The 101 Association, Inc.
For the preservation and enjoyment of 1928 to 1931 Indian Scout Motocycles
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Engine/transmission vibration

  • 27 Jul 2017 4:46 PM
    Message # 4998643

    Hi, I have a Scout 1928 37 "with an engine or transmission problem. If I drive at the highest gear and over 60-70 km / h, a vibration feels in the footplates and feel on the gear lever. If I'm on the throttle Quickly, I perceive it as it disappears during the actual accelration.

    The engine and transmission including the clutch have been recently refurbished but not the gearbox itself.

    Does anyone have a tip where the problem is?

    Regards

    Jorgen Fritz

    Visby/Sweden


  • 29 Jul 2017 12:33 AM
    Reply # 5001058 on 4998643

    Vibration at high revs or heavy load, as it is up on high speed can have a multiple of reasons. Something surely changed from previous. What is done? My 37ci was smooth almost as an electric engine (well, compared with the 45ci) all the way up to it didn't pull anymore...You say your engine is newly refurbished..

    Of course bad gearbox ball bearings with too large in-and-out or up-and-down play at the gearbox mainshaft is a common reason for rumble or vibration, the gearbox can also be shimmed with too small or too large gear play between the clutch/idle gear. The clutch basket can be oval or mainshaft bent.

    But higher compression alone can render harsher vibrations. New bored cylinders, pistons and rings can do that. Even just a change of rings might result in higher comp and vibrations. Heavier or lighter piston assemblies can alter the balance factor. Did someone tamper with the crankshaft? Just a hammer fisted method of knocking on a puller for release a gear can make the crankshaft crooked. Slightly crooked crankshaft will add vibrations... 

    Or..Something is loose or cracked. Check with a magnifying glass the gearbox 4 lugs and pinbolts to the engine and frame, and the forward and aft mount to the frame. 

    While at it with the magnifying glass, check thoroughly the entire frame and fork for cracks or anomalies.  Bad or too loose headstock or wheel bearings can render an involuntary sick leave...eventually...

    Further fetched troubles just in general...

    Ignition. Weak magnet or jumping points can make the engine miss at high revolutions especially at high load as the plugs need extra juice to jump the gap then.. That can happen on one cylinder only and you may not hear or notice that except added vibrations..

    Carburetor. A heavy float can't keep the float level stable at high frequency of vibrations so the level rises and the carb is giving too much. The ever recurrent problem with manifold leaks can give odd symptoms.

    I trust your wheels are straight and rebalanced. 

    Last modified: 29 Jul 2017 12:38 AM | Carl-Erik Renquist
  • 30 Jul 2017 12:48 AM
    Reply # 5002410 on 4998643

    The rear Most mount point  if loose  can give vibration issues ,That is the 7/16 Bolt at the rear of transmission .It needs to be tight .


  • 07 Aug 2017 3:46 PM
    Reply # 5017060 on 4998643

    Visby 7/8-2017

    Hi, I have now disassembled my primary transmission. Seems like I have a pretty big up and down play if I'm testing on the clutch basket.

    I have new bearings on my spare part shelf so I'll see if I can change the bearings without removing the engine/gearbox from the frame. 

    Return with the result if it were the bearings in the gearbox which was the problem to the vibration.


  • 07 Aug 2017 7:58 PM
    Reply # 5017397 on 4998643

    Both mainshaft ball bearings should be changed, especially the thin bearing is exposed to tremendous forces from chain whip and jank. When pulling/pressing or knocking in/out those ball bearings, heat up the case with something ( I use a hot air gun) to when spit sizzles on the case. 

    When mounting a new bearing, some do, but I don't bother to freeze the bearing as I have the opinion that steel contracts little with temperature compared to aluminium and the cold steel does chill the aluminium quickly. I do lube a little with a light oil like WD40. See to that the bearing has seated right at bottom of the seat. Be careful not to press or knock on the bearing too hard when the case is heated because the aluminium is soft, best is to use some kind of anvil to support the backside. Abort and pull out if it looks like the bearing isn't sliding in with constant force.

    Same shims or distance washers at both ends between the bearings and the shaft splines should be used for the mainshaft, the thicker one at the clutch end. Perhaps thicker if you must reduce side play, but do not reduce the thickness. The reason I mention that is...

    I made the mistake once to think the sideplay was reduced with new bearings, so I reduced the shim thickness to get proper sideplay. What I didn't check was if the clutch side bearing had seated fully. Assembled and out on my first test ride, the gearbox warmed up and when I pressed the clutch pedal, the bearing sunk in. The result of that was that the clutch disengaged suddenly, pedal went to the floor and the bike lurched forward. I had no chance to engage the clutch or stop the bike so it was a heart stopping moment there for a good while...  

    And check play in the cluster gear bushings, both in the gear and in the case, large play makes gears whining. Loose or oval worn gear case bushing on the right hand side is particular bad as the bushing seat is fairly weak, and gear vibration that will result can pound the bushing loose in the case with more elaborate restoration in the waiting..

    The cluster gear shaft is constructed a bit daft, it really should have a threaded hole at the clutch side end, in order to fit an extractor. Because the far end bushing should be so tight that you have to lightly knock in the shaft.

    There is a tell-tale hole below the chain sprocket. The reason for the hole there is two-fold. One is so you can insert a thin mandrel and knock out the cluster gear shaft. The other is that it tells when the bushing is getting worn and the shaft starts to rattle, then oil seeps past the bushing and the leak is visible on the outside. That is the reason why I advocate don't seal up that hole.

    Last modified: 07 Aug 2017 9:17 PM | Carl-Erik Renquist
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