I recently purchased a Yamaha outboard F150. I have logged 45 hours on it. There is an small rectangular opening on the back of the motor, just below the cover and above the oil drain. I recently noticed that there was a black oil residue (soot) below the rectangular opening and on the cowling at the join to the lower unit. Is this an exhaust outlet and is this soot built up normal? Thank's in advance for your response.

https://www.thehulltruth.com/boating-forum/388307-shift-cable-adjustment-yamaha-f150.html

The amount of RPM's on a boat motor depend on a number of things...
1. Height if the motor in relation to the bottom of the boat. - too high and it over rev's.. Too low and it won't rev..
2. The correct size (O.D.) of the prop. Too large and you have mechanical interference or too large for the motor to spin...- ..Too small and the motor will over rev!!
3. The correct pitch of the prop. - Too large/big of pitch and the motor can't spin it..-.. too small and it will over rev the motor.
..... these are the big three...
Now, Make sure the carburetor butterfly(s) are opening exactly flat or vertical ...and not under/over opening..if this is a fuel injected motor...have the dealer check it out..

didnt specify unit --generic answer --best way is to buy a rebuild kit [should have instructions]--carb cleaner --disconnect carb --and tear down the carb becareful their are small parts inside using cleaner cleal all parts and housing --in the rebuild kit should be instructions on putting it back together
15 minute Carburetor Cleaning Rebuild Yamaha 20HP outboard 4 stroke How to Clean an Outboard Carburetor Outboard Motor Carburetor Cleaning

2003 Yamaha 115 4 Strokes Operation and Idle Demo ...

? 2:29
www.youtube.com/watch?v=sMmionreldM
Dec 29, 2012 - Uploaded by Peter Richards Engines for 1989 Larson boat with dual 115 Yamaha 4 StrokeEngines. ... Outboard Yamaha 150 HP DIRTY idle JETS, Motor will not idle .

Try contacting Yamaha direct or a reputable outboard dealers for the address, they will point you in the right direction, like all motors manufactured the makers keep a record of the serial number, i recently tracked this down for a friends Perkins Diesle engine that is running a generator in the middle of Spain, it turns out that his engine came from a 1912 lorry.

Good luck
lacala69

check your thermostat is working ok.

New plugs and service

You got it :

http://www.yamaha-motor.com/outboard/service/viewmanuals/outboard_manuals.aspx

here !