Glen-L.com

Hi, i have a 90 HP merc, 2 stroke 2001 on a 18 foot boat. Motor idles very rough and sometimes makes a terible sound like metal on metal. Sound goes away at sliughtly higer RPM.

Also tachometer reads 1200 rpm when key is off. When key is turned on reads normal.

Can anyone advise on problem

many thanks

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Kiwi boater from downunder.

First off is your tach set to the right position? It could be giving you a false reading overall.

The metal on metal sound could be the flywheel scraping another component.... I'm not sure though. Have you ever ran the motor really lean before? Meaning the incorrect oil to gas ratio. I assume its 50:1 for that motor. I doubt is a shot piston but hey you never know.

Another suggestion is your propeller nut is too tight. Thats a last call though.

Has the motor ever been rebuilt?

The carburetors might need cleaning. I know the 90hp 4 strokes are notorious for bad carbs getting gummed up.

If you want to do it yourself best thing to do is to get a repair/shop manual from Ebay. It'll provide all specifications on everything for your motor. Very handy and it'll go over some trouble shooting too.

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Tuffy with '75 Merc 650 SS.

Thanks, I've been told the metal noise atbidle is the gears and common on mercs with stainless props and nothing to worry about. Down loaded a manual. Compresions all good, motor only done 70 hrs so may simply need a good clean of carburetors and tune. I'll have a play in the weekend

Cheers

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Kiwi boater from downunder.

Really at idle? I've ran stainless on various Merc gearcases and there isn't whining nor chatter of the gears. What size is your prop, you could be overloading your engine/gearcase. And what fluid are you using in the gear case? The manual you downloaded might have a prop chart using the variables - weight & rpm to give you your correct prop diameter and pitch sizes and theoretical speeds.

Ya a tune up is always good in the spring. Make sure you check the condition of the fuel lines since the alcohol in the ethanol in 87 & 89 octane will kill the lines.

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Tuffy with '75 Merc 650 SS.

Not sure who told you that but I'd be hesitant to ask for any further advice from them. It's not normal. Now the trick is to find out where it's comming from. The noises in these motors telegraph and are hard to isloate. I start by pulling the lower unit off and putting a hose on the water tube that goes up into the mid section. This eliminates the L/U from the equation but it also tells you somethings really wrong in the powerhead.

If the problem goes away, then get yourself a rebuilt lower unit and you'll be back in business. If it persists, look for the easy stuff first. Pull the flywheel and look for anything that looks like it might be loose or damaged. Your symptoms could be a lose or broken flywheel magnet.

If everything looks good there then proceed to the oil pump. I doubt this is the problem because they rarely go out and when they do, you usually lose the motor due to lack of oiling. You don't seem to have that problem.

That's about it for external noises. If it's still prevelant then you need to pull the front end off and look for a broken reed. A broken reed will cause a rough idle that will smooth out with accelleration. It's usually accompanied by hard starting and sneezing so if you have none of that then you can probably rule it out.

Personally, I think you're going to find a lose magnet under the flywheel. If it is the problem, and the magnet and stator aren't damaged then you can eaisly epoxy it back in and be done with it.

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Steve Schefer
Santa Rosa, Ca.

New Years Resoluiton - Never leave something for someone else to do when I should be doing it myself.

I had a 75 3 cyl that knocked at idle.

the sleeves had moved in the block .....a ring eventually caught on the edge and broke a piece off the piston and jammed the piston.

I was running fine(?), shut it off, then wouldn't start.

It didn't lock up the engine, but you could only rotate the motor about 345 degrees in either direction

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