First ride almost complete success.

Finally finished putting the 28cc McCulloch on the old Schwinn Continental. After pulling a Mac 2816 weed wacker out of someone's trash a few years ago, I left it in the corner of the garage until about a month ago. I got it running, but the compression seemed like it might be a little weak, so I took it all apart (which I would probably have done anyway).

Some of the things that got done on it:

Cleaned all deposits from piston/cylinder (was hardly any).
Got a new ring from Frank Bowman. TIGHT fit - virtually no gap.
Trimmed flywheel fins - removed starter pawls.
Flipped carb over so the choke and mixture screws are where you can get at them. But this made more mods necessary.
Fabbed an adjustable choke lever.
Also made a new throttle lever - the tiny metal "tab" on the carb throttle shaft.
Bend in fuel line would have been too tight, so I made a plastic elbow.
New fuel lines and filter.
Aluminum spindle that fits over shaft and is tapped for threads by flywheel. Spindle is 5/8 OD for drive rollers and outboard bearing.
Tapped square hole in end of shaft for fine thread, hi-grade bolt. This prevents the spindle from screwing off when bump starting.
Made new outboard bearing holder/roller guard.
Steel plate for mounting.
Two tension spring holders. Each one pivots and holds two springs. When I had one tight spring on each side, I'd go through a few springs a year. Using two springs that aren't stretched as much solved that problem.
Cut all new gaskets. I even used real gasket paper this time. When I rebuilt my Homelite, I used the cardboard from a case of Old Milwaukee. You could see the Old Mil label on the intake gasket. Those gaskets worked fine (but I'd still recommend the real stuff).
Made throttle cable so it's easy to adjust, and can be taken on and off without having to re-adjust.

Etc... You get the idea.

It's a bump-start friction drive. So I pedalled up to about 10mph, dropped the motor, and she fired up. Had to fiddle with the choke for a bit, of course. I also was adjusting the high mixture screw. I was able to do this while riding because I have a brass tab soldered in the screw slot. That is, It WAS soldered in the slot. Popped off and fell out of my fingers. By then I had it adjusted pretty good anyways. So I was taking it kinda easy to break in the new ring and listen for anything amiss.

Well, she runs great! It's already faster than my old Homey, and I'm sure it will get a little stronger as it breaks in. I know it was getting around 30mph. Someday I'll have to check it with GPS (I don't have).

I was going to take more pictures during the build, but that didn't happen. I'll shoot and post some of the finished product. Someday. I'm tired.

Thanks for the good words, guys!

Here's a comparison of the old Homelite and the new Mac.

Close ups of motor. Notice the carb screw extension. I hate having to stop for adjustments. This one ain't falling off! But I may trim it down a bit.

I did take a few pics during the build...

Here's the motor bolted to the mounting plate. (1/8 inch hardened steel)
You can see one of the pivoting double-spring holders.

In the close-up of the spindle, you see the flywheel was trimmed down. The starter pawls can still be screwed back in if I ever need to run it on the weed wacker for tuning. Probably will never do that and I'll end up throwing away the trimmer head. The motor shaft had about a half inch of threads past the flywheel nut, so my aluminum spindle is treaded to screw on tight. I threaded the hole in the end of the motor shaft and ground down the washer to be a hair smaller than the spindle diameter. That spindle ain't goin' nowhere.

Next pic shows how it mounts on the bike. A Billet aluminum block clamps around seat stays. Block has hole and steel inserts. 5 1/2 inch carriage bolt is bolted on mounting plate and passes through block to pivot. Not shown is outboard bearing holder installed.

In my hand is the bearing holder with tension springs attached.

BTW, cannonball2, roller is 1 1/4. Homemade from hockey puck and steel spacer tapped for two set screws.

This thing was flying today! I actually eased her back a few times because of a slight speed wobble. Now I have to go over my wheels/tires and get everything nice and true. I can't believe 28cc goes this fast. I don't exaggerate (proud member of the 20mph club). Mind you, I'm not complaining about surplus speed.

And here's the other side with bearing holder installed. I have a stainless guard over the roller.

Thanks for the suggestions, guys. I don't think cooling will be a problem. Here's why:

There was a minor amount of fin trimming on the front of the "frywheel" (pun). Half of the fins are on the back of the Mac's flywheel. My Homelite has no fins on back and I trimmed it much more, nearly finless, with no problems.

My steel mounting plate doesn't cover up much of the original aluminum mounting plate (shroud). Of course, when you take the motor out of the trimmer housing, the flywheel doesn't force air past the cylinder like it might have. But the trimmer also had this black safety cover [PHOTO].

I don't think there was more air blowing across that covered motor while standing still cutting weeds than while cruising all naked at +20mph (naked motor, not rider). When I get to the edge of town, I shut off motor and pedal around town, motor cooling nicely. Every time I get home, I blow everything down with the air compressor. FD's get dusty. Motor is cool enough to touch, but I wouldn't press my cheek on it; any of my cheeks.

