I've now torn down a dozen or so Poulan/Weedeater featherlite engines for conversion (please, save your comments on their low performance! -- you can check my posts from several years ago discussing their porting limitations, and they do actually work quite well on slow floaters like Telemasters and such). (Besides, these things just keep appearing at my house and showing up in my truck bed now that my friends know what I do with wayward weedies! ) All of mine are 19cc or 25cc engines. As a mechanical engineer, I am surprised at the many subtle differences in the design of the crankcase castings (and I'm only talking of the 4-screw backplate models here, not the latest 2-screw backs). I don't know if this was just evolutionary design as the bean-counters forced the engineers to squeeze more $ out, or if there were real design issues. All crankcases seem to interchange with all of the trimmer plastic parts from the other trimmers. All cases also seem to align well with the two-transfer port jugs as well as the 4-port ones. Given that it's not cheap to machine pressure-casting casting molds, I'm curious why so many variations to accomplish the same basic function. Have any of you who have messed with the featherlites noticed this? I first noticed it as I was grinding a case down, and realized it was going much slower than the engine I had done a few months earlier. The main difference was in how the fan shroud was attached to the outside tubular bosses where the back cover screws go in, although there were other changes made on the coil mount and upper crankcase also.

Have any of you put a 25cc jug on a 19cc case and run it? The bolt pattern is the same, although the flow into the transfer ports may be blocked somewhat on the smaller case if it's not opened up some. I'm going to give it a try, although there may be vibration issues due to counter-weight differences -- haven't checked the counterweights yet. The reason I'm doing this is I was given a 25cc weedeater blower that was almost new, but it has a very long shaft on it (like 6", vs around 4" on my Featherlite blower) that is not very suitable for mounting a prop driver since the shaft has a flat down most of its length. So, I'm going to rob the 25cc jug and piston and use the 19cc case. This particular engine I am going to try on the gas/glow blend with glow plug. If you haven't read the "gas fuel with glo plug no ignition" thread you should. These guys are to be commended for having done some fantastic experimentation.

Try replacing the crankshaft of the 19cc in place of the 25 CC (long one) they interchange easily. That way you can still use the crankcase of the 25 with its own jug and piston!!!

Excellent thought, and I actually considered this, but I've already got the 19cc cases ready to fly and I'm tired of being covered with aluminum dust! Also, this particular 25cc case is quite different -- It has a solid (unvented) fan shroud that's continuously cast arond the case. Its also got a cast aluminum back cover. They used the cast cover (it has two mounting lugs on it) as the rear mount of the engine in the plastic clamshell housing. FYI, the blower this came out of -- although a Weedeater brand -- was not labeled as a featherlite model. Before getting the plastic clamshell peeled open I expected to find one of their wierd-configuration reed-valve motors that aren't much use for planes. I was surprised to find this featherlite variant lurking inside.

Will the 25 crank fit in the 19 case? Any additional volume / mass of 25's crank would just serve as 'stuffing'.

I bent the crankshaft on my 25 on a crash and someone gave me an 18cc or 19cc and it looked the same as the one in the 25 so I took the crank off and replaced it for my 25 and that's what I'm using on my nitroplanes Ultimate bipe and it works "FABULOUS"

Hi Volture,
I have a few of these Poulan engines myself. I have two 21cc engines, 2 FL-25s and an extra jug and piston but no rings for the piston. Someone overheated the engine and cracked the rings but the piston and jug seem to be ok. The only reason to mention this jug and piston is because it has 2 rings and the piston diameter is between the 21s and 25s I have. I was surprised to see a Poulan with 2 rings. I might buy some rings and put them to use. The 21s at first only turned 6k so with nothing to lose I ported them and they now turn a 16x8 right at 7k. I got one of the 25s to turn the same prop 8200RPM. I will soon test the other 25 to see if I can duplicate the RPMs of the first one. I don't intend to mess with many more of these, but they are nice running engines for what they are. On the long shaft, with a Lathe, you can cut and rethread it. Doing it with a hand die set is tough to do accurately, as you can guess. (There is too many engines and not enough time.)

The featherlite 25cc has no power?! Look at this 1/4 scale sopwith Pup fly

http://media.putfile.com/Sopwith-Pup

I have a Weedeater with the plastic crankcase cover that doubles as the case and carb manifold. Also holds the reed valve. Anyone make a aluminum backplate with rotary intake port for this model? I think that would go a long ways to improve performance.