Merc 2001 model 90hp 2 stroke question

[cll53]

Hoping somebody out there can help with this...I have a 2001 model 90 hp Merc 2 stroke that I just recently put a Turbo SS prop on. I've had it out 1 time since and I am hearing all kinds of noises that I've never heard from my motor before. The noise, presumably mostly gearcase rumbling, only seems to happen at idle speed. When I speed up a little it goes away and, it performs very well. With the aluminum prop on I don't get these 'noises'. I've seen one letter in Bass and Walleye Boat mag where this guy had the same motor with the same or very similar complaint. Anybody out there ever hear of this and whether or not it's detrimental? I can't believe this motor won't support an SS prop!? Heard a partial fix is Blue Marble gear lube, this is supposed to decrease the noise. One suggestion was that it is simply 'clutch dog rattle'. Any input/experiences much appreciated!!

 

[Big Al1]

Re: Merc 2001 model 90hp 2 stroke question

Could be the outer hub of the prop is scraping the inside of the lower unit. My brand new 15hp Merc was barely scraping the hub of the prop as I spun it with my hand, I think mostly paint. After running it once it scraped away the excess and has been running smooth ever since. Does it make this noise as you spin the prop by hand while in neutral?

 

[ODDD1]

Re: Merc 2001 model 90hp 2 stroke question

Cll..you have clutch rattle, a classic case ....only makes noise at idle , dissapears just of idle, and only started after putting on a HEAVIER stainless prop....what you are experiencing is normal, and a different gearlube wont help....a lighter prop, or a softer hub in the prop are your remedies.

 

[cll53]

Re: Merc 2001 model 90hp 2 stroke question

Thanks for the replys BigAl and ODDD1. Clutch rattle huh? What you described ODDD1 is exactly what happens, goes away just above idle. Does it hurt anything?

 

[cll53]

Re: Merc 2001 model 90hp 2 stroke question

Big Al, it doesn't make the noise while spinning it in neutral. Thanks again for the reply!

 

[Bob Kimber]

Re: Merc 2001 model 90hp 2 stroke question

Hi cll53<br /><br />As the owner of a 2000 model 90 hp 2/s merc I would totally agree with ODDD1 about the heavier SS prop, I ran my motor for a long period from new with a 13 1/2 X 15 pitch SS prop that I had swapped from my old 90 hp inline six where no gear noise was evident. On the new 3cyl 90hp the motor would go into gear (either F or R) with a very solid clunk and at idle speed would exhibit the same noises you are describing, it did not to seem to cause any problems but may have over a long period. At this point the motor was well run in and was beginning to exceed recommended WOT by 600 rpm, so I tried several different props (dia and pitch) both SS and aluminium, all SS props had the same behaviour, I am currently using a standard merc aluminium prop 13 1/4 X 17 pitch that shifts into either gear as smooth as a hot knife through butter with no noise or vibration at idle speed, it is giving me great performance and fuel economy. I don't know if this noise is partially caused by the gear ratio of 2.3 on the 75 & 90 as the only other motor that is similar is the 40hp four stroke with 2.4. the local mechanics say the problem can be more obvious on the four strokes.<br /><br />Bob

 

[cll53]

Re: Merc 2001 model 90hp 2 stroke question

Thanks, you guys are great. What you described Bob is right on the money, even the 'clunk' when putting in gear. I'll be going back to an aluminum prop.

 

[KIWI MERCMAN]

Re: Merc 2001 model 90hp 2 stroke question

cll53<br />these guys are right on, this is what happens when then prop starts acting like the flywheel and overuns the engine, these models were very bad for it. Caused Merc to build different props and hubs. We never found any long term problems, just very noisey. If your boat go's ok with a alloy prop stick with it.<br />Kiwi

 

[KIWI MERCMAN]

Re: Merc 2001 model 90hp 2 stroke question

cll53<br />Just an add on, this problem is also alot worse if your engine is running cold.90's have a 120deg stat std and we aways found that the run temp on these engines were around 90 to 100,deg's. Get it checked and if it's low then get them to check that your poppet valve is not bypasing water. There was a mod for it from Merc. <br />When you get these engines up to 120 deg at idle they are sweet. If you are running in very cold water you can also put a 143deg stat with no problems. <br />Get rid of the lumpy running and the prop rattle is better as well.<br /><br />Kiwi