Sparkplug diagnosis

[LaborOfLove]

https://photos.app.goo.gl/NWEJdxVUX1KgyUCm9

Here is a video of the engine running. Let me know if you here a knock. Thanks again for all your help guys. It’s greatly appreciated!

[mmwjr]

Originally Posted by LaborOfLove

I’m no expert, for sure, but I don’t hear anything that sounds unusual to me. Ugh on the starboard side. Hard to get back there! Do they go bad? Engine was sitting for 5 years before last year. Thus far lots of parts have needed replacing.

No they typically don’t go bad.

[mmwjr]

No I don’t hear any knocking. If you have the engine serial number you can go to Mercruiser parts on line to look up part numbers.

[LaborOfLove]

Thx. Quick update. I was down today and tested the knock sensor. Connected volt meter to the purple/white wire that also is used to ground out for base timing. That had 9 volts on it. I compared to the other strong running motor which also had 9v. I also swapped the icu’s, and everything checked out the same. Thinking that means my issue is not related to the knock sensor or system.

[jrsick]

Now that you know the knock sensor is getting voltage, the next step is to see if the knock sensor works properly. Knock sensor test is in the service manual in section 5B.
Have you tried 89 or 91 octane fuel? Higher octane fuel may smooth out the engine some.

[LaborOfLove]

Thanks again. I was thinking if the sensor was failed or was detecting a knock I would see less than 8 volts on that test? Is that how it’s supposed to work? I kind of moved on once I saw it was 9v since I couldn’t find the darn thing and can barely get my head back there. I will get some octane booster and put it in. Just topped off so it looks like I’ll need 4 or 5 bottles of the stuff.

[jrsick]

Take a look at the Thunderbolt V Spark graph, the knock sensor doesn't kick in until 1000 rpm. You can't check the operation at idle or static.
I don't think the knock sensor is part of your low rpm/low power problem. You plugs don't show signs of detonation, which is what the knock sensor corrects. If your knock sensor is giving erroneous signals that should be obvious at 3600 rpm.
Have you checked your carburetor? Does the throttle lever move through full range? Does throttle cable operate smoothly and fully? Have you tried the 8 point carburetor check? A restricted needle valve, faulty float, or weak fuel pump could give you the symptoms you describe.

[LaborOfLove]

Originally Posted by jrsick

Take a look at the Thunderbolt V Spark graph, the knock sensor doesn't kick in until 1000 rpm. You can't check the operation at idle or static.
I don't think the knock sensor is part of your low rpm/low power problem. You plugs don't show signs of detonation, which is what the knock sensor corrects. If your knock sensor is giving erroneous signals that should be obvious at 3600 rpm.
Have you checked your carburetor? Does the throttle lever move through full range? Does throttle cable operate smoothly and fully? Have you tried the 8 point carburetor check? A restricted needle valve, faulty float, or weak fuel pump could give you the symptoms you describe.

I’m thinking it’s not the knock sensor, tested up to 3k rpm and was still 9v the whole time. Carb throttle cable opens all the way. Not sure how to test fuel pressure. I’ll have to do some research. Next step will likely be clean/rebuild the carb. Was hoping to try the easy way and fill the fuel filter with seafoam first, but the darn thing is stuck on there and I didn’t have an extra one available if I destroyed it pulling off. Does anyone think it’s worth adding seafoam to the gas tank? Just filled up and have 100 gallons right now. So I was thinking I’d run it down a bit closer to 75 gallons and put in 6 cans of seafoam to see if that clears anything up. Really need to pull the carb and clean, but with vacation this week timing isn’t right to start tearing things out since it still runs strong up to 3500 rpm and I plane out just fine. Fuel burn is about 1 mile per gallon, so not bad right now. Any thoughts on that plan of action?

[jrsick]

I recommend you put the seafoam in the fuel filter. You should be able to find a filter tool or strap wrench that will get the filter off for you. I think you need concentrated cleaner running through your carb to get it clean. I also recommend a compression test.