"Spark Knock" or Detonation - Prevention/Maintenance

[nospark1196]

On my last post about the "soft" jingle @3500 rpm you mentioned "spark knock"<br /><br /> 1) Will it hurt the engine ?<br /><br /> 2) How can I prevent/repair it ?

 

[nospark1196]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

If you don't remember, my motor is a '02 50 hp Tracker. Thanks again, Nospark.

 

[The Marine Doctor]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

Spark knock...or detonation does do damage to the engine. The airfuel mixture is igniting too soon. As the piston is still moving up the cyl it is met with the burning fuel charge...but in your case...its way to early. This will eventually wear away the piston.<br /><br />Firing early is what is supposed to happen...but your engine if detonation or Pinging...its happening too soon. <br /><br />TMD

 

[nospark1196]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

TMD, is there anything I can do to prevent. My Merc. manual says 2 things. 1) Set up the engine correctly.(too vague) 2) Diligent maintenance is applied to combat the causes. So, my plugs are NGK and are gapped @ .040. I will use a higher octane gas. My prop is turning 5500-5600 rpm(so that is perfect) My cooling system is working. I've used Seafoam to De-Carb the combustion chamber. I don't know about my timing, but this sound , or detanation, only started after I rebuilt my carbs. Another thing I could check is my fuel mixture @ WOT. Well, TMD am I on the right track? Any additional help/guidance would be appreciated.

 

[The Marine Doctor]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

There should be no need to run with a higher octane fuel. In some cases this might make it worse.<br /><br />IF the problem with your engine is indeed detonation...I would consult your local dealership and have them Resync everything on the engine.<br /><br />Are you using the correct spark plugs?<br /><br />It is also possible that the detonation is caused by a lean condition on the top end.<br /><br />TMD

 

[nospark1196]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

I just purchased NKG bp8h-n-10 plugs from my local Merc. dealer. Gapped @ .040. <br /><br />How do I remedy the "lean condition @ top end"<br /><br />The only Merc. service Shop in my area(+/- 75 miles) is backed up for at leased 2wks. I've got the manual, can I just do this myself ?<br /><br />Thanks for your help TMD, and your time, with this matter.

 

[The Marine Doctor]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

While operating the engine on the lake...flip the choke while running at higher RPM this will throw some extra gas at the engine.<br /><br />If the sound goes away..you will know its lean.<br /><br />TMD

 

[NEECAPRS]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

nospark: I like TMDs advice here, as always. If this detonation only showed up after rebuilding the carbs, you have to suspect that it's running too lean now--and you know that brings all kinds of trouble. By the way, the noise you get as detonation, pinging, knocking, etc. is because of an explosion rather than a burn on the flame front.Typical flame speeds are 350-400 ft./sec. Explosions are estimated to be 5000-6000 ft./sec-- sort of like hitting the piston with fast buckshot. That's what they usually end up looking like!! Good luck with the carbs.

 

[nospark1196]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

Thanks guys, I'm headed out to run the boat now. I'll post the results when I get back.

 

[nospark1196]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

O.K. guys, I ran the boat and I still hear that noise even when I used the fuel enricher at higher RPM. When I returned to the boat ramp, I pulled the cowling off and started looking for loose parts. The only thing I found was when I messed with the starter (moved it w/ my hand) the piece that engages the flywheel sounded similar to the sound I hear @ high RPM. I still don't know that it is indeed Detonation because the engine runs smooth. I plan to take it in to my local Merc. dealer and wait the 3-4 wks for them to check it.

 

[The Marine Doctor]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

It is possible that the sound you hear is the rattling of the bendix.<----the piece that spins the flywheel to start the engine.<br /><br />Try to eliminate this some how and try it again.<br /><br />No need to spend money if you do not have too.<br /><br />TMD

 

[nospark1196]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

I found the problem , it is the steering cable connection. There are 2 pcs. of metal that touch and vibrate when the motor is at higher RPMs. I noticed this morning when I was out on the water. When I turned the steering wheel, the sound would stop. I can't believe it was something so simple. Thanks for all your help!

 

[clanton]

Re: "Spark Knock" or Detonation - Prevention/Maintenance

I will check my motor later this week to verify what you say. I hope thats my problem also. I dont run the boat that much, but did notice that it did it at 2 different rpm ranges the second trip, after I clean the steering cable at the tilt tube.