WeedEater Engine

Even if it's no good for airborne use, you might be able to put it to some land-based use. There's plenty of larger cars/buggies that use petrol engines these days.....might fit in one of them?

MrBonk

Here you can find conversion goodies....

My son and I are converting a 25cc Homelite weedwhacker engine for a 1/4 scale Citabria. We'll let you know how it goes. So far, it runs well, other than being a bear to start (got to really hit that prop hard to get a spark) and producing some vibration (flywheel's a bit off after removing cooling fins). Some experimentation with balancing should make a decent powerplant of it. Might even work up a battery boost for the ignition, similar to the systems used in some full-scalers like the Lasar ignition booster.

Dan

BALANCE that flywheel !

I have two Ryobi 31 engines - one that I've converted using a C-H ignition and a second that I am in the process of doing so. The C-H unit starts by hand quite easily. I've been told that hand starting even a Zenoah without electronic ignition is next to impossible! The trick is getting the necessary speed of the magnet in the flywheel past the coil pickup arms to generate a sufficient spark

Anyway, back to balancing. For the second engine, I had the flywheel cut down to the flat sections of the face. I then put it in my Dubro prop balancer - major out of balance! I ended up drilling 5 or 6 holes about 1/4" deep on the heavy side to get it in balance. I tested it by chucking the balancer shaft in my Dremel and spinning it at about 5000 rpm - very little vibration.

David

Is it not possible to arrrange the heaqvy side of the flywheel to be opposite the piston in these engines ? (Genuine question - I've never seen one).

I made a special trip out to my shop to look at the engine to come up with a possible amswer......

I say no. I looked at the relationship of the piston position to the area of the flywheel where I drilled exactly 4 holes 1/4" in diameter varying between 1/4" and 3/8" deep. The piston has not reached TDC when the 'heavy' section of the flywheel has passed BDC. They are out by approximately 50 degrees - and cannot be changed sinced the flywheel is keyed to the crankshaft.

David

There's already a balancing weight on the crankshaft inside the case. Dan

In a word, NO. The magneto flywheel is too far off the rotating plane of the piston/counterweight to do any good. Balance it by itself is all you should do. If you want it to run any smoother, you'll have to invest in an ignition system that has automatic timing advance.

BTW, the bloke that couldn't get his Homie chainsaw engine to run right probably just didn't have any clue as to how to adjust the Walbro type pumper carb.

I have not mounted and run the second engine, so I do not know how much it vibrates with the machined, balanced flywheel. The C-H equipped engine does not vibrate much, considering its size, until I tried to take the idle way down to 1200-1300 RPM, then it shook and thumped considerably. I settled on 1500-1600 RPM.

I just tore apart another Ryobi 31 for the engine - I'll try to remember to test the balance of a stock flywheel and report the results.

David

I have to politely disagree with you on this point, Volfy.

For years, I used the heavy blade of the prop to offset some of the unbalance that was present in glow engine's piston/rod combo and it worked noticably. It is tantamount to the same thing that we are talking about with the utility engines.

Ed Cregger

I have heard about people doing that. Personally, I have not seen a convincing case of it. The problem is that the attempt is to use a rotating imbalance with a relatively huge moment arm to counteract two modes of vibration: (1)an essentially linear or axial vibration due to the piston travel and (2)the secondary vibration caused by the rotating crankshaft counterweight which attempts to balance the linear power pulse(1) but introduces vibration of its own. Still, you might have a slim chance of making it work if the prop rotates in the opposite direction of the crankshaft counter-balance, but unfortunately, that is not the case.

Of course, none of this is even taking into consideration the offset between the prop plane and the piston/counterweight plane, as I mentioned previously.

To me, it's far better to balance perfectly any driven load (prop, magneto flywheel, spinner, etc.) and just let their added weight and inertia help lower the peak magnitude of the engine vibration.

Then again, who am I to argue? If you like *your* results, that's all that matters. :-)