I have a 2001 Yamaha 250 OX66 with 225 hours. Well maintained and runs great. Unfortunately I have begun to notice a tapping / knocking noise at idle. It continues through the power band. Took it in to a dealer introduced me to the engine salesman even before he heard the motor run!
Disgusted, and decided to do my own diagnostics. I checked the following;
-O2 Sensor draw tube (carboned up, replaced it, but no fix)
-Lower unit
-Compression 110/115 all cylinders
-Max RPM 5200-5300
-No metal visible in the cylinders
-Plugs fine (some carbon but fine)
-Removed the plugs and using a dowel tried to move the pistons to check for wrist pin movement. Nothing.
I listened with a stethescope and it seems to be coming from the bottom of the engine perhaps louder in the lowest cylinder starboard side.
A couple questions;
-Rodbearing?
-Torsional Dampener?
-Carboned up since I don't use ringfree or yamalube?
-Would I do damage just to run it into the ground?
-Rebuilt or New motor
-I am a pretty decent backyard mechanic. Could I swap the powerhead myself?
Skidoc,
I am no rodbolt or a professional mechanic but the same thing hapened to my littel 25 hp 2001 yamaha at only 50 hours. Knocking sound from the lower/center part of the engine. Luckily a Yamaha tech stoped by the dealer, and listened to it with a stehescope. Not sure what it was, he took the powerhead back to the factory for further analysis. his suspision was out of balance crank shaft. Whatever that means. Anyway, now I have close to 250 hrs on it an I have other noise( Just plain noisy engine) but no kncking sound like before. I was given a new short block under waranty. The head is the original.
start with some decarbonizer (combustion chamber cleaner, quick clean, engine tuner) and go from there. if it still knocks start looking into breaking it down possible collapsed piston skirt, or ron bearing.
were the ends of the O2 sensor draw tube shiney? if so the draw tube could be bouncing between the holder and the block. it will sound just like a wristpin.
it could be the torsoinal damper but I doubt it.
but its not a hard motor to rebuild, just time consuming and heavy.
you also may wish to check the oil condition of the lower unit.
noise tends to telescope up the mid section.
I've got another question since power head swap was talked about.... I wanted to pull the head on my 99 F100 to clean out the pan. I removed the six bolts going up into the head down low,two others up high the four short ones going down it the lower case and lifted up on the head .... it didn't separate, the boat just started to lift up off of the trailer. Did I miss something that my book didn't talk about or is the shaft frozen in?
Rodbolt, I actually removed the draw tube and ran it. Still makes the noise. Also remived the lower unit and ran it. Still made the noise. I put it in the water and it now sounds like a sewing machine. It's running great though.
I am pretty certain that I need a rebuild.
If it is a wrist pin or piston skirt issue can I just find someone to swap the entire piston assembly? Most guys I talk to want to replace the entire powerhead.
you can find people out there that will just replace the bad wrist pin or piston have done this myself for rental fleets a lot in the past not so much anymore did this once to an old johnson and ran it for 300 more hours before selling it
(had to sell it med bills or I would still have it)
I took the motor to a "reputable" shop today. They pulled the heads off and pointed out the problem. I have scuffing at the bottom of the #5 cylinder. Otherwise, the motor looks like new.
They are going to send the block out to bore the #5 and hone the rest. I will get a new piston for #5 and new rings for the rest. They are sending the injectors for testing as well.
Rodbolt (or anyone else) - I have 2 questions.
(1) What could have caused the problem? I did not use Yamalube. The motor sat on a regular basis as it has less than 200hrs in 6 years.
(2) What else should I replace since I am already looking at a total disassembly / assembly?
Rodbolt - Engine Knocking Yamaha ox66
Linear Mode
