Help needed for backfire ? issue

As stated, I need help! I am not sure if you call this a backfire or what. Let me describe the issue. When running boat at idle for extended time, once you power up, the engine pops and "backfires" bad. It will not clear itself out. I have done everything I know of. We changed the plugs and wires. New Pertronix setup to replace the old one bc I know they have had some issues. Rebuilt the carburetor as last resort bc thought that might be the issue. Nope, still does it. If you start it up and take it right out, she runs amazing. The problem seems to only present itself after long idle (i.e. cocktail cruising), Not sure what to do. Help please. Thanks in advance, Mark

Could be one of three things. Air fuel ratio, bad spark plug or the timing. Since you change over to a Pentronix Setup on the distributor, you might have moved the timing some. I want start by rechecking the timing first.

Also, if you can, connect a vacuum gauge  and check for steady needle at idle. (when hot)

If the backfire you mention is only a pop out the carburetor and not a true backfire that makes a bang out the exhaust?   I would look at the accelerator pump on the carb, if you don’t get a goos squirt when the throttle is cracked up you can get a pretty good pop back through the carb.  No idea why it would only do it after a very long idle, heat soak maybe?

MrMcD wrote: If the backfire you mention is only a pop out the carburetor and not a true backfire that makes a bang out the exhaust?   I would look at the accelerator pump on the carb, if you don’t get a goos squirt when the throttle is cracked up you can get a pretty good pop back through the carb.  No idea why it would only do it after a very long idle, heat soak maybe?

You had a low speed "shudder" a few years ago, is this a continuation of that issue or did that get fixed along the way?

KENO wrote: You had a low speed "shudder" a few years ago, is this a continuation of that issue or did that get fixed along the way? Like Morning Wood suggested a vacuum gauge will give you some good information to decide where to go from here. I'd get readings right after you start it and again later when hot and having been run long enough so that you're having  the problem What eventually makes the problem go away, a long slow idle home and you let the engine cool off............or something else?

If you don't know what the vacuum and timing are doing, you'll just end up throwing parts at it till you get the right part.

Your symptoms sound very similar to what we experienced a couple years ago on my dads 86 2001. I think KENO has provided good advice about the coil, that turned out to be our problem. The final diagnosis was placing a hand on the coil when it was experiencing those symptoms after a long idle, it was scorching hot. Went to local auto parts store and bought a plain black coil, no “hot spark” or performance coil needed, in fact the failed coil on our boat was a “fancy” coil that was only a few years old.

For $30 it’s worth a try, especially considering just about anybody can swap out a coil in 15 minutes. To make the project especially fool proof just take pictures of the wiring connections before you disassemble, use those photos as a reference to ensure the wires are hooked up correctly with the new coil.

Thank you guys. I will report back.

It sounds like the coil is getting hot and that the extra airflow allowed the coil to stay cool enough to not shut down.  Even a new coil can fail if they get too hot.  A buddy has a 1984 S.N. and his coil started acting up in a similar fashion.  New coil.  Bingo.  Problem solved.  

I’ll keep the wood 🪵 handy😂 I agree I think the coil is the problem. KENO, you mentioned buying generic coils above? Will they work with my Pertronix set up? Has anyone ever wrapped their coil with the header heat wrap? I wonder if it would make a difference? I’m just glad it’s working at this moment. I will try the extended idle with the board under the box for air flow.

Yes the heat is not coming from the engine- it's the coil making the heat from internal resistance. 

Thanks guys. Makes perfect sense about the heat issue coming from within. I will order a new coil from Rock Auto and see if it makes a difference. First I will try to reproduce the issue again by idling w the box closed. Then once the problem occurs I will feel the coil and see how hot. Then I will sit for a while and let it cool down and then see if the problem is gone temporarily. Thank you

Good idea. I do t know anything about resisters or such.