2 stroke Lawn boy engine

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Bob Campbell

Bob Campbell

New Member
Jul 11, 2012
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Colorado Springs, CO
Hi Guys,

I have an old 2 stroke lawn boy lawn mower with a verticle shaft. Since this is a 2 stroke and uses premix fuel for lubrication, can I turn it 90 degrees and run it as a horizontal shaft engine? Also would anyone have a clue as to how big these engines are? It's on a mower that has a 20 or 21" blade.

Thanks,
Bob C.
 
2door

2door

Moderator
Staff member
Sep 15, 2008
16,302
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63
Littleton, Colorado
Yes, 2 strokes will run in any position. No oil in the crankcase. The only thing you'll have to consider is the carburetor. If it has a float it will need to be mounted 90 degrees from where it is now. If it is a reed type carb position is unimportant.

The only way to be certain of the displacement is to remove the cylinder head and measure (accurately) the bore and stroke. http://en.wikipedia.org/wiki/Engine_displacement
If you can find a model number of the mower a Lawn Boy dealer might be able to offer some input on engine size.
You might want to check out our DIY section for ideas on mounting, sprocket and clutch ideas. Good luck.

Tom
 
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M

mat_man

New Member
Jan 29, 2011
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athens ga
Hi Bob

I have a couple of the earliest motors. I have also not been able to find the size, my guess is over 100cc. The newer Silver Series (2-stroke) motors (4.5 HP +/-) have larger CC's. The newest 6.5 HP Duraforce (2-stroke) would have the largest motor.

The Early and Silver Series (2-stroke) were produced by OMC that made Evinrude / Johnson outboards.

Most motors have a problematic tapered shaft. See 1st pic.
Friction drive would be the easiest.

This guy has converted his from vertical to horizontal:
2 stroke lawn boy engine
http://www.youtube.com/watch?v=708t50Qy6x4

custom lawn boy carb mount
http://www.youtube.com/watch?v=48OT9kCMx9k&feature=plcp

From 2nd pic, from a newer mower, looks like you would also need to build an adapter plate.

From 3nd pic, looks like the exhaust would also be problematic.


A lot of good pics:
Lawn Boy Engine Rebuild
http://s160.photobucket.com/albums/t165/MlrosePlant/Lawn Boy Engine Rebuild/

Mat
 

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Bob Campbell

Bob Campbell

New Member
Jul 11, 2012
10
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1
Colorado Springs, CO
Hey Tom, Mat,

I thought that they would run in any position, I just wanted confirmation on that.

The carb. mount wont be a problem and the tapered shaft will also be a small issue. I'm a machinist by trade and have done race car fabrication for many years. Another plus is I also work in a bike shop in the evenings.

My bike will be board track style. I've been eying the red Power Plus Indian on Sportcar Pat's website. It's calling my name and saying build me!!

I'm currently doing my home work and rounding up parts for the build. Currently I have some tan Schwalbe Fat Frank's and some Alex DX32 rims.

I will maybe use a spare rear hub from my downhill bike. the hub is 135mm wide with a 20mm through axle. The hub is designed to use a floating rear caliper. I can build an adapter to mount both the rotor and left side drive sprocket. If I add 15 mm worth of extra spacers, I should be able to mount the caliper and sprocket without chainline issues. This would add up to 150mm rear spacing which is very common on modern downhill bikes.

Back to 2 strokes,

Question about the intake manifold, I'm thinking that a longer intake will give better bottom end?

Thanks again,
Bob C.
 
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MotorBicycleRacing

MotorBicycleRacing

Well-Known Member
Jul 28, 2010
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Bob Campbell said:
Hi Guys,

I have an old 2 stroke lawn boy lawn mower with a verticle shaft. Since this is a 2 stroke and uses premix fuel for lubrication, can I turn it 90 degrees and run it as a horizontal shaft engine? Also would anyone have a clue as to how big these engines are? It's on a mower that has a 20 or 21" blade.

Thanks,
Bob C.
Click to expand...
The lawn mower engines use the blade as a flywheel so you may
have to add some kind of flywheel to your engine.

msrfan is the expert here on putting these kind of motors into bikes
and he normally does not use vertical engines.
 
Y

young grease monkey

New Member
Sep 20, 2011
362
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Chicago
Sounds cool! A few years ago, I got a lawn boy for free and wanted to build something with it but didn't have the skills or tools so I ended up junking it. Those mowers had goofy exhaust systems, some exhaust under the deck near the blade. First upgrade would be to make an expansion chamber exhaust pipe, could adapt one from a small dirt bike like a 65 or 85 kawi or ktm. And grind off all the covers and junk the doesn't look nice down to the bare engine, for the vintage look. Maybe try a different carb like a mikuni 22mm. Sounds like you got the wheel figured out, idk if you can get 11ga spokes in that hub/rim though, you will want at least 12ga.
 
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supercub

New Member
Nov 1, 2012
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New London, CT
Most mower engines use a bronze bushing instead of a roller bearing for crankshaft ends. Your setup shopuld minimize the side force on the bushing to prevent fast wear.
 
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