knocking noise

Have a 93 Searay with a 4.3L, put a new rebuilt motor in it this spring. Been running great through the break in period. Finally got enough hours on it to take it over 4000 RPM's. At around 4200 rpms after a few minutes its making a slight knock or ping sounding like a metal to metal coming from the engine. Only does it at or above 4200 rpms, can run it all day at 4000 rpms with no noise what so ever. Was wonmdering if it was a spark knock, but its way inconsistant.

Re: knocking noise

No one have any thoughts on possible cause or where to look further??

Re: knocking noise

metal coming from the engine. Only does it at or above 4200 rpms, can run it all day at 4000 rpms with no noise what so ever. Was wonmdering if it was a spark knock, but its way inconsistant.

Click to expand...

Ayuh,... Some people call it Spark Knock,...
Detonation is what it is,...
The motor is possibly timed to advanced,...
Or,.. More likely,...
There a fuel restriction,+ it's goin' Lean...

Fix it or it'll be an Anchor in a few Minutes doin' that...

Re: knocking noise

If you think it is pinging/ detonation try retarding the ignition.

Re: knocking noise

Fuel restriction since its only doing it at high rpm's and it is starving for fuel? Theres no real way to check fuel delivery other then checking filters, lines and carb is there? We had the carb rebuilt last year.

We purchased the rebuild from Michignan Motorz, and they recomend setting the timing at 26 advance at 4000 rpm's. As part of the rebuild we replaced the distributor and went with a Delco Voyager instead of the Thunderbolt IV.

Bond-O is it true it will be an anchor in a few minutes doing that? What is its run at an RPM where it doesn't spark knock?

Re: knocking noise

Maybe try an octane booster in the fuel tank and/or some high test(93 octane) gas in tank. If it stops the noise then you'll know more. Check timing and compression ratio on that motor. It may require 93 octane.

Re: knocking noise

Can you tell by the compression if it needs higher octane gas and if so what compression dictates that?

Also is it safe to run the boat at dock at 4200 to 4400 for any lenght of time? Much easier to hear the knock when its sitting still, but thinking it may not do it without the extra load of being inn gear.

Re: knocking noise

Richardson, I think Bond-O is correct with it being an anchor after doing that for a time. When I had my marine engine overhauled, I was told specifically by my rebuilder to set the timing exactly as the manual states, and not to advance/retard it. Being this is a marine engine, he explained to me that with the excessive strain on the engine, and basically being a direct drive, you must set timing correctly or you will destroy the engine. So, wish I could help you with the timing issue with the new distributor, I would contact perhaps the manufacturer for additional details on how they recommend setting the timing.

Re: knocking noise

Did you receive the timing shunt with the distributor? you must have this to disable electronic advance without it you will not set the correct base timing.

Re: knocking noise

Yes, I did recieve the timing shunt with the new distributor, and did set it exactly according to the instructions that came with it. One question on that it they say to use the shunt to set the "initial timing" I assume that to mean the base timing but when you check and adjust it at 4000 rpm's you would remove the shunt because you would want the advance working??

If thats the correct procedure thats what I did, my paperwork that came with the distributor says 26 at 4000 rpms, In a link that Bond-O posted on another post it says 25 at 4000 rpms, I may back it down to that and see if the problem is corrected.

also when you check the timing at higher rpm's should you do it with the boat in neutral or crusing? Didn't know if the added load of being in gear would effect the reading??

Re: knocking noise

I pesonally would set the timing at idle then check it at 4000 rpm to see if it is advancing. Im asuming being a 93 that it has a carb and no cumputer. That said typicaly marine distributors advance based on a predetermined ignition curve and the distributor only knows the engine rpm ie no vacum advance like an older model car or no cumputer like a newer model car so as long as you are at 4000 rpm shoulnt matter load/no load. I dont like the idea of running at 4000 rpm with no load for an extended period of time, but I dont like the idea of my hand/timing light being close to the engine belts at 4000 rpm on a bumpy lake!! Pray for smooth waters. Good luck!

Re: knocking noise

So headed north with timing light and octane booster to try and solve the knocking at 4200 rpms. Decided to put in octane booster and top it off with some good 93 octane fuel and run it awhile to make sure it was all through the system. After half hour of moderate running decided to open it up a bit and see if maybe it was just the fuel, well after about 30 secs of running at 4200 rpms got the knock.

Decided to run it at 4000 to 4100 and see if it would knock at that rpm, ran great no knocking for a good half hour. Then with no notice and a sound like it was throwing a rod the boat quit, just shut down completely. But started right back up and sounded like nothing had happened ran great again back to the dock.

Any thoughts? To me it sounds electrical other then the noise since it shut down completely and then started right back up. I would think if it was mechanical it would continue to make noise?? Going to warm it up and check the timing, wondering if I should do a compression check?