Author Topic: Rotax 793 engine in to pilot  (Read 21724 times)

lee1969gb

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Rotax 793 engine in to pilot
« on: November 17, 2006, 03:19:07 PM »
Hi. I have not been on this site for a while, forgot my password so have had to start again, my original  user name was lee1969uk.
I have just purchased a rotax 793 engine with exhaust cdi carbs etc, i am told that this engine has the facility to run in reverse (I think it advances the timing, backfires and continues to run in reverse until switched back.)
I am thinking of removing the pilot transmission as reverse is not required and probably wont take the 133 bhp of the rotax engine and using a layshaft to reduce the speed of rotation or use a small reduction gearbox. for a cvt drive or clutch i am thinking of using the clutch and belt from a mxz renagade rotax 800 sledge which uses the same engine as i am using (I think, we do not have snowmobile,s in england so i am not familiar with them)

e-bay link


e bay link

This is a picture of the engine i am using
I am also unsure if this clutch will work with the engine in reverse?
I am told rotax do not do a electric start for this engine although it does have a power output to recharge a battery. I wonder if the snowmobile manufacturer fits an electric start to the engine.
As you can see i have a lot of things to sort out and i do not have access to snowmobil,s. Any advice on this project would be very welcome.
Thanks Lee.

nuke em

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Rotax 793 engine in to pilot
« Reply #1 on: November 18, 2006, 11:14:03 AM »
Sweet, keep us posted with progress! Take pictures too, I enjoy the pics.

atvnut

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« Reply #2 on: November 22, 2006, 05:44:31 PM »
the engine should be drilled in tapped in the front for an electric start there would be 2 bolt holes just to the inside of the out put shaft. I would leave the pilot trans in because inorder to get your gearing right with a layshaft your sprocket will need to be very big you need at least a 5:1 ratio.  getting a good fit for the exhaust pipe will be your biggest challenge.
Thank you
"The Nut"

lee1969gb

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Rotax 793 engine in to pilot
« Reply #3 on: November 23, 2006, 12:56:06 PM »
Thanks for the advice. Do you think the pilot gearbox will take the power, this engine has about 140 hp, and will the torque clutch take it even with stronger springs. Has anyone tried putting this kind of power through a pilot transmission. I was thinking of removing the transmission and putting in a lay shaft to reduce the speed in two stages and use the reverse on the engine.

FL670R

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Rotax 793 engine in to pilot
« Reply #4 on: November 23, 2006, 09:16:48 PM »
I run the stock pilot tranny behind a 670 rotax.

I believe stock the 670 was around 115 HP
I don't know what mine is putting out but it's has been mildly ported...

Last Sping for two weeks I ran it all over Killpecker WY and St Anthoys ID and
even thou I managed to push the motor close to its limits (over heat issues) the tranny never flinched...

I kept checking the clutch pac as others have said it may get hot - mine never did - we were mostly on sand thou...

Later at St. Anthonys I thought maybe I lost my clutch pack during one ride
We even took the cover off to check and it was perfect - no blue or heat related / slipping issues at all

It actually turned out that I lost the splines in the adaptor collar I made to allow me to run an arctic cat driven on the stock honda tranny input shaft...

Long story short - Keep the Tranny and forget about running your engine backwards...

The fun stuff is figuring out how to setup the clutches, exhaust, and other stuff
ps mine doesn't have electric start although kits are available (I have one) you can make most any one work with some custom brackets...

PS
Nice motor will you share what rotax charges for something like that ?

Later
Dennis

lee1969gb

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Rotax 793 engine in to pilot
« Reply #5 on: November 24, 2006, 10:37:05 AM »
If that is the case then i will have a look at using the pilot tranny, did you use stronger springs in the clutch pack (I take it what you call clutch pack is the same as torque clutch, the multiplate one in the tranny).  Not sure about the space available to get the rotax engine, pilot tranny, and clutches etc  in, the one thing in my favour over the 670 is my carbs sit a bit higher than the crank case induction fuel injection 670, should give more clearance.

