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So I bought a used Yamaha 5C earlier this year and have been having a lot of trouble getting it running well since. First of all, a bit of background - the guy I bought it off said the head had been replaced by the previous owner, but he hadn't used it as it came with a boat that he already had a motor for.
The outboard will eventually start after a lot of pulling, however it runs a bit rough and for only a few seconds if you give it a little throttle before abruptly stopping.
I started with the usual spark and compression tests which were all up to scratch. Thus I assumed that it was a carb issue, so I ended up just replacing the entire carb with a brand new one as it wasn't expensive, plus a new spark plug just to play it safe. However the issues still persisted!
Next I noticed that the compression was a bit higher with oil introduced to the cylinder, so I decided to replace the piston rings and thought I might as well replace the piston while I was at it... Lucky I did as the piston had a small gash in the side of it. But that got me thinking, how did that happen? So I then decided to completely tear it down, replace all the gaskets and seals, and put it all back together with gasket maker where necessary as per the service manual. Meanwhile, after clearing a lot of gunk built up behind the cylinder head cover where the coolant water flows through, I noticed that the cylinder/head had clearly been damaged and welded back together (note that the head and cylinder are a single part for this model, thus no head gasket)... That explains the damaged piston. From the inside of the cylinder no damage was visible, so perhaps the head had cracked due to overheating? Anyway the compression is good now and the same with and without oil introduced to the cylinder (95PSI, good for a 25-30 year old engine), so I am struggling to believe that that is part of the problem.
IM BAFFLED! If anyone could provide any further advice that would be extremely appreciated.
In summary, I have replaced all of the following parts and the issues still persist:
Many thanks in advance!
The outboard will eventually start after a lot of pulling, however it runs a bit rough and for only a few seconds if you give it a little throttle before abruptly stopping.
I started with the usual spark and compression tests which were all up to scratch. Thus I assumed that it was a carb issue, so I ended up just replacing the entire carb with a brand new one as it wasn't expensive, plus a new spark plug just to play it safe. However the issues still persisted!
Next I noticed that the compression was a bit higher with oil introduced to the cylinder, so I decided to replace the piston rings and thought I might as well replace the piston while I was at it... Lucky I did as the piston had a small gash in the side of it. But that got me thinking, how did that happen? So I then decided to completely tear it down, replace all the gaskets and seals, and put it all back together with gasket maker where necessary as per the service manual. Meanwhile, after clearing a lot of gunk built up behind the cylinder head cover where the coolant water flows through, I noticed that the cylinder/head had clearly been damaged and welded back together (note that the head and cylinder are a single part for this model, thus no head gasket)... That explains the damaged piston. From the inside of the cylinder no damage was visible, so perhaps the head had cracked due to overheating? Anyway the compression is good now and the same with and without oil introduced to the cylinder (95PSI, good for a 25-30 year old engine), so I am struggling to believe that that is part of the problem.
IM BAFFLED! If anyone could provide any further advice that would be extremely appreciated.
In summary, I have replaced all of the following parts and the issues still persist:
- All gaskets and seals
- Carburetor
- Piston and rings
- Impeller
- Thermostat
Many thanks in advance!
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