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4Lw Measuring Revs


Chris Pink

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running the engine again at vibration speed with particular regard to the mounts, the amount of vibration on the front mount is noticeably more than in the engine bed at that point indicating they are not fully coupled.

 

With the rear mounts which, now I've got them unburied, look far more messy, the vibration is the same on the mount or on the bed.

 

So I'm going to work toward beefing up the front mounts as a first step.

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I would suggest that the engine mounting arrangement is fundamentally incorrect. To prevent vibration being transmitted to the boat it is a good idea not to have a through bolt from engine to bearer and engine bed. We generally supply engine feet that are bolted to the engine with a stud with an oblong plate rebated into a hardwood bearer. The bearer is bolted to the boat with ordinary through bolts. So the engine is bolted to the bearer but is independant of the hull and there is no direct vibration path.

 

From what you have said I would be surprised if there is not an underlying mechanical issue there. Are you sure that it is firing on all of the cylinders? We don't find 4LWs prone to vibration unless a cylinder is dropping out.

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Well it's very very smooth and sweet at normal cruising speed, 5mph on a 24" x 21" prop - as the OP I don't know what revs this is at. I would be surprised if a cylinder was missing and there's certainly no discontinuity when increasing revs. It just gets to a point and everything rattles like hell. If there is an issue it's something like the flywheel or drive plate loose.

 

But the mounts, now I've looked carefully, don't seem to me to up to the job for such a heavy engine. For instance I have an HA2 bolted through two Russian railway sleepers widthways and those bolted to 10mm angle and that doesn't shake at all. This, bolted through a piece of 10mm steel and 12mm rubber onto 8mm C section, is not the same thing at all.

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