Re: Water Getting in Top Cylinder Yamaha 40hp Twin

[Don]

Well, I really don’t think you have a fuel pump issue, so I wouldn’t spend time ripping that apart. Keep in mind that you seem to have a lean condition while idling, a bad fuel pump diaphragm would cause a rich condition in the crankcase it is connected to. You could diagnose a weak fuel pump by pumping to fuel line primer bulb while the engine is running to see if RPM picks up.
Did the engine have more WOT power RPM when you first ran it, before the reed problem occurred? Or, do you just feel the engine should have more power….
Again, I just don’t want you going crazy looking for a water leak that I’m thinking is not there. When I get frustrated with stubborn projects like this, I have to remember (force myself) to return to the basics. Most problems are basic ones, we create our own confusion by skipping over basic issues and looking for/sometimes creating complex problems that aren’t there.
So, the engine doesn’t idle real well, but idle quality can be improved by feathering the choke, correct? Again, I’m not too familiar with Yamahas, but am assuming it has two carbs, am I correct? If so, you problem could be as simple as a synchronization/linkage problem. On the OMCs, both butterflies must be completely closed when idling….If one is cracked open ever so slightly, the engine will run lean, just like yours. You have had this thing apart several times, it surely is possible the synchronization is messed up. Do you have the Yamaha manual showing the step by step synchronization procedure? Are there any sort of adjustable needle valves for idle, or does it have fixed idle jets?
Let’s just say you check the synchronization and it is spot on…Which cylinder is running lean at idle, or are they both? Manually tickling the choke valves on each carb (disconnect choke linkage between two carbs) can help isolate a problem cylinder. Let’s just say you find that the top cylinder is the problem cylinder by tickling its choke valve to keep RPM up. So, now we are wondering is it a carb problem, or some sort of internal problem causing the lean condition….Well, we could just swap the carbs at this point….Let’s just say the bottom cylinder now runs lean with the carbs swapped, so now we would conclude that there is a carburetor problem seeing that the problem followed this carburetor to the bottom cylinder.
If the problem stayed with the top cylinder after swapping the carbs, then we would conclude we have some sort of problem with the top cylinder/reeds/intake manifold causing the lean condition….
See what I’m getting at here? Let’s try to keep things simple and perform tests that will provide us with fairly concrete conclusions….So, your next step should be rechecking the Yamaha synchronization/linkage procedure thoroughly. I’m sure this is step by step procedure, not unlike OMCs, it is very important to perform every step in the order spelled out in the manual.
Please know I am not trying to "minimize" your problems here, but I just don’t want you to get even more frustrated looking for a major problem when the issue could be something very basic. And, you can safely assume I harp on these things only because I have "relearned" this lesson several times in my life!
Hopefully, a Yamaha guy can chime in if you don’t have the OEM manual showing the step by step synchronization process for this engine.
D