GM really screwed the pooch on this one.
 

Whats the story on the GM, Vortec head, flat cap, spider cap, How ever you call it cap? It blows in exactly the same place. Coil to #3 cylinder. Distributor replacement is a NO-GO because the hardware involved. Is there
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a solution other than keep a spare onboard?

Ive always had problems using NON GM parts....went back to AC Delco and problems went away. Not sure if thats your case here or not...

Looks like burn thru, so all the tsb’s on corrosion problems with these dist’s/caps won’t help with this.

What cap/rotor plugs and gap’s are you using ?

What is the acronym TSB's? I know there are platinum plugs in it. What ever the manual said. Changed them about 3 yrs ago and not many hours on them because of COVID. Don't know number off hand. As for caps, I've had Mercruiser and aftermarket All last about the same. Going with Blue Streak (Standard) now. Has life time warranty. I guess I'll buy 2 for a spare so I can change while on the water.

Technical
Service
Bulletin.

Yup, sorry. I hate acronyms too. Guilty.

Larger gaps create higher voltage from coil. Higher voltage = more likely for cap or rotor to have issues like burn thru.

Mercruiser has been using automotive plugs and gaps like GM on the Vortec 4.3-8.1l for years. It’s too hard on the ignition system. Been preaching this to the 496 crowd for years. Mostly deaf ears. Oh well.

Anyway, anything over .040” causes too much resistance, making coil produce more current than the cap can handle.

.035”-.040” will always be the standard for chevy small and big blocks. .050-060” is jost oem emissions settings that burn stuff up. Even individual coil engines like the 496’s.

What ever the gap in the manual said to set them, I did. Can't see the plugs eroding that much with the amount of hrs on them. If the Manual has the higher gap #, I'll lessen it.
I guess that's the only thing that can be done?

OF COURSE
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Do yourself a favor and buy a GM cap and rotor. The one in my 99 Tahoe lasted 250k miles and really did not even look that bad when I changed it then. I would personally have a good part instead of a lifetime warranty :)

Unfortunately even the factory parts quality is not that good all the time now. OEM parts only for me when I can get them.

True but still usually better than aftermarket. Also you have to watch where you buy them. You can get AC Delco at WalMart but is it real ?? Same with stuff online. Dealer or AC Delco distributor only for me.

I agree with you 100%. Factory parts only when possible.

I wouldn't buy parts off ebay and especially Amazon. There are so many fake/counterfeit parts out there, it's crazy. Some look identical. Very hard to tell the difference till you find out the part doesn't fit or work. It's not worth trying to save a few bucks on.

i have had a few of these flat caps break the tang off the center of the rotor it can be hard to find as you have spark but it will not fire

I agree with SB , knock the plug gap to .035 , way less stress on the ignition.
Assuming your plug wires are good.
You will also notice that the build up on the inside of the terminals in the cap is alot less.
Marine engines run rich enough , they dont need a huge spark gap to light a emissions lean cylinder.

Any chance it's a ground issue,