8 speed 79cc revival

xXNightRiderXx

Active Member
Jan 12, 2017
416
63
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Boise
Hey guys, I know it's been a while since I posted here. Ive been working on that mongoose with that 79cc predator and shift kit a lot lately, and just wanted to share some updates:

I've removed the transmission plate that comes with the kit to reduce friction and noise. Ive also moved the 23t sprocket to the jackshaft, and am replacing the current cassette with one that has a 42t-11t range. The stages are now as follows:
10-23
9-44
36-38\11 soon to be 36-42\11

I have also moved the exhaust pipe through the center of the rack to clean up the look some. It certainly looks better and more thought out.

I have gotten a new carb with a slide, but need smaller jets for it. The ones it came with either seem to be too large, or the same size (65-85, increments of 5, same amount of exhaust smoke, other than the stock jet). Regardless, throttle response has improved.

I finally sanded down the cylinder head. There turned out to be a gouge near one of the pins, so I sanded with 1000 grit paper and cutting oil until that was gone. Compression has definitely increased. I can feel it in the starter rope. Along with the head sanding, I replaced the valve springs with 18lb springs. I will soon replace the pushrods if I can find em, just to be safe. The lifters shouldn't be an issue.

I will be fabricating a fairing for it. On this fairing will be a speedometer, a tachometer, and, of course, all the switches I don't need on the handlebars, such as parade lights, headlights, and ignition switch. The kill switch, high/low beam switch, horn button, and turn signal switch will be on the bars at the grips.

I will also be building new wheels for it. I am going to upgrade the tires to maxxis hookworms (so I can have a chicken strip. Like seriously, that tread is almost halfway down the sidewall), and use a 36 spoke double wall mtb rim, and of course, mtb hubs. What can I say, mountain bike wheels are built for abuse.

Oh, I almost forgot about electrical. The last post I made, I was going to put an alternator on it. I've since ditched that plan. Instead, I'm just going to attach a quick connector to the battery for a trickle charger. That keeps the build cleaner, and lets me plug it in wherever I find an outdoor outlet, or can run an extension cord.

41312217-68c7-4c9b-86de-9f0888c636d7.jpg
20190822_160221.jpg
20190822_160233.jpg


Don't worry about the paint damage. That's getting replaced. I'm having the frame chrome plated so I can add a transparent emerald finish for that anodized look. The headset, tank, and fenders will get a green metal flake finish. The box is getting retorched, and sealed with high gloss spar varnish, and the seam will be sealed using black silicone caulking. Oh, and that little bottle opener on the box? That's getting a green LED put behind it.

All in all, it sounds like a small motorcycle now, when it just sounded like a lawn mower before. It is also beginning to look a lot cleaner. I just have to reroute the cables and heat shrink them together.
 
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xXNightRiderXx

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Jan 12, 2017
416
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Just got my 30t in yesterday. Haven't mounted it yet, but I'm going to. The thing is a BEAST! I swear, this one sprocket weighs as much as one and a half centrifugal clutches! Time to go get my chain resized again to fit the additional 7 teeth (I'm going to need quite a few links, haha).
 

waynesdata

Active Member
Jul 10, 2017
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Did removing the transmission plate give you anymore speed or acceleration? Is that the CNC short head? What is the box for battery for lights horn etc?
 

xXNightRiderXx

Active Member
Jan 12, 2017
416
63
28
Boise
Did removing the transmission plate give you anymore speed or acceleration? Is that the CNC short head? What is the box for battery for lights horn etc?
Removing the plate reduced chain noise significantly, however, I need to compensate for the loss of gear reduction with a larger jackshaft sprocket, which is why I got the 30t. As soon as I can get the chains installed, I can do a 0-30mph time test and a top speed test.

Yes, it is the short head. I need to put a vent in it though. I think it's causing some emissions issues cause I'm getting some cold vapors coming out of the air filter.

The box is essentially a fuse box with a battery compartment. My entire electrical system stems from that box. I have my tail light bolted onto it because it was convenient, and gave the bike a high spot to mount it, increasing visibility.
 

waynesdata

Active Member
Jul 10, 2017
254
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42
I am still trying to design a electrical system. If you post your work other users can build on your design. Been looking at youtube videos on how to build battery packs.
 

waynesdata

Active Member
Jul 10, 2017
254
107
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42
A few users have posted that some of the jackshaft plates have crap bearings in them. I will be spending the extra money to get a GTC plate. I wonder if the bearing in your place can be replaced? Will you be adding any kind of chain guards for extra safety?
 

xXNightRiderXx

Active Member
Jan 12, 2017
416
63
28
Boise
A few users have posted that some of the jackshaft plates have crap bearings in them. I will be spending the extra money to get a GTC plate. I wonder if the bearing in your place can be replaced? Will you be adding any kind of chain guards for extra safety?
Yes, the moment I get a metal brake, I will be fabricating some chain guards. As it is, I already have to cut, grind, and drill an 8" steel disc to separate my right side chains.

