01-30-2011, 04:33 PM #1
Well, here's my latest project, while I wait for my knee to heal from knee surgery. A 1984 Surfjet model 236 Freestyle. It's got a Fuji (!) 255cc 2 stroke, 2 cylinder motor, mounted vertically. Only puts out 17hp, so they're kinda slow.
Anybody have any suggestions on modifications for more power? I'm a member over on deckjetwatercraft but their form is pretty much empty. Lots of questions but nobody with answers...
Last edited by delagem; 08-20-2011 at 12:16 PM.
01-30-2011, 05:51 PM #2
- Join Date
- Jul 2007
- near Toronto, Canada
If you did get more power out of the engine, could the impeller and jet pump handle it?
255cc across two cylinders? That's 128cc and 8HP per cylinder - lawnmower territory.
Compression boost and porting come to mind - how much effort you planning on putting into this thing
01-30-2011, 06:51 PM #3
My understanding on these motors (as well as the SL650 Fuji motors) is they were built to be industrial irrigation motors, not high performance motors. They were meant to run for a long time, not a fast time.
I checked the squish before disassembly, 2.0mm! But check out this head, this design doesn't exactly lend itself to tightening the squish... You could skim this thing an inch and it wouldn't make a difference!
So I guess I've gotta skim the block instead. Porting would undoubtedly help, too, but the exhaust manifold on this thing is another "unique" feature. I don't have a pic but I'll get one. Unlike anything I've ever seen before....
I need to compare the exhaust porting numbers to my notes from Bell's book, 2 Stroke Performance Tuning. I'm sure there's a lot of gains to be had all around.
I'm sure the engine and pump could definitely handle an performance increase. According to the service manual, peak HP is achieved at 4500-5500. You're not kidding about lawnmower territory!
I'm still hoping to find someone who's already driven down this road!
02-02-2011, 09:39 PM #4
Anybody know of any really small carbs? This thing used to have a 22 mm Mikuni of some sort (and no flame arrestor as stock! Guess things were different back in the early 1980's).
Anyways, the carb on this one is missing. I've been told that a particular Walbro carb is a significant improvement over the stock one. But nobody seems to know the model numbers.
Walbros were common on rototillers, lawnmowers, etc.
The studs are spaced 42mm center on center. The opening in the intake manifold is 26mm.
Anybody have any suggestions? Or know where to find some fitment charts? I've got a few emails out to Walbro and parts suppliers. I thought I had an old Keihin 28mm from the Jetmate, but it was actually the larger 34mm size.
02-02-2011, 10:29 PM #5
- Join Date
- Jul 2007
- near Toronto, Canada
02-05-2011, 08:52 AM #6
No, I tried them too, no help. The only guy who had a clue about what I needed wanted $400 for a single used carb!
Random google searches finally clued me in. I searched for "Robin BV24" (BV22 was the stock carb), this led me to a gocart forum, which led me to a 1988 EZGO golf cart, that used a BV24 carb. Problem solved!
My next battle is with waterlogged foam. Surprisingly, taking out the old foam is really easy if you use the right tool....
02-05-2011, 07:30 PM #7
Here's a pic or 2 of the exhaust setup. It's just bizarre, the exhaust comes out the port, and then down thru the hole out the bottom. The "manifold" is really just a cover.
166 degrees of exhaust duration! Briggs & Stratton called, they want their port timing back!
02-05-2011, 11:00 PM #8
What about a 300sx pipe? Or a 300sx motor, go bigger and shoehorn a 550 in there! I had a neighbor that had one of these, never got to look inside at space...
Could the pump handle a slightly tweaked 550, they will spin up to the stratosfer...
02-06-2011, 10:11 AM #9
02-06-2011, 10:47 AM #10
That board does look awesome.
I like the idea of a motor swap, but look at the size of the cyl's in that pic. It practically fits in the palm of my hand! Even a Kawi 300 isn't this small, and the engine compartment has absolutely no room. That's why there's no flame arrestor, there isn't even an inch of extra space. Besides, I'm not convinced the Kawi is any better from a starting point than this Robin engine. With good mods, I'm confident I can exceed the power of a js300!
Regarding the Powerski; 50 hp from a 50 cc engine? Geez, I think the guys af F1 would like to have a look at this design.
I'm calling BS.
"A proprietary two-stroke, water-cooled engine that puts out between 30 and 50 hp, depending on displacement. The 6.5-inch thick, 50-cc engine weighs about 40 lbs, but use of a metal matrix composite material should drop the weight to about 28 lbs, Montgomery says"
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