Need engine advice
 

I purchased a large piper cub (90 inch wing span) at an estate sale. It is in excellent condition. Don't even think it's been flown. Got it with a futaba 4 channel radio for $100.00. It has a gas 2 stroke engine on it. The engine has no brand name or numbers on it at all. No idea what size it is. It runs fine but barely has enough power to get it off the ground. My question is what size gas 2 stroke engine would i need for a plane this size? Also I don't know if it has the correct prop since I have no idea what size engine it has. I was using 50:1 mix that i use for my weed eater. Any advice would be great.
Thanks
Budman

Some pictures would certainly help a speed response.

I'll get some pics tomorrow

Here are two pics of the cub. I will get close ups of the engine tomorrow

How about pictures of the engine? We can't really tell much about the engine from looking at photos of the airplane .... other than it is a piston ported engine. Also, what size is that prop?

The photos look like it might be a Great Planes CUB.

A better prop sure would help. as far as size goes, prop it to run around 7,000 rpm.

True on the prop. Those black plastic props do a better job of turning the available engine power into vibration than thrust.

Where are the wing struts .Cubs usually need them.
BC

ya it has hardwood struts. I just didn't have them on when i took the pictures. It was so nice today that I was at the flying field till dark and didn't get better pics of the engine and prop. maybe tomorrow

The prop is 18x8. Here are some pics of the engine.http://www.rcuniverse.com/forum/atta...mentid=2081703http://www.rcuniverse.com/forum/atta...mentid=2081704http://www.rcuniverse.com/forum/atta...mentid=2081705

looks like a weed wacker engine probably somewhere in the 25 to 32 cc range. If you cant get around 7000 rpms with the 18x8 prop try a 16x8

Looks like a "Weedeater" brand engine. In my experience they are a very cheaply made engine that is low on power. One thing you can do to help is pry apart that muffler, and remove any and all baffling or noise suppressant materials inside. Then provide one 5/8" or larger tube outlet, or two tube outlets of about 1/2" diameter. Then try a 16 x 8 prop as mentioned. That engine will never do well with an 18" prop.

If you want to go a step farther, look into the carb throat and see what number is cast into the throat. Then we can tell you if a larger carb would help as well.

Of course, all this is assuming the engine is in good shape to begin with. I have seen engines that actually ran quite well gain 800 rpms (which is a lot of power!) when the worn piston ring was replaced....

AV8TOR

Thanks for all the replies. I will try a smaller prop. What about gas mix. I have a really old weed eater that used 16:1. I have a newer stihl weed wacker that uses 50:1.

32:1 is a good mix for that size engine. Regular gas is fine, premium gas would actually cost a little performance.

AV8TOR

Hi!
First or all...The plane is just around 230cm in span and thus a suitable engine for it would be a 15-17cc (.91 -1.00) four stroke swinging a 16x4 APC or 16x6 prop. And with those engines it whould be aerobatic...
The gas engine you have looks way to large and bullky and the prop is no good performance wise!
The best plastic props on the market are APC, RAM and Graupner G-Sonic (not the old grey blunt tip version though)..
A suitable gas /oil mix is 2-3% oil , that is 2-3 centiliter to one liter of gas..
.

As av8tor1977 said it is a "weedeater" brand, most a likely 25cc engine.
As stock they are not very strong but they are very light and can be improved. As mentioned the easiest way to improve is by:
1. Changing the muffler, or at least remove any internal baffles and increase the size of the stinger (outlet).
2. Changing the carburettor, the standard venturi size is often around 8mm (size 20), the correct size is 11mm (size 28), but size 24 (9.5mm) and 32 (12.7mm) will also work.
3. Suitable props for this airplane (floater type) and engine are 16x6 (fastest - around 8000 RPM), 17x6 or 16x8, even an 18x6 will do better than an 18x8, but it will be more responsive with the smaller props.
4. If you are a bit technically minded you can also do some ports work, in standard form this engine is very mildly ported.

Here is a link to tips here on RCU how to improve performance of Homelite 25cc. Some may apply to your Weedeater. http://www.rcuniverse.com/forum/engi...iques-how.html

Good luck. Richard