View Full Version : Reversing engine rotation- Lawn Boy, 2 stroke

Nashville Geoff
10-11-2008, 10:23 AM
I'm into some extreme projects, and one of the more insane ones is mounting a 2 stroke Lawn Boy engine on a bicycle. It would be a huge help to have the engine running in reverse of it's original configuration- I wouldn't have to fabricate a jackshaft assy if I can make it run in reverse. I want to run a centrifugal clutch to the front derailleur, which would give me a lot of gear selection at the rear.
I have a great running older 5 hp Lawn Boy engine. I will probably use a Tillotson carb of some sort. (If anyone has a carb that would work well that they'd sell me, let me know)
I have a lot of fabricating experience, as I build custom motorcycles as a hobby, and have a fairly well-equipped shop. Any suggestions would be greatly appreciated. Thanks, Geoff

10-11-2008, 10:38 AM
It would be complicated. Russ will chime in on this.

Why dont you mount it on the opposite side and turn the wheel around?

10-11-2008, 11:30 AM
Is possible in theory. Would require reversing polarity of flywheel magnets and relocation/flipping around of CDI pack which would then introduce timing issues. Flywheel and/or crankshaft would have to be re-keyed.
Lawn-Boy CDI packs are different from most in that it's not only responsible for producing spark, but timing as well.
Really 2 coils in one package. Start and Run.
The start side of CDI works at rpm's under 800 and spark is advanced by about 6 degrees or less.
Past 800 rpm, the run side of CDI takes over and spark is advanced by 24 to 27 degrees.
If start side of CDI fails or is otherwise disabled, engine will be impossible to start.
If you could get a mirror imaged flywheel and CDI pack, your problem would be solved.
Sounds like you have a model "F". Look here (http://www.youtube.com/watch?v=U_fapdfrMyc).
Had some bugs in it first time out (4 cycling) that have since been corrected. I recommend this carburetor (http://cgi.ebay.com/NOS-Tillotson-HL239B-Carb-Go-Kart-Minibike-Mini-Chopper_W0QQitemZ270284556895QQcmdZViewItem?hash=i tem270284556895&_trkparms=72%3A1205%7C39%3A2%7C66%3A1%7C65%3A12%7C 240%3A1318&_trksid=p3286.c0.m14).

Nashville Geoff
10-11-2008, 02:50 PM
After Russ's comments, I don't think that reversal would be practical for the amount of work it would require, unless a "mirrored" flywheel & CDI pack were readily available. I wonder if the factory has built any sort of application where they had the motor reversed in rotation.
I'm not following you about mounting on the opposite side & turning the wheel around. The only options that I see are to either reverse the rotation or run a jackshaft to get the power to the front derailleur turning correctly. What am I missing here? Thanks, Geoff

Nashville Geoff
10-11-2008, 02:54 PM
Thanks for the input. You've saved me a lot of grief. The video that you linked me to is what got me started with the idea of adapting a Lawn Boy engine. I'll buy the Tillotson you suggested, and probably start designing a jackshaft into the build. Thanks, Geoff

10-11-2008, 03:10 PM
I wonder if the factory has built any sort of application where they had the motor reversed in rotation.Never done by Lawn-Boy. Only 2 small engine manufacturers have done it as far as I know.
Clinton Engines, Inc (http://www.clintonengines.com/). (closed doors in '80's) and United States Motor Power, Inc (http://www.usmotorpower.com/820.htm).
The only options that I see are to either reverse the rotation or run a jackshaft to get the power to the front derailleur turning correctly.I can see no other way.

10-11-2008, 03:46 PM
I mean if you literally TURN the engine around and use it on the other side. So if you mount it on the right and it turns counter clockwise, It will move forward, where as on the left, it would move the bike backwards.Not sure which way the LB's turn, but it would work.

Heres a crappy pic of what i mean.

10-11-2008, 04:14 PM
he want to use the drive train as is but put a motor where the pedals would be. so he can use the same shifters and shift as if he were pedaling.

Nashville Geoff
10-11-2008, 04:28 PM
That does make sense now! It could work. Thanks, Geoff

10-11-2008, 04:35 PM
Not sure which way the LB's turnCounter clockwise, same as Briggs, Tecumseh, etc. Standard stuff.
he want to use the drive train as is but put a motor where the pedals would be. so he can use the same shifters and shift as if he were pedaling.Sounds reasonable. Not sure if drive train can handle torque of L-B. Something will require some beefing up, me thinks.
What ever the final design is, much time, math and labor will surly be the order of the day (weeks, months?).

Nashville Geoff
10-11-2008, 08:19 PM
Actually, I plan on putting the engine behind the seat and either run a chain up to the front deraelleur, so that I can use the gears, or to the rear wheel. Geoff

10-20-2008, 11:06 AM
I wouldn recommand running a chain to the front derailleur because you will loose some mechanical advantage (you need to gear the motor down not up) and you will end up with a bicycle crank spinning very very fast.

I did exactly what you want to do and I used a 3/4 bore 12 tooth clutch (#35 chain) and a 72 teeth sprocket on the rear wheel for a 6:1 ratio. I've machined the crankshaft to 3/4 but you could buy some kind of piping to make an adapter. This ratio on a bicycle with 26" wheel had a top speed of about 84-90 km/h with a stock carburetor.

To mount the sprocket I've welded a steel plate on the rear wheel and then bolted the sprocket on it. Centering the sprocket was very hard. Now I'm currently rebuilding my bike on a smaller bicycle (20" wheel) and to mount the sprocket I plan to build my own wheel with a front disc brake hub.

The pics are from my first setup, I'll post pics of my newer setup when I'll be back home.

Pic 4: Details of my first adapter (welded)
Pic 5: My current bicycle
Pic 6: Clutch assembly
Pic 7: New adapter (bolt on)

10-20-2008, 03:28 PM
I'd like to see a DuraForce on there.
Pic 1 is Model 'F' (same as yours) Note installed stock muffler.
Pic 2 Is DuraForce Model 'E'.

10-21-2008, 07:10 PM
Can I chime in on something that might help? On my bike project I eliminated the front derailler and just used the rear with its 5 ratios. There was plenty of range to choose from, and with a motor like you lawnboy, it won't need more than the 5 ratios you'll have.

10-21-2008, 07:35 PM
Back in the dark ages, a friend of mine mounted a Briggs 2.5 on a 26" bicycle. Big black Schwinn with shock/spring front suspension. Perfect for delivering newspapers.
He got some heavy gauge spokes to replace originals on rear wheel. He purchased a large sprocket for #35 chain and mounted it to wheel. It seems to me that sprocket was at least 2/3 the size of wheel. Biggest I have seen. I haven't seen another since.
I can remember this thing doing at least 40 mph with engine never really coming up to anything near full speed.

10-30-2008, 12:33 AM
I am trying to do the same thing. Don't try attaching the pedals, as previously mentioned, they will revolve very quickly. I believe you should use the original gears, as you could then travel at moderate speeds, because these things are not road legal, not in Australia anyway, and you will then have less chance of the fuzz confiscating your ride. I am trying to use an old Victa 2-stroke, and figured I could run the chain in a figure of 8, because the motor turns counterclockwise when mounted inside the bicycle A-section. I can still use the derailleur to change gears, and would use some kind of separator halfway between the engine and the gears to separate and not wear the chain. Due to the various pictures posted, I am not fully aware of your issue, but I am sure it is not dissimilar to mine.

(BTW, if you turn around a bicycle whhel, you will have to alter the freewheel mechanism because it will be freewheeling while you accelerate).