 44s. I thoroughly cleaned them and rebuilt them. Made a custom throttle cable mount for flywheel side mounting as well as drilled and tapped 2 new threaded holes to put it on that side - since most 760 intakes are the wrong side.
I'm too crafty and cheap to buy t-handle adjusters. So I made my own with some 3/32 316L tig filler rod. For the low speed screws I just did a 25amp fusion zap with the tig. For the brass I used a 45T (high silver content) brazing rod - worked amazingly well.
Dual fuel lines, dual fuel pickups.
Tossed it in, with some fresh hose.
The carbs hit my lowered hood. Simple enough to fix... cut out a football shape and utilized my underwater basket weaving skillz.
Some other tidbits on the engine include some NOS Boyesen reeds I found, 1 screen in the flame arrestors and a stock 760 intake box.
Ride Report
I started with the following jetting - with the presumption that it would likely be rich (I'm at 5280ft). This was after looking at way too many jetting posts on similar engines. Wax says repeatedly - aim for around a 240 combined count for main and pilot - this is obviously bigger. This is also slightly richer than what factory pipe says for the blaster 2 pipe.
- Mains: 132.5
- Pilots: 117.5
- NS: 1.5
- Spring: 80gr
- Popoff (measured): 40psi
- LSS: 2
- HSS: 1
I primed the fuel system, hit the primer a few times and it started right up!
Took it to the loading dock, did a quick strap down test and it was idling and moving through the RPMs pretty well - I know by now that neither of these will really tell you how a motor runs while moving in the water though...
Took it out and after a brief no wake zone crawl it was so loaded it wouldn't get on pipe. I hit the beach quick and turned LSS in 1 turn, HSS in .5 turns. BRAAAP. It now runs and can climb in RPMs to hit the pipe. Ran the rest of the 32:1 break in tank through and noticed it does indeed drink fuel. I knew this was going to be the case - so I'm just going to order the slightly oversized Wax tank and get some more volume. Like I said - adventure jet skiing. For those of you that ride dirt bikes with oversized tanks like I do - same idea.
I've since bumped it down to 115 pilot - the 115 pilot is very good and with the LSS set at 2.5 turns out - it's idling like a dream but not loading up. Keep in mind the weird yamaha oem SBNs use the fine threaded low speed screw - so 2.5 turns is like 1.25 regular turns - or something like that I'm imagining. My understanding is that if your less than 1 turn out on the LSS - go down a pilot size so you can fine tune better. If your over 3 turns out, go up, etc.
So for now:
- Mains: 132.5
- Pilots: 115
- NS: 1.5
- Spring: 80gr
- Popoff (measured): 40psi
- LSS: 2.5
- HSS: 0.5
There's a slight mid RPM flutter/burble and it doesn't climb in RPMs as high as I would imagine it could - both making me think the main is still a little rich. Don't get me wrong - it revs up but I feel like there might be a little more on the table. I gotta do a zero HSS turns test next but I'll likely go for a 130 main next. It really isn't much of an issue as I'm usually not cruising at the exact throttle position to cause the issue, so I can totally live with it. Another rich indications is black plugs with a slight oil left on them. The pistons are also a wet black with no real visible piston wash pattern. It never falls on it's face with a WOT stab and climbs very nice in the RPMs so I'm probably just in-between jets.
I know a lot of guys do the reverse jetting but I'm just trying to get this thing to run good for mid to top fun with enough low end to pull myself out of the hole and recover from a slide out. It's there at this point.
In the end - the power is very strong and linear. I'm very happy with the result given I've never done this type of build before. When sliding/slashing the ski remains hooked up the whole time - sort of like it has more traction now. I'm not sure why this is but It's actually fun and in the last few hours I've been able to really enjoy this new riding style knowing that I can really get slashy and grab throttle and jet off again. There's still enough bottom end that if I'm at least moving a tiny bit I'm going to be able to jump out of the water without issue. Definitely a different powerband vs the 61x cylinder but I'm enjoying every bit of it.
