Jazzy

I am going to be converting a Mac 2815, gas, 2 stroke weedie into an aircraft engine.
The plan is to do a step by step walk through to show those that my be daunted by such an endeavor that it isn't that hard to do.

I plan on using much of the original equipment as a baseline and substituting aftermarket components to show possible advancements in reliability and power output.

In addition, I will attempt to do some mapping and porting of the engine to "clean it up" for added efficiency and again, power.

The beginning:
The neighbor across the street was tossing out a weedie and leaf blower. Of course I walked the dog over that way to get a better look. Within moments both were in my garage.

The Mac would not run past idle so I took the ZAMA carb apart to see if I could find the problem. There were some small bits of debris but nothing I saw would indicate a clog. I blew out the high and low speed jets, put it back together, and stuck it back on the engine. A few prime pumps later I had it up and running. The top end was lean so a little tweaking brought it up just right. It ran so well I hated to tear into it.

Below are before and after pics of the initial deconstruction.

Note that the only tools I used are a pair of needle nose pliers and a T25 Torx.

I don't have my workshop set up so the going is going to be slow. Bear with me please.

bluejets

Mufflers get clogged up too, especially the spark arrestor gauze, might be worth checking. Can affect top end and acceleration.

Easiest way to clean out is build a fire and throw the muffler in for a while. (assuming it is steel)

Jazzy

I finally had some time to tinker a little more.

I wanted to get an all-up weight of the engine-muffler-carb-magneto combination before I started cutting metal and plastic.
To do this I had to get rid of the gas tank. To remove the gas tank either cut the plastic back plate or remove it and re-install.
I chose to remove and re-install.

During this process I also took off the heat shield for the carb and the muffler. The heat shield for the carb also doubles as a gasket. You can easily see the hole in the case and the gasket for the pressure pulse that allows the carb to pump.
Below are the weights with all components minus the gas tank and the weight without the two heat shields.

It started out right about 5 lbs. Removing the two heat shields dropped 2 oz.

w8ye

My Feedback: (16)

I was given one of these some 12-14 years ago. parts of the cylinder were broken by the arms of a bucket loader on a tractor.

With the help of the CH Ignition people, I was able to make a 32cc engine with electronic ignition.

it is a great runner 18-8 at 7600 rpm.

The original CH ignition of Wyoming is no longer in business.

Jazzy

Thanks w8ye!

I wonder if changing the head and piston could make this one a 32cc engine...
I don't know if the cranks or connecting rods are the same for the two different displacements though.

I'm amazed at the size and overall mass of the magnet/flywheel. This was a direct drive trimmer (had no clutch) so it does make some sense.

rangerfredbob

My Feedback: (13)

changing the piston and jug is all that's needed to make it a 32, the cranks are the same, the crankcase on mine started out as a 28cc.

They aren't light engines, but they're good runners, the crankshaft is most of the weight of the things, you can shave off all of the aluminum you want and it will barely make a dent in the weight, the crank threads are 7/16" instead of the normal 5/16" on trimmer engines, and after the taper they just get bigger...

w8ye

My Feedback: (16)

I have a 28 and 32 as weed whackers. Hard to tell the difference

spaceworm

So, unless you needed to replace the piston and cylinder, it would not be worthwhile to convert from 28cc to 32cc? Thanks, I have a 28cc, that I am planning to convert, and it still runs well as a weedie.

rangerfredbob

My Feedback: (13)

one of these days I'll test the difference, I think I have 4 usable mac engines on the shelf, two of each...

Jazzy

More tinkering:

The first thing I need to do was remove the drive shaft adapter. As it is a CCWSE rotating crankshaft, the threads are standard: "Lefty-Loosy."
A screwdriver and curve-jawed pliers loosed it right away. See pic.

The next step was to remove the flywheel/magnet. I had a set of 3-jawed gear pullers that popped it right off. The little woodruf key got put in a jar with all of the other screws and small components that have a tendancy to go on a walk-about and never return. See pics.

I removed the shroud which also doubled as the magneto/spark holder. Three screws and off it came.

After shooting the gummed up parts with Outer's Crud Buster (for firearms) I blasted everything with CRC Mass Air Flow sensor cleaner. The CRC cleaner is safe for plastics and apparently rubber as well. Between the two the outside of the engine and all other parts removed are clean and oil-free.

The nice thing is the overall weight is now down to 3 lbs, 2.8 oz with the original muffler and the carb still intact.

av8tor1977

My Feedback: (6)

Just curious.... what mods were done to your engine and which brand prop did you get the 7600 rpms with? I've got a couple of these, but never bothered with them as Ralph C. mentioned one time that he was not very impressed with them. Got lots of other engines to play with, so I just set the Macs aside....

AV8TOR

Jazzy

Ranger,

Interestingly enough, the crank threads are 5/16-24 on this engine.

The rolling-thread screws used to hold pretty much everything else together are 10-32. This will make for an easy change to socket head cap screws.

Jeff

w8ye

My Feedback: (16)

Most of those with clutches were 3/8"

Jazzy

Gents,

I thought I had a link to find a prop hub adapter but now I can't find it. I tried searching but didn't have much luck.

