I just came back from the garage where I tried the glow running again. Its late so I only ran it on tickover but by turning out the slow running jet a turn further than with spark ign I got it running. I'll try more again tomorrow after work hopefully when I can make more noise!

I want to stick with straight petrol/oil if poss as straight glow fuel is about £10/UK gallon here, where as petrol is £4.50 and I can buy it at the end of my road!

Cheers,
Rich

We in US are 'lucky' to have methanol for less then 3USD a gallon when the gasoline price climbs almost to the same level[:@] If this goes that way I will be converting my Toyta Tacoma 4x4 to glow fuel soon.

RysiuM

Just remember, the price is misleading. It takes twice as much methanol to run an engine as it does gasoline. So until the price of gas hits $6.00 a gallon....

(Some day soon I'm going to wish I never said that...)

AV8TOR

... for the same engine size WOT. It is because gas-air mixture must about twice leaner than methanol-air for proper combustion.

However the engine will make much more power. So the methanol mileage is much lower than 2-times of gas mileage. That brings the equotation to maybe 4-5 dollars per gallon.

(Some day I'm going to wish I never said that too...)

RysiuM

Hi RysiuM,
Here is a picture of a Weedeater 25cc piston that I modified for better breathing. As you can see the skirt is releived to allow the transfer ports to breath at bottom dead center.

Dave

Very nice job. Now let's see if you got more power.

RysiuM

I ran it tonight on an OS type F plug powered all the time and straight petrol/oil mix at 50:1, and it ran 9200 on the APC 16x8. It ran not too bad with the glow power off either but spluttered a bit and would not tick over below about 2500.

I also tried it on a mix of 30% prosynth 10% nitro fuel, and 70% petrol. This mix gave 70% petrol, 21% methanol, 3% nitro and 6% oil. On this it ran 9000 rpm but ticked over nice and pickup was better, together with the off power running. Curiously it ran less revs than the straight petrol/oil mix though.

Running spark ign instead of glow ran 400 revs lower on both fuels.

Cheers,
Rich

I believe with the electronic ignition timed right you should get the same or maybe better results than glow. The fixed magneto timing is limiting engine's rpm. Especially on the hight end. Trimmers were not intended to tun high rpm. Imagine you run trimmer WOP and you are out of string. In such situation gas engine might rev up beyont it's safe limits. With limited ignition timing unloaded engine will not run beyond safe rpm.

With yourmethanol-nitro-gas test did you power glow plug all the time?

What ebout the difference on the same setup between full-skirt piston and the cut at transfer ports?

RysiuM

I have not cut the transfer pockets in the piston, only the inlet port pocket as detailed elsewhere. I won't do the transfer pockets for a while yet as I have to take the engine out of the plane to remove the backplate etc etc and just want to fly it for a while now. Hopefully tomorrow I'll have another engine and that one will be fully tuned and I see no reason why it would not get 9500 or maybe more on the 16x8 APC. I'll update in a couple of weeks on that one once I've had chance to do the work.

Regarding the magneto restricting revs I'm sure that is the case, one other thing I want to look at is variable magneto timing, and also building an electronic ign with auto dwell control for it. All stuff for the future though.

With the petrol-methanol-oil-nitro mix I ran it with and without glow powered. With the glow powered or unpowered it ran the same top and mid revs and pickup, but the low end pickup and tickover was better with the glow powered. I've got a couple of 3000mAh 1.2v cells that I'm going to make a pack from to run onboard glow, at first all the time, and later just for maybe half thorttle and less. All this work to do and not enough time!


Also I'm tempted to try a smaller carb sometime, I think the WA200 is perhaps a little on the large size for the engine and sometimes it stutters when on tickover and you go WOT quickly. Either that or get the same size but with a throttle pump to give the extra fuel it needs for the transition.

Cheers,
Rich

Just set the low end a little rich. As long as it doesn't die on idle it willbe OK. The extra fuel in the engine helps with transition.

I setup my engines low end that way, that when I bump the throttle from idle I got immiediet respons in rpm. Even if it's to rich for long idle I don't care as I learned to fly throttle all the time,. So when I fly idle (in example on aproach) I still bump the throttle every second or so. This also confirms that the engine is still running (on noisy field you don't hear engine idling, an to faar to see the prop). Then I'm not surprised with the dead stick at the last moment.

RysiuM

I've got the bottom end rich right now, 1 3/4 turns on low, 1 1/4 on high jet but it might take more. The problem with too rich on the bottom end is it just will not clear into a two stroke going flat out. I'll try some more fiddling though, and maybe the smaller carb too but of course I can get round it right now just runnign tickover at maybe 2500. Its still early days for the poulan 25 for me, all my previsou 20 years of work has been on 100cc racing two stroke kart engines, this is my first venture into smaller ones so I'm still getting used to them. So far I'm pretty happy how its going, but of course there is plenty more to go.

Happy tuning whilst I'm away, I expect 9500 from you Rysium until I'm back in 10 days! Calling yourself a tuner is a lot more than just guessing and hacking a bit off here and there and getting lucky, its about starting with an aim and working to it. I'll be buying a 17x6 in a couple of weeks too to give more thrust. Well, you can never have too many props can you...?


