View Full Version : Spark Plug choices

06-04-2005, 12:39 AM
My TR3 plugs are very fouled with fuel, and I've determined they're too cold. I was cross-referencing the number and it didn't come out to make sense. Currently I'm using Autolite 425...but I checked the site for a cross reference for the original Champion L87Y and the Autolite match is 273. There's also the NGK BP6H5.

I'm black and fluffy across all plugs, and my timing is perfect, my carbs are tuned and I have a new sports coil and I even checked my point gap.

What plugs should I be using!?!?!

06-04-2005, 01:24 AM

Spark plug numbers are a mystery to me; however, I will share with you my limited experience. I attempted to buy a set of Champion L87Y for my TR4. The parts store didn't have them, but came up with an Autolite 4123 as a cross reference. I actually have had very good luck with the Autolite plugs. No fouling are missing problems at all.

I believe that the last two digits of the Autolite's numbering system indicate heat range. 4125 plugs would be hotter than 4123. However, I don't know if this carries across different types. In other words, I don't know if your 425 plugs would be hotter than my 4123 plugs (nor do I know what the difference between the 4125 and the 425 plug is).

Champion uses a similar system to denote heat range. The L92Y plug would be the next step up in heat range from the L87Y. My car actually had L92Y plugs in it when I bought it 4 years ago, but since my engine rebuild, I've run L87Y or the Autolite 4123 plugs.

I hope this helps.

06-04-2005, 07:15 AM
There is usually a lot of confusion about spark plugs and their heat range. The following site will help everyone understand the differences and might clear up some confusion. <dansmc.com/sparkplugs1.htm> Hope it helps.


06-04-2005, 08:09 AM
Hey Sammy,

One thing to check is engine running temp. A motor that never gets up to normal running temps can also cause poor combustion. On the other hand raising plug temps can cause detonation when the hot summer days come around. Unless there is an oil consumtion issue or the motor has had carburetion or compression changes I have always found the stock recomended heat range to work well.
Is it possible a slightly stuck choke could be to blame?

P.S. I want your garage /ubbthreads/images/graemlins/laugh.gif

06-04-2005, 11:39 AM
Hello Sam,

from your description of the plugs, the engine was running rich when you pulled them out. However, plug reading can be deceptive so ideally do