Chainsaw Repair

Chain - Grinders - Filing - Wood Milling - Tools - Welding - Machinist - Mowers - Tillers => RC Radio Control - other small engines => Topic started by: Spruce_Goose on August 02, 2014, 01:21:03 pm

Title: Poulan 46cc Porting Plane conversion
Post by: Spruce_Goose on August 02, 2014, 01:21:03 pm
Hi all. I am currently converting a 46cc Poulan into an rc airplane engine. I am pretty new to engine work, so be advised some beginner questions are coming atchya.

This engine seems to have quite a bit of potential with some real simple mods. SO i would like to do some real introductory porting (i.e. staying away from messing with the timing too much) In the picture of the exhaust port you can see a metal insert. What is this for and can I remove it to do some smoothing? You can see the exit hole where that insert sits is pretty large but the actual exhaust port entering the jug is much smaller. Can I simply open that up to match the larger opening ON THE SIDES and stay away from top and bottom?

Is there concern in general with porting of flaking off the chrome when filing/grinding the very edge of the port/cylinder?

Is that little brass nub on the intake side the equivalent to the impulse line, running the diaphragm pump? It hooks up directly into the carb. Is there a difference between this (or an impulse line) and a 'breather'?

Last question: While it is common to completely replace the muffler for rc conversions, if I were to mod this one, would opening up those two smaller holes on each side of the larger one where the screen imprint has been left be a pretty typical mod? I hear people talk a lot about punching out holes or enlarging holes in the muffler... I assume they are talking about right where the exhaust finally exists (i.e. the holes the spark arresting screen covers)?

Thanks for any help. I realize its a lot of questions. I have been reading up about mods, but want to be sure my understanding is up to par thus far. I am doing this in part to to gain confidence in working on chainsaw engines for when I need/want to work on my more expensive cutting saws.


Goose

Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 02, 2014, 01:50:38 pm
Goose
Hi all. I am currently converting a 46cc Poulan into an rc airplane engine. I am pretty new to engine work, so be advised some beginner questions are coming atchya.

This engine seems to have quite a bit of potential with some real simple mods. SO i would like to do some real introductory porting (i.e. staying away from messing with the timing too much) In the picture of the exhaust port you can see a metal insert. What is this for and can I remove it to do some smoothing? You can see the exit hole where that insert sits is pretty large but the actual exhaust port entering the jug is much smaller. Can I simply open that up to match the larger opening ON THE SIDES and stay away from top and bottom?

Keep in mind, and (this is for chainsaw guys reading this post), model airplane engines are usually set up for maximum power no greater than 8000 rpm for prop tip speed considerations. When messing with the exhaust port, the shape where it enters the cylinder is very important to prevent the ring from snagging the top of the port. If it were me, I would definitely remove the steel insert and, as you said, enlarging at least the width of the exhaust port. It does no good to lower the bottom of the port because it is normally level with the piston at bottom dead center anyway. If you raise the top of the port, you risk raising the rpm at which maximum power is produced. Also the outside of the port where the muffler is attached is rather limited by the bolt holes.

Is there concern in general with porting of flaking off the chrome when filing/grinding the very edge of the port/cylinder?

If using a rat tail file, I would approach the hard coating from the inside out to prevent flaking of the Nikasil coating.

Is that little brass nub on the intake side the equivalent to the impulse line, running the diaphragm pump? It hooks up directly into the carb. Is there a difference between this (or an impulse line) and a 'breather'?

Yes, it is the impulse hose connection for the pump diaphragm in the carburetor. It is not a breather to the outside atmosphere like is on a 4 cycle engine.

Last question: While it is common to completely replace the muffler for rc conversions, if I were to mod this one, would opening up those two smaller holes on each side of the larger one where the screen imprint has been left be a pretty typical mod? I hear people talk a lot about punching out holes or enlarging holes in the muffler... I assume they are talking about right where the exhaust finally exists (i.e. the holes the spark arresting screen covers)?

I would completely file out the depressed area that was behind the screen to make it a big rectangle opening including the area where the two holes are/were. Also, the screen cover should be opened up to allow free flow. The screen is normally left out on model engines. The side of the muffler that connects to the engine exhaust port cannot be opened significantly because you are limited by the bolt holes on each side of the port. but match the exhaust outlet, exhaust gasket, and the muffler where it attaches to the engine.

Thanks for any help. I realize its a lot of questions. I have been reading up about mods, but want to be sure my understanding is up to par thus far. I am doing this in part to to gain confidence in working on chainsaw engines for when I need/want to work on my more expensive cutting saws.

