My Johnson has a weak stream

[chrisgt]

I have a 1981 35hp johnson that has an intermittent, spurting stream at hot idle; see video:

This doesn't happen at cold idle, but perhaps because the idle drops down some after the engine is up to temperature?
I don't know if this is an overheating engine, blown headgasket, normal behaviour, or what.

I installed new spark plugs and after a few hours of running they're a nice chocolate colour, maybe just a hair rich, but it's pretty close.
Compression checks out to just under 125psi both cylinders.
Looking in the cylinders with a boroscope I don't see evidence of steam cleaning from a blown headgasket.

I have replaced the impeller, housing, gaskets, etc. It is possible I screwed something up and have an internal leak, but advise on what to look for would be appreciated.

I really have no idea why this is happening, so thanks for the advise.

 

[chrisgt]

Well due to the fact that I'm aggressively impatient I went digging and found the problem (or at least part of it).
The thermostat was broken and wedged sideways in the thermostat bore:

Of course no matter how much I heated the bolts half of them broke off (the bottom half that are a pain to access). I know I can pull the cylinder head to do it on the bench but i really, REALLY do not want to do that.
I was able to extract three of the broken bolts, I'll tackle the rest tomorrow, but it gives me hope that I'll be able to fix it up.

Unless of course when I put it all back together if it still does the same thing and needs a headgasket, I'm hoping really hard that isn't the case...

 

[chrisgt]

Just in the event you're wondering if it's fun to take these stupid things apart: it isn't.

No amount of heat or swearing prevented half the bolts from snapping off, fortunately with a bit of patience I got them all out without damaging the threads: