I am having the exact problem.
I have a 1978 Evinrude 75 hp on my 1969 Steury. It worked great for about 3 days. Over a week or so, the engine began to slow under load. Gradually the problem got worse, and now is very slow at full throttle, and won't idle unless the choke is engaged. Also has begun to not start often.
Early on I changed the plugs, and have noticed that the lower cylinder's plug shows no sign of combustion. It is wet, not dripping, but otherwise shiny and clean. The other plugs show signs of normal wear. Easy, right? You should try replacing the plugs if you haven't yet, as it is relatively cheap and might show you if all cylinders are firing. Are your plugs wet?
So, since this discovery I've done a pressure check and everything looks good; spark test and spark is strong; just like you.
Next, I replaced the carburetor (rebuilt, but from a highly rated eBay seller). It was easier than cleaning out myself (and I am not a pro at carbs), but less expensive than having someone else do it for me. I only took this step be she I know it's one cylinder, and had exhausted most other avenues. This also didn't work.
One more thing: in searching for a solution, I ran across the diagram linked below. Part # 49 hose isn't listed in the diagram, but I think it's supposed to link the nipples on the intake manifold to the lower cylinder. How is yours set up?Doing so should create more airflow to lower cylinder, I think. On mine the lower cylinder nipple was rubber plugged. So thinking I might connect the two to in read air intake to lower cylinder. But I doubt this will fully solve the problem.
http://www.marineengine.com/parts/j...&manufacturer=Johnson§ion=Intake+Manifold
