Flathead Honda build- it’s happening

Yellow Perch

Member
Joined
Jul 7, 2017
Messages
134
Ok guys I’m back after a year lol, I’ve got the Honda, and I’m ready to start my build. I’ve gained a lot of mechanical skills in the last year so I think I’m ready to tackle it. I’m starting out with a steel frame bike, and I’m making my own motor mount with angle iron and sheet metal. The Honda g100 runs good but needs an oil slinger, probably will have to replace the connecting rod assembly as a whole. I’m going to fabricate a throttle linkage, but I am not sure what to do for a transmission, the bike I’m using has too small of a rear hub so I’ll probably have to go with the rag joint setup for a sprocket...
going to be a fun project
 


5-7HEAVEN

Well-Known Member
Joined
Jun 22, 2007
Messages
1,154
Welcome back, Yellow Perch.
Lotta builders use Torque Convertor(TC) system.
Mount your engine first, then see if you have room.
Remember that the TC multiplies your rear wheel gearing about 2.7 times, then reverts to 1:1 or less.
So, if you use a 12tooth(12t) jackshaft gear and a 48tooth(48t) rear wheel combination, that's (48/12) x 2.7 = 10.8:1 at a standstill.
However, when the TC upshifts, you've only got 4:1 gearing, which is too high for your G100 engine.
So, you'd have to go with TC, plus a large 72t wheel sprocket and a 9t-12t jackshaft sprocket,
which gives you 8:1, 7.2:1 or 6:1 final drive, when the TC upshifts..
Orrr, you could use all sprockets with a jackshaft:
12t sprocket on the clutch chained to a 24t at the jackshaft,
then 12t jackshaft to a 48t wheel sprocket.
That gives you 24t/12t = 2:1; 48t/12t = 4:1.....
2 x 4 = 8:1.....
If that's too stiff, replace the inboard 12t with a 14t sprocket.
2 x (48t/14t) = 6.86:1.....
or an inboard 16t to get 2 x (48/16) = 6:1 gearing.

I'm building a 212cc bike. However, I'm using pulleys, belts, a jackshaft and a 7-speed bike gears.
My 7th gear will be 6.34:1.
 
Last edited:

Yellow Perch

Member
Joined
Jul 7, 2017
Messages
134
Welcome back, Yellow Perch.
Lotta builders use Torque Convertor(TC) system.
Mount your engine first, then see if you have room.
Remember that the TC multiplies your rear wheel gearing about 2.7 times, then reverts to 1:1 or less.
So, if you use a 12tooth(12t) jackshaft gear and a 48tooth(48t) rear wheel combination, that's (48/12) x 2.7 = 10.8:1 at a standstill.
However, when the TC upshifts, you've only got 4:1 gearing, which is too high for your G100 engine.
So, you'd have to go with TC, plus a large 72t wheel sprocket and a 9t-12t jackshaft sprocket,
which gives you 8:1, 7.2:1 or 6:1 final drive, when the TC upshifts..
Orrr, you could use all sprockets with a jackshaft:
12t sprocket on the clutch chained to a 24t at the jackshaft,
then 12t jackshaft to a 48t wheel sprocket.
That gives you 24t/12t = 2:1; 48t/12t = 4:1.....
2 x 4 = 8:1.....
If that's too stiff, replace the inboard 12t with a 14t sprocket.
2 x (48t/14t) = 6.86:1.....
or an inboard 16t to get 2 x (48/16) = 6:1 gearing.

I'm building a 212cc bike. However, I'm using pulleys, belts, a jackshaft and a 7-speed bike gears.
My 7th gear will be 6.34:1.
I like the CVT idea, but since the motor is only like 2hp I’m afraid of how much power it would rob. Another option I was considering is somehow making a jack shaft to be able to use the rear gears but then I’d need a free wheel setup and that would be expensive. What would be the best option for a centrifugal clutch/ jack shaft single speed setup?

EDIT: I am planning on doing a couple mods like fabricating an adapter to use a pod style filter, as well as a different exhaust setup. I have literally 5 carbs for this motor so I can do some experimenting with drilling out jets to get more fuel. I was also planning on removing the governor, not sure how many rpms are safe to run a flathead at but I figure the valves will float before it blows
 
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