Author Topic: 38 LaSalle engine vibration  (Read 920 times)

Offline Tom Beaver

  • CLC # 22002
  • Posts: 373
  • CLC Number: 22002
  • Name: Tom Beaver
38 LaSalle engine vibration
« on: May 25, 2013, 11:58:03 PM »
I have an engine vibration in my 38 LaSalle which I can't seem to fix and am now out of ideas.  The engine has approximately 500 miles on it and starts immediately and runs smoothly at idle and low speed but when the rpm's get to approximately 1500 a vibration starts (you can feel it inside the car and through the steering wheel).  The amplitude of the vibration increases up to a peak at approximately 2500 rpm and then starts to diminish and feels to be almost gone by 3500 rpm.  The engine internals, crankshaft, rods and pistons have been dynamically balanced, so, I thought that the problem could be caused by carburetion.  Therefore I bought a good replacement carburetor, rebuilt it, and put it on the engine.  Result - absolutely no change in the engine vibration.  Tried something else, since the flywheel and pressure plate had not been previously balanced I took them out and sent them off for balancing.  Turns out they were both out of balance.  I thought my problem was solved.  However, I just got them back in the car and started it up and the vibration is still there.  The character of the vibration has changed in that it starts at a bit higher rpm and if I hold the rpm constant at what appears to be the peak amplitude, it feels cyclical.  In that the vibrations build up to the peak then die away and then build up again in a period of a few seconds.

So now I don't have any idea what is going on and am looking for help.

Tom Beaver

Offline Glen

  • Posts: 2789
  • CLC Number: 727
  • Name: Glen Houlton
Re: 38 LaSalle engine vibration
« Reply #1 on: May 26, 2013, 01:51:29 AM »
The cyclic action is typical of two things rotating at slightly different rates and they are out of balance.  The cycling is caused by the two out of balance items adding and then then subtracting as they go in and out of phase. 

I don’t know how that will help you.    But you might start by disconnecting the fan or other belt driven items.  See if that changes things. 
Glen Houlton CLC #727 
CLCMRC benefactor #104

Offline harry s

  • Harry Scott 4195
  • Posts: 1920
Re: 38 LaSalle engine vibration
« Reply #2 on: May 26, 2013, 10:05:20 AM »
Tom, If you still have the problem after disconnecting the drive belts try rotating the pressure plate position by position and if the vibration imroves but doesn't go away try adding flat washers to one of the pressure plate bolts (one then two or even three) and rotating from bolt to bolt until the best spot is found. This is kind of tedious but has worked for me in the past. Good Luck,   Harry
Harry Scott 4195
1941 6733
1948 6267X
2011 DTS Platinum

Offline Tom Beaver

  • CLC # 22002
  • Posts: 373
  • CLC Number: 22002
  • Name: Tom Beaver
Re: 38 LaSalle engine vibration
« Reply #3 on: May 26, 2013, 11:42:11 PM »
Glen and Harry thank you for the replies.  I will disconnect the drive belts and give it a quick test as the engine heats up pretty fast, especially without the water pump. 

I think I did find part of the problem today while checking the timing.  The engine has a 1940 distributor with vacuum advance and I noted at idle the timing was where it should be but by 2000 rpm I couldn't see the timing mark anymore.  The darn thing is hard to see in the first place but it must have been advanced 20 degrees or more.  I also noted that at this rpm the exhaust note sounded rather irregular.  I disconnected the vacuum advance and slowly increased rpm through the operating range.  The mechanical advance seemed to work correctly, the engine vibration was markedly diminished and the exhaust note also sounded better.  I could probably live with the vibration that is left but it still isn't right so I will do some more testing with the original distributor and as you suggested with the drive belts.  I also think it would be interesting to check the operating air/fuel ratio because it may well be running a bit lean after the carburetor transitions over to the main jets.

Harry, I might try you suggestion if all else fails, but the engine has been all dynamically balanced.  The flywheel and pressure plate were both balanced separately and the combination checked out to be no more than one gram off.  However the flywheel was balanced separately from the crankshaft and before I took the flywheel in for balancing I measured the run-out on the flywheel mounting boss and found it to be out approximately 0.002 inches.  I have no feel for what that would do to the balance, the guy who did the work wasn't concerned, but that is an awfully heavy flywheel.

Thanks again.

Tom Beaver

Offline rodger

  • Posts: 23
  • Name: Rodger
Re: 38 LaSalle engine vibration
« Reply #4 on: May 27, 2013, 11:08:47 AM »
I had the same thing happen on a '38 LaSalle years ago.

Turned out when the guy resurfaced the flywheel, he didn't have a clean table on the flywheel grinder so the flywheel had a slight taper across the clutch surface..
Rodger Craig

 

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