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One thing about our 6640 VFW. They watch that if you drink a beer someone will remind you to also drink a bottle of water.
In the past this post had a lot of WW2 guys who would get to the point sometime one of them would fall off the bar stool.
Back then the WW2 guys did not want any Viet Nam...
There is a tool to test the springs on the heads without removing them.
There may be one that can be rented from one of the big box auto stores.
Speedway motors #54562390 @$142.99
Your studs and valve settings appear to be good. Try to get the spring testing tool.
You may have either a broken ot some weak valve springs. To replace them you will have to remove the manifolds. They are easy to replace with the KD tool. Are your rocker arm studs positive stop?
Make sure none of the studs have pulled out.
I cannot remember having a leak at that point with a new stainless shaft and bushing. Make sure the shaft does not have a scratch where the seal rides.
You need sierra marine #18-2150 stainless steel shift shaft &18-7153 shift shaft bushing.
Google mercruiser shift shaft bushing removal tool.
Make sure the tool is completely engaged on the bushing. If you have an impact screwdriver 3/8” drive and a good 6 point 3/4” socket you should be...
At this point you should order new lines. Removing the drive should give enough room to get to the fittings. Take some pictures of the lines as they are mounted before removing them.
Also a good time to replace the bellows and grease the joints.
A 3/8” flare nut wrench will help. If the...
When I had the Donzi I found some pieces in the check valve. It was raw water cooled and the check valve was located just below the circulating water pump. In your model it looks like it is low on the port side.
The distribution housing should also be checked.
Depending on your boat these...
Does the noise happen with the belts off? If not then you may not have the power steering or sea water pump mounted out of alinement with the brackets.
One other place to check. Look inside the intake manifold runners just to be sure there are no restrictions.
There may be a rust or dirt build up in the fuel rail to the cylinders giving the problem. Try blowing out each part of the rail.
Have the fuel injectors flow tested.
Try disconnecting the throttle cable at the throttle body then reconnect the shift cables and see if there is a difference. If that works then back off the throttle body cable where it just closes the throttle body. There may be too much closing pressure on the throttle stop.