I'm a huge fan of rotary engines. I own an rx-7 (and in the past owned another), and if these engines were more affordable, every dinosaur powered tool I have would be rotary powered, They're smooth, high revving, and just sound great. Not to mention a lot of power in a small package. So it should go without saying, it's been a dream of mine since I bought my ody that I would one day rotary swap it. That day has come.
Turns out, Johnson and Evinrude had single rotor powered sleds back in the 70's and I just so happened to be able to pick one up for dirt freaking cheap. Now, I am not a snowmobile guy and I realize beauty is in the eye of the beholder, but even new, these things were not what you would call attractive or well optioned. The only good thing about them in my opinion is that powerplant, so no guilt in my mind ripping it apart and putting it to good use.
The good: This project will solve many of the problems the fl250 has. 1.) It will give it electric start (for less than a third of the cost of a new GOKI). 2.)If I can figure out a way to get the driven to fit to the odyssey axle, it will have reverse. 3.)It will be beefed up to nearly 45 HP. 4.)It will be a more smooth running Engine with less vibration that wants to shake the machine apart. 5.) An actual charging/lighting system that can power more than a single red LED at half strength. 6.)And it gets extra badass points for a goofy Engine that sounds amazing. Still can't fix the lack of rear suspension haha.
The bad/Hurdles: 1.) I am gonna need to figure out how to mount the thing, from some rough baseline measurements, it looks like it will just barely fit. I may need to modify the frame and move stuff around. Obviously will need to weld up some mounts that bolt to the frame, would be nice to incorporate the original rubber Engine mounts. 2.) 45 hp from 17ish is a ridiculous jump and there may be some structural issues occurring with that kinda power. Axle and frame reinforcements may be in order. 3.) When I have to rebuild the thing, it's gonna be a nightmare due to lack of parts availability. From what little I have read, they're pretty reliable all things considered. 4.) Getting the driven to fit to the axle will be....fun. If it simply won't work, no way no how, then I will see about fitting a comet system to it and live without reverse.
The Engine does run! very well in fact, cleaned the carb and it started second crank and smooth as butter. First order of business is removing an auxiliary air valve on the bottom of the Engine that apparently has a tendency to get sucked in and make your Engine go from running, to junk, instantly.
This is an exciting, ambitious, and unique project for me and I welcome any comments, concerns, advice and ridicule. And if anyone wants the shell when I am done with it, it will be sitting for free in west chicagoland.

does that actually use a rotary (wankle) Engine or is it just a rotary valve Engine?

My opinion is, that sled looks to be in prime condition for a restoration, I would check on the value of it restored before you start tearing into it, if you do, there looks to be quite a fey hard to find parts that someone restoring one might like to buy.

what HP is that rated for?

It is a wankel rotary Engine yes, not a radial Engine. The sled is pretty rusty and not in the greatest shape, it just photographs well, you do have a point though, there may be some hard to find parts that someone would like to get their hands on. May look into that. As far as HP rating, the Engine is rated at 45hp on a 528cc Engine. Not sure if it would actually hit that on a dyno or anything, but that's what the documentation says. I believe it, the liquid cooled dual rotor mazda engines are 150ish at 1200cc.

Did start tearing into it, though mostly to get to that air valve which happens to be on the bottom of the Engine, in an impossible to reach spot. So, Engine removal is mandatory. They didn't make this thing the easiest to service, and my dumb ass ended up breaking the fan shroud a little trying and failing to get to a bolt. Its not that bad and could be easily fixed with some jb weld. I found someone before me broke it in another spot as well trying to get a different bolt out, that was seized. I may do away with the fan shrouding entirely due to that fact it would give the Engine a smaller footprint, and I have an "ebay special" radiator fan that I plan on mounting to the back of the ody seat, to be actuated by a clixon type temp switch. This was a plan i had for the original 250 Engine, that is a good idea for this one too. Those apex seals don't like getting too hot. If it doesn't provide adequate cooling, I will think of another plan or shoehorn the fan shrouding back on.

The shrouding and tins are really bulky



I read the fuel pumps are very plentiful as they are the same on most OMC engines. So that's good. Has a nice mesh filter on it too.


Primary and secondary engine-driven fans off



This is the air valve cover off, the screws on the valve itself were loose, so I'm glad I am doing this, not sure how much longer it would've survived. To get it out completely the Engine needs to come off the mounts so it can slide out.

