4.3 L Spark knock *** Help please...

[Happy Days]

I had my old 4.3L Mercruiser replaced with a new block with a 4 bbl carb and roller cam. I have TB IV ignition with the control module mounted on the distributor. The engine runs great and has lots of power. The problem is a light spark knock that occurs in the mid power range. I know it's not my imagination because my wife can hear it too. <br /><br />Retarding the timing makes no difference except it runs worse. If I advance the timing, the spark knock gets severe at WOT. I'm running 87 octane now.<br /><br />Is it possible I should be running higher octane fuel or maybe my problem is in carburation??<br /><br />Any advice would be appreciated.

 

[Haut Medoc]

Re: 4.3 L Spark knock *** Help please...

Higher octane can't hurt......J

 

[LubeDude]

Re: 4.3 L Spark knock *** Help please...

Good chance that your problem is in the prop, what are your RPMs at WOT? Posibly luggung the engine.

 

[robfogle]

Re: 4.3 L Spark knock *** Help please...

I thought I read somewhere that the 4.3L needed 89 octane or higher.

 

[Bondo]

Re: 4.3 L Spark knock *** Help please...

If the Carb. is going Lean at the transion, or in the 2ndary circuit,..<br /><br />It can also cause Dentonation........

 

[bluewater19]

Re: 4.3 L Spark knock *** Help please...

If you replaced that old motor with one that says vortec on the valve covers there is your problem.<br />The vortec motors have higher compression and require 89 octane minimum. The vortec motors were designed for fuel injection and will live off the knock sensors in a car with low octane fuel, but your TB-IV ignition doesnt retard timing when detonation happens. Try using the good stuff.<br />I had this same problem when I switched my 1992 over to vortec heads. If this doesn't help listen to bondo, He is the man when it comes to the 4.3 chevy!

 

[Happy Days]

Re: 4.3 L Spark knock *** Help please...

Thanks guys.. My WOT RPM is 4600... and no detonation. The pinging occurs between 1400 and 3000 rpm. Below or above those speeds, all is well. I'm thinking I may have a vacuum leak but it runs real good. just a little pinging. <br /><br />I wonder if I need cooler plugs.. What do you think guys?

 

[Limited-Time]

Re: 4.3 L Spark knock *** Help please...

See Bondos post