Hi Guys,

Its been a long time, but my engine/boat problems aren't over

My previous thread where I replaced the engine is here:

http://www.correctcraftfan.com/forum/forum_posts.asp?TID=51037&PN=3&title=1989-sn-water-in-oil-wet-plug" rel="nofollow - https://www.correctcraftfan.com/forum/forum_posts.asp?TID=51037&PN=3&title=1989-sn-water-in-oil-wet-plug

Since fitting the "new/used" engine and getting it running last year, I only used the boat for 3 days and everything was great.

Then a week later (still in December 2023) the boat was running great, but then while cruising for about 10 minutes at ~2500 rpm I heard a knocking sound.

It was a deep knocking sound, at a rate of about 2 knocks per second. I slowed the boat down, and it idled fine, and the knock slowed down to about 1 knock per second. I sounded like it was coming from the back of the engine (transmission side) but it is hard to be sure as the cover was closed.

I shut the boat down, but couldn't see anything wrong. I tried to restart, but it puffed and spluttered and would not start. So I towed the boat in. I am not sure if the starting problem is directly related or maybe it was totally heat soaked?

I have been so busy this year that I haven't had much time to diagnose.

- Plugs look fine

- Oil looks good - no metal shavings

I have pulled all the plugs, disconnect ignition and raw water pump, and cranked the engine. It turns over smoothly, no noises.

I then did a compression test, and all cylinders were around 140 psi, but what was strange is than during the compression test of cylinder 4, it made a puffing sound (almost like a backfire) at the intake area. None of the other cylinders did that.

Any suggestions or ideas?

As a starting point, maybe pull the valve cover off and look at the rocker arms and springs for #4 especially the intake because of your description.

leak down test on number 4. and maybe the rest. i had one do that a number of years ago. turned out exhaust valve stem retainer was not installed correctly and it dropped the valve on top of the piston.

Managed to remove the valve cover, and this is what I found. No. 4 exhaust rocket bolt has sheared off.

https://ibb.co/LZRt06Y" rel="nofollow">

- Is it possible to pull the stud and replace without removing the head?

- I believe the studs are press fit?

- Could this have caused the knock sound? I would expect a tick not knock?

I saw a video of someone that tapped the broken thread on the stud a bit longer and use a nut and washers to extract it. Fitting a new one would probably require a press? Or tap the hole and use a threaded stud to replace?

I do have my previous engine's heads as spares, but it is a bit different (newer style with bolt down rocket gear). I assume mixing different heads is a bad idea? These are the spares I have.

https://ibb.co/b5nb9Dp" rel="nofollow">