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Motorized Cruiser Bicycles The beach cruiser has always been great bicycles to motorize. They just look good with a motor. Use this section to share and discuss about motorizing this classic.

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  #21  
Old 05-19-2017, 02:31 AM
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Tony01 Tony01 is offline
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Default Re: 79cc Predator Build

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Originally Posted by Kletus View Post
Yeah, I really have to baby it till it reaches 25 MPH. If I don't the whole bike shakes and vibrates. Feels like the clutch is locked because the engine really labors. The magic number is 25 mph then I can give it throttle. When it hit 43-45 mph I was on a long level stretch with a light wind to my back. I like the low RPM's at 30 MPH but I'll probably increase the number of teeth to 36 on the rear sprocket to improve the low speed performance. My Max Torque six shoe clutch came Saturday and then I found the 415 chain was to narrow to fit on the #41 clutch sprocket so now I am awaiting the #41 chain to arrive.
It's shaking because your setup is wrong. The magic number is 25 because your gear ratio is probably close to a 8:1 which puts you at about 100rpm per mph and 25mph is about the speed that would be your clutch locking up. Put the green spring in it and it will get better. A stock clutch works great because most karts and minibikes that use a Max torque have gear ratios set for about 30mph. To go faster on one gear your clutch needs to slip a lot more. Think of it as driving a manual car in 1st gear vs 3rd only. In 1st you lock it pretty much at 10mph, but starting in 3rd you need to slip it till about 25mph or else risk under revving the motor. Also when you slip it starting in 3rd you rev the motor to about 2800 to 3000, because your foot instinctively knows the best torque is right there. This is why you need a heavier spring. Setups that have worked for me have been black spring with 3 heavy shoes, black spring with 6 heavy, and green spring by itself. My local karting shop doesn't carry heavy shoes so I just get the one spring. Then when your clutch shoes and bell get really worn like .050" each, you can go back to stock spring for almost the same effect. Get a short piece of pvc about 1.25" ID to make the reassembly easier. The acceleration will still be slow in general terms, you might not beat a Prius or even a bus but you'll beat a bicycle off the line. You need horsepower to accelerate fast, and to go fast, and a 79cc doesn't have much of it so the setup needs to be tits.

Basically; gear the bike for the speed you want to go (and if motor can handle it) and set your clutch to lock up at around the torque maximum of the engine. For example, my bike is geared 7:1 to top out around 55mph at 5k with the clutch locking up at 30mph (green spring). Even the green spring is a bit too close to stock for me, but it's good enough and my motor has enough power to force the clutch through its chatter.

Also cancel your order for that #41 crap and get the #420 at your local mc shop.
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  #22  
Old 05-19-2017, 05:37 PM
Kletus Kletus is offline
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Default Re: 79cc Predator Build

Thank you Tony01 for that comprehensive theses you wrote on the clutch modifications. I have decided to go that rout instead of increasing the gearing ratio and loosing top end speed. I can live with slower take offs. I ordered a green spring for my Max Torque clutch today. I noticed that the silver spring in the MT clutch is made with much larger wire then the wire in EBAY clutch I am now using and I wonder if a green spring in stalled in it might improve its performance in my application, might be worth a try.
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  #23  
Old 05-19-2017, 09:54 PM
TheNecromancer13 TheNecromancer13 is offline
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Default Re: 79cc Predator Build

If you remove the governor you can get a lot more top speed while still running a better gear ratio.
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  #24  
Old 05-19-2017, 11:20 PM
Kletus Kletus is offline
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Default Re: 79cc Predator Build

If I had a billet rod and a billet balanced flywheel and stronger valve springs I would consider removing the governor. Also these engines have dipper oiling for the rod bearing, no oil pressure, you could end up welding the bearing insert to the crankshaft throw. What we need is a needle bearing rod. I know people are doing this and getting away with it but its risky. It might be that the governor could be modified to let it build another thousand RPM's. Say 5000 RPM's for short bursts. If the engine flies apart with you sitting on the bike you could be seriously injured.
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  #25  
Old 05-20-2017, 12:04 AM
TheNecromancer13 TheNecromancer13 is offline
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Default Re: 79cc Predator Build

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Originally Posted by Kletus View Post
If I had a billet rod and a billet balanced flywheel and stronger valve springs I would consider removing the governor. Also these engines have dipper oiling for the rod bearing, no oil pressure, you could end up welding the bearing insert to the crankshaft throw. What we need is a needle bearing rod. I know people are doing this and getting away with it but its risky. It might be that the governor could be modified to let it build another thousand RPM's. Say 5000 RPM's for short bursts. If the engine flies apart with you sitting on the bike you could be seriously injured.
I've been running mine for over 6,000 miles with the governor removed and the stock rod, stock flywheel, and stock springs, and it hasn't blown up. The valves start to float and limit the rpms before they get high enough to do any damage.
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