That is a COOL Engine.
I used own a race a rotary race car so I know all about those rotary's.
Apex seals are very fragile. I ran carbon apex seals because the steels would break up over 12000 rpm.
There used to be a rotary dirt bike in the 70's if I remember right -- Suzuki ???
Rotary's run super hot so you better get all the cooling to that Engine as possible.
Looking forward to this build but would really like to see it in a FL350 frame.
CO

Edit: The FL250 frame is rather weak so I would mount that Engine on a shear plate and then bolt the plate to the frame. I think I would pass on the welding because the frame will most likely crack. Just my opinion. Cool project.
CO

Edit again: If then Engine don't start right up first hit of starter then that is a sign the apex seal springs are toast. A rotary MUST start up right away, if it don't that is a dead give away that the Engine is weak. You can still get them started even if a seal is broken or springs are shot. You dump some oil down the intake so the seals have a good seal at start up. It will start but if you shut it down you have to do the procedure again. Engine won't have much power but it WILL run. That's why these are good aircraft engines in my opinion.
CO

Wow CO, that's an amazing car, looks like a blast. I am personally pretty familiar with rotaries as well. I was unaware of the dirt bike, but I know there was a road bike that had a rotary, I think it was a 'suki' and had a big ugly dash that looked like a can of beer. The heat is certainly a concern that will require close monitoring in the beginning. Honestly, the crappy rad fan I bought moves a surprising amount of air, but like I said, will monitor and amend my plans as needed. I am unsure what kind of apex seal this Engine uses, but I believe they are in good shape. I checked compression of each rotor face and the readings were 47/47/50 cold. Initially thought this was real bad and the Engine was toast, but I looked it up and the general consensus from the enthusiasts is that anything over 40psi is 'A+'. In my opinion, it does start right up, right away, certainly doesn't need the oil/atf trick for compression boost at all. Maybe you're right though, the youngest it could possibly be is 45, it could very well be clapped out, but I am confident in its health at the moment. Thanks for the advice on Engine mounts/welding! I wish I could find/afford an fl350, but the 250 was hard enough to find. In my area, if you can find them at all, they're typically beyond repair or crazy expensive. My fl250 was literally a frame and a box of parts when I got it, had to reassemble the entire machine haha.

Here is the dirt bike I was talking about.
You have to watch the video to the end as they throw in the history of the bike and other model at different spots in the video.
You can start at the 30 second mark to knock off some time. It's about 8 min long.
https://www.youtube.com/watch?v=G_7aDjfUN-8
I remember seeing that machine in a dirt bike magazine in the mid 70's when I was a teenager. I was big into dirt bikes at the time. I'd buy a new bike every holiday pay. Wish I still had some of them as they would be worth real money. The one I really want back is my 1977 Can Am MX3 Pumpkin 175cc motocross.

That's a very rare machine you have there. You might want to investigate value.
CO

Hadn't had time to work on it lately. My ATC 200x was screaming at me to finish the cam chain replacement I started and never finished months ago.
But did manage to get the Engine out of the sled and on the bench. It needs a quick bath I think.



From the couple centimeters I can see through the plug hole, the apex seals look good

The belt this thing uses is massive. Has a big beefy clutch too.


For those that were worried about tearing into a potentially rare and valuable sled, I looked into that. Rare? Yes, but apparently not very sought after or worth a damn. If these guys' opinions are worth anything these sleds are "kinda worthless" https://www.hardcoresledder.com/threads ... th.540255/

be careful when cleaning not to blow through any seals or gaskets

Finally got the auxiliary air valve out of there


Drilled and tapped the hole.


And stuck a brass fitting in there with copious amounts of red locktite.



Now the hard part, which is going to be finding a way to get that beast mounted into the ody. First gotta make some room and remove the stock Engine.

Have been putting some thought into the mounting of this new powerplant and I have a few questions, maybe someone can help.
One of the big obstructional challenges is going to be the big horizontal plate that is welded to the rollbar. The one with the two holes in it, one of them for filling the gas tank

Is that a huge structural component of the rollbar? I would think so. Is it unwise to cut it and move it forward, or add some bracing to compensate for its removal?

Also was thinking about the idea of making the output of the new transmission essentially a jackshaft that pulls a sprocket mounted to the ody axle, the downside of this plan being, that if the chain breaks, I have no brakes except the e-brake. Is there any way to get a sprocket that fits the axle splines? I don't know if its a common size at all. Also would like to minimize the chance of chain breakage if I go this route, what load rating would I want to look for in a chain that would be subjected to approx 45 horses?

For my swap, I cut a large piece of it away for clearance. IMO I wouldn't call it a huge structural component, though I'm sure it adds something. I think it's just as much for providing an anchoring for other stuff, such as the tail light, the driven clutch shield, and the reduction unit, the latter which I think is pretty important. But it sounds like you're not using the reduction unit anyway.

Took a look at your build thread, interesting stuff there, gives me some things to think about. You cut quite a bit away, makes me feel better. I am not planning on using the stock ody transmission in anyway so should be good.

Could I just crack open the case of the reduction unit and take the final drive sprocket out and throw in on the axle? Sounds like it could work.The tooth count/ratio might be funky though. If not, does anyone know of a proper sprocket that would fit?

I don't see why not. If you're not going to use the reduction unit, may as well take it apart and try it out. The only issue I can think of is that since it's connected to the axle by splines, you're limited with where you can mount it -- it's going to be pretty far offset to the left.

I'm watching your build with interest. Post updates as you can. Unfortunately there's not much activity on this forum but there's a few people around. Probably other people that doing builds and posting :)

@OddFella, sorry for not responding to you, I went AWOL for a while...

