I've found that my broken starter probably resulted from water in a cylinder.
Where could it come from?
The boat ran fine when I stored it last spring. During the summer, the CMI hearers were removed so that the Silent Choice could be repaired, then reinstalled. It was not run for 6 months.

The first time I started the motors, one moter was very difficult to start: allot of "bucking and grinding." It did start, and ran fine after warming up. Every time therafter, I had the same starting difficulty, until the starter broke its housing.

Also, the last time it ran, the oil pressure was down about 15 pounds from normal.

Could the header removal be related!

Thanks for any insights.

Unless the headers are cacked and leaking into a cylinder. Just removing them shouldn't cause any trouble.
Maybe a cracked cylinder or head is causing the leak. Possibly a bad intake manifold or head gasket.
Unfortunately you'll have to tear it down to find the answer.

Thanks.

Unless the headers were deteriorated
badly internally, I doubt that just removing
and replacing them would cause them
to develop an internal leak.
Your lower oil pressure may be coming from
water diluted oil, which could be either a
gasket failure, or cracked component ( head,
header)
Start with the headers, check them for leakage.
my .02
jt

Remove the spark plugs and turn the engine over to remove water from the cylinders. This will also tell you which bank is the culprit. Check compression on suspected cylinder and compare it to the others. A bad head gasket will usually show low compression but a leak-down test is sometimes more revealing.
Note......water injestion can and will cause severe engine damage because water will not compress and bent connecting rods and ballooned cylinders are sometimes the result along with damaged bearings and valves.

Its always hard to make a call without looking at the parts: However, you talk about fixing the Silent Choice and no one has questioned that ,,, what about reversion from the exaust system?? The above advice is good, start with plugs and find the bad hole, then remove exaust and leak test , it should not be hard to find.

RHC

the problem for me was a broken exhaust valve spring

As said there could be a number of issues, if you did HYDRAULIC then the head could be in trouble, Rod Bent (I did that once in a HP500) , etc.

Check the compression or leak down. I know it is quite a bit of work, but I would pull it and go through the motor.

I have recently one through something similar with my 350. I still am not 100% sure what caused it and there are numerous things that can cause it. Some of which have been mentioned here already.

Things I tried; New exhaust manifolds and risers, adjust timing, check for cracked heads, cheak for blown head gaskets, check for cracked block. I am currently doing a complete rebuild. Hopefully the problem will disapear.