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Spark Plug Gap
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Tim Donahoe
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PostPosted: Wed Sep 19, 2018 10:51 am    Post subject: Spark Plug Gap Reply with quote

For those with stock carbureted engines, what gap do you use on the spark plugs?

Bentley provides mixed messages, according to where you’re looking in the book. At one point, they tell you .024, at another point, they say .28.

I’ve always understood that the larger gap was for fuel injected engines, starting in 1975, but my original 1974 Owners Manual says my gap should be .028.

I’ve always gapped my plugs at .024. However, my 1600 dual port with a Bosch 034 distributor, coupled with a Volkzbitz-restored Solex 34 pict 3, has always given me plugs that have been a bit rich looking when I pull to change or replace them. I’ve never gotten that “tan-gray” perfection, but more like a gray electrode with a thin black layer in the cup.

The engine runs extremely well with lots of power, and I get 28 mpg on the highway.

My question is this: could my issue be the result of gapping my plugs at .024, instead of .028? Would the larger gap provide better combustion?

Take into account that my carburetor is adjusted properly. I use a tach when adjusting, and I do it exactly as Bentley, Haynes says to do it. My timing is also spot on at 7.5 BTDC. Advance is perfect.

Tim
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73SlowBug
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PostPosted: Wed Sep 19, 2018 11:23 am    Post subject: Re: Spark Plug Gap Reply with quote

I use .32, with electronic ignition. Works great. No knock, better fuel economy. I was using .28 with points and condensor, and when I first switched to the points replacement device. The bosch plugs come pre gapped at .28. I Immediately noticed smoother delivery when I switched to .32. I haven't seen any adverse effects, and no plug fouling. I'd imagine your results would be similar going from .24 to .28 with points and condensor. I just pulled my plugs to replace last week, and they looked pretty good, but I'm replacing them yearly anyway.
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gt1953
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PostPosted: Wed Sep 19, 2018 11:57 am    Post subject: Re: Spark Plug Gap Reply with quote

Wider gap is the way to go. The smaller gap is great for starting when it is really cold.
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Cusser
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PostPosted: Wed Sep 19, 2018 2:32 pm    Post subject: Re: Spark Plug Gap Reply with quote

0.025 inch for me.

For both:
1600 DP 2016 rebuild with Solex 30/31 PICT carb and German 009 with points

1835 DP with single Weber 40DCNF and German 009 with points
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Last edited by Cusser on Wed Sep 19, 2018 4:45 pm; edited 1 time in total
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mukluk
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PostPosted: Wed Sep 19, 2018 2:45 pm    Post subject: Re: Spark Plug Gap Reply with quote

I use .028" on my basically stock 1600sp, but you may need to tailor the gap to the needs of your specific engine and its setup/environment.

http://www.globaldenso.com/en/products/aftermarket/plug/basic_knowledge/spark/index.html
https://www.eng-tips.com/viewthread.cfm?qid=158647
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PostPosted: Wed Sep 19, 2018 2:46 pm    Post subject: Re: Spark Plug Gap Reply with quote

With a good coil .028 is a good bet, and honestly up to .035 should work well too.
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Bob Loblaw
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PostPosted: Wed Sep 19, 2018 3:23 pm    Post subject: Re: Spark Plug Gap Reply with quote

I learned to gap them at "a loose 0.028"
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gt1953
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PostPosted: Wed Sep 19, 2018 4:55 pm    Post subject: Re: Spark Plug Gap Reply with quote

According the Without Guesswork manual...it says electrode gap .028 inches. That is what I have followed all of these years.
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PostPosted: Wed Sep 19, 2018 5:40 pm    Post subject: Re: Spark Plug Gap Reply with quote

I set them at .028. No bigger, no smaller, no matter what engine I work on. The Bosch W8AC’s came preset at that spec. I would still check them but it was so nice to not have to bend the electrode.
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Tim Donahoe
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PostPosted: Wed Sep 19, 2018 11:26 pm    Post subject: Re: Spark Plug Gap Reply with quote

Thanks for the replies, folks. Next time I take out my plugs, I’ll gap them to .028 and see what happens.

Tim
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PostPosted: Fri Sep 21, 2018 8:57 am    Post subject: Re: Spark Plug Gap Reply with quote

Quote:
I’ve never gotten that “tan-gray” perfection, but more like a gray electrode with a thin black layer in the cup.


This is more of the plug's heat range than the gap. A spark plug is an electrical device. Certain characteristics 'help' the plug need less voltage to 'jump' the gap. One is for the center electrode to have square edges, not rounded. Platinum plugs have such a small diameter center electrode that having square edges doesn't matter so they need less voltage to make the 'jump'. The output voltage of the coil is what really determines how large a gap will work.
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Volks Wagen
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PostPosted: Fri Sep 21, 2018 1:02 pm    Post subject: Re: Spark Plug Gap Reply with quote

Tim Donahoe wrote:
However, my 1600 dual port with a Bosch 034 distributor, coupled with a Volkzbitz-restored Solex 34 pict 3, has always given me plugs that have been a bit rich looking when I pull to change or replace them. I’ve never gotten that “tan-gray” perfection, but more like a gray electrode with a thin black layer in the cup.


Ah, the quest for perfection... Thin black layer - is it sooty? If you do a lot of tootling around town and idling u might get that anyway. Also short trips will mean you'll be running a bit rich at the start... If that's your general type of usage, then maybe getting a 'hotter' plug would be better.

If you take it out for a good blast before the next reading, get it up to temp and give it a bit of a run, then it might self clean a bit more. It's got the signs of being a little bit rich or a bit cold.
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Tim Donahoe
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PostPosted: Fri Sep 21, 2018 1:29 pm    Post subject: Re: Spark Plug Gap Reply with quote

Thanks, Volks.

Yes, the bug doesn’t get much highway driving. A few times a year, I’ll get it on a long freeway drive; but, for the most part, I’m just driving in town.

A hotter plug may be the way to go.

Tim
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