Yes, the stock bearings are crap. My 79 shattered the shell of the left bearing, forcing me to get a new set of bearings. I have to get a new set now for high power applications.
 

xXNightRiderXx

Active Member
Jan 12, 2017
416
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Alrighty then! I got my 30t sprocket installed, got a new chain to fit it (had to go #40), and now she rolls like a champ! I think I could further optimize with a 32t, which would also take the slack out of the clutch chain. I've almost got it pullin wheelies. Next is the wide ratio cassette (40-11t) and derailleur hanger extender.

I still need to do some ACSheriff approved tests, as my rear brake isn't put putting as much power into the ground as it should be, and idk why. My front works great. I think hitting it with some brake cleaner and sanding down the pads will help. Hopefully the pads aren't contaminated. If they are, I won't be riding for a while, cause contaminated pads means destroyed pads. I can't afford new ones right now. I think I'll have a caliper/rotor guard 3d printed soon so chain oil doesn't get flung onto them until I get some chainguards. I will be making these fully functional with a shallow oil bath. It will help keep the chain clean while also keeping it lubed. The guards will be 3/4 enclosure to prevent spray from flung oil, but allow easy access to replace the oil with a hinge, which will also be kept lubricated by the remaining flung oil.

Aside from the malfunctioning rear brake, everything is working wonderfully. Chains are aligned just right, so there's no more binding on the right side, and shifts are smooth. I'm getting what feels like 35mph out of it now, but I need a proper gps speedometer to truly know.

I still need to sand down the cylinder head some more. The sanding I did feels more like a honing than anything functional. No noticeable power increase. I will be using 400g with cutting oil and finishing with 1000g with cutting oil.

Edit: Pads are contaminated. Gotta get a new set.
 
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xXNightRiderXx

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So, I just realized something when talking to my dad yesterday. My valve lash is off. When I did that last bit of sanding, I tightened the valve lash via material removal, so that may, in concert with the film left by the cutting oil, be contributing to my performance problems. I'm going to do a full disassembly and clean the parts, remove the governor, ensure piston ring alignment, set my valve lash, and put every part that needs it into an oil bath so they aren't dry when I reassemble the engine.

And when I say "clean the parts," I mean taking an old toothbrush and some degreaser and scrubbing down every single part in that engine. This way, I know there will be zero grit causing wear in my engine, and I can be sure that my oil won't collect that grit and send it everywhere.

Anyone know the bolt torque values for the conrod, case, and flywheel?
 

xXNightRiderXx

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Jan 12, 2017
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I've decided that I NEED MORE TUNING! Gonna swap the Piece of Shallot Runtong for a pbhg 19mm, so that I not only have a mixture adjust screw, I can also supertune for any mods I do in the future. Not to mention, I would FINALLY be able to use my 35mm ram air filter (looks like a high bypass turbofan with the cone in the middle). I haven't quite figured out how I'm gonna mount the carb yet. Maybe I'll slip it on over the intake (19mm od) and secure it with 19mm ID silicone hose and hose clamps (stretching it over the carb). I feel like that would also eliminate any air leaks. Any thoughts from the peanut gallery?
 

xXNightRiderXx

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Jan 12, 2017
416
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Ok, my headlight finally came in, and I got it mounted with a rough fit. Once I get another pair of screws and nuts, I can finish mounting it and fine tune the location so it doesn't blind anyone.

JPEG_20200517_204054.jpg
JPEG_20200517_204145.jpg


It also doesnt look quite as silly as I thought a 7 inch headlight would look on a gas bicycle, lol.

I'm thinking of maybe drilling a bunch of small holes into my cooling fins, with the aim of increasing surface area and air flow. I might also try one of those nt carbs, see what the needle lets me do compared to this flat slide thing.

One more thing: that front end is getting a stanchion upgrade, and a strong fender.
 

LR Jerry

Well-Known Member
Dec 19, 2011
1,211
117
63
58
Rockwood, TN
Hey guys, I know it's been a while since I posted here. Ive been working on that mongoose with that 79cc predator and shift kit a lot lately, and just wanted to share some updates:

I've removed the transmission plate that comes with the kit to reduce friction and noise. Ive also moved the 23t sprocket to the jackshaft, and am replacing the current cassette with one that has a 42t-11t range. The stages are now as follows:
10-23
9-44
36-38\11 soon to be 36-42\11

I have also moved the exhaust pipe through the center of the rack to clean up the look some. It certainly looks better and more thought out.