Cheers,
Rich

The lowest reliable idle I got was 1700-2000 (that's what my tacho shows, swaping the numbers like at the bingo table). My MVVS 1.6 gas also runs about the same numbers and with APC 18x6 the PowerLine Hobbies Edge 90 handles OK on the ground (taxi OK, but does not want to stop). As I'm going to use this engine on bigger (heavier) high wing floater any idle below 2000 will be perfect.

With some engines (glow) I had a problem that reliable idle was to fast for taxi. I used 'Idle down' on my futabe then. I keep reliable idle while I'm flying, and for taking off and landing I switch to low idle (then I work thottle to no tlet it die).


RysiuM

I just measured the engine with degree wheel. My numbers are close:
Inlet: 122 deg
Transfer: 84 deg
Exhaust :136 deg

This is after the cylinder was moved down by 0.5mm. I guess it's time to get a file and dremel.

One more thing: my MVVS 1.6 has walbro with 11mm venturi and it runs great. Hmmm... still not sure what to do about the carb.

RysiuM

Hi,
You did lose a little on the exhust and transfers. My 25cc is 100 degree transfers. 150 degrees exhust. 132 degrees intake. You can cut a 45 degree shanfer around the piston 1/2 mm deep. This will give you back your timing without loseing the compression gain. When I cut ports I use masking tape to protect the cylinder walls where I don't want to cut. It is not a matter of if you slip, but when you slip.

Dave

Hi
one week ago,last Sunday I make my firsts flights with my Weedeater, nice engine, 3 flights with more than 35 minutes accumulated.
Diablo is right , no interferences, but for extra care I added a coax shield twisted over the High tension lead, my spark plug is RCJ6Y, this came with the engine.
I am reading your posts and learning, very useful.
Many Thanks Diablo and Rysium

Diablo
Do you think to dremel the wall between both transfers to make only one big transfer?
I have seen a transfer like it in a Zenoah G26 and I believe from photos the only one transfer have near 25 to 30 mm wide.
Many thanks for your time
Jose

Hi Folker38,
If you remove the web in the transfer port you may allow the ring to catch the top edge of the port. I beleive that the ring is pinned which allows a greater unsupported area, but be careful. I would not have a unsupported span wider then the exhust port. Here is a picture of the two types of weedeater cylinders by Farley9.

Dave

The other thing is that two transfer channel stream the mixture in patricular way for better distribution in the combustion chamber and pushing out the exhaust gasses. If you change the shape of the transfer channels you may loose performance as the the flow of the mixture will be different. Poulan engines are flat cylinder type so transfer ports (shape and angle) play a big role in the mixture distribution.

RysiuM

Hi
Dave and Rysium
Yes , but i have seen big transfer ports in the Zenoh G26 photos and also in a 3W 24 photos in a Model airplane news test.
In my readings from Gordon Jennings , he say the wide limit in the exhaust port is 0.6 times the piston diameter.
Also in a moped and in this engine the transfer was directly pointed to the spark plug , wide transfer and good performance engine.
In the weedeater the transfer are pointed back.
My engine is like the left in your photo, Dave
To prove my theories I need other Weedeater, but here for 20 U$S more I can buy a reconditioned Poulan chainsaw at 135 U$S , 2.1 cid.
But the Weedeater is so sweet! Yesterday I have flown 1 hour more with the Weedeater, soloing for first time my first gas engine, what a experience.
Many thanks for your time Dave and Rysium.
I am learning from you.
Sorry for my english
Jose

Hi Jose,
I did some calculations and found that the 25cc trimmer transfers are less then 50% of the diameter of the cylinder bore. It should be OK, but I would recomend opening the exhust side transfers only. this will leave the center ring support while giving more cross section area. I am attaching a pivture of a G26 cylinder and piston. By the way your english is great.

Dave

hi
Dave
Thanks for your photos and answers.
When I catch other weedeater engine I will open the transfer ports like in Zenoah.
Thanks for your time
Jose

After enlarging transfer ports the next bottle neck will be piston skirt covering the inlet to transfer posts from the bottom. In Poulan 25 the front transfer is blocked by counterweight and the back transfer port is blocked by backplate.

RysiuM

Hi Rysium,
Notice the G26 piston and transfers. It also has a full disk counter weight to stuff the case.

Dave

Yes, and the piston has shorter skirt where transfer ports are.

RysiuM

Hi
I forget it.
I will remember the piston skirt and backplate when dremel the new engine.
Many thanks for your time.
Jose

I'm back from me holidays now, good to see some more Poulan 25's getting chopped up! Get it done and post some RPM figures[8D]

Regarding carbs, I've flown mine some more and am going to try a smaller carb, I think the WA200 is just a bit too large for low revs throttle response. Flat out screamer, yes, its the kiddie, but it just does not carburate 100% clean every time from below 2000rpm when fast throttle up.

Nice pic of the G26, it looks like a full crank engine, IE two halves, so heavy. Who needs a G26 when a $20 poulan runs 9200 on the same prop though (8600 quoted for G26 on APC 16x8)

I think I've broken my engine though as I dropped the plane upside down onto a concrete floor and it landed straight on the flywheel breaking off the coil mount. Looks like this one will be glow only until I get another engine sorted!

Cheers,
Rich