Can someone post or send me some links please?

Thanks,
Jeff

av8tor1977

My Feedback: (6)

www.wackerengines.com
www.lambertsrc.com
www.carrprecision.com

AV8TOR

w8ye

My Feedback: (16)

While you are converting the engine, change out all the soft Torx head self tapping screws to Allen hex socket head bolts.

Jazzy

Finally had time for more tinkering. Sorry for the long pause but 'work' does pay the bills! LOL

I ordered a prop hub, velocity stack, and resistor spark plug from Carr.
In the meantime I disassembled the engine completely. What I saw is indicative of restricted exhaust and/or low CR (compression ratio).
Future mods may include opening the exhaust port depending on the timing and removing the intake restriction.

Jazzy

Using Outers Crud Cutter, Mass Air Flow Cleaner, and Scotchbrite the crank case cleaned up nicely. The piston took well to the scotchbrite. I got a little over zealous on the piston crown but I'm not terribly concerned. The spark plug got a good rinsing too. I'm still working on cleaning up the head fins and flywheel.
Upon closer inspection I noticed the four transfer ports are tapered in as they progress up the cylinder... Seems odd that the cuts aren't parallel but I'm no 2-stroke porting guru.

Jazzy

Another mod is the back plate and possibly the back surface of the crack case. The back place WILL need to be trimmed. See silver outline on the back plate. A common mod to the case is to take some 0.040" off the length of the case. Why? Well, with the supplied gasket between the back plate and case the inset is around 0.050" from the crank pin. I guess the thought is that by reducing the internal volume (which is compressible) the fuel/air charge will be more concentrated and more apt to provide better combustion. Just a SWAG (Scientific Wild Arse Guess).

Doing some simple measurements with both my digital and dial calipers I determined the stroke, cylinder diameter, and squish. To get the stroke I secured the head on the case with the piston installed. Turned the piston to BDC and placed a 5/16" washer over the plug hole and set the end of my calipers on the hole. I ran the depth rod of the calipers down the hole until it touched the top of the piston. The case back was still off so it was easy to visually verify BDC and TDC. I did the same with TDC and subtracted the measurements to get the stroke.
The squish was almost as easy. I took the head off and removed the piston from the case. After liberally applying synthetic 2-stroke lube, again, I gently pushed the piston all the way up into the cylinder. I took this depth measurement and subtracted it from the TDC measurement.

Cylinder dia.: 1.378", 35.0mm
Stroke: 1.129", 28.677mm
Squish: 0.038", 0.965mm

Now with some simple math, (Dia/2)^2 x Pie x Stroke, the displacement of the piston turned out to be:

Disp: 27590.5mm^3 or 27.6cc

That is about right for a 28cc engine.

The squish on the other hand should be about half of what it is. It wasn't made to be a HiPo engine so it is what I expected.

Given that the squish is large for this displacement, the exhaust appears to be restricted, and the intake is restricted, I understand better why there was so much carbon build-up inside the head. Oh yeah, the muffler. After removing it I noticed the outlet hole is quite small. Expansion muffler or no with an outlet that small (about the size of the outlet on a standard .46 glow muffler) I would also guess the engine couldn't rid itself of all the spent fuel charge. Another mod in the works...

Maybe tomorrow I'll check the combustion chamber volume and figure out what the CR (compression ratio) is. That will tell me how much I need to convince the machine shop next door to my workplace to take off the bottom of the head.

Cheers!

av8tor1977

My Feedback: (6)

I recommend that you don't take the "nub" out of the intake port. It is there to keep the ring from bulging, and catching the port top when the piston is at BDC.

You will also probably find the exhaust timing quite low. In many of the Ryobis, it is only around 120 degrees. 150 degrees seems to work nicely.

Machine things so that you get a deck height, or piston to head clearance of no less than .020". Do your machining before you start your port timing work, as lowering the cylinder changes the port timings.

The biggest gains you will see will come from raising compression, adjusting exhaust port timing, upgrading to an 11mm venturi carb, and using a free flowing muffler. I wouldn't get too excited about increasing port volumes, nor go too high on the port timings, as you don't want a Ryobi to run more than about 7500 rpms static due to the relatively weak stock rod.

Good luck,
AV8TOR

rangerfredbob

My Feedback: (13)

av8tor, this is a Mac 32, built a little beefier than a Ryobi, but agree with everything else...

av8tor1977

My Feedback: (6)

Whoops, my mistake. But yeah, everything still pretty much applies. You can run more rpms safely with the Mac though.

(Gotta quit answering these things before I get a couple of sips of coffee down in the morning!)

AV8TOR

Jazzy

Thanks for the tips!
I kinda figured the restriction in the intake port was there to keep the ring concentric. I'll leave that nub alone.

The exhaust port has a similar center "column" in it. It was obvious why that one was there due to the width of the port. The overall port area seems small though.

I'll have to make a timing wheel at work.

Thanks,
Jeff

av8tor1977

My Feedback: (6)

AV8TOR

w8ye

My Feedback: (16)

If you pinned the ring you could probably take out those port dividers? Pin it so the gap is not where a port is.