The limited mounting of the sprocket may be an issue, I will have to see as the build progresses. As it is, I might be able to make it work, but it will require the new transmission to be mounted up into the air a bit. Think of it like an an "8", where I will need to set up brackets and mounting to the bottom of the "8". So with the stock configuration, if the belt travels at 90 degrees, with this setup the belt would travel at 45-ish degrees, in the air a bit. As I said, this might be an issue, but I might be able to make it work.

I mounted up a pillow block to hold the axle. Its actually pretty straight and true, needing only minor finagling. I still need to cut down the little sleeve that goes over the axle, to hold the sprocket in place. Also please ignore the complete disaster my garage has become...




Then I set about getting the Engine mounted in the spot that I want it. I needed to set it as far forward as I could while still leaving space for the cooling fan that I want to stick on the back of the seat. Also needed a bit of space for the exhaust. I threw down some garbage-grade welds, to hold it steady while I get everything mocked up. I think I am happy with where it is. I am going to need to cut and braze/weld the little tower sticking out for the pull start. I wont need that much with the shrouding removed, and it sticks off the machine as it is. Some aluminum brazing should hold it well enough.





As you can see, I literally just took the stock oddy Engine mount and welded the johnson mount to it. I had to flip the johnson mount around backwards and cut a notch out for electric starter, otherwise it wouldn't have fit right. Again, we will see if this is an acceptable solution long term, but its good for the time being to get things all put in place. Next, I think I am going to try and tackle mounting the transmission as that is going to probably be the biggest hurdle here. From just me holding the big heavy thing up to the Engine, it looks like the jackshaft may need to rest in a pillow block bearing above the frame, as opposed to the axle which is sitting below.

Cool, keep at it man!

Looks like you got the Engine mounting pretty good. One thing I would point out is that with a smooth rotary, you could probably solid mount that motor (( Internal Combustion Engine ? )) and still have less vibration than the stock Engine w/ rubber mounts. W/ rubber mounts you probably won't even feel that Engine running!

I think the cacophony of the exhaust right by my head, should be a suitable indication that its running haha.
Another question to throw out into the aether: Does anyone have any idea as to the efficacy of the first gen "Half cage" present on the yellow machines? Personally, I really like the look of them, and it would be such a hassle to create a full cage on my machine, as there wasn't one, stock. Obviously Honda engineers/lawyers thought a full cage was a better idea, but has anyone had a near death experience after rolling their half cage machine?

I guess it all depends on how crazy (or not) you intend to get driving it around and where you'll be riding. Personally, I'd feel a little naked without the full roll cage but I drive through trails with trees.

But aside from safety, the full cage also adds stiffness to the chassis to keep it from twisting and bending/drooping in the middle. If your new Engine is heavier than the stock Engine, you may want to at least add some additional bracing to the frame. When I put the V-twin on my 250, I welded some 1/4" plate steel along the "H" area of the frame behind the seat to provide extra strength.

After staring at it, brainstorming, staring more, getting frustrated and abandoning the project, then doing some more staring, I decided that I needed to go back to the drawing board and rethink the positioning of the various drivetrain components. My original placement of the Engine made it actually impossible to mount the driven, and after an "Ah shit" moment, I realized that the way I would've had to mount the driven anyway, would've been flipped around. This means the pulleys would be pulling the belt in opposite directions as they "changed gears".
So I abandoned any self righteousness and delusions of grandeur around not extending the frame and decided to just do it. The frame is being extended along with the half cage so it can fit the longer form factor. Now, with the freedom and space to do what I thought would be the best way to mount this stuff, instead of trying to fit 20 pounds of shit in a 15 pound bucket, I more or less mounted the driven in the same way the honda engineers did with their much smaller one. I cut the top of the C channel frame to let it drop down as far as possible, squeezing every centimeter I could out of things. Later, supports and fortifications will be added to strengthen and stiffen the frame despite this. Many hours of cutting, and shitty welding later, I managed to get it mounted up. Started to feel good/motivated about this project again.




I cut that notch out to make sure the piece with the yellow sticker would fit, I was a little over zealous and will fix it later.

Then came the issue of mounting the Engine. It would interfere with the jack shaft if it was mounted as low as i would like, so instead of a 2-3 inch drop, I had to give it a 1 inch rise. Despite the increase in power, this Engine is not really much heavier than the stock one, if at all, so it shouldn't be too tippy...well, more so than these machines already are. If it is, I can always revisit and reposition.
Admittedly, it looks a tad silly mounted up this high, but i think with all the other stuff crammed in there (Exhaust, battery, electronics, gas tank), it will look less so.


More shitty welding. This will be welded more completely, and with more care, once the position of everything is finalized and the machine is in driving condition.


Now that I have the Engine in place, I know how much i need to extend the frame by, and I can put the back of the machine back on. Once that's done, I will need to put a support from the back that grabs the Engine mount so that it doesn't pull and twist the one side of the frame. Next on the list is aforementioned frame reassembly, and then jack shaft brackets and mounting.