Looking for a large displacement conversion for a 25-30 lb aerobatic decathlon type plane. $800++ is just too steep for me.

I am not sure if any one has done one before but how about a snow blower engine. Just the other day I picked up a new/broken Yard Machine from the local small engine shop for $30.00. It needed a new piston and ring which was $30.00 so I have $60.00 into a new snow blower. It is a Tecumseh 98cc that would be easy to convert and not too heavy with a CH ignition. It will need a Walbro carburetor also. My .02
Tim

Regardless of what you may hear, big singles are going to be big shakers. The bigger they are, the more they shake. There is just no getting around it. While a big 100cc thumper may work fine in a lawn mower, where all that weight of the mower frame help make the vibration less felt, the same engine mounted on a comparitively featherweight model aircraft is going to shake the bejeezes out of it.

This is why starting at 70cc or so, single becomes rarer and rarer and twins starts to appear, and by 100cc, they are completely taken over by twins. Some over-square engines shake less, but I suspect most industrial engines aren't built to be revvers. They need to have good torque for the kinda of jobs they do, so they are more likely to be strokers.

Still, if you find a good big bore engine, by all means give it a shot. Still, I would favor one from a "handheld" device (eg. chainsaw, etc.) than from a "ground-based" tool (mower, generator, etc.). The engine is more likely to be designed with minimizing vibration in mind.

Good luck.

I have converted a number of large saw engines and they all worked very well. The comments about being a shakers however are true. The physics of that much mass moving at the RPM they ture will create vibration. I will say this; if you take care and make sure all shaft items are striaght and true the vibration will be reduced some what. I have converted Stihl, Huskies, Sacks,Poulands in the 75cc to 125cc range with good success. One thing is for sure these big singles will make a lot of power.
carl

An old Sachs model 133(5.2) or 143(5.8) chainsaw will made you a conversion with plenty of power for a 30 lb plane....

Anyone ever think or try using a gear reduction to increase the prop rpm's, then go down in prop size and pitch?
Then the engine wouldn't rev as much.
Also possibly use a soft mount.

Been done...Mick Reeves has a website, makes a belt reuction for G62 and G23 engines...The G62 will turn a 30 inch prop with his reduction...Engine turns 8000, prop turns 4000..Major thrust, less speed....

I have a Tecumseh 50cc, 98cc and 138cc on the shelf now. Since the cold weather is fast approaching, I'll have time to get back on these. As far as singles shaking, my Brison 5.8(95cc) is VERY smooth. Maybe it's due to the ignition being retarded, not sure, but it is definitely smooth. If I could get the conversions done and that smooth I'd really be happy!

Mick Reeves proves that it will work.
You can gear down and put a bigger prop.
The only thing is, what prop would you use if you took a 100cc engine and reduced down to 50% prop speed?
Probably not a prop big enough.
Engine would definatly have less vibration.

Certain rpms give more vibrations than others. You could have vibration at 4000 rpm's and not at 6000. It all depends on the engine.
FYI, girls love 4200 rpms when they are on the back of your Harley fat boy. It vibrates the most.

See if ya can find one of the motors off of a military drone...BIG two stroke twin cylinder something pushed it right along...wings didn't vibrate off..