The complete radio control model airplane FAQ.
Converting a 32cc McCulloch weed wacker for model airplane use. This string trimmer Mac 32cc engine is a great conversion engine, with good power and moderate weight.
Here are some tips from Ron:
I mill off the rear of the block until the plastic back plate fits up very close to the bottom of the rod and crankpin. I only leave about .010 clearance there. Than I mill off .030 from the top of the block to move the cly. down. Now this is hard to explain, but I'll try. Look at the side of the piston, the area above the ring, from the top of the ring to the top of the piston is approx 0.140" I chuck up the skirt of the piston and turn it and machine off 1/2 or .070 of that material, and of course taper it back on top of the piston. I hope you understand.
Next I remove all the seals from the bearings except the front seal on the front bearing. I make up my own ignition advance coupled to the throttle for the C&H and set it for 30 degrees of total advance at full throttle. I use a Walbro "WT9" carb that I get from C&H. The carb is mounted with a 1 and 1/4 inch long, tapered, velocity stack that I make myself. I don't think the stack helps performance much, but it sure looks cool.
The biggest problem with this motor is the muffler and the way it is mounted. I use the stock mufflet and grind one side of the crimp off so I can open it. I gut it and braze the stock outlet hole shut and then braze in two 3/8 in copper stacks. I make these stacks 2 to 3 in. long, depending on the installation. Then the muffler is reinstalled with the stock springs and strapes. A real sorry setup, I wish I could improve on this. I have found that #10 x 24 SAE. bolts will screw into the metric holes on this case, I do not use any motormount instead I use 10-24 allthread into the holes on the rear of the case. I cut them long enough so that I can put a flat washer and then a plain nut on to hold the bachplate and than drill holes in the firewall to match the bolt pattern on the blackplate and then just slide the allthread "studs" thru the firewall and use flat washers and lock nuts on the inside of the firewall. Be sure to use plenty of locktight when installing the studs into the engine case. The 10x24 is not a prefect fit into these metric holes, but it is very close. It seems to hold OK with the locktight.
these mods also work well on the Homey, as well, except I do not use a
throttle coupled advance on them. Just set it to about 27 to 30 degress (static)
and let'er rip, if you are using a C&H ignition. Also for the Homie
guys there is a different carb mount block that is available from Homelite
dealers the will realign the carb so that it is a straight shot for the throttle
linkage, and therefore eliminate the need for any linkage. I forgot the part
number right now but I got it from the boat guys. It works real slick. From the
sound of things it looks like if I want a 2.7 I will just get it from BME. 4
4 4 4 stroken ron
All of the information on
this web site is copyright 2000 by rcfaq.com. All rights reserved.
Revised: October 05, 2001 .