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TOPIC: best spark plug results for a 305 V8

best spark plug results for a 305 V8 04 Oct 2017 00:48 #1

  • floatyerboat
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After installing electronic ignitions on my two US Marine 305's I changed out the plugs with NGK which required a .031 gap.
I'd like to try a resistor type plug which a wider gap.
The engines start well, run well at all rpm's ,the timing is on , the carbs are on but the exhaust has a rich gas smell to it.
I was hoping a wider gap might clean that up a little. better.
Does either AC Delco or Champion require a .040 gap?

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Dan
Frostbite Falls, Minnesota
Claudia V. III
1988 - 3218
Gas Drives
Last Edit: by floatyerboat.

best plug results for a 305 V8 04 Oct 2017 01:05 #2

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Gap is gap. That’s set by the engine manufacturers. Fwiw, I used E3 plugs in my 4.3’s. I found that I had a slightly lower throttle setting and they wintered over better. They don’t have a direct marine application and were surprised when I asked, telling them that I had been using them for several yevars. The only thing they could recommend is to keep doing what’s working. I don’t remember what number I used, but it was equivalent to the ngk callout.

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P/C Pete
Edmonds Yacht Club (Commodore 1993)
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best plug results for a 305 V8 04 Oct 2017 01:10 #3

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I read that before on the recommendation from the engine manufacturer. I don't have the manual and cant find the engine tag. Would anyone know what it is for a US Marine 305?

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Dan
Frostbite Falls, Minnesota
Claudia V. III
1988 - 3218
Gas Drives

best spark plug results for a 305 V8 04 Oct 2017 02:41 #4

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Gap is gap based on the plug itself. Delco or Motorcraft platinum is where I go. Since you changed ignition, it is possible the coil isn't as strong as before. And if you truly are overfueling then adjust the carb. To me all inboard gassers smell overfueled, but I think it's comparing to new catalyzed cars. Smell a cars exhaust from the 60's and they smell the same. If there is no sheen on the water then you should be fine.
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1985 3870 175hp Hino's

best spark plug results for a 305 V8 04 Oct 2017 02:44 #5

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P/C Pete
Edmonds Yacht Club (Commodore 1993)
1988 3818 "GLAUBEN"
1980 Encounter Sunbridge "Misty Blue" (Sold)
MMSI 367770440
1972 Chevrolet Nova Frame off Resto-mod in the garage
Boating on the Salish Sea since 1948

best spark plug results for a 305 V8 04 Oct 2017 11:51 #6

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Your absolutely right. Being from a older generation the first thing I thought of was what cars smelled like thirty / forty years ago with no pollution devices. I've had a few folks on board that have commented on the richness of the exhaust. My sister sat on back for a three hour cruise with a beach towel over her face to filter out exhaust. I just got pulled out yesterday and I'm changing out to lifting pot mufflers hopefully improve the exhaust problem. The old style mufflers are very loud and that is a problem of its own. I think the baffles may be gone and was thinking maybe the cooling water is running on the bottom of the muffler and the exhaust fumes are passing over the top.
The new ignition systems came with a new coil pack.

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Dan
Frostbite Falls, Minnesota
Claudia V. III
1988 - 3218
Gas Drives
Last Edit: by floatyerboat.

best spark plug results for a 305 V8 04 Oct 2017 15:04 #7

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the plug gap could possibly be opened a slight bit more, but to some small extent, the heat range of the plug is what needs to be changed to cause a richer or leaner burn.
but if the carbs are just allowing too much fuel in, the plug heat range will make no noticeable difference.

at what rpm and boat speed do you notice the smell worse?..... do you have twins?..... is it a dry or wet exhaust system?...

on a marine engine, its a fine line between too rich and too lean... one causes smell and a higher risk of CO poisoning, where the other causes severe engine damage. its very rare that a carbed engines runs close to that fine line at all engine speeds.

then one needs to look at where the exhaust exits the hull, because above the waterline exhaust will have a much higher tendency to be sucked up and over the transom and into the cockpit as the boat moves thru the water. and has already been said, even a proper running carbed engine will smell rich at times...

lastly, you say the carbs are working correctly... but are you basing this opinion on the performance of the boat?.... a carb that has a problem that allows too much fuel in to the power circuit, may seem like its operating properly, but the fact that it is passing too much fuel thru it proves that its performance could be improved. you could have a loose jet, or a piece of debris that is causing the spray pattern to be interrupted, the carb could be worn out, or maybe the jets are too big for the best performance from that carb....

there are a number of things that should be studied that have a greater impact on the rich exhaust rather than the spark plugs...
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1989 Bayliner 2556, 5.7 OMC Cobra
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best spark plug results for a 305 V8 04 Oct 2017 15:33 #8

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All things I will note for next springs launch. The richness seems to be from start to stop. no power or full power. Both carbs were rebuilt 2 years ago.

Thanks

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Dan
Frostbite Falls, Minnesota
Claudia V. III
1988 - 3218
Gas Drives
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