RE: Poulan 25 from FL trimmer
 

No worries, mate

7k - 8k rpm at pitch 6 is around 40 - 45 mph. Even park flyer can handle that:D

RysiuM

RE: Poulan 25 from FL trimmer
 

[8D]

RE: Poulan 25 from FL trimmer
 

I got another engine and just finished converting if=t for the plane. I still need to turn my prop hub and engine mount but the whole 'conversion' is done. It took me less then 2 hours. This time I took pictures for 'step by step' instruction that I posted in the thread [link=http://www.rcuniverse.com/forum/m_3192059/tm.htm]Step by step converting Poulan 25cc FL trimmer ($25 on e-bay)[/link]

Enjoy.

RysiuM

RE: Poulan 25 from FL trimmer
 

Thank you for sharing your experiences & for your Step-by-Step thread. Very useful information.

In Aug 05 Model Aviation Magazine, column The Engine Shop by Joe Wagner, there is a good discussion on Glow Plugs. Rossi, Fireball, & OS make plugs in different heat ranges. (I think I saw elswhere that Enya has plugs in 4 different heat ranges?) Also: plugs may not fire due to contamination even though they may still glow. Also: effect of moisture in glow fuel.

In Aug 05 Model Airplane News, article Engine Maintenance by Dave Gierke, Demon-Clean varnish remover is available from Cooperfuels.com

RE: Poulan 25 from FL trimmer
 

Congratulations on your beautiful Machine Work & Fabrication.

Am not familiar w/ Motul Oil. What is the intended usage: Model Fuel?, Weed Trimmer? Motorcycle?, Marine? Is it Synthetic, Petroleum, or Castor Oil? Also, do you have address of Manufacturer or Supplier?

Thank you

RE: Poulan 25 from FL trimmer
 

Thanks. It is amazing what you can do on simple minilathe (Homier 7x12). It is my favorite tool.:D

It is synthetic 2-cycle oil. I was looking for it becasue it was suggested by Pe, as the oil for MVVS 1.6. I found it in jet ski store, also in Honda motorcycle store. I think it is made in France.

The name is:
MOTUL 800
Factory line
Off Road., 2T
100% synthese

http://www.otorohonda.co.nz/_shop/im...2t800large.jpg

RysiuM

RE: Poulan 25 from FL trimmer
 

Thanks, I will keep it this way.;) Now I got my next trimmer head (with same engine). This time the carb is different. It doesn't have a choke plate (but still has choke arm and shaft), and choke just openes the throttle butterfly. Strange - looke like something is missing or wery well hidden iside :eek:

Anyway this one is going to be 'converted' for performance. I want to get the best possible power/torque ever. Should I:

1. replace carb with 11mm one?
2. cut oprts?
3 shave the cylinder for higher compression?

I may use it for glow plug or electronic ignition. I just want the best reliable power from this little bugger:D

RysiuM

RE: Poulan 25 from FL trimmer
 

Hi Rysium,

I'm in the UK but over the last couple of weeks did a Poulan 25 converion for my Cermark Pitts biplane, and it had its first flights last weekend. I have done quite a bit of tuning of two strokes in the past so did this just for fun really, I did a pretty mild tune on it and still run the std carb and a modded std exhuast and it turns a 16x8 at 8500 running 2% synth oil in normal pump petrol and the standard magneto ignition. I wanted a nice running super reliable motor, but I'm trying to get another so I can do a balls-out tune, I reckon 9500 on the 16x8 is possible but I might break one or two engines on the way to getting there:D

If you want a step by step how to do this its here:

http://www.*************.co.uk/4um/i...?topic=19374.0

sorry to post a link to another forum here, but it you want the info you'll need to read that thread. Forget the first few posts as they cover a different engine, but from about post 10 to the end is my testing and development including all calcs to determine the porting etc.

Happy reading, let me know what you think.

