4.3 Mercruiser Spark knocking at high rpm

[Jpwendt94]

Hello,

I am new to forum and have had my boat three years. It is a 1997 VIP Valiant with a 4.3lx gen+ thunderbolt 4 ignition. My boat as had the same issue the last year of when it get to operating temp 160-180 and under load it will start spark knocking at full throttle. You can back out of the throttle and maintain speed for a little while then it will start doing it again until you are down to no throttle and have to stop and shut the boat off and let it cool down. Also the manifold on the left(looking at front of engine) is significantly hotter the the one on the right. I always run 89 or 91 octane if available. I had a full tune up done on it last year new plugs, wires, cap, and rotor they also adjusted timing, cleaned carb and told me everything was set to Merc specs. I have had another mechanic look at it recently with out digging real deep he thinks it could be the ignition control module. But with a new one being so expensive im curious to know how to test this one. Or if anyone has any other opinions on what could be the cause of this issue.
Thank You

 

[Lou C]

It’s possible your fuel mix might be excessively lean; high running temps can also cause this. What do the center electrode insulators on the spark plugs look like? Before changing any parts I’d want to see what your total timing advance is at the specified rpm. Spark knock can also be due to excessive carbon build up in the combustion chambers; this effectively raises the octane requirement of the engine.

 

[Scott Danforth]

spark knock can be from many things

high operating temperature (180 is high)
quality of fuel (check contents of your fuel filter)
lack of fuel flow to the engine (AS valve/ plugged fuel pump, etc causing it to running lean)
carb gunked up
Carbon deposits in the combustion chambers
improper timing

that little rattle of pre-combustion is your motor eating itself. I would fix it asap.

 

[Lou C]

PS both gas docks I use sell only 93 octane; I’m not saying the engines were designed for that but in all the years I’ve had this boat I have not had pinging or detonation.
PPS if you have the one piece Merc manifolds on that V6 think about getting rid of them and converting over to the 2 piece units. If they are like the similar OMC one piece units they way they had to cast them results in smaller internal cooling passages and that can raise operating temps.

 

[Jpwendt94]

After reading other posts and forums I am also realizing that I am reving to high when WOT the tach on the dash is showing right at 6000 rpm I am curious if with the engine over reving like this, if this could be the root cause. And curious why it’s not limited out at 4800 like I’m reading. I have a 19 degree prop on which I feel is not to much for my boat

 

[nola mike]

4800 is what you *should* max out at. There is no rev limiter. But I seriously doubt you're actually at 6k. You need to verify your readings with another tach.

 

[Lou C]

Agreed it is a bit frightening to imagine what would happen to a 4.3 V6 with the offset crank throws at 6000 rpm!
I have mine propped for 4900-5000 rpm lightly loaded, that makes it perfect with a normal load.
The fact that they adjusted the timing makes me think that at least that should be re-checked, that and total timing advance at the specified RPM. Then, look at your spark plugs, the center electrode insulator, to see if your fuel mix is excessively lean. If so that must be fixed.