I have been examining a Poulan 18cc weedie motor for possible conversion. I have read here that the motor itself is low on power. Just looking at it, I can see a couple of reasons why. Both the intake and exhaust ports are pretty small. However, it has some good points too. The opening for the intake is more than large enough and it tapers down to the opening in the cylinder. It can take a much bigger carb if the port opening itself can be opened up. Same goes for the exhaust. The opening for the exhaust is huge. The transfer ports also seem to be plenty big enough. The crankcase can be carved down to a nice size and is very light. There is plenty of space on the crankcase to mount a sensor for electronic ignition. And finally, the piston ring is pinned. The gasket for the cylinder is very thick also. Seems that compression could be increased by eliminating the gasket and using RTV to seal.

Now to my questions. How would I go about opening up the intake Cand exhaust ports. Looking at the cylinder, looks like the best way would be to grind down on the intake port and up on the exhaust port. This would keep the timing the same for when these ports close. However, this will increase the timing on when they open. Here is my question. How will that affect how it runs? I cant see how this would change much except to allow more fuel in on the intake side. But it could cause a loss of power on the exhaust side if the exhaust opens too much. Am I right in this? If so, what is the limiting factor? Last question, what tool would I use to make these changes? I have a dremel tool. Can I use a grinding bit in my dremel to do this?

I would like to make this motor powerful enough to fly a .60 size warbird. I have a set of plans for a Zero. With the short nose on the Zero, I am sure it could use the extra weight up front if I can get enough horses out of it.

Thanks
Jim

The first step on a hop up on one of these conversion engines, besides perhaps gutting the muffler, or buying one and rounding up a suitable carb, is to lower the cylinder for more compression if the deck height will allow it. (I also recommend either Gas/Glow or electronic ignition to get rid of the mag and to have the ability to optimize the ignition.) You want a deck height of .015 to .020". You want to do this first, because lowering the cylinder affects the port timing. It raises the intake timing and lowers the exhaust timing.

So once you have your cylinder to head distance (deck height) dialed in, you can then start looking at port timing. I would aim for at least a 150 degree exhaust port timing, about a 120 degree intake timing, and if you can get to the transfer ports and raise them without changing their angle(s), I would make them around 130 degrees. You want to raise the exhaust and transfer ports, and lower the intake port to get these numbers. Be careful when lowering the intake port that it doesn't become uncovered or open, when the piston is in the area of bottom dead center; effectively not closing completely. You can also widen the ports, but be careful when widening the exhaust port that it doesn't get so wide that the ring can bulge into the port when passing by. You can do all this porting with a Dremel tool and carbide cutters. Don't try to use grinding stones, as the aluminum will quickly clog the stone, and this can lead to the stone overheating and exploding. When you are done with the ports, be sure to give them a slight bevel to keep them from scuffing the piston or damaging the ring(s).

BTW, I don't recommend enlarging the ports without first getting the port timing set. A larger port won't help if the engine doesn't have the "time" to use the larger port.

I would try a 14 x 8 APC prop when you are all done, and experiment from there. It should make a good engine for a .60 warbird if propped correctly. You may end up using a 13 x 8 or 10 prop on a warbird for good performance. You can also check with "Aeronut", as I believe he has direct experience with this particular engine, and was happy with it once modified.

Have fun,
AV8TOR

P.S. More on the exhaust port... You will probably find that the bottom of the port will be about even with the top of the piston when the piston is at bottom dead center, especially if/when you lower the cylinder. Lowering the port any wouldn't help because then the piston would be in the way of flow.

AV8TOR

I just modified one of these, haven't ran it and it has fairly low compression and a scored cylinder, so it might have just been practice...

as for taking the gasket out completely, there isn't room, the piston will hit the head (mine was that way anyway), just use a really thin gasket material. probably best to get a frank bowman ring, as my ring gap was pretty big. to modify the muffler, just take out the two plates in the middle that are it's form of restriction, if you're going to make a different outlet for it, just braze/weld the pieces together and make it how you want it, and if you don't like the springs, I saw one in a local club members museum that had bolts through the factory spring holes and to the head where the springs normally mounts. I'm planning on keeping the springs and the factory outlet as mine is planned for a 40 size US Aircore coroplast biplane that I'll just make the exhaust exit in a decent place

as far as porting, the exhaust port sucks from the factory, way too short, if I remember right the timing was around 110 degrees, and around 90 for the intake. I think I widened the exhaust port along with raising it, and I just lowered the intake, I think it was pretty wide, just had sucky timing... the transfer ports are a pain to work on since the cylinder is so small, I slipped a few times...

a removed factory carb from a 25cc should be fine for this tiny thing...

Thanks for the input guys. I just did some calculations on motorcalc to see what kind of rpm I will need to fly a 60 size warbird. If I can keep it around 9-10 lbs. I will need on a 14x10 8400 rpms and on a 13x10 8700 rpms. I plan on using gas/glow. With porting is this attainable on this engine?

Thanks
Jim

don't know about the rpm's as I haven't ran mine and it's not in top shape, I do know that I got mine at just under 2 pounds prop included (I think I had a 15-6 MA plastic) if that helps any, I might be able to reproduce the prop hub I made if you need me to...

Put the head back on today to check head clearance. Without the gasket, there is no head clearance. Piston strikes the top of the head. So will make a trip to the local auto parts shop this afternoon and see if I can get some gasket maker to make a gasket. Then see if I can map the timing on this bad boy.


What are the ideal timing measurements for intake, exhaust, and transfer ports?

Thanks
Jim

Quote:
So once you have your cylinder to head distance (deck height) dialed in, you can then start looking at port timing. I would aim for at least a 150 degree exhaust port timing, about a 120 degree intake timing, and if you can get to the transfer ports and raise them without changing their angle(s), I would make them around 130 degrees. You want to raise the exhaust and transfer ports, and lower the intake port to get these numbers. Be careful when lowering the intake port that it doesn't become uncovered or open, when the piston is in the area of bottom dead center; effectively not closing completely. You can also widen the ports, but be careful when widening the exhaust port that it doesn't get so wide that the ring can bulge into the port when passing by. You can do all this porting with a Dremel tool and carbide cutters. Don't try to use grinding stones, as the aluminum will quickly clog the stone, and this can lead to the stone overheating and exploding. When you are done with the ports, be sure to give them a slight bevel to keep them from scuffing the piston or damaging the ring(s).

BTW, I don't recommend enlarging the ports without first getting the port timing set. A larger port won't help if the engine doesn't have the "time" to use the larger port.
end quote.

AV8TOR

Went by the auto parts store yesterday and picked up some 1/64 paper gasket maker and made myself a new head gasket. Now I have some head clearance. I then proceeded to map the port timing. These are the results.

exhaust 110
transfer 73
intake 112


Really tame it looks like. Lots of room for improvement. What do yall think? The intake and exhaust ports look fairly easy to change as they are a rectangular shape and can be raised or lowered using just a flat file. The transfer ports are going to need a dremel.

On another topic, how much clearance does the backplate need for the crank? Asking this to see if the back plate needs any stuffing.


Jim

Any advice on the porting numbers I recorded?

Jim

As you said, they are VERY tame and there is a lot of room for improvement.

Back plate clearance should be around .015 to .020", but make sure there is no fore and aft movement of the crank before you make it that tight.

AV8TOR

Keep checking your progress with a degree wheel as you port. It is easy to take too much material off and end up with timing higher than you want. With a piston ported two stroke, you are changing both the opening AND the closing of the port. So a small mistake in removing material is actually a "double" error.

AV8TOR