Reproduction Parts for 1916-1964 Chevrolet Passenger Cars & 1918-1987 Chevrolet & GMC Trucks


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#471922 07/12/22 11:54 PM
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Grease Monkey
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Grease Monkey
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so my questions today would be pertaining to valve lash and spark plug gap for my 1925 coupe

Spark Plugs - Champion W89D
i set my gap to .030 as per my manual
i kept seeing people online talk about .040 spark plug gap so decided to try it and it definitely increases my pep in the car and has a ton more power to it and seems to run a little bit smoother (original coil)
now the question being why would i get so much better performance despite the manual stating a lower amount?
and should i keep increasing it until i get a miss or less performance? or is that ill advised?

also
i have 2 other sets of plugs wondering if they may be a better fit. most of the talk for the spark plugs seem to be for 28's and mention the 3077's which are no longer made
should i try gapping these to .040 or would it be futile

Autolite 3076 set at .030 they would run okay but lack of power and occasionally missing.
Autolite T7 set at .030 as well. they caused misses / backfires at high speed. i cant seem to find almost any info on these plugs but they came with the car.
i have a roughly 1-3 mile loop with hills nearby that i do to test the car when adjustments are made depending on which turn off i take, can be long or short.


as for the valves for a 25 with a 25 head single port exhaust my manual states set the valves at .010 exhaust and .008 intake when hot yet i see all references to the valves online say .008 exhaust and .006 intake.
is it recommended to set the valves at the later gap or stick to what my manual says?


my ultimate goal currently is to get the idle RPM low enough to set it at the 300RPM the manual calls for when retarding the spark. currently runs at about 620RPM and usually will stall out when drawing fuel.

Last edited by MrPuffin; 07/13/22 12:42 AM.

My Car: 1925 Chevrolet Superior coupe K series Green & Black
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MrPuffin: The increase in plug gap will make your coil dissipate its built up charge faster and make it work harder. The pick up in power and smoothness may be due to a lean mixture. In the '70s when cars were leaned out for emissions purposes the plug gaps went from .030" up to as high as .080". (most at .060") The larger gap provided a larger spark front for the lean mixture and the HEI coil powered by full system voltage was able to increase the coil/condenser oscillation duration eliminating a lean miss. Without a more powerful coil you are making a trade off on performance vs coil life. The spark plugs that your car takes are AC 'B' plugs. They are very hard to find but work excellent. (I have a set in my '25 sedan) So starting with an incorrect plug may allow you to experiment some with plug gaps etc. Set your valves per your manual. Good luck with your project.

Last edited by SSG26K; 07/13/22 06:16 AM.

Steve
'25 Superior "K", '79 Corvette , '72 Corvette LT-1 & 1965 Corvette Coupe
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Hi MrPuffin

I consulted an engineer (my son) to be a little more sure of my answer here.

Increasing plug gap will indeed result in the coil discharging the secondary windings quicker.
This of course means that the actual duration of the spark will be shorter.

But, because the amount of energy in the discharge remains the same, that shorter duration spark will actually be “hotter”.

The end result is a “hotter” spark jumping farther through the air fuel mixture for a shorter period of time.

Because your mixture hasn’t change any (neither leaner of richer), we can only surmise that you had less than perfect ignition of the fuel charge at .030”, and improved the situation with a .040” gap and the resulting “hotter” spark jumping across more of the air fuel mixture.

Increasing the plug gap shouldn’t affect the life of the ignition coil in any way UNLESS…
you increase the gap to the point that the spark no longer reliably jumps the plug gap and instead finds a place to discharge INSIDE the coil itself.
This can and will happen.

With all this said…
I have tried increasing the gap on my 1946 truck with 10mm plugs from .035” to .040” to improve the way the engine ran during off idle and light acceleration and saw no change what so ever.
Maybe due to the little 10mm plugs? Don’t know.

Disclaimer:
Nearly all of the above depends on my ability to understand what my son was telling me! :-)

PS
As Steve mentioned above...
You may be running a little lean, and running a little richer might eliminate the ignition woes AND improve your performance at the same time.

Last edited by Stovblt; 07/13/22 04:34 PM.

Ole S Olson
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Grease Monkey
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Grease Monkey
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well that's all good to know from both of y'all!

although a bit of conflicting info that would be good to sort out or have others weigh in on too
SSG26K mentioned it will shorten the life of the coil and Stovblt mentioned that it shouldn't effect coil life.
i might swap to a modern coil for going out and about to not worry about that while swapping to the original coil for stuff like car shows to minimize stress on the original one.

i tried putting the Autolite 3076's back in at .030 and runs about the same but surges on my test loop today.
widened the gap to .040 and it still surged slightly while driving and backfired once when decelerating but not much of a difference at all

definitely a noticeable difference when using the Champion W89D Plugs at .040 vs the .030 and there is no surging or backfiring what so ever

haven't tried to swap back to the T7's as i still know almost nothing about them.


now with a lean mixture any advice on that? i have a Carter C RX-0
i know i have the air adjust screw to approximately 1.5 turns as the default. i have tried everything from completely closed off (rich) to quite wide open lean (as much as 3 turns)
none of that seems to change performance much at all.
what other than the air fuel mixture and well choking the engine could i do to richen the mixture? i have not messed with the jets at all in the carb as i don't know what i would need to do to tweak it properly.


My Car: 1925 Chevrolet Superior coupe K series Green & Black
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Your symptoms do sound like you are probably on the lean side.

I really know nothing about the Carter C RX-0, but you might try raising the float level some.

I had to raise the float level between a 1/16" and 1/8" above spec on the W1 on my '46 to make the idle circuit work.
It was always too lean, and adjusting the needle never seemed to make a lot of difference and would never solve the lean condition.
Now it runs much much better.
And because the idle circuit on the W1 isn't completely over taken by the main nozzle until about 20 miles per hour, the over all performance is much better as well.

It's probably an easy adjustment on the C RX-0 as well, so well worth trying.


Ole S Olson

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