Sixty-Seven Fifty, 79cc Predator/Q-matic Build

[MotorbikeMike]

Hi all, it has been a long time since my last post, and quite a while since I have built anything other than Whizzers, both Vintage, and New Edition. It had come to my attention that a 79cc Predator could be made to run at a similar level, and with similar results as a 1946 Whizzer H-motor kit bike. I have a 1946 H, but mine is not a good example of what to expect, since it has a High Lift Cam, High compression Head, big bore, big manifold and pipe, and big valves with appropriate porting to make it run (did I mention a different carb?). So, what are the expectations? No pedal needed at takeoff, better that 30 mph with heavy rider on a 26 inch bicycle. No sweat, I figure!
The Start:
67.50 spent on an Electra steel framed stretched Cruiser at Flea Market
whatever the 79cc Predator cost me
a new Q-matic, specially drilled for HF Engine 2.5 inch primary pulley, AX belt
a custom sprocket adapter made for us, allowing me to use cheap steel sprocket
44, 48, 50, 56 and 60 tooth sprockets in my stuff
Pedal extenders
Flex pipe exhaust
custom tak (repurposed)
a cheap mountain bike suspension fork
V-brakes
Steel wheels spoked in .105 Coaster rear, plain front will use V-brakes front and rear
I will make a motor mount using some factory parts and some stuff, as the frame is stretched, it will not be able to use a stock motor mount. I made one already, and have an easy way to show.

My goal here is to build a good, strong performer that can be done at home without the advantage of a machine shop to build custom parts. I like all the fancy custom made parts, but I know that they are not within the average guy's budget or abilities

MotorbikeMike

 

[curtisfox]

Sounds good , should be a nice build.........Curt

 

[MotorbikeMike]

Thanks Curt! The next step will be to put that seven speed on a repair stand, and convert it to front suspension, single speed

Mike

 

[zean]

Hello Mike, The Q-Matic with the 2.5 inch primary pulley, could you please tell me the pros and cons of using the more expensive adjustable pulley on the transmission? Thank you.

 

[MotorbikeMike]

Hi Zean, thanks for asking! I have the stock 2.5 iron pulley that is available, BUT I also have a new in box adjustable that I bought a few years ago and never used. So my plan is a digital speedo so that I can "prove" what it will (and won't) do. I weigh more than I will now that the Doctors have got a hold on me (about 340) so we will have a good test of stock ratio, with heavy rider. After I get those tests done, and am comfortable and used to the way it rides, I will begin a few little mods to see the effects.
In terms of your question. The adjustable pulley starts at the stock 2.5 inches. Now as you screw the outer half of the pulley in closer to the fixed part (pulley body that is set screwed to the shaft)the inside of the pulley gets smaller, which raises the overall gear ratio meaning, perhaps slower takeoff, BUT higher top speed. Top speed is ultimately determined by the engine not having enough torque to spin the wheels any faster. The Torque limitation of course being negatively affected by more weight, headwinds, or by going uphill.
This is going to be fun!

Mike

 

[Tony01]

Yep, qmatic is a good setup I think. Any setup that puts the clutch on the jackshaft is a smart idea. Keeps the thickness of the bike at a minimum at the motor. And in the case of the qmatic, allows them to run a stock mt with the stock weights and springs for more slip before lockup. Makes it easier to pedal with the motor off. I would run a bigger engine though, I think a 140 or 160cc ohv is about the perfect size for a bicycle.