79cc Predator Build Progress

GoldenMotor.com
Jan 21, 2015
610
25
18
Portland, Oregon
So I'm replacing the 66cc chinagirl on my bike with a 79cc predator because I got tired of the chinagirl breaking down every 3 weeks. Some people said they want to follow this build so I'm making a new thread in the proper location on it. Here's the pics so far, as you can see, the engine barely fits into the frame, I won $10 in a bet with my dad on that, only won by 3mm of clearance though lol. http://imgur.com/a/MguAc

So far, I have taken apart the engine and removed the governor, and have begun fabricating a really over-engineered mounting system for the bike. I have ordered the parts I need which should be arriving sometime next week, but for now the to do list includes finishing the mounting system and figuring out some sort of throttle linkage, as well as drilling some holes in the air filter cover.
 

curtisfox

Well-Known Member
Dec 29, 2008
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minesota
Looks Kool got to watch how you stuff it in, LOL Where there is a will there is a way.....................Curt
 
Jan 21, 2015
610
25
18
Portland, Oregon
Update! Tonight I finished over-engineering my mounting system. I think it will work. On another note, I have too many projects going right now (in the last picture you can see the bike engine swap, a refrigerator repair, a welder I'm learning how to use, a bunch of seed for growing a meadow in the backyard, and the beginnings of a gigantic transforming scythe-gun from my favorite anime, yea, way too many projects), I don't have enough tools (jk), and I don't have enough space for the tools I already have :p

Here are the pics: http://imgur.com/a/pWtEP
 
Jan 21, 2015
610
25
18
Portland, Oregon
Update! Finalized mounting system, adding lock washers and using up half a bottle of threadlocker, that motor's not going anywhere. Also designed and built wide pedals, those are going to take some getting used to, but then, I don't plan to be doing much pedaling once this build is done! Also fired up the engine for the first time, forgot to tighten the bolts on the side cover and had to shut it off really quick and frantically tighten the bolts while oil dribbled all over the floor of my garage. Then I fired it up again and my god it's loud (the pieces I ordered to build the exhaust pipe have not arrived yet and the stock pipe doesn't fit, so there isn't one) Here are some more pics: http://imgur.com/a/WYZ8T
 
Jan 21, 2015
610
25
18
Portland, Oregon
Did the throttle linkage this evening, engine doesn't idle properly right now but that should be an easy fix. Still need to find a suitable spring, tested it with rubber bands instead lol.
 
Jan 21, 2015
610
25
18
Portland, Oregon
Today my jackshaft (at least, I think that's the right word for it) and exhaust pipe arrived in the mail. I attached the jackshaft, found a suitable spring for my throttle linkage, and rolled my own chain tensioner using the roller from the stock one, 9 washers, and leftover metal bits from my motor mount and wide pedals. Going to have to do some slice and dice on the exhaust pipe, right now it doesn't quite fit in the frame, time to break out the acetylene torch, might be able to use it to heat up the metal and then bend it while it's hot, maybe avoid actually cutting anything. Centrifugal clutch is supposed to arrive tomorrow, assuming I've gotten the exhaust system figured out by the time it gets here, all I'll have left to do is slap it on, fire up the engine, take it for a spin, and fix the few dozen inevitable problems which cause me to crash horribly after my bike catches on fire and explodes 100 feet into the test ride and causes me to break my face. :p

Here are some more pics, sorry the first one is a little blurry: http://imgur.com/a/mQQlW
 
Jan 21, 2015
610
25
18
Portland, Oregon
Gear ratio is exactly 8:1 After test riding the bike today, I definitely need a larger rear sprocket, but how much larger? EDIT: I forgot to mention, even though the bike currently has crap acceleration and hill climbing, once it gets going it can really get going. I clocked it on one of those radar signs in my neighborhood at over 45mph, not sure exactly how fast it went, but after I got above 45 the sign switched from reading my speed to telling me to "SLOW DOWN" in huge flashing red letters.
 
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Jan 21, 2015
610
25
18
Portland, Oregon
On second thought, putting a bigger rear sprocket in this frame might be a problem, unless I can figure out a way to put it closer to the wheel so the chain doesn't hit the frame... time to put on my thinking cap...
 

