Author Topic: Poulan 46cc Porting Plane conversion  (Read 1299 times)

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Offline Spruce_Goose

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Poulan 46cc Porting Plane conversion
« 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.


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Offline 660magnum

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Re: Poulan 46cc Porting Plane conversion
« Reply #1 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?


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Offline 660magnum

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Re: Poulan 46cc Porting Plane conversion
« Reply #2 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.
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That knowledge can live after us... and that "Pays It Forward".
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Offline Cut4fun .

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Re: Poulan 46cc Porting Plane conversion
« Reply #3 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.
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Offline 660magnum

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Re: Poulan 46cc Porting Plane conversion
« Reply #4 on: August 02, 2014, 02:43:10 pm »
THNX
We should share what we know... someone may learn...
That knowledge can live after us... and that "Pays It Forward".
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Offline JohnG28

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Re: Poulan 46cc Porting Plane conversion
« Reply #5 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!

Offline aclarke

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Re: Poulan 46cc Porting Plane conversion
« Reply #6 on: August 02, 2014, 10:35:09 pm »
Jim, anyone use a propeller speed reduction on the bigger r/c motors?

Offline 660magnum

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Re: Poulan 46cc Porting Plane conversion
« Reply #7 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
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Offline Spruce_Goose

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Re: Poulan 46cc Porting Plane conversion
« Reply #8 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?

Offline JohnG28

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Re: Poulan 46cc Porting Plane conversion
« Reply #9 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.

 

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