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Author Topic:   Bad Vibrations
baltica posted 11-18-2001 11:19 AM ET (US)   Profile for baltica   Send Email to baltica  
I just had a new 90 hp Mercury in-line 3 cylinder (saltwater model) with a 19 pitch prop installed on my 1970 Katama (16 foot), and am breaking the new motor in. The old motor was a similar Merc 90 (6 cylinder), so I thought switching out the motors would be a cinch.

Problem: There is a strong vibration coming from the motor at idle and at low throttle (below 10 mph) - the hull and even the bow rails on the bow shake strongly. The vibration seems to lessen at higher rpm once the boat gets out of the hole. At 5,0000 rpm it seems to calm right down. However, at lower rpm there is still a fair amount of vibration carrying thru the hull. Any way to dampen the vibration from the motor? It seems to weigh as much as my old motor did.

Any suggestions before I head back to the marina?

Jerry Townsend posted 11-18-2001 12:30 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
I do not have experience with the engines you mention - and am just an retired engineer with a lot of vibration experience. As such, realize that the 3 cylinder engine will be "rougher" than the 6 cylinder. Isolating the vibration will be difficult as such techniques require, for example, attaching the engine to the boat via rubber pads, et.al. I suggest you go back to the marina and have them check the engine to ascertain that it is in proper balance - the engine is the source and virtually all engines employ a balancing device to lessen the vibration. --- Jerry / Idaho
FISHNFF posted 11-19-2001 12:27 AM ET (US)     Profile for FISHNFF  Send Email to FISHNFF     
I owned a 1996 Merc 75 (Same displacement as your new 90) and it would shake at trolling speeds like it was on the verge of dying. A new switchbox helped but it still shook. Always started and ran. Others with similar motors had it the same. Yamaha's and OMC's were smooth, just the Merc's are rough. Like I said, it would shake and rattle, but kept on going.
Clark Roberts posted 11-19-2001 09:10 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Baltica, I have owned about a dozen 3 cyl Mercs and none had a vibration problem at all, in fact ALL have idled and run very smoothly! Smoothe, strong and steady idle is dependent on, among other things, the idle fuel/air mixture being set correctly after break-in. This is a must on Yamaha and Merc 3cyl engines and if not correctly adjusted engines will idle rough (and sometimes caugh and die at idle). The adjustment screw is right on the carbs and can be done easily (check with your mechanic or e-mail me). Since you say it's rough at high rpms I'd bet on prop/prop shaft balance! You may have a prop shaft that has been bent(new engine/no prop droped on prop shaft in shop or backed into obsticle or an unballanced/damaged prop -same reasons!) Is this a new prop or did you have this one on the in-line 90? Isolating the vib is not the solution.... getting rid of it is! happy Whalin'... Clark.. Spruce Creek Navy

baltica posted 11-19-2001 10:09 AM ET (US)     Profile for baltica  Send Email to baltica     
The 19 inch prop is new. The vibration is worst at idle and low trolling speed, so I don't think the prop shaft is bent. However, there's still a bit of shaking goin on at cruising speed so I'll have the shaft checked.
I will try adjusting the carb after break-in (I'm still running with the first tank of 2x fuel-oil mix).
Clark Roberts posted 11-19-2001 11:08 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Baltica, here are the things to do after break-in (10-20 hr. adjustments) : the carb idle/air ratio on each of the 3 carbs; drain and refill lower unit lube; move oil injection actuation arm from breakin to run position (there are two scribe marks on the oil rod bracket thing on the oil pump, set for high volume oiling at factory)simply move linkage to other mark... ask you mechanic if in doubt. Most motors run their entire lives on the breakin setting.. doesn't hurt but makes idle rougher and you will use more oil...etc...!!! Clean and gap plugs (I think your engine has surface gap plugs so can't be gapped just clean). Since there are no head bolts to torque just check all under cowling visually...etc... done deal! Happy Whalin... Clark.. The Old Man and the Sea

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