It is something to consider. Now you've got me thinking. I might do a muffler mod that would be more like a pipe and expose the jug a bit more. I'm sure there will be more tinkering to come and I'll keep you posted.

Oh, btw rusty. This motor is likely going on my next bike, soon to come. It will be more laid back, without the "curly" road bike handle bars. But I don't see an mtb. Skinny tires for me. More friction-free. Agree?

Glad you like it, pocdragon!

Thanks for the tuned pipe info:

From tuned pipe link:

HPI Baja Aluminum Tuned Pipe Part Number:
Part #: 86690 - HPI Aluminum Tuned Pipe Set
MSRP: $139.00

Click to expand...

Yikes! 140 bucks - Increase net horsepower. Decrease net worth. Decent looking pipe, though.

Yeah, these skinny tire bikes aren't for everyone. I use them because I happen to have collected a bunch over the years. I prefer them for pedalling, even though you have to treat them like they are made of breadsticks. Mountain bikes are better for motorizing in many ways. But I bet a lot of people who are disappointed with the power of a wacker motor on a mtb would be surprised how well they push a road bike.

Throttle cable would be shorter on top tube. But the carb needs the cable to pull from the bottom, so it would have a funky loop. Also, before I painted the frame 5 years ago, I added homemade cable bosses under the down tube. Both derailleurs and the throttle run along there nice and neat, and there is even a spare empty boss.

Whistle Tip, glad to have another diy-wacker member.

I've never heard from anyone successfully using the existing clutch from a weed wacker. Those tiny cent clutches must burn up quick. It would be nice to have a cable operated throwout clutch just for starting and idling.

In the page 1 pics of this thread you can see how the mac is mounted. Motor mounted on steel plate. Big carriage bolt through plate (secured firmly with nut). Bolt goes through mounting block on frame's seat stays. So when roller is lifted off tire, plate rotates (vice versa).

This system worked well with my homelite motor. Not as well with the Mac. I'm in the process of changing everything now. I'm doing a belt-driven, larger-roller, friction-drive, using-too-many-hyphens, setup.

The way the mac is tilted put too much motor weight over the roller. At top speed it wants to get into a bouncy harmonic. If my wheel and tire were closer to perfectly concentric, it would probably be OK. And I could redo the plate with less motor tilt and/or the pivot point closer to the motor shaft. But the new design should be better.

Most hardware stores (Lowes for sure) sell "steel spacers". Basically a short piece of pipe. I drill and tap the spacer for a set screw. Using hole saws, I cut a rubber donut out of a hockey puck, epoxy the donut on the spacer. Slide spacer/roller onto shaft and tighten set screw.

Thanks for the tip, cannonball.

I was thinking exactly the same thing and tried increasing spring tension. The homey worked good with just under 30 lbs roller downforce on tire. I used a fish scale to measure how much pull to lift roller. Those springs might not be as light as they look (and there are 4 of them). I had put on stiffer springs and maxed out the fish scale (>50 lbs) without solving problem. The roller never actually hopped off, but you could feel a sort of rhythmic pulsing.

I really think it's just bad geometry with my design. I tilted the motor for a better fuel tank angle and clearance above spark plug. This put too much weight (inertia) over roller. If I wanted to keep this design for the mac, I would take off the fuel tank and position motor straight up or even tilted forward.

But I'm now working on the belt drive idea. I was looking for the flatter, timing/cogged belt style. Industrial supply places around here are too expensive and poorly stocked. I need to find an online source - any suggestions are appreciated. So I'm using v-belts. Pulleys are 1 3/4" drive and 3 1/2" driven. So roller diameter should end up around 2 1/4". I'll start with 2 1/2" - it's easy to sand them down smaller.

I've been wanting to do a belt/FD for a long time. Looking back in my old posts, me and Neat Times were talking about Zipcycles three years ago. I'm getting old.

Thanks Dave. Proud to be on the video.

CB2, glad to hear someone is using v-belts successfully. Would have gone with 3/8 if I could find them around here. This kinda stuff is getting harder to find in the hardware stores. I think people rarely fix their own appliances anymore, and if they do, these new gadgets probably take their own "special" parts, not a pulley or bearing from Ace hardware. You can find a million things online, but I don't want to add $9 shipping for a $9 belt (and wait 3 days).

So the pulleys I could finally find take 1/2" belt. I hope that fat belt bending around the small (1 3/4) drive pulley doesn't make more heat energy than kinetic energy.

Oh, and the quality of some stuff is sad. My 3 1/2" pulley from Chicago Die Casting makes me ashamed of the "Made in USA" label. Thin aluminum with rough casting flash and not very round. Roundness is a quality you might expect in a pulley, don't ya think? Do a google search on them and see how crappy they look. I'm glad it will only be spinning at half motor speed.

The spring loaded idler is a good idea. We'll see how the belt tensioning and pulley changes go. I expect after everything is worked out, I'll only make adjustments for belt and roller wear occasionally.