Dreaded spark plug question.

[Hickory]

I guess to start this thread I will say that I'm not looking for personal opinions on the best plugs. I see that question asked with a lot of mixed answers and bashing done. So my 29 DeSoto had autolite 389 plugs in it. Not sure yet what belongs in it yet.  I'm looking more fo facts from people who can tune with plugs. First is heat range. I have two cylinders running a little warmer than others and I know you can mix heat ranges to help even this out. So do hotter plugs lower or raise the temps in the cylinders. Next is resistance of the plugs. Which works better in our cars. And how about wires, I have solid core beldin edge wires. I'm not a fan of napa ignition parts but do these wires work ok on our cars. On my car (remember updraft carburetor) #1 and #2 are a little sooty, #3 and #4 are black and damp, #5 and 6 coco brown. #5 and #6 run hotter. #3 and 4 probably are rich with updraft carb. What could help even these out.

[Sniper]

2 hours ago, Hickory said:

So do hotter plugs lower or raise the temps in the cylinders.

 

Neither, the heat range refers to how hot the plug itself runs.  You want them hot enough to keep from fouling but not so hot as to cause preignition.

 

2 hours ago, Hickory said:

#3 and #4 are black and damp,

 

I assume it's damp from gas?  Does it smell like gas.

 

Tuning the carb is the best way to fix the rich running, changing plugs won't do that.  But it sounds more like a distribution issue. 

 

I suppose it could be a weak ignition system. 

 

Originally, your plug wires were solid core.  Those work fine for your application since you have no electronics to interfere with.  Non Resistor plugs were standard for you too, again, they work fine for you application for the same reasons. 

 

No idea on NAPA wires though, I prefer cut to length, spiral core wires myself.  Here's how I did my 51

 

http://www.yourolddad.com/plug-wires

[Hickory]

I feel it's poor fuel distribution or atomization. So the autolite 386 plugs have a resistance listed of 3000-12000. Maybe different plugs and wires may help. I thought maybe a hotter plug may keep the plug cleaner and less likely to flood under full throttle.

[maok]

The distributor cap and the shaft bush can cause these types of issues.

 

Also shaft cam wear.

Edited August 19, 2023 by maok

[Sniper]

46 minutes ago, Hickory said:

So the autolite 386 plugs have a resistance listed of 3000-12000

 

That seems off, I would expect the resistance of 6 same part number plugs to be a lot closer in value than the highest being 3x the lowest reading,  In fact I would expect the resistance to be a heck of a lot closer to zero than the lowest plug is reading.  Well, come to find out the Autolite 386 is a resistor plug and it's resistance is supposed to be 5k, lol.

 

I can't find a listing for a 29 DeSoto, but for 1930 they list an Autolite 388, which is a bit hotter than the 386.  But that is still a resistor plug.  The '30 non resistor plugs are Champion 516 or AC C88L.

[9 foot box]

   I looked at the Tod Fitch ply33.com site. I assume that you have 18mm plugs. A Champion W16Y/561 plug might work better for your new engine. The number doesn’t cross over to an Autolite plug. It appears that the Champion plug has a 5/8” reach, and your 389’s are 1/2” reach. Plug gap would be different with resistor or non resistor plug, so maybe regapping your 389’s would give you a hotter spark. On a P15 it’s .028” non resistor and .035” resistor. Does a 29 Desoto have a spark advance on the steering column? Just some thoughts Hickory. Rick D.

[desoto1939]

Question: are you adding any thing to the gas tanks such as LEAD Additive or Marvels Mystery oil or any other product? Also check the gap on all the plugs.

 

Also run the engine in your garge at night time will all the lights turned off and open the hood to see if you have any tracers coming off the wires. If you see blue streaks then the wires are leaking and then replace all the wires at the same time.

Just my 50 cents worth of input.

Rich Hartung

Desoto1939@aol.com

 

Here is information taken from my Autolite sparkplugs catalog which starts at 1930:

 

 

[Dartgame]

Colder plugs will help the engine to run cooler, but tend to foul. I've tested this in my small block mopars. I'd guess you might want to look at going hotter on those that are running cooler (fouling) as an option as well. All plug manufacturers have their own heat range codes, champions are colder with a lower number and get hotter as they increase in the plug number. Such as RN9 is colder than RN12 etc.

 

I'd take some time and read your plugs as you change heat ranges to determine what's best of the available plug heat ranges.

 

AS for plug manufacturers I've found NGK's to be very consistent, with 5Kohm resistance, compared to others. I tested resistance values for a number of plug makers for the same application. NGKs just work better in my opinion, but I'm unsure if they make them for your engine. Of the other plug manufacturers I tested , autolite and motorcraft are second in consistency. Look for a crossover chart on the web. Champions, and AC's run all over the place on resistance values.

 

 

[Sniper]

35 minutes ago, Dartgame said:

Colder plugs will help the engine to run cooler

 

Absolutely not.

[Dartgame]

Well thats strange. It did with my car.

[Hickory]

My research came up with an agreement with sniper and hotter run cooler combustion temps but I also found that a longer plug can have .5 degrees timing advance figure compared to the shorter plug.

[Ivan_B]

Here is some good brief info on spark heat range: https://us.gsparkplug.com/shop/spark-plug-heat-range? Should not affect the engine temperature, unless your sparks are too hot and you are pre-igniting. I do not believe that mixing different plugs is a good idea; the same is true for using incorrect length, resistor, etc. If the cylinders are running unevenly, as previously mentioned, the ignition system (and then the engine mechanics) should probably be inspected to ensure proper operation.

Edited August 20, 2023 by Ivan_B

[Hickory]

So I'm sorry my plugs are 386 not 389. 9footbox I do have a dash timing advance by cable. I only have mechanical on the distributor no vacuum.