Re: 1974 Evinrude 9.9 won’t idle at low RPMs, need help

[Don]

Well, this particular engine might be in pristine physical condition due to lack of use because it has been a problem child since new. The 74-76 9.9-15s were not OMC’s best efforts and had several update kits to improve running problems. All that being said, I don’t want to just condemn your engine because it is a 74-76 9.9hp.
Like everyone says, start with simple stuff. Compression, spark, etc…. The engine sounds like it is running on two cylinders, just dying out due to lack of fuel, but start with new plugs gapped at .030". The last time I knew, these engines used Champion UL-81J spark plugs, I have never tried the NGKs, but guys swear by them. Perhaps the fuel pump is weak, or there is an air leak in the inlet side of the fuel system. You mention using an old tank that appears to be unvented. Pump the fuel bulb up hard before starting the engine, then have an assistant pump the fuel bulb while it is running. A fuel supply problem is indicated if the idle improves using this method. Please do not attempt to rebuild this fuel pump, there are many tiny pieces that are tough to reassemble properly.
This could be a carb synchronization problem, a throttle plate that is not closing properly/improperly adjusted/partially stuck open would cause the exact running symptom you are experiencing. These early engines had some funky lockout linkages that were connected to the throttle butterfly, I have seen many that were stiff causing the throttle plate not to close fully. Someone may have messed with the synchronization and messed it up as well, causing the throttle plate to be held partially open at idle.
You have had this carb apart, and it looked clean inside. Generally speaking, these are pretty rugged/simple carbs, but it surely is possible to have a plugged idle passage that can only be investigated by removing the welch plug. This is not a difficult operation, but the mixing pocket is very shallow, so extreme care must be used when drilling through the plug. The simplest way to rule out carb problems is by just trying out a different carb on the engine, but I am assuming you don’t have access to another carburetor.
You may have an intake manifold problem, bad gaskets, or broken/bent reed valves. Does it look like the intake manifold has been off before, do the bolts look tampered with? I don’t recall upper/lower seals being a big issue with these engines, but anything is possible. But, like Casey says, you would probably see evidence of oil drooling down under the mag plate if there was a problem.
You mention that you just bought this engine, so I am assuming it has never run properly for you, is that correct? Did you get any service history when you bought it?
Remember, stick to the simple stuff, most problems are simple until we confuse things by digging into bigger issues and creating more confusion. You have picked a real challenge for your first project!