This engine is a clamshell design so you cannot normally decrease the squish.  But you could look around for a piston with a higher deck? The edges of the piston would need to be lowered to stock height but this would leave you with a pop-up piston that could reduce squish to the typical .020" plus maybe a little height into the combustion area in the center?


Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 02, 2014, 02:16:39 pm
Later on, keep in mind that other clamshell engines also make good model airplane engines.

For example:
Stihl 025/MS250. These essentially already have beam mounts on the lower crankcase.
The Stihl 029/039 - MS290/390 as well as the MS310 are all the same on the outside.

The old Echo 520 is a excellent model airplane engine as it was the basis for the BME 50

The Stihl clamshell chainsaw engines are copied by the Chinese and you can buy a more or less new complete one from the Hong Kong people that will run decent for a model airplane.

Also the Stihl FS38,45,46,& 55 series weed whackers have a model 4140 - 27cc clamshell engine with beam mounts. This engine has a separate lower crankcase top half like a Husqvarna 350 where you can adjust the squish height. Also you can get Chinese pop-up pistons for them from DLA engines in Greece. This pop up piston is a derivative of the Zenoah G26 design made by Chung Yang of China as a F270 which is available from Dave's Discount Motors.
http://www.davesmotors.com/zenoah-t208841110-t2088-41110-piston-34mm-g260rc-g260pum-g270rc-fg-07385-02.html

This also brings up the Husqvarna clamshell engines where there are Chinese cylinders available that offer much flexibility in making up a 45-50cc model engine.
Title: Re: Poulan 46cc Porting Plane conversion
Post by: Cut4fun . on August 02, 2014, 02:40:37 pm
Jim I tried to make your 1st post easier to read. I removed the quotes so the lettering would be bigger. Hope you dont mind.
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 02, 2014, 02:43:10 pm
THNX
Title: Re: Poulan 46cc Porting Plane conversion
Post by: JohnG28 on August 02, 2014, 09:27:29 pm
http://chainsawrepair.createaforum.com/husqvarna/husky-350-mod-options/

Here's a thread from a while back where these guys walked me through my first build and hand at some light porting work. Sounds like you want to do about the same as I did. Hope you can make it work!
Title: Re: Poulan 46cc Porting Plane conversion
Post by: aclarke on August 02, 2014, 10:35:09 pm
Jim, anyone use a propeller speed reduction on the bigger r/c motors?
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 02, 2014, 11:35:03 pm
Yes!

There's a company in England/Germany that markets a Gilmer belt reduction for the Zenoah G62. This is a open port engine that likes to run faster than any 7000 rpm.

2.5 : 1 reduction turns a 34 X 20 at 2900 which means the engine is turning 7250 rpm. The unit costs more than a new G62

http://www.vogelsang-aeroscale.com/geardriveschlundt.html
Title: Re: Poulan 46cc Porting Plane conversion
Post by: Spruce_Goose on August 03, 2014, 03:18:58 pm
Thanks for the help! I think I will try to do some smoothing and figure out something to do with the muffler.

Would anyone ever consider adding an impulse LINE off that brass nipple to replace the current design of being routed through the intake boot?
Title: Re: Poulan 46cc Porting Plane conversion
Post by: JohnG28 on August 03, 2014, 04:25:09 pm
If you have the carb setup so that you don't need the intake block then no reason you can't run an impulse line to it, least not that I can tell.
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 03, 2014, 09:22:34 pm
With the Poulan 46 you usually have to make a carb insulator block that attaches to the cylinder with set screws
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 3000 FPS on August 03, 2014, 11:09:13 pm
I have several Poulan 295's , 46cc chainsaws that I have worked on.
First I have removed the metal piece or heat shield in the exhaust and then widened the exhaust port to eliminate the step.
Like 660 said I would not raise or lower the exhaust port opening.   
On the stock muffler I usually weld the back holes up and then make a new one on the clutch side.
I then cut away about half of the center of the baffle out and leave the spark arrestor in place.
I have done plenty of cutting and with the mods and have had no problems.   
Also when ever I grind on a port I either sand the edge smooth or bevel it just a little to prevent the ring from hanging up on it.

I did bump up the compression on one but it was done in a rather unconventional method and so I will not go into that.
Like 660 said you should look for mods that give torque but not RPM.   
Title: Re: Poulan 46cc Porting Plane conversion
Post by: Spruce_Goose on August 04, 2014, 09:14:33 pm
Quote
First I have removed the metal piece or heat shield in the exhaust and then widened the exhaust port to eliminate the step.