I may have done a deal for a ski doo primary and secondary clutch brand new he tells me with starter ring gear $600, Is that a good price or has anyone got one to sell me. I dont suppose the secondary will go stright on to the pilot tranny and not sure how the ratios work out.

You ask about the engine, there is a bit of a story about that, actually i  have 4 of them, if you have ever seen the james bond film die another day at the begininng there is a hovercraft chase, 2 of my engines are from the hovercraft that get blown up (they were taken out before they were blown up complete with carbs electrics exhaust etc) and the other 2 are brand new that were spares for the hovercraft. I have one for sale on ebay.com i would consider selling one with cdi carbs exhaust etc. they have this RER Rotax electronic reverse which alters the timing and makes the engine run backwards, saves fitting a reverse gear.

FL670R

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Rotax 793 engine in to pilot
« Reply #6 on: November 24, 2006, 10:57:19 PM »
Yea I was talking about the multi-plate torque clutch on the tranny.
I am actually using a totally stock pilot torque clutch and plates.
I have heard that others simply add a washer under each of the springs to increase the torque applied.

I'm actually running a water craft downdraft intake on my rotax which gives me tons of clearance with the tranny...

I really have no clue what a good price is for new cat clutches - I get most of my stuff from ebay or sled salvage sites...

Correct the cat driven is 1" keyed and the stock pilot input shaft is 7/8 splinned...
You will have to either modify the cat clutch, replace the input shaft, or build it up and turn it back down with a lay or something...

I went the route of cutting a stock pilot clutch driven splined collar off and welding it to a arctic cat secondary.
As you already know my first attemp of this failed - It was actually done by someone before me and they only used 1" of splines...

I just rebuilt a new one this time with the full 2+" of splines and I have yet to put it to the test...

If it fails again I will get a stock pilot tranny input shaft and have it welded up and turned down instead...

Another word of caution running a Cat secondary will hit on part of the rear brake caliper and you will have to clearance it in a couple of places

I have posted pictures of most of this on the various pilot/odyssey sites.
If you can't find them and would like to see my set up let me know...

007 - COOL...  ( I haven't seen that one yet )

So you are from the UK ?

I actually have had that ebay auction bookmarked and have been watchng it...

3K is a little steep for me but it does look like a nice powerplant...

Keep us posted of your progress

Later
Dennis

lee1969gb

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Rotax 793 engine in to pilot
« Reply #7 on: November 25, 2006, 04:25:36 AM »
You say cat clutches, I am after clutches for a ski doo, are they the same? As long as the primary shaft size is the same and is designed for that power the secondary spline does not matter, it needs to be modified any way!  
Could you please tell me where to find pictures with the clearance problems on cv boot and caliper.
Yes i am from the uk this is why i do not know much about snowmobiles it is often wet here but not much snow just plenty of mud.
you say your torque clutch is standard, i am suprised it does not just burn it out, you would think it is set up to slip when it gets a shock load with the standard 40/ 45 hp of the pilot engine, so you are putting over double that through it.
How does your gearing work out and what sort of top speed can you expect whilst keeping good low speed/ standing start performance. I know you can get the best of both worlds with different makes, springs, ramps, belts, angles, helix, etc, but again here in england i just no not have access to that sort of stuff and need help from you guys to get it sorted out.
Which takes me on to the engine i have for sale if you are interested i will sell it to you cheaper as you are giveing me lots of good advice, but i will keep the inlet rubbers/reedblock as my used ones have split which does seem to be a problem if you look at some of the ski doo forum sites, just fit v force items which are cheaper than the standard ones and cures the problem. you will buy a cdi, carbs, exhaust, etc much cheaper in canada/ usa from a ski doo. let me know if you are interested.
Thanks Lee.

