Mercruiser 7.4 Horizon compression and hard warm starts

[DGAM]

With these Starboard engine compression test numbers.... what should I do??? .. the motor start hard when warm.. but if you goose the throttle a few times it does start and runs well.

I have about 1300 hours on these Merc 7.4 Horizons 2001 vintage!

2 165
4 124
6 145
8 120

1 105
3 159
5 150
7 160

 

[scoflaw]

rebuild the heads at the very least. If the cylinder bores look good and it wasn't burning a lot of oil.

 

[Wanderer 290]

Additionally, do another comp check and spray a small amount of fogging oil in each cylinder just prior to checking.
If the comp numbers increase then there may be a ring / cylinder sealing issure. If the comp numbers do not show an increase then a cylinder head rebuild will probably cure the problem.

 

[hottoddie]

Take the boat for a really good hour long run at 80% throttle and do a compression test before they cool down. Then you will get a good set of readings.

 

[Seapig]

I’d redo the test. Make sure it’s been run for a while at sea first. Pull all the plugs and test one hole at a time with the throttle wide open. If it isn’t spinning like a cat chased by a dog you may have a starter/battery issue that can affect the compression readings.
Any weak holes that are found should be oiled and retested. 3-4 squirts with a pump oil can. If the compression returns to normal it indicates leaking rings.

 

[northern]

Long time ago we had a 28 footer at water line with twin 3.8 V6. It was at the north end of Vancouver Island. One engine lost 50% of its power. We went into a small government dock and the person beside me was a drag racer he showed me picture of his car long with little wheels on the front. He had his mechanic on his boat. Told him my problem. His mechanic offered to look at engine. Told me to start it. Then said that cylinder is not firing. No test equipment. Told me to remove spark plug electrode was flattened to plug he said it was a brass fitting from carburetor that probably got dropped into engine. I bent electrode back and engine sounded fine to me. He then said you have ring problem some are carboned up. Asked if I wanted it fixed. He said I will either blow your engine up or fix it. He told me to take engine to 1,500 RPM and over 5 minutes poured a quart of water into the carburetor. He said they do that on old drag engine to loosen the carbon. I had done a compression check on engine year before and one was 40 PSI lower than the other. When we got home I checked and all were within 10 PSI. It is you choose if you do this as he was very blunt it could damage your engine. In my case it saved me doing engine work.

 

[Seapig]

Long time ago we had a 28 footer at water line with twin 3.8 V6. It was at the north end of Vancouver Island. One engine lost 50% of its power. We went into a small government dock and the person beside me was a drag racer he showed me picture of his car long with little wheels on the front. He had his mechanic on his boat. Told him my problem. His mechanic offered to look at engine. Told me to start it. Then said that cylinder is not firing. No test equipment. Told me to remove spark plug electrode was flattened to plug he said it was a brass fitting from carburetor that probably got dropped into engine. I bent electrode back and engine sounded fine to me. He then said you have ring problem some are carboned up. Asked if I wanted it fixed. He said I will either blow your engine up or fix it. He told me to take engine to 1,500 RPM and over 5 minutes poured a quart of water into the carburetor. He said they do that on old drag engine to loosen the carbon. I had done a compression check on engine year before and one was 40 PSI lower than the other. When we got home I checked and all were within 10 PSI. It is you choose if you do this as he was very blunt it could damage your engine. In my case it saved me doing engine work.

Im a retired auto/truck mechanic. This method of de-carbonizing is very effective and really not risky if done correctly. The engine MUST be fully up to operating temp. Hotter the better. It’s best done immediately after a workout when the carbon is still hot. The RPMs need to be well above idle (2000-3000) and the trick is to feed as much water as you can without stalling the engine. It’ll slow it down some but you don’t wanna dump it in quickly and cause a stall and hydro lock that could potentially bend a rod.

 

[RollerCoastr]

This method of de-carbonizing is very effective and really not risky if done correctly.

I once mentioned how Sea Foam brought my genny back to life to a friend who manages a fleet of diesel city buses. He said the same thing about water. I don’t have the nerves or skill to try it myself though.