That's about what I had planned. The only issue I see is that the top step will still be there. I assume this is not ideal for it will create eddies. Is there a way to smooth the step without actually removing it entirely causing timing changes? Maybe just a bevel. Or adding material in that little crevice even? I didn't plan on touching the intake unless it makes sense to do some sort of balancing...

Quote
Like 660 said you should look for mods that give torque but not RPM.

Does widening ports actually increase torque or is it only RPM really?
8,000 rpm isn't too far off cutting-saw peak performance is it. Don't many saws peak power at around 9,000 rpm? For planes is it that they want lower rpm simply because a super high speed prob is less efficient than a larger more consistent speed one with more load/force output?

In regards to the intake... this probably will show my ignorance, but do some saws use one way valves and others don't? As far as I can tell there is nothing stopping flow back out the intake of the cylinder through the carb. No valve that is. But I've heard of some of that physics stuff that describe why most of the fuel air doesn't blow back out...:)
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 04, 2014, 11:13:13 pm
The piston skirt closes off the intake port at the proper time. The intake port begins to open after the piston starts up and closes after the piston starts down.
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 3000 FPS on August 04, 2014, 11:50:30 pm
To answer your question about the step in the exhaust port.   Yes I did taper mine, but I did not raise the port.   That will drop compression, lose torque, and raise rpm,   

The point of widening the ports is to increase flow through the engine.   If the exhaust flows out better then it will also allow the incoming fuel air mixture to increase.

  In reality I think anytime you make any changes to these engines that helps to increase the HP rating there is also going to be a small increase in RPM.   
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 05, 2014, 12:32:31 am
Though I've seen several 46cc Poulan model airplane engines, I had two different iterations of the 45cc Timberman Homelites of which I originally intended to make model airplane engines but never did. The Homelites did not turn the rpm in the cut that my Stihl 025/MS250's do. They miss the mark by a couple thousand rpm. Stihl 9000 rpm / Homelite 7000 rpm. These Homelites have never given any trouble but fuel filters over the years. One of my kids still uses them for limbing.

But if you are used to Husqvarna 346 and 357XP's, you get the impression that the Homelite is not running full throttle.

I don't really know how the Poulan and the Homelite compared in the cut?
Title: Re: Poulan 46cc Porting Plane conversion
Post by: Spruce_Goose on August 08, 2014, 12:05:56 pm
"But if you are used to Husqvarna 346 and 357XP's, you get the impression that the Homelite is not running full throttle.

I don't really know how the Poulan and the Homelite compared in the cut?
"

Yeah, for the short test I did do with the Poulan before ripping it apart, it felt a lot more sluggish than the Husqvarna XP models. Kind of like you said: not feeling wide open when it actually was. Not sure what this means in terms of airplane engine performance or how it will change if I do any porting or muffler mods, but we'll see!
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 08, 2014, 09:23:16 pm
They run pretty much like a 46cc gas model engine should. They are more powerful than the US41 and Zenoah G38's but less power than a Brison 3.2 (Dolmar 115).
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 1manband on August 10, 2014, 05:40:54 am
first thing is, this is an insomiac reponse, so tread lightly.

sounds like the torque curve of the motor needs to come down in rpm, and the powerband needs to narrow up.

similar to this idea: http://www.epi-eng.com/aircraft_engine_products/ls_crate_engines.htm

maybe a smaller venturi carb, or better yet a smaller carb plus less intake port x-sectional area/duration may help?

compare the airplane optimal torque curve from the first link to page 19 of the second link here: http://www.vintagesnow.com/SledU_Folder/delivery_ratio-1.pdf

epoxy will be your friend.  don't know if rc folks do any of this?


regards
-joe


edit:  would help if i could spell and actually include the links



 
Title: Re: Poulan 46cc Porting Plane conversion
Post by: Spruce_Goose on August 10, 2014, 05:26:38 pm
For these clamshell style cranks, anything to know about resealing it? Should I be using something like Yama bond?

I also ground some of the fins down a tad bit (just to round them out aesthetically) and so now there is exposed fresh aluminum. Is there something (and should I) to cover this with to prevent corrosion. I know some people paint or powder coat the heads. I would prefer to avoid any process too involved or costly. Really if I could just throw some bluing agent, or oil or seal, etc. on real quick that would be ideal.
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 10, 2014, 06:16:12 pm
I use the Stihl Dirko (Like Yamabond) on the joint between the top and bottom. Don't make a mess with the Yamabond. Just spread it real thin and neat. Let the sheen dry off before putting the two halves together. I then let the engine set around until the next day so I don't get into the squeeze out and then it gets all over everything.

