Detonation issues...rebuilt mercruiser 165

[WOEISMEIGOTTA470]

Thank you guys for all the input, all very good different points. Just heard back from the shop, the carb is ready to be picked up. Turns out it had .061" orifice, quite a huge difference of about .020"!! I'm thinking this was my problem, going to pick it up today and will retest soon!

 

[WOEISMEIGOTTA470]

Alright, so last week I was able to do another water test after putting the carb back on. The new jets didn't make any noticeable difference, the knocking sound still remains under engine load. So I was a bit disappointed to say the least, but athe least I know it's not a carburetion issue.
Then I did some digging on the forums and came across someone else who had the same exact symptoms going on, so I posted the link to the thread below. Now I'm curious if my problem is indeed a head gasket (or similar), which is what fixed the problem for the OP in the other thread.

I also conducted both a compression test and a leak down test.

Leak down test results:
Cyl 1.* 11%
Cyl 2.* 28%
Cyl 3.* 17%
Cyl 4. 9%
Cyl 5. 9%
Cyl 6. 9%

Compression test results:
Cyl 1.* 140psi
Cyl 2.* 120psi
Cyl 3.* 152psi
Cyl 4. 160psi
Cyl 5. 157psi
Cyl 6. 157psi

I think my best course of action is to remove and inspect the cylinder head. I'll be looking closely at cylinders 1-3.

 

[WOEISMEIGOTTA470]

Here's the link to the thread I found of the same issue I'm having.

 

[WOEISMEIGOTTA470]

Update:
I decided to pull the head off and have it checked out. Machine shop found some valves leaking and also discovered ring material in a few of the valve seats. Shop suggested I take a look at my lower end and pull all the pistons, but I'm thinking the discovery might be from a previously blown engine. The bottom end seems to be in OK condition to me. So I went ahead and had them install hardened seats for all the exhaust valves and now the head is completely rebuilt. Looking to get it back on with a fresh head gasket and possibly another water test this weekend and see how she runs.

Here are some progress photos thus far...

Removed Head:




Close up...a few dings indicating a blown engine in a past life...


Head after the rebuild:






Cylinder walls are nice and clean...standard bore too




We'll see I'm really hoping this works...

 

[FreeBeeTony]

Just a thought...........I think the initial timing on an IL6 is 4* BTDC.

 

[NHGuy]

Check the capacity of the combustion chambers and the pistons. Add in the head gasket thickness to learn the total capacity and figure the compression ratio. Mark #1 TDC while she's apart. If he decked the head you are going to want a thicker head gasket.

 

[RobertThoreson]

Another thought, those dings could have been creating hot spots same as a carbon deposit does. Good call on not pulling the bottom end though. I too would cc the heads to find exact compression ratio