MEASURE TWICE

Well-Known Member
Jul 13, 2010
2,757
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CA
I bought the 19 tooth Martin sprocket (about 3 1/2 inch dia) 2 years ago for my 5/8 Azuza Jack Shaft, as I needed a real big gear down. I found the Martin sprocket on Amazon. Azuza Engineering did not have the larger one so I found the sprocket on Amazon. Azuza includes a few sprockets with the jackshaft kit they sell and they have been handy. If I really want to gear down I use a 9 tooth that came with the jackshaft kit and use the short chain made up to connect to the 19 tooth on the twin jackshaft set up I use. I've mixed and matched other sizes sprockets that equate over all 20:1 ratio. With overall ratio that is made the 26 inch rear wheel propel the bike at 15 to 20 mph on the Briggs 3hp 4 stroke. This given my sheave and pulley working with the twin jack shaft I like that for riding in the woods.

Azuza Eng

http://www.azusaeng.com/catalog/catalog.html

Azuza does not sell direct, but there home page can find nearest distributor. I found it cheapest when I went to a local go-kart shop and bought there.

The 12 tooth

http://www.amazon.com/Stens-260-083...=1446867876&sr=1-12&keywords=5/8+#35+SPROCKET

28 tooth

http://www.amazon.com/gp/product/B0045JJWE2?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00
 
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Tony01

Well-Known Member
Nov 28, 2012
1,783
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sf bay area
Gear ratio is exactly 8:1 After test riding the bike today, I definitely need a larger rear sprocket, but how much larger? EDIT: I forgot to mention, even though the bike currently has crap acceleration and hill climbing, once it gets going it can really get going. I clocked it on one of those radar signs in my neighborhood at over 45mph, not sure exactly how fast it went, but after I got above 45 the sign switched from reading my speed to telling me to "SLOW DOWN" in huge flashing red letters.
Go to a 9 to 9.5 by changing the big sprocket on your JS. I rolled with 8:1 for a long time and tolerated the bad acceleration for the enjoyment of bike response and top speed above 20mph. Currently running 4hp on a top gear of 7.6.

You can smoothen your clutch engagement by putting heavier weights along with a higher stall spring.

That bike looks fast; I'd be scared going 45 on what is basically a cx bike with those drop bars!! xct2
 
Jan 21, 2015
610
25
18
Portland, Oregon
I rode that bike down a 2000 foot tall mountain on a narrow, twisting, pothole-filled mountain road in the pitch dark at 5:00 AM at 45mph every day for a year before I put the motor on it, so I'm perfectly fine with going really fast.

Will 9.5:1 really be enough to climb hills well? BC right now with 8:1 it is pathetically underpowered.

BTW it's not an ordinary cyclocross bike, I built it myself from the frame up, and then when I added the motor, rebuilt it with the durability to withstand the motor. It's got a super strong layered carbon-aluminum frame with wheels I made myself with 10 gauge spokes and kevlar tires, and the front fork is made of pure carbon fiber which acts as a shock absorber, and a bunch of other really cool stuff that make it a lot better to motorize than it looks... I love it.
 
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MEASURE TWICE

Well-Known Member
Jul 13, 2010
2,757
1,252
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CA
I now understand the sprocket you used that was 36 teeth was the one direct attached to the free wheel cluster?

This a less expensive sprocket than the keyed 5/8 jackshaft type. For lower gearing the engine pulley or sprocket is small connects with to larger pulley or sprocket. Then again with small gear connects with chain to largest gear at the rear wheel so it can fit for what ratio you want.

Can you tell there is more torque? A lower ratio.

MT
 

Tony01

Well-Known Member
Nov 28, 2012
1,783
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113
sf bay area
With single speed and a weak engine it's a trade off between hill climbing and top speed. My low gear is a 12:1 and it will climb steep hills but it is turning approx 4000rpm at 25mph when it shifts.

Also my recommendations are based on a 26" OD wheel; if yours are bigger then adjust. If you did not have to climb hills regularly and were ok with 35mph top 9.5 or 10:1 might be ok. I like to use the torque of the motor to move, not high rpms. 7.6 on a 26" wheel is 100rpm per mph.