Help needed for backfire ? issue

Todays update. Took the boat out and again ran w zero issues. I will mention that there is the slight shudder after running hard and then coming back into the no wake zone. This is the shutter I had mentioned in another post. Could be the motor isn’t balanced 100% internally, could be a number of things, most of which I am clueless about. For now I am focusing on the current backfire popping. None of that today. She ran fast and strong. I did notice some smoke at initial startup though. I made a point of feeling the coil once back at the dock. I don’t have an IR thermometer. All I can say is it was very hot. I could basically put my hand on the valve covers and the exhaust manifolds. I could not keep it on the coil, way too hot. Is this normal or indication that the coil is running too hot. Should I order the coil mentioned above from Rock Auto? Thank you

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

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

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

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.

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.  

Thank you guys. I will report back.

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.

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.

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?

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?

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?

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?

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

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.