I have gotten a new carb with a slide, but need smaller jets for it. The ones it came with either seem to be too large, or the same size (65-85, increments of 5, same amount of exhaust smoke, other than the stock jet). Regardless, throttle response has improved.

I finally sanded down the cylinder head. There turned out to be a gouge near one of the pins, so I sanded with 1000 grit paper and cutting oil until that was gone. Compression has definitely increased. I can feel it in the starter rope. Along with the head sanding, I replaced the valve springs with 18lb springs. I will soon replace the pushrods if I can find em, just to be safe. The lifters shouldn't be an issue.

I will be fabricating a fairing for it. On this fairing will be a speedometer, a tachometer, and, of course, all the switches I don't need on the handlebars, such as parade lights, headlights, and ignition switch. The kill switch, high/low beam switch, horn button, and turn signal switch will be on the bars at the grips.

I will also be building new wheels for it. I am going to upgrade the tires to maxxis hookworms (so I can have a chicken strip. Like seriously, that tread is almost halfway down the sidewall), and use a 36 spoke double wall mtb rim, and of course, mtb hubs. What can I say, mountain bike wheels are built for abuse.

Oh, I almost forgot about electrical. The last post I made, I was going to put an alternator on it. I've since ditched that plan. Instead, I'm just going to attach a quick connector to the battery for a trickle charger. That keeps the build cleaner, and lets me plug it in wherever I find an outdoor outlet, or can run an extension cord.

View attachment 101974 View attachment 101975 View attachment 101976

Don't worry about the paint damage. That's getting replaced. I'm having the frame chrome plated so I can add a transparent emerald finish for that anodized look. The headset, tank, and fenders will get a green metal flake finish. The box is getting retorched, and sealed with high gloss spar varnish, and the seam will be sealed using black silicone caulking. Oh, and that little bottle opener on the box? That's getting a green LED put behind it.

All in all, it sounds like a small motorcycle now, when it just sounded like a lawn mower before. It is also beginning to look a lot cleaner. I just have to reroute the cables and heat shrink them together.
I too, like using a techometer in that you can be in a gear where you can sustain max horsepower rpm at full throttle. Thus giving you max average speed. Along with keeping you from over reving by being in a gear too low or when going down hill.

If you're pulling a load and/or climbing a really steep hill, you can be in a gear where you're able to at the minimum max torque rpm. Thus reducing engine strain from being in too high of a gear.

I also like the techometers that have an hour counter on them. This is great for maintaining a maintenance schedule.
 

xXNightRiderXx

Active Member
Jan 12, 2017
416
63
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Boise
20200523_102230.jpg

Turn signals and Gas tank finally arrived. Still waiting on wires, inline petcock, fuel line, and M10x1.0-6mm barb tank fitting. I also still need to increase brake rotor size to 180mm from these terribly insufficient 160mm rotors... I've got the rear rotor at least, so it's just a matter of taking off the wheel (and supporting the back end while I'm at it).
 

xXNightRiderXx

Active Member
Jan 12, 2017
416
63
28
Boise
ALRIGHTY, I finally got the wiring done, spiffed up, and connected. Running lights, turn signals, and brake lights work fantastically! I'll be sure to take some photos of the lights beam when it gets dark.

JPEG_20200527_181330.jpg

JPEG_20200527_222336.jpg

JPEG_20200527_222421.jpg
 
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5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,574
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What gear sprockets are you using?
You never mention the chainring sprocket that drives the cassette.
What is it?
And does the 79cc have to be spun so tightly?
Idk; that's why Im asking.
 

xXNightRiderXx

Active Member
Jan 12, 2017
416
63
28
Boise
What gear sprockets are you using?
You never mention the chainring sprocket that drives the cassette.
What is it?
And does the 79cc have to be spun so tightly?
Idk; that's why Im asking.

My cassette is a 32-11. The chainring setup I have is 44/36. So, that would be (30/10)x(44/9)x(38/36) giving me a 1st gear ratio of 15.48, and an 8th gear ratio of 4.48. I'm thinking that I should swap that 36 for a 28, which would change those values to 16.76 and 5.76.

What do you mean by "spun so tightly?"

Tomorrow, I'm going to explain my wiring job, and post links to some of these items on amazon, if they are still available. The main thing is the controls. They are standard motorcycle controls, but they do not come with a diagram or any kind of explanation as to what wire goes where, because they are meant to be plug and play. It is the same with the headlight. The turn signals were pretty straight forward, connect the negative, test the other two leads to see what turns on.

