Spark knocking diagnosis

[wellcraftJoe]

I have a 1995 mercruiser Weber 9770s carb 5.7 mag alpha one gen 2 combo with the thunderbolt 5 ignition original engine serial number 0F423587. I had to install a new engine this year because the other one blew the head gasket and ate a gutter between 3-5. I went with a new vortec and i got it all together and took it out this past Saturday it felt like a turd and the spark knock is back albeit not as bad but I still didn't want to chance anything so I just putted into the cove and swam with the family. My buddy whose is a good mechanic and was on his boat rode with me and his thoughts were just rebuild or get a new carb. My father-in-law who is a mechanic suggested a new distributor at this late in the season I'm going to call it but maybe next spring I'm thinking about just replacing both now.

So this all started the very last time I went out on my boat last year high pitched metallic rattle noise I thought it was the out drive so I went through it, before finding out it was the engine. Before I knew what was actually wrong I tried plugs in the old pre vortec and I remember thinking these plugs are saturated with fuel. Now I haven't pulled my current plugs because on the way home the trailer hub burned up and i left my boat at my friend's house as it was closer then mine.
So when I sat base timing on my new vortec at 10° and I turned my digital timing light to 10° and lined everything up it did not want to run. In fact the light jumped around a little maybe a degree or two before and after but I locked it in the best I could and if the motor increased in rpm the light would hold steady. I mentioned this to my father in law and he said he mostly likely thought it was the distributor saying it was probably worn out and that was why it was jumping around, he acknowledged it could be the carb but said if the more it ran the better it idled it then it was the dizzy. Couple that with the rich plugs on the old motor it seems like it but then would that also cause it have such low power ? Granted I never did hammer down because the knock. My final question is it even worth replacing the distributor on the thunderbolt 5s or should I just be looking at the AC Delco marine est units?

 

[Scott06]

I have a 1995 mercruiser Weber 9770s carb 5.7 mag alpha one gen 2 combo with the thunderbolt 5 ignition original engine serial number 0F423587. I had to install a new engine this year because the other one blew the head gasket and ate a gutter between 3-5. I went with a new vortec and i got it all together and took it out this past Saturday it felt like a turd and the spark knock is back albeit not as bad but I still didn't want to chance anything so I just putted into the cove and swam with the family. My buddy whose is a good mechanic and was on his boat rode with me and his thoughts were just rebuild or get a new carb. My father-in-law who is a mechanic suggested a new distributor at this late in the season I'm going to call it but maybe next spring I'm thinking about just replacing both now.

So this all started the very last time I went out on my boat last year high pitched metallic rattle noise I thought it was the out drive so I went through it, before finding out it was the engine. Before I knew what was actually wrong I tried plugs in the old pre vortec and I remember thinking these plugs are saturated with fuel. Now I haven't pulled my current plugs because on the way home the trailer hub burned up and i left my boat at my friend's house as it was closer then mine.
So when I sat base timing on my new vortec at 10° and I turned my digital timing light to 10° and lined everything up it did not want to run. In fact the light jumped around a little maybe a degree or two before and after but I locked it in the best I could and if the motor increased in rpm the light would hold steady. I mentioned this to my father in law and he said he mostly likely thought it was the distributor saying it was probably worn out and that was why it was jumping around, he acknowledged it could be the carb but said if the more it ran the better it idled it then it was the dizzy. Couple that with the rich plugs on the old motor it seems like it but then would that also cause it have such low power ? Granted I never did hammer down because the knock. My final question is it even worth replacing the distributor on the thunderbolt 5s or should I just be looking at the AC Delco marine est uni

at idle the TB 5 will advance the ignition and monitor the rpm increase and keep advancing until rpms stop increasing, called mean best timing. So you will see it hunt at idle when not in base mode. Shouldn't move much when in base mode. Were you able to see the total advance by reving it up to 2800 or so, I think total should be maybe 28-30 degees. if you set it at 10 and it wont run something is off maybe dist stab in is off a tooth

Would check the condition of the plugs and compression first to set a baseline. The pre vortec and vortec use different spark plugs vortec is I think MR43-LTS.

I would look to see if they are lean and more prone to detonation. If they are way white or look hot then definitely rebuild the carb. The TBV when in good condition is an excellent system , only reason to replace would be if the module dies and is way $$$ or is NLA

 

[Lou C]

I’d be looking at fuel mix and also what’s the max RPM you get? If it’s over propped (prop pitch too high, max rpm too low) you will eventually get that overheating of the combustion chamber and valve burning/blown head gaskets.
If these have a knock sensor is it in fact functioning?
Lastly I have always used higher octane fuel than the manufacturer advises due to eventual carbon build up, at least 89 octane…