Cheers,
Rich

RE: Poulan 25 from FL trimmer
 

Hi Diablo

I have read your entire topic in *************
Very nice, I have played with two stroke moped engines.
Also read Gordon Jennings articles.
Congratulations for the Pitts.
I have a converted Poulan like yours but not flown yet,all is standard,except enlarged carb venturi to 9.5 mm
May be next weekend.
Questions:
Do you have problems with RFI, I see in the photos you have original high voltage lead to spark plug.
What spark plug?
What radio control you have?
Thanks for your time
Jose

RE: Poulan 25 from FL trimmer
 

Wow, it was some reading:D Good stuff. However I don't see clear conclusion tho. Let's bring the whole mod down to the pieces:

1. Compression increase by removing 0.4mm from the bottom of the cylinder. There is no really clear result of that - did or did not it improve engine performance. There is other issue not covered here: the engine was manufactured for low compression, so how does the increased compression impact on the bearings and rod. Both bearings are quite close together, so the force form the crank has big leverage on them.

And question how did you machine it? I mean the aluminum is not a big deal, but the liner is quite hard, isn't it?

2. Timing and port area. I also believe that this is the main restriction. I understood all the calculations (I mean the simply way) you put together so I have some idea where did you go. However again I have machining question. How did you open ports in the aluminum and cylinder liner.

As the final result is excellent with over 8500 rpm on 16x8 (is it APC or MA?) it's difficult to draw conclusion where is the source of that improvoement. I mean my engine turns 7400 now, - it is over 1k less but what I see there are four major elements that are different:

1. Exhaust. You are running kind of 'tuned' exhaust. This may mean a couple hundreds rpm more alone.
2. Carburator. I don't know if you changed it. I'm running stock Zama which is I believe 8mm venturi. Most 25cc conversion run 11mm venturi. It might be place to gain some power.
3. Compression. Mentioned before.
4. Port timing and size. Also clear explained by you.

I wish, we could compare the result on the same free-flowing exhaust (unrestrictive but not tuned) and the same carburator (I think maybe 11mm venturi) before and after the cutting the engine interior.

Thanks for the great info.


RysiuM

Edit: I just found: ZAMA C1U-W18 is 8mm venturi

RE: Poulan 25 from FL trimmer
 

How do you do that?

RysiuM

RE: Poulan 25 from FL trimmer
 

I use the standard Poulan coil, plug wire, plug lead and plug. I have absolutely no problems with RFI but I took some time to eliminate possible problems:
1. Mount the Rx as far back as possible in the airframe
2. mount the throttle servo at least 10" back from the engine and link to the engine using a plastic link rod (I used a sullivan gold'n'rod) as a metal rod will transmit RFI
3. keep the rx aerial well away from the servo wires and battery wires and twist all servo cables to prevent RFI.

I use a futaba 138 dual conversion Rx, and a futaba T7CAP Tx, nothing fancy.

The engine has a normal Champion RCJ7Y, the one the engine came with.

I have machined out carbs before, you just make a mandrel to mount them on the back face in a lathe then get turning. Careful though, the carb still needs a venturi shape to help fuel into the airstream. Often better to just bolt on a bigger carb.



I have a WA200 carb (28 in the throat) that is 15.8mm bore at the engine end, 11mm at the narrowest point. Its unmodified as it seems fine but does need the bellmouth.

Onto your other questions:
Compression. This engine is still very conservative, it gave 200rpm but of course it also dropped the port heights 0.4mm doing this. The smallest chage probably, but you need it once you get the ports modded to get the best out of the engine. Tuning is a balancing act, there is no point just doing, say, the ports, or the compression, or the carb, as each will not make a lot of difference, but together they work to help each other give you more power.

As far as the bearings go, the engines are fine, I've run 20hp 20,000prm engines on rubber sealed bearings for hours with no problems. My main concern would be that the engine is designed to run with no end load on the crank hence has ball bearings. With a prop on you all maybe 11lbs of end load and ball bearings are not designed to take end loads.......

