Backfire?

[LateNightSalami]

My wife and I have been hearing a funny thump come from the boat at random intervals this summer. It is very difficult to describe as it isn't particularly loud and doesn't occur super regularly. It almost sounds like something thumping against the aft hull. It seems to happen more when driving slow but even then it is maybe once every 10 mins of run time (?). Yesterday I threw out the word "backfire" and my wife jumped and said that it might be it. But it isn't particularly loud at all and it doesn't happen all that often. Could there be a very subtle timing issue that builds up slowly causing a mellow backfire after a while? Is this a thing that is possible? I am hoping to have it in another month then get a bunch of work done but maybe I need to pull sooner than I would like?

 

Edited September 14, 2020 by LateNightSalami

[justgary]

Perhaps it is time for a tune up?  I would include new plug wires as well....

[Bozboat]

does this noise happen on acceleration or just idling along?

[LateNightSalami]

13 hours ago, justgary said:

Perhaps it is time for a tune up?  I would include new plug wires as well....

I put in new plugs and a new cap and rotor last year. Not new plug wires though.

 

13 hours ago, Bozboat said:

does this noise happen on acceleration or just idling along?

I have only heard it once at speed. My wife has heard it a few times at speed but we mostly have been noticing it lately at slow not wake speeds. I don't think I have every heard it when dropping the hammer to get out of the hole or when going WOT.

[justgary]

4 minutes ago, LateNightSalami said:

I put in new plugs and a new cap and rotor last year. Not new plug wires though.

Consider changing the wires.

In the mean time, check all of your plugs to make sure they are tight.  A loose one can allow fuel/air to bypass the threads, and occasionally it will be enough to ignite.  Good plug boots can help prevent that, but the plugs need to be tight either way.