Engine vibration on stock CT70 engine

Discussion in 'Tech Area' started by Etown, Jul 2, 2018.

  1. Etown

    Etown New Member

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    I have a 79’ CT70 that has a complete fresh rebuild. Everything was nice and tight when I installed it. I cracked the case to check the transmission and removed and cleaned all the components. I used an aftermarket head and cylinder. My question is, is it normal for engine vibration at higher speeds? It is a 3 speed and as soon I hit a high rpm (25mph in 3rd gear) the motor has a noticeable pulsing vibration. If that is not normal any ideas of what it could be. Thanks
     
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  3. racerx

    racerx Administrator
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    First thought was a loose flywheel but, you've covered that. So...second idea is relatively uncommon however, I've seen it on "rebuilt" engines...mismatched primary drive gears. A `79 engine should be fitted with 18/67. FYI K0 & early Z50 motors (all 3-speed/semi-autos) had 17/69 primaries. Based on your description, I'd remove the clutch cover and count the gear teeth on the crank (drive) and transmission primary (driven) gears; if you find either17/67, or 18/69, combo, there's your problem. BTW, this isn't casting aspersions on anyone's work. That said, a lot can happen in 39 years' time...with an unknown number of unknown previous owners & mechanics.:cautious:
     
  4. Etown

    Etown New Member

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    Sounds good, I have researched since I posted and someone in another forum corrected what seemed to be an engine vibration issue just by checking the exhaust tightness. Seems to me that there is no way that could be the issue but I might as well start with the easiest first and work my way to the less easy
     
  5. racerx

    racerx Administrator
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    You should definitely start with the easy stuff first. A loose fastener can have what seem to be wildly disproportionate effects. And, the exhaust is a long piece with lots of potential resonance. Take a close look at the motor mount and swingarm pivot bolts...as well as the chain, sprockets, chain alignment & tension. Support the bike such that the rear wheel can be spun, then look for wildly varying chain tension. A certain amount of variance, i.e. a relatively "tight spot", is normal as the chain cycles through a full rotation. The rear sprocket mounting/attachment setup leaves a bit to be desired but, is durable enough to make it well into 5-digit mileage. (I replaced the rear hub on my daily rider after ~18,000 miles...no idea how many miles it had seen before I bought it...and it was still okay for a stock bike, just getting too loose for my liking, on a 65mph bike.) See how much motion you can generate, by hand, between the sprocket & hub, paying close attention to radial and axial slop. Believe it, or not, lateral wobble is the least critical parameter. The rubber dampers should fit snugly inside the hub pockets and on the sprocket drive dogs. If they're loose, treat yourself to a new set...on a stock bike they'll likely last another 4+ decades.
     
  6. Etown

    Etown New Member

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    Good points, I guess I could start with the bike in neutral and bring it up to a high RMP, that should tell me if it is actually the motor or not? If I’m not getting the vibration at that point I would have to assume it is in the clutch or chain/chain alignment /chain tension. Unfortunately I will be at work for the the next couple days and won’t be able to diagnose till closer to the end of the week. I will let you know regardless though what I find out.
     
  7. Etown

    Etown New Member

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    Had time to wrench on it yesterday. Apparently when I reassembled the bike I just had the motor mount bolts lightly snugged. I torqued them down and I’m good to go, the vibration was reduced significantly. Sometimes when I have problems with the bike I’m not sure if it’s becasue the bike is 40 years old and has not been (treated with kids gloves) or it’s becasue (it is what it is) and that’s how it rolled off the factory floor
     

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