For machining I did the exhuast and inlet with a series of coarse and fine files and the transfer ports with a dremel 35000rpm hobby tool with a rotary burr in. Took about 2 hours in all. The liner is just the same casting as the barrel, its an AAC engine so not hard at all.

As I said, there is no particular magic change, you need to do them all for it to work. Again bear in mind I did this as a mild tune and the PTA and PAA numbers are just for a normal engine, not a full race version.

You can make the same exhuast I did the 8500 tests with, just take the outer off your standard exhuast. Putting the long exit pipe on took 300 revs off the engine but I've been making a new exhuast tonight, pics will be on the RCMF site later.

Hope that helps. Just a note, I could not get mine to run with glow power though on 98% gas, 2% oil, but it seems good on ign so I'll probably stick with that.

Cheers,
Rich

RE: Poulan 25 from FL trimmer
 

Hi

Rysium

I must thank you the time spent in the forum to show your conversion steps with photos. Very clear.

I have older model of Poulan, four bolts on plastic back.
The carb I have is a Walbro 1 (one) needle, High, like yours, not easy to access and tune, the low needle is pressed , imposible to reach. The Walbro have two numbers , upper is "308" and lower is "WT 631".
Over the venturi have a big (relatively) hole, I can see from outside, and after the venturi in the same line with the bigger hole it have 2 or three ( dont remember well) very small holes only seen from the back (engine side).
I cover with strip of tape , 1/4 wide , all the holes, this is the "base" or bottom of a circle of 9.5 mm diameter (3/8 approx) I made with a file to enlarge venturi. All other holes of carb also taped.
I choose 3/8 approx from readings of posts of the other gentleman in this site RCIGN.
Lot of patience, I believe the Dremel is better but not easy to control.
I do not remove the buterfly, and the stroke of file is small, patience.
After it it have the venturi shape still , very important!
After it I wash with a lot of gas .
Work well
I tested in a test stand.
Good transition. Do not hesitate. Extra gas at 5% oil for break in, after one liter of gas burnt the compression is better.
Good for one needle carb.
I compare results with other Homelite 38cc I have with electronic ignition and the transition is good for one needle carb.
Sorry for my english.

Diablo
Ok for the sullivan throttle linkage
Ok for the Rx, near 3 foot from engine
Ok for running wires away from Rx.
I believe it have the RCJ7Y, forget to check , I am outside of my house
I have a Hitec Flash with original Rx with GWS servos , 4.8 V and 1000 mAh Nicd pack
Make test with colapsed antenna in Tx at approx 200 yards and Poulan running and no problems, not obstacles between.
I make a coil over the high tension lead with coaxial recycled coper (braided copper from RG 213 coax)
One side to ground , work effectively in a Curare (yes) with a 18 cc Poulan reconditioned, shaker before the copper solution , but this Poulan worked with CDI ignition.

Many thanks for your time
Jose

RE: Poulan 25 from FL trimmer
 

The Zama carb I used on a 25cc Weedeater was a C1U also, with M29D & 86A on the side. This Zama has a 14mm throttle bore at the engine side, 9/16" for reference. My carb has adjustable needles, meaning they have screwdriver slots on them. I got 7800 RPMs out of my 25 with a 16-8 M/A Classic prop. It would go all the way to 8k but getting close to being lean. I discovered later that the spark plug gasket was leaking a bit. I fixed that and haven't run it again. My 25 was new and still not broken in yet. I did increase the intake port significantly. I don't think 8K would be possible without this porting. I cleaned up the exhasut port some also, but not as much as the intake port. It is a very smooth running engine.

So with my 25's history for reference, I would definitely try the carb you have. Try and get the needles out and cut a slot, this should be possible. Your exhaust port looks large compared to my 25, but your intake port also seems very small in comparison to your exhaust port. Your carb spacer is different and more complicated than mine was. I was able to bore mine to 1/2" with not much of a problem. I could not take the carb spacer all the way to 14mm, but maybe I could make a new custom one. (Your carb spacer looks like more of a challenge to modify.)