lee1969gb

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Rotax 793 engine in to pilot
« Reply #8 on: November 29, 2006, 03:39:33 PM »
I have now bought a set of clutches from canada, I also got a jack shaft for the secondary clutch to fit on to. I am strongly considering not using the pilot tranny as long as i can get the RER to work  :?  the engine has the 2 pickups for timing forwards and backwards so that is ok and i am told the cdi has the facility to work the rer but I do not know how to wire it up, if any one can help please let me know before i rip out all of my hair :x . I plan on using the jack shaft with a small sprocket on it driving to a big sprocket (about 3 to 1) on a lay shaft then another small sprocket on the lay shaft driving to a big sprocket (again 3 to 1, giving 6 to 1 overall) on a pilot final drive shaft, mounted in the same place as it is in the pilot tranny, this way I can change the gearing and i have worked out a way to tension both chains by different amounts. This way i do not have to worry about the pilot tranny taking 140 bhp from my rotax engine. I will mount the engine, jack shaft, lay shaft, and final drive shaft on a common frame, rubber mounted to the pilot chassis, hopefully with no mods to the pilot chassis. :)

FL670R

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Rotax 793 engine in to pilot
« Reply #9 on: November 29, 2006, 05:13:09 PM »
Sorry for the delay....

Yea most all snowmobile clutches are very similiar whether it be Arctic Cat or Skidoo or whatever
(Actually the Skidoo TRA clutches are very good)
Just make sure the motor and clutch are for the same taper usually 30:1

The Pilot tranny torque clutch really only takes a beating when you jump and land while on the throttle...
It is made to slip to save drive line and tranny wear and tear
Although I do love to jump I rarely if ever land under throttle...

Currently mine will spin the tires at launch and pull around 75 MPH top end.

Thanks for the offer on the motor but I'll have to pass at this time...

Another big advantage of you not using the stock tranny is that the jack shaft will be the correct size for you driven clutch (usually 1" keyed)
That way you won't have to mess with that odd ball input shaft on the stock pilot tranny....

I don't know anything about RER so others will have to chime in
Your best info on that may come from the sled sites....

Included is a pict of the clearance issue I had with a larger Arctic Cat driven on the stock pilot Tranny
If you look close you will see the top half of the upper rear brake caliper bolt has been clearanced to allow the driven clutch to fully open...

PS
Good Luck and Keep us posted - WE LIKE PICTURES...

atvnut

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gearing
« Reply #10 on: November 29, 2006, 10:03:59 PM »
do you have access to some giant sprockets as you will need at least 6:1 gearing to make it run right the pilot trans is 9:1
Thank you
"The Nut"

LiveWire

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Gear reduction
« Reply #11 on: November 30, 2006, 05:47:30 AM »
That 9:1 number has been going around for a while. Somebody said it and it stuck. The service manual on page 1-4 says 9.857. You'll need at least two chains and three shafts to get that kind of reduction.

lee1969gb

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Rotax 793 engine in to pilot
« Reply #12 on: November 30, 2006, 01:30:21 PM »
I plan to use two 42 teeth sprockets from a gsxr 1000, one on the lay shaft and one on the final drive with two 14 teeth sprockets, one on the jack shaft and one on the lay shaft. 14 goes in to 42, 3 times, so thats 3 to 1, twice = 6 to 1.
Hopefully that should be somewhere near but easily changed with different size sprockets. I am hoping to get my pilot to have a top speed of about 90/ 100 mph and do wheelies, but not at the same time  :shock:  Another thing to consider is my rear tyres are about 3 inches lower in overall hight than original pilot ones so that will lower the overall gearing quite a bit. I am using these tyres because they put less strain on the tranny and more importantly I bought 26 of them very cheap :D I just hope they do not come off the rims at 100 mph.

LiveWire

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gear reduction
« Reply #13 on: November 30, 2006, 01:54:32 PM »
You multiply, not add so that would be 9:1. 9 turns of the input shaft will turn the intermediate shaft 3 turns which will turn the final shaft 1 turn.

That should be fine. The final drive on the Pilot clutches is .7:1 (per service manual) and I think the cat clutches are 1:1. .7 x 9.857 is 6.9. So with 9:1 final drive, you'll actually be geared a little lower.

lee1969gb

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Rotax 793 engine in to pilot
« Reply #14 on: November 30, 2006, 03:06:41 PM »
Thanks Livewire you are correct. Now Wondering if I should use 15 teeth small sprockets to make up for the smaller rear tyres.
I was wondering how the different clutches ratios would work out. the clutches I have bought are from a ski doo 800 with rer, when they show up and I sort out the rer I can make a start.