av8tor1977

My Feedback: (6)

Hi,

Well, my latest project is a Stihl 25.4cc engine, and so far I am pretty impressed. It is light, and only weighs 1 lb. 13 oz. as shown in the pics. It's a nice looking, and compact engine, and I think it is going to have good power. The combustion chamber is very small, so it will have a good compression ratio. Original port timing is 157 degrees on the exhaust, which is very good. Intake timing is a bit conservative at 120 degrees. It has two gaskets under the cylinder, and a fairly large deck height of .040". I will probably leave one of the gaskets out, which will close up the deck height a bit, and also increase the intake timing just a touch, (at the expense of a few degrees of exhaust timing). Ports are all good sized and I think it is going to run very well with basically no modifications other than what I just mentioned.

It actually came with an 11mm carb, but it is a Zama. I considered using it, until I looked up the price of a carb kit!! $19.00 plus shipping!!! Not in this life time my friends.... That's utterly ridiculous. So, in the junk box it went, and I will put one of my new Walbro 11mm carbs with accelerator pump on it.

It will be a while before I can give a running report, as I still have to make a muffler for it, a prop hub, probably a "twister" intake adapter, and buy an ignition system and a Bowman ring. Time and money....

But I think it is going to be one sweet engine!

AV8TOR

spaceworm

Very nice. I will be waiting for a test run report.

Has anyone converted a Stihl 4-MIX engine. I have not been able to find much about it. What was/is it used in and was/is there just one size of it. TIA

av8tor1977

My Feedback: (6)

There are several 4-Mix engine sizes in the 30cc range, and a 65cc. I have a 32cc and a 65cc I am going to convert whenever I get the time. (Don't know when that will be...)

Do a search here in the forum and you will find some threads. The 32cc appears to be considerably more powerful than the Honda GX-31 from the posts I read, and much lighter as well.

AV8TOR

j_t01

I have that same engine on a Bridi stick. I haven't flown it yet but with a 16-8 prop on gas glow it 's max rpm is 7300 and idles at 1700

Jim

av8tor1977

My Feedback: (6)

Nice looking plane. Did you make the carb insulator/adapter? Mine mounts 90 degrees off to how yours does, and I am probably going to make a twister manifold to better align the carb linkage and not have to use a bell crank.

AV8TOR

j_t01

Thank you for the compliment. The carb adpt, is a Zenoah 13810 kit. I had to open up the top and bottom of the intake port on the cylinder slightly to match but it was a good fit on the carb.

Jim

av8tor1977

My Feedback: (6)

Ok, thanks. Maybe I will buy one and save me the trouble of making one.

AV8TOR

Twin Star

I picked up a Stihl FS74 trimmer today that is a prime candidate for conversion. The cutting head is trash and the shaft looks like it's been run over. Feels like it has good compression.

Does anyone know what the displacement may be. The only info I could find was for a FS75 and it said it was a 25.4cc

av8tor1977

My Feedback: (6)

Does it look like mine in the beginning post? If it came from an FS-75, it is a 25.4cc. I haven't been able to finish mine and run it, but I am positive it is going to be a good "runner". Nice size ports and port timings.

Here is a copy of the owners manual for the FS-75: http://www.stihlusa.com/stihl_owners...S75_Manual.pdf

In the future, to get displacement, just measure the bore and stroke. Then use this forumula: (bore x bore) x stroke x .7854 = cubic inch displacement (If you measured in inches.) Then just divide that result by .061 to get cc's.

AV8TOR

av8tor1977

My Feedback: (6)

Well, I just spent a half hour searching, and can't come up with info on the FS-74 either. Stihl makes really good, high quality equipment, but they don't want anyone but the licensed dealers having access to the technical data, which is a huge pain in the *****!!

AV8TOR

Twin Star

Yea, The power head looks the same but mine is a straight long shaft (maybe the reason for the different mod. num.)

I've had to measure a couple in the past to be sure and even been pleasantly surprised a time or two. Hope my luck holds.

w8ye

My Feedback: (16)

I have a Bridi Stick/Old Timer with a Homelite 30. The engine is stock except for open exhaust and a WA 167 carb from a G-23. The engine turns a 18 X 8 around 7400 rpm and it has decent performance on the bridi. I originally had a Kalt 22 (Old Stye G-23) on the plane and it just didn't have enough power.

j_t01

I have tech info on the Fs75 but not on the Fs74. I'll see what I can find at work tomorrow.

Jim

j_t01

Here's the specs on the FS74

Jim

frenchie79

Av8tor-Thats one fine engine-I've put together several Stihl engines and that model ran the strongest with least amount of vibration. I posted it under "don't forget stihl"
Good runner-you'll like it.

av8tor1977

My Feedback: (6)

Thanks. Yeah, it looks to be a good one. Just need to finish it!

AV8TOR

Twin Star

Thanks j_t01
Well, It appears that the engine in the FS74 trimmer is a 23.9cc. It would probably be ok for something like a H9 Ultra Stik but not something in the 80" range as I had hoped. I don't have a Ultra Stik though.

av8tor1977

My Feedback: (6)

I put a little Stihl 20cc on my brother's Big Stik and it flew great. (We set it up on Gas/Glow) I think it would look a little better with the engine inverted, but he wanted it upright.

I think the 24cc would fly something like an 80" Giant Stik. It wouldn't have a big excess of power, but it should fly ok. The whole "power option" had soared upwards on models nowadays. What we considered sufficient 20 years ago would be considered way underpowered today. But it sure is fun to have all that power!!

AV8TOR

Kostas1

When you say ' gas/glow'

you mean that you mixed glow fuel with gasoline and burned it through your engine ???[]

av8tor1977

My Feedback: (6)

Here is all the info on Gas/Glow: http://www.rcuniverse.com/forum/m_80..._1/key_/tm.htm

AV8TOR

Scota4570

Speaking of Stihl, I put one of my 23cc on a 60 size four star. Hopefully it won't be too much.

Kostas1

Thank's for the link..........

spaceworm

I looked there but did not see my possible conversion. I have a OS Max 80 (13 cc) dual plug rear drum induction glow engine from thje 60's. The carb flange is broken, although repairable. But, I am thinking that the rear induction and flange mount carb is ideal for fitting a Walbro carb and running gas/glow. What do you all think? Appreciate all comments good or bad. Here is a picture.

Regards,

Richard

av8tor1977

My Feedback: (6)

Why Gas/Glow for that engine? Gas/Glow is a special conversion for engines that were originally configured to burn gasoline and oil. A glow engine has bushed rod and wrist pin bearings that need a lot more oil than the Gas/Glow mix provides. Better to run those engines on glow fuel unless you are prepared to change to roller bearings on the rod, as has been done on some Super Tiger .90's for example.

AV8TOR