Vibration at 2500-3000 rpm with drive off

Sikboats

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Apr 7, 2010
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7.4 mpi , customer drove with no oil pres because of blown oil line. fixed line and told him It prob will not live. Went to lake and had vibration at 2500-3000... told him it was probaly main bearings which it was they were burt up. Replaced motor and still have vibration, motor mounts tight and thru bolted alluminum 1/2 L angles. Removed drive and ran at shop and vibration still there but only at rpm's stated. When on dyno someone stated they saw coupler slight runout , it has the H/P coupler. Question has anyone ever heard of a coupler causing vibration before I pull the motor back out?
 

CharlieB

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Apr 10, 2007
Messages
5,617
Re: Vibration at 2500-3000 rpm with drive off

When you replaced the motor was it a long block or complete motor assy?

Spark test all plug wires, a poor wire or dist cap can cause erratic misfire at RPM.
 

Don S

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Re: Vibration at 2500-3000 rpm with drive off

What I would do is pull the engine, block it on the floor and run it and confirm you still have the vibration. If you do, pull the coupler and run it again.

This is also assuming the engine is running good and you don't have a miss. But I know I have had customers come in say the engine runs great at idle, only to find zero compression on 2 cylinders.
 

Sikboats

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Messages
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Re: Vibration at 2500-3000 rpm with drive off

Motor is running perfectly , idles great and raps up to 5000. Have you guys ever heard of a bad coupler?
 

Sikboats

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Apr 7, 2010
Messages
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Re: Vibration at 2500-3000 rpm with drive off

Actually a short block because Mpi heads are a lil diffrent
 

Don S

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Re: Vibration at 2500-3000 rpm with drive off

Sure there are bad couplers, never heard of one causing a vibration with the drive off, but it's possible.

PS, an engine will rap to 10,000 rpm without a load and sound good if you took half the spark plugs out.
 

Pete104

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Apr 30, 2011
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1,439
Re: Vibration at 2500-3000 rpm with drive off

You can't do anything without the motor on floor!
Sometimes they don't replace the "dowel pin" in the crank for the flywheel. The flywheel is the balance along along with the "harmonic balancer". Was this replaced? Fresh engine, fresh balancer!
As far as the coupler, anything is possible but not very likely.
If the dowel pin was NOT there, you 6 possible positions for the flywheel to be installed!
 
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