I will be using the kill switch on the right side control to operate hazard lights. My headlight has an amber ring with the white DRL, and I'm going to hook that into the hazards. Because I will be climbing mountain back roads, I'm going to be moving quite slowly in a 50mph zone, and I need people to see me in time.
 
Last edited:

5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,574
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63
Your present gear ratio in First gear is. 13.04:1, not 15.48:1.
Small math error. Like you said, you have a 32-11 cassette.
No big deal, if your engine doesn't mind.
So maybe your engine isn't wound up so high.
 
Last edited:

LR Jerry

Well-Known Member
Dec 19, 2011
1,211
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Rockwood, TN
Your present gear ratio in First gear is. 13.04:1, not 15.48:1.
Small math error. Like you said, you have a 32-11 cassette.
No big deal, if your engine doesn't mind.
So maybe your engine isn't wound up so high.
Good catch on that 5-7.

I also believe the engine max hp rpm is 4200 for 3.5 hp. At 2500 rpm the max torque is 3.5 ft/lbs. The 36:11 seems to be to low of a reduction for this engine unless going down hill. On 26" wheels the 36:11 would give a speed over 72 mph. I personally wouldn't recommend going below a 17 or 18 tooth rear sprocket.

I don't know what kind of overall weight you're dealing with or how steep of a grade hills are that you encounter. Still if you go for a reduction range of 25:1-7.21:1; you should be able to climb any hill and have a top level ground speed of 45 mph at full throttle. I personally wouldn't bother with down hill assist gearing.
 
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LR Jerry

Well-Known Member
Dec 19, 2011
1,211
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Rockwood, TN
David Staton can make you a shift kit in which you can put 3 chainrings on it. If you used the following reductions you could achieve the reduction range around 25-7.21:1. It'd be 10:45; 9:44; chainrings [28, 36, 44] and the 32-11 cassette.

Your shift pattern would work like this:

Hill climbing 1(1,2)
Take off and leasure riding 2(2-4)
Top level ground speed 3(4-6)

I'd only use (7,8) to obtain extremely high down hill speeds.

I think you're doing a great job on your build.


3398-1327198739-b13e4a5f980204de8c20b6515674f787.jpg
 

xXNightRiderXx

Active Member
Jan 12, 2017
416
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Good catch on that 5-7.

I also believe the engine max hp rpm is 4200 for 3.5 hp. At 2500 rpm the max torque is 3.5 ft/lbs. The 36:11 seems to be to low of a reduction for this engine unless going down hill. On 26" wheels the 36:11 would give a speed over 72 mph. I personally wouldn't recommend going below a 17 or 18 tooth rear sprocket.

I don't know what kind of overall weight you're dealing with or how steep of a grade hills are that you encounter. Still if you go for a reduction range of 25:1-7.21:1; you should be able to climb any hill and have a top level ground speed of 45 mph at full throttle. I personally wouldn't bother with down hill assist gearing.
The bike weighs, empty, around 85 lbs. With my planned fuel capacity of 6 gallons, it will have an additional 48lbs. This will also give me an estimated range of more than 500 miles. As it is, I need to do a dyno test to really see how much power this engine is putting out with the cylinder head sanded down and the carb I have installed. I removed at least .018", so compression ratio should be closer to 9:1 now, instead of the stock 8.5:1. I have no intention of going any faster than 65 at any time, this is meant to cruise at 50mph on back roads. Right now though, this bike will not go above 5th gear and 45mph. I believe High Desert Harley Davidson would be able to accommodate the dyno test, and confirm the top speed.

Cassette tooth counts:
32-26-21-18-16-14-12-11
I aim to cruise around in 6th gear.

Average hill grade here is 3-4%, but I expect to encounter at least 6-7% on my travels. This thing should putt right along at 30mph on those steeper slopes, but considering the average speed on those back country roads, that's why I'm installing hazards, operated by the kill switch on the right side control.

Here's a basic representation of my wiring:
Control wires:

Turn signal:
green-white = right = green supply to rear = orange
green-black = left = green supply to rear = blue
orange = hot

Headlight switches:
yellow = hot
yellow-red = to high beam switch
red-white = to high beam flasher
light blue = low beam/DRL = yellow to rear = yellow
blue = high beam

Headlight wires:
blue = high beam/DRL on turn signals
red = DRL
green = low beam
yellow = amber DRL - disconnected, unstripped, reserved for hazard lights
black = negative


Brake light switch:
red = hot
bottom red = supply

Tail light wires
orange = right signal
blue = left signal
yellow = running lights
red = brake lights
black = negative

SAE waterproof plug:
black = negative battery terminal
red = positive battery terminal

Hazard lights:
red = power from flasher relay
black-red = supply to rear turn signals and amber DRL on headlight.
 
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