rc4j

My Feedback: (103)

I just saved a used weedie my brother in law was getting ready to trash. Just curious if anyone could tell me what size it is, and if it could be converted for rc. I'm new to conversions, and I'd like to try it out. It's a Stihl FS48 trimmer. I took it apart today and cleaned it up, went ahead and trimmed off most of the extra metal that wasn't needed. I found some numbers on the side of the cylinder that may mean something to someone. M540 and H18 . It sort of resembles the head on a G23, except it's only held by 2 screws instead of 4. It has a walbro carb that also says Stihl, and some numbers WT 45A D8 .
Also, the pull start rotates the engine clockwise when viewed from the front. Is this a problem, or can it run ccw just as well?
Any advise is appreciated!

Jai

rc4j

My Feedback: (103)

Oh, I forgot to mention it has 2 piston rings. Here's a pic.

jwv3

My Feedback: (1)

Stihl's website doesn't list that one, but most current models are 25 to 30cc. You can take some measurements of bore and stroke and calculate it.

Flypaper 2

My older weedwacker book doesn't show that model. the lowest it goes is FS 50 which is 16cc and Komatsu motor in a Stihl weedwacker.

ZAGNUT

My Feedback: (1)

what is that little black box on the end of the wire? some older transistor ignitions had a separate pickup and ignition box, if thats what this is it should be grounded. you might be able to retime and run ccw but your best bet is battery ignition. the current fs-45 is 27.2cc but there doesn't seem to be any correlation between model number and dsplacement.

dave

rc4j

My Feedback: (103)

I finished getting a prop and spinner ready for this little guy today and fired it up! I cranked it cw initially, and just run the prop backwards. I got the needles dialed fairly well, and yep, it sounds just like a weedie. Next, I tried to crank it ccw. Boy, it complained, but managed to fire and run, but not nearly as well. So I decided to try to figure out how I could get this thing to run in the correct direction. I studied where the magnets were positioned when the piston was at TDC when it runs cw, and figured I needed to do this in reverse for it to run ccw. If I could have only flipped the flywheel around, it would have put the magnets in just the right position. But because of the offset in the flywheel, this wasn't possible without major modifications. So I decided to make a new slot in the flywheel for the woodruff key in the new position. It wasn't fun, but I was able to get the slot cut and luckily it came out in the right position. Put it all back together and fired it up ccw, re-tweaked the needles and whutdoyaknow, it now runs just as good ccw. I checked it with the tach and it's turning a 15X8 Master Airscrew right at 6500 max. This engine has seen some use in it's day, and the compression doesn't feel like it's all that it should be. But there doesn't seem to be any real problem starting it and keeping it running. The bearings feel ok, and it appears to turn true enough, no endplay or slop in the crankshaft.
This was the only prop I had for testing, so I'm curious what else to try. Does this sound suitable, for a 1/4 scale cub or something similiar to fly in a scale fashion? I've got an old Taylorcraft w/ 104" wingspan, that I've never finished. I'm not too sure what the weedie weighs, but it feels pretty heavy. Everyone says light wing-loaded ac are best suited for the weedie conversions.

Oh yeah, the little black box I connected to one of the engine mount screws on the stand to make electrical connction. Not sure what it is exactly.

Any ideas as to how to make it run faster?

ZAGNUT

My Feedback: (1)

good going! 6500 ain't too great but sounds about right for an older engine. you could try opening up the muffler , this usually helps with the small stock ones. these engines are low compression to start with but new rings might help if you can find them. i wouldn't load the engine any more, maybe a 16x6 but this would fly real slow at that rpm.

dave

jwv3

My Feedback: (1)

I have a stihl weedie engine waiting to tear down. I has a small 'box' type thing also connected to the coil grounding wire. It's mounted on the bottom of the engine/flywheel housing. The only thing I can figure is it may be a klixon. Designed to short the ignition and kill the engine if it gets too hot. maybe?

Flypaper 2

The little black box contains the electronics for the ign. the coil is just a coil that could be used with point or electronics. Most of todays coils have the electronics built into the coil. In fact there is a small device that will convert any points mag to electronic.

bdtsr

My Feedback: (5)

That little black box to convert from a point type system works great. I've got one on my older Quadra 40 and it eliminated my glitch problem. The points arcing was driving me nuts!

jwv3

My Feedback: (1)

In this case, though, I don't think it is ignition components. Only one wire and its in parallel with the kill switch. I think it just grounds to the case when overheated. i'll test it with a heat gun, see if thats it.

rc4j

My Feedback: (103)

Any ideas on what I could try to get more out of this engine?

I haven't tried running it without the stock muffler yet. It also has a small screen over the outlet on the muffler. I'll try without that also.

The walbro carb has 24 stamped on the inside. I read another post stating this was the size of the venturi opening. On another walbro carb I have it says 34, and is quite a bit larger. Would it benefit anything to try the larger carb? Also, the stock carb spits gas quite a bit, especially when throttling up. Would it help to put one of the carb stacks on it?

I have never tried modifying the ports or anything internal to an engine before, but if someone has a suggestion and it seems simple enough I would be willing to give it a try. After all, this was a freebie, and I haven't spent the first cent on it.

How would I go about finding replacement rings for this engine. I still don't know exactly what size it is. I'm guessing 18cc since on the side of the cylinder head at the bottom it has an S with a circle around it, M540, and H18 stamped on it. Someone said to calculate the displacement. How exactly do I do this? I saw somewhere there's a fellow that makes rings. Anyone know how to contact him? If their not too expensive, I could give them a try.

Jai

Flypaper 2

Jai:
Get rid of the screen first. Tach it with it and without to see the difference. try the bigger carb too, but do one thing at a time to see how much of an improvement you get. The velocity stack won't give more power just less messy and better fuel economy. A lot of it is trial and error stuff. Might improve one engine and not the other.

ZAGNUT

My Feedback: (1)

the stock carb at 9.5mm seems pretty big for the power you're seeing now, the other carb at 13.5mm is much too large for an engine this size (?) work on the exhaust first like gord said and also try it without any muffler. these little weedies make their best power at high rpm so don't bog it down with too big of a prop.

dave

rc4j

My Feedback: (103)

I finally got around to trying some of the suggestions made. Here's what I got:

stock exhaust w/screen MA 15x8 6500
stock exhaust w/o screen MA 15x8 6700
no exhaust MA 15x8 7200

stock exhaust w/o screen Zinger 15x6 7600
" " " " Zinger 14x10 6400

It looks like making some mods to the exhaust is the ticket, somewhere between not to loud and not to restricted. Also, the
15x6 prop shows the most promise so far.

I have another question. This engine originally ran cw as a weedeater, and I have modified the timing so that it will run ccw.
It seems to have greater compression when turning it cw, as opposed to ccw. I can even hand start it in the cw direction. Can anyone explain this? Is there something I could change to get more compression turning it ccw?

jwv3

My Feedback: (1)

There should not be any change in compression CW vs CCW.

The only thing I would be concerned with is the prop hub coming unscrewed. Make sure you clean it well and use highest strength loctite you can get.

ZAGNUT

My Feedback: (1)

if it still runs reasonably well cw then the timing might be able to use some more advance. if you have access to a tming light put some marks on the flywheel a check the timing, 25-30 deg. btdc should work the best.

dave