I have several carbs that I just checked with C1U on them. C1U must be a basic casting number for the main body of the carb because the carbs are significantly different.

RE: Poulan 25 from FL trimmer
 

It is 9mm venturi. see [link=http://www.zamacarb.com]http://www.zamacarb.com[/link] - they have great info about all ZAMA carbs.


I just stripped another Poulan trimmer. This will be my experimenting one. I see - ports are quite small. I'm going to do major cuts:

1. Increase compression (I need to shave about 0.5mm from the bottom of the cylinder. I should be able to fit it on my lathe.
2. Open intake port from the bottom. Allmost all the way to the pump hole.
3. Open exhaust port from the top.

I need to check if transfer ports are not blocked by the backplate. I may try to make a little smaller the space under the piston by moving backplate forward.

The carb on this engine is a little different. It has a brass choke shaft (the other were plastic), but there is no choke plate. Closing the choke (this one has a spring) will open the throttle a little. That's it.

I need to look for 11mm venturi carb. Any ideas what may be usefull?

RysiuM

RE: Poulan 25 from FL trimmer
 

Check this out at Dave's Motors Walbro 167-1
http://www.davesmotors.com/store/product546.html
I have them on 21cc Echo, (possibly abit too big), Homie ran good on stand with one, and have another on Ryobi.

Hope someone can suggest a 10mm venturi Walbro part number!

RE: Poulan 25 from FL trimmer
 

It took me almost a hour to mount the head on 4-jaw chuck. But I got the bottom surface running true with accuracy less than 0.001 inch (0.025mm). Then in 10 minutes on low rpm (to not introduce any vibrations) I shaved 0.5mm of that (+/- 0.01mm)

Next problem. The top of the cylindr had some burned oil so the piston didn't want to go higher. Carb-cleaner, wooden stick, and paper towel did the job cleaning the top of the head.

I checked compression - I don't have tools but I can feel it is much better. Feels like MVVS 1.6.

Tomorrow I will replace the head (I didn't do any porting yet) on my first magneto ignited engine. I will see if I can get better rpm. Can I just swap heads on these engines? Just for testing?


RysiuM

RE: Poulan 25 from FL trimmer
 

I just tried a new home made exhuast on my poulan 25 yesterday, and with a bit of playing about with the angle of the air into the bellmouth it now runs 8700 with an in cowl type exhuast on the std magneto and straight gas/oil mix. I hope to have anotehr engine by next week and I'll tune that one further, next stop 9000.


Rysium, to skim the bottom of the barrel I made a mandrel fr 20mm bar, put an M14x1.25 thread on with the lathe and screwed the head onto this mandrel using the plug hole. Make sure you check the squish (maybe you call it the deck clearance.?) with a bit of solder before running the engine just to be sure its okay.


Cheers,
Rich

RE: Poulan 25 from FL trimmer
 

diablo,

Your engine was pretty extensevly modified I recognize it from another site what prop are you using and what was the gain just from the exaust? I have been playing with some exaust designs trying to figure out what will work best. I liked your engine build thread on that other site. Nice job.

Pete

RE: Poulan 25 from FL trimmer
 

FWIW: I would try to keep the piston/ring matched to the same head. You went to trouble of wearing in the ring to a particular cylinder, why not keep the match?

RE: Poulan 25 from FL trimmer
 

Hi Blueberry,

I'd not say extensively modified, I only did the ports and squish to a mild tune, I've never even touched the bottom end. There is plenty left in it yet. Its just I did all the calcs first and set off with a goal in mind rather than just hacking a bit of here and there and hoping it worked.

The prop I'm using a German "Super 16x8" but I've ordered a 16x8 APC today so that will provide a very relaible benchmark for everyone.

Exhuast, have a look on RCMF and you'll see the guts of the exhuast. I found it was good on the one shown in the pic above bit it certainly needs the baffle in to provide back pressure otherwise the low speed pickup when you bang the throttle flat out is terrible. Full open pipes just knacker the bottom end performance. With my 2 exits if I block either one takes 400rpm off though!