You don't want any sealer squeezing into the engine.

The fins will be just fine to leave them bare metal after reshaping.
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 3000 FPS on August 11, 2014, 08:11:35 pm
I have always used Yamabond when sealing the crank cases back together on a Poulan clamshell.   Some other makes of clamshell crank cases use an oring and on those I use nothing.
Title: Re: Poulan 46cc Porting Plane conversion
Post by: aclarke on August 11, 2014, 11:25:31 pm
Joe makes a good point on the port velocity in regards to improving power at lower  rpm which would be advantageous for direct drive props and keeping the tips in an efficient subsonic range.  Bringing the power curve down in rpm won't necessarily require bigger ports...
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 1manband on August 12, 2014, 11:05:44 pm
re-reading my post including the the torque link to the paper was not very informative.  the paper is a  dry read, but has some vgood points, imo.

delivery ratio causes the torque.  torque output is the result of delivery ratio.  the curves look like one and the same. shapes look the same, etc.  so, by changing the delivery ratio curve, you change the torque curve to the same shape, vice versa as well.

-joe

Title: Re: Poulan 46cc Porting Plane conversion
Post by: Spruce_Goose on August 14, 2014, 11:23:52 pm
thanks all for the help. I'll have to look into this lowering of the rpm in the power curve. As of now its a bit over my head, but I'll try to find some time to look into it.
Title: Re: Poulan 46cc Porting Plane conversion
Post by: Al Smith on August 16, 2014, 08:42:47 am
For what it's worth McCulloch used either Indian head gasket shellac or later RTV to seal the crankcase halves .It must work because I've never found a leaker .
Title: Re: Poulan 46cc Porting Plane conversion
Post by: Spruce_Goose on August 16, 2014, 06:06:00 pm
Looking to the muffler... it is sort of looking like zinc coated steel? would that be correct? If I want to close up the old holes and attach a pipe to this thing can I braze with mapp gas? (I have no welding set up)
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 16, 2014, 06:41:02 pm
No!  You will need to really rotary wire brush that cadmium hard to get it off. Brass will not stick to cadmium or even the black oxide coating
Title: Re: Poulan 46cc Porting Plane conversion
Post by: Spruce_Goose on August 29, 2014, 07:26:10 pm
Is it normal for the bearings to just slide around? I can literally just slide (not even using any force to speak of) the bearings along the shaft (off in the case of the flywheel side and into the worm gear on the clutch side). Was like this upon opening the case...
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 29, 2014, 07:38:08 pm
Something is wrong? They are supposed to be a slight interference fit. Maybe .005 interference with both the case and the crank when the cap is on the case.

I just took apart and Stihl MS250 clamshell engine tonight and the 6202  C3 bearings were made in Turkey. The clutch side bearing fell off the crank. I had to use a bearing pulling clamp and puller on the flywheel side and that bearing fit tight on the clutch side. The new bearings were tight on both sides.

Look at the crank and see if it is shinny in the bearing seat area or galled? My Stihl's bearing area on the crank looked normal though the bearing fell off.
Title: Re: Poulan 46cc Porting Plane conversion
Post by: Spruce_Goose on August 30, 2014, 09:56:36 am
"Look at the crank and see if it is shinny in the bearing seat area or galled"

It is kinda shiny. Actually has that kinda glazed brown look, presumably from burnt oil.
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 30, 2014, 10:38:06 am
All I know to do is to try a new bearing on the shaft and see if it is tighter?
Title: Re: Poulan 46cc Porting Plane conversion
Post by: Cut4fun . on August 30, 2014, 11:01:25 am
Jim the guy that owned the K3500 454 plow truck I looked at Bucyrus airport was getting into rc. Seen them in background in hanger. 

He said he crashed a 4 engine ????  cant remember plane learning. Now only single engines.  I told him if he needed any info text me and I would hook you two up.
He said the guy that taught him to fly them was from Shelby too. 
Title: Re: Poulan 46cc Porting Plane conversion
Post by: 660magnum on August 30, 2014, 01:40:43 pm
OK.

There is a guy in the Galion model airplane club that lives in Bucyrus that flew at the Bucyrus airport for years that could help him also.