79cc Predator Build

Discussion in 'Motorized Cruiser Bicycles' started by Kletus, May 12, 2017.

  1. Tony01

    Tony01 Member

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    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.
     
  2. Kletus

    Kletus Member

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    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.
     
  3. TheNecromancer13

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    If you remove the governor you can get a lot more top speed while still running a better gear ratio.
     
    Dan likes this.
  4. Kletus

    Kletus Member

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    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.
     
  5. TheNecromancer13

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    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.
     
  6. Kletus

    Kletus Member

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    Installed a stock Max Torque clutch with a silver garter spring and 46 gram light shoes. Improvement. Now I can give it throttle at 10 MPH with no shaking. I installed a green spring with not much improvement. I am awaiting a black spring to arrive. I am going to try that before I adjust the gearing. One thing at a time. The no name clutch that was previously installed that shook up to 25MPH had 60 gram shoes and a garter spring that had a wire diameter the same as the Max Torque green spring. The only differences I saw between these two clutches was shoe weight, a brass spindle bushing and a replaceable sprocket. The shoe weight being the most important as the 60 gram shoes would cause the clutch to engage at a much lower engine RPM. I'll let you know how that black spring works out. My gearing is 6.6:1. Clutch sprocket=10 tooth, jack shaft sprocket = 22 tooth, jack shaft output sprocket =10 tooth and the back wheel sprocket is 44 tooth. Many Thanks to Tony01 for pointing me in the right direction with respect to the clutch engagement and Necromancer13, help also.
     
  7. TheNecromancer13

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    6.6:1 gear ratio is kinda steep even if you were using a 212cc.
     
  8. Kletus

    Kletus Member

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    I have a 212cc predator on a I have a Schwinn Meridian trike and its geared 4.2:1. It does standing starts well. I have no idea how fast it will run because the front wheel starts to shake and vibrate and hop around at 30 mph. These both are King motorbike kits.
     
  9. Tony01

    Tony01 Member

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    6.6 is far too tall for a 79cc and about the limit for a 212. When I was running a 2-speed my top gear was a 6.6. I run a 7 now and have friends with 7.5, and 8. However the tooth counts you posted add up to probably about a 9.5. With a 9.5 a spring change won't do much because the hearing is well suited to the engine. Black spring with regular shoes was too high engagement for me. Engaged about 2800 and locked around 3400. Too much slip. Black spring with mixed shoes worked well. I used MSExcel to calculate lockup Rpms for different springs and shoes. I need to find that screenshot again.
     
    #29 Tony01, May 28, 2017
    Last edited: May 31, 2017
  10. Kletus

    Kletus Member

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    Installed the BLACK Max Torque spring today and went for a ride. I like it. Now I can give it full throttle from a dead stop with no shaking or vibration. The amount of slipping depends on how much throttle I give it. Full throttle it slips to about 30MPH and starts to lock.
     
    #30 Kletus, May 30, 2017
    Last edited: Jun 10, 2017
    Dan likes this.
  11. Kletus

    Kletus Member

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    Today I alternated 46 gram shoes with 60 gram shoes along with a black spring in the Max Torque clutch. I get shaking and vibration at standing starts and up to about 20 mph with full throttle but if II give it 1/4 throttle at a standing start and ease on the throttle it is smooth and vibration free. The clutch seems to completely lock somewhere around 25 mph. Acceleration is better with this setup than with the black spring and 6 light shoes due to the light shoe setup slipping badly all the way up to 30 mph. I have a 44 tooth back wheel sprocket and I'm thinking a 48-50 tooth rear sprocket may solve or improve some of this full throttle vibration issue. I may also take Necromancer's advise and remove the governor as I don't think the gearing or me getting chicken at any speed past 40 mph would allow a bad over rev to occur.
     
    #31 Kletus, Jun 10, 2017
    Last edited: Jun 10, 2017

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