Gain just from the exhuast, difficult to say as I said, it all works together but my new exhaust would probably make very little difference to a std engine, maybe even make it worse, same with the carb. I always get asked "what if I just did this, or just did that" well the answer is usually either stick with what you've got, or do the lot, but don't mess about doing a half of the job. If you are not sure about the calcs to get the port angles and areas, just get on amazon, buy a book and get reading. If you can do simple maths you can do the port calcs, and if you can use MS Excel it just about does it for you.

With you americans having a supply for these engines at $25 each go for it! I wish I could get them at any money, I've been searching for 2 weeks now and cannot get another at any money and the ebay seller won;t even reply to my mails.



Cheers,
Rich

RE: Poulan 25 from FL trimmer
 

For the carb link: The carbs that have the built in primer bulb like that can be easily changed to the type without. There is just one hole you have to plug after you remove the primer bulb. Then you can install a regular flate plate regulator cover on the carb. I use JB Weld to plug the hole, but you could drill and tap it for a small set screw in case you ever wanted to change back. This is important for those who are running completely cowled engines, as then you can solder a brass tube in the regulator cover, and run a hose outside or into the fuselage to reference static pressure instead of the ram pressure inside the cowl.

Mike, if you go to the walbro.com website, you can find carbs and their venturi sizes. From the home page click on "Service & After Market". Then click on "After Market Parts & Service". Then it gets a little weird. It won't show the venturi size at first when you find a carb you want to check out. But if you click on the number of the carb you have selected in the box of parts listing on the right, it will open another window with just the carb and it's price. Above that click on "Detail", and it will open another parts listing box, but this time it will show the carb venturi size.

Hope that makes sense,
AV8TOR

RE: Poulan 25 from FL trimmer
 

Was easier for me to replace the head on glow plug version. For comparision I run the engine first on original head and then on the shorter one.

Enhine: Poulan 25cc short shaft (weed whacker)
Exhaust: Original gutted out and drilled the muffler case with two holes 14mm dia. This is more like no muffler at all.
Carb: Original Zama C1U with 8mm venturi.

Fuel: 47.6% Methanol, 46.2% 91 oct gasoline, 6.2% oil (Motul/Castrol mix). I made the fuel : 40cc methanol, 40 cc gas/Motul mix (1:33), 4cc castrol oil. First I mix Castrol oil with methanol, and then I add gas/Motul mix. Shake it well and it looks OK.

Propeller: APC 17x6

Now the performance:

Standard cylinder.
Engine can easy start by hand. Peek 7020rpm , reliable idle 1760rpm

Shorten cylinder.
Engine can easy start by hand. Peek 7160rpm , reliable idle 1820rpm

Low end in both test was uneven. Engine was slowing down to quoted rpm but from time to time jump to 2000. However was idling like that for about minute and when I opened throttle full it immiedietly reved up without hesitation.

BTW, the glow plug needs to be powered only at start up. For running glow starter was unplugged.

Next before I mess up with ports I will try bigger carb. I need to find some cheapo walbro that will have 11mm venturi and will fit into the place.

RysiuM

RE: Poulan 25 from FL trimmer
 

I got hold of an APC 16x8 this evening, balanced it up and put it on the Poulan 25. It ran 8850 before it got too dark for my optical tacho to pick up the prop.

Rysium, what glow plug are you using as I tried an OS type F and could not even get the engine to fire. I'm on straight petrol/oil as methanol is not so easy to get hold of over here.

Cheers,
Ric

RE: Poulan 25 from FL trimmer
 

I can say ony WOW[8D].

As I recall, I hve OS F. The bottom of the glow plug is at the level ob the bottom of the gutted spark plug.

Do you have glow fuel (0% Nitro) available? You can mix that 50:50 with gasoline. You will have way to much oil but it doesn't hurt.

RysiuM