Preignition knock under a load 2000 Thunderbolt 5

[Stetson]

Preignition knock under a load 2000 Thunderbolt 5 with knock sensor. Can’t seem to find any threads on this exact issue. I’ve ran new hi octane fuel, checked cap, rotor and a couple of plugs ok. Tried to check the timing but not sure how to ground out the purple/white wire with out shaving it. It shows 12 degrees before tdc with out locking base timing. I moved the distributor a little while under way with little improvement. I would like to be sure I’m not burning a hole in my pistons! Could I have a bad ignition module or pickup sensor?

 

[Bondo]

Ayuh,..... Welcome Aboard,...... Check the Contents of the fuel filter for anything but clean fresh gasoline,.....

Boats tend to get water in the fuel, 'n a lean condition is a sure fire way to massive detonation,.....

 

[Stetson]

I did remove the filter and dumped it in a jar found no water or trash.
I found the wire to lock base timing, rpm’s dropped a little when I grounded it but still advanced when I throttled up if that tells you anything. Thank you so much for the quick response, you all are amazing! I’m having difficulty finding an experienced boat mechanic. I’ve learned so much just reading the threads.

 

[dubs283]

check the total timing advance, should reach total between 2000-3000 rpm.

not sure which engine you have but total should not be more than roughly 32 degrees for a sbc

if total advance is okay you most likely have lean issue, check fuel pump pressure throughout rpm range, also may be time to rebuild the carb

 

[nola mike]

check the total timing advance, should reach total between 2000-3000 rpm.

not sure which engine you have but total should not be more than roughly 32 degrees for a sbc

if total advance is okay you most likely have lean issue, check fuel pump pressure throughout rpm range, also may be time to rebuild the carb

I'd be curious about what the timing does when it starts to knock

 

[Mcfltfyter]

I think I've read that you have to ground the timing wire before you start the engine, and shut it off before disconnecting it from ground. I could be wrong though, as I've never messed with a thunderbolt.

Is it pinging only at wot, or just cruising too?

 

[Stetson]

I think I've read that you have to ground the timing wire before you start the engine, and shut it off before disconnecting it from ground. I could be wrong though, as I've never messed with a thunderbolt.

Is it pinging only at wot, or just cruising too?

It was pinging at @ 3500 and sooner under a load, it seemed to get worse the more I messed with the fuel system ie. Dumping fuel filter, using new fuel etc. at that point it started pinging at 2000. So I rebuilt the carburetor and now it’s back to pinging at 3000. I now have some one to drive the boat while I check the timing when it starts to ping. I’ll let you know what I find. Thanks so much for your quick reply, my vacation is running short.

 

[Stetson]

check the total timing advance, should reach total between 2000-3000 rpm.

not sure which engine you have but total should not be more than roughly 32 degrees for a sbc

if total advance is okay you most likely have lean issue, check fuel pump pressure throughout rpm range, also may be time to rebuild the carb

The engine is a 5.7 2 barrel I’ll check the timing while under way and let you know what I find. Rebuilding the carb didn’t help but it was dirty and needed done. Thanks

 

[Stetson]

The engine is a 5.7 2 barrel I’ll check the timing while under way and let you know what I find. Rebuilding the carb didn’t help but it was dirty and needed done. Thanks

Ok I grounded the base timing wire, started the motor, could here a couple of knocks at start up put it in gear got up to 1600 rpm @20 degrees before tdc and it knocked so load I was afraid to take it any higher. It seemed to do the same with out grounding the base timing wire. FYI noticed the water pump is leaking at the shaft drain hole and the belt is worn.

 

[Stetson]

It was pinging at @ 3500 and sooner under a load, it seemed to get worse the more I messed with the fuel system ie. Dumping fuel filter, using new fuel etc. at that point it started pinging at 2000. So I rebuilt the carburetor and now it’s back to pinging at 3000. I now have some one to drive the boat while I check the timing when it starts to ping. I’ll let you know what I find. Thanks so much for your quick reply, my vacation is running short.

The engine is a 5.7 2 barrel I’ll check the timing while under way and let you know what I find. Rebuilding the carb didn’t help but it was dirty and needed done. Thanks

Ok I grounded the base timing wire, started the motor, could here a couple of knocks at start up put it in gear got up to 1600 rpm @20 degrees before tdc and it knocked so load I was afraid to take it any higher. It seemed to do the same with out grounding the base timing wire. FYI noticed the water pump is leaking at the shaft drain hole and the belt is worn.

 

[Lou C]

What do your spark plugs look like? Light tan or grey or blistered white? If the latter that’s a clear indication of a lean condition