4HPs produced from '69-'76: Cooling -- Plugs -- Tuning

[Good Trout To You Sir]

Have been testing a 4906b on a square stern Old Town canoe (rated for 4HP). After a few uses with significant time at WOT, compression is holding at 90psi per cylinder. Per Ramblings, bearings in this vintage require an oil mix of 24/1 and use JC4 plugs (which in fact I found installed when I acquired the motor).
Cleaned old plugs and contacts simply to assess operating status after addressing a fuel leak in carb. Seems to run OK --- but back lower metal cowl becomes much hotter than I expected and on one occasion the flywheel became hot to the touch (much more than "warm").
Lower metal cowl in back burns me if I try to keep a hand on it after running WOT for 5 minutes. Some posts on this model point out the exhaust is not fully cooled. Any advice from owners of this model run of what's normal heat vs too hot?
Before installation of new impeller, forced water up water tube and observed it running back down the inside of leg from location other than water tube insertion point ---- but I could not see clearly where it was emitting. Was I observing water flowing through the cooling circuit or is it possible it was emitting from an overflow and that I have a blockage?
Particularly since I'm running it 24/1, I'd like to swap out old plugs for the JC6 now recommended by BRP, but that's a hotter plug and I'm reluctant to make the swap prior to addressing a potential overheat situation. Is this a legitimate concern?
Once I address the above, I'd like to tune (adjust timing) so motor runs its best across all speeds. It's OK now, but if there's room for improvement with a procedure that does not require a timing fixture, I'd like to know.

 

[racerone]

Use a timing light to check the timing as it is now.----From 1964 on these can run at 50:1 mix.

 

[F_R]

You wrote "Before installation of new impeller, forced water up water tube and observed it running back down the inside of leg from location other than water tube insertion point ---- but I could not see clearly where it was emitting. Was I observing water flowing through the cooling circuit or is it possible it was emitting from an overflow and that I have a blockage?"

Good question. Actually, it is slightly common for the water tube to become bent from pushing from the bottom when reinstalling the lower unit to the exhaust housing (use lube to prevent that). Point is, the bent tube may rub on the spinning drive shaft, wearing a hole in the tube. Then some of the water spills out the side of the tube instead of going on up and through the powerhead.

 

[Good Trout To You Sir]

You wrote "Actually, it is slightly common for the water tube to become bent from pushing from the bottom when reinstalling the lower unit to the exhaust housing (use lube to prevent that). Point is, the bent tube may rub on the spinning drive shaft, wearing a hole in the tube. "

Never considered this! There was a slight bend in the water tube I noticed first time (only time) I removed lower unit. I hope this is not my problem, but can add it to my troubleshooting list.
When I forced water up the tube it seemed to emit exclusively from an area very close to the short tube (exhaust escape?) on "other side" of powerhead relative to water tube.
2 questions: Is cylinder head removal enough for inspection/cleaning of cooling circuit from point of entry to powerhead? Where can I find diagram of water flow for better understanding of pathway?
Thanks for your help!

 

[F_R]

Notice the "bump" on the top of the flat exhaust cover on side of powerhead. There is a horizontal water passage behind that bump, with a small hole in about the middle of the horizontal passage. That is the most frequent obstruction point----if it is indeed obstructed.