Hi Raymond,
Yes, I had a very similar problem when I bought my engine ('85 9.8hp).
Short version at the end for those who don't want the story...
Long version:
New to me engine, low flow at tell-tale. Opened up my pump, pulled out a broken impeller fin, cleaned up, replaced the impeller, and buttoned it up. Tested OK in tank. Took it to a lake, and things looked OK, so went out fishing. I stopped-and-started a bunch of times before checking again, and by then I noticed the flow was reduced but still safe. By the end of the day I was getting worried since flow was down to 2 drops per second - I returned at an idle to keep temperature down - the dribble was warm but not hot.
Back at home, I took the pump apart - all clear, no problem!
I used a pipe cleaner to poke up the water intake until I could see the pipe cleaner at the bottom of the pump housing - no problem.
With the lower unit removed, I then used a can of compressed air to try to blow in through the tell-tale hole - NO flow. I tried the other way, blowing up the copper tube in the leg, but again NO flow.
Started simple and removed the rubber hose exiting the top of the power-head. I blew air into the power-head - good flow through the system. I then blew air down the outflow hose towards the tell-tale hole - NO flow. I removed the rubber tube from the nipple at the tell tale, and could see through the tube so that wasn't the problem. But when I took a second look at the inside nipple for the tell-tale, there was a large oval of rubber plugging the tell tale from the inside. It appears a chunk of the impeller had travelled through the power-head, and eventually plugged the hole in the last inch of the cooling system. I don't know how the thing could have travelled through the system, it looked so big!
If I were you, I'd start with a similar test procedure. Assuming you're getting NO air flow through your power-head, you'll have to start taking things apart..
Short version: I had a piece of impeller that had travelled through the cooling system, and was blocking the outflow right at the tell-tale hole. Disconnect the easy-to-remove stuff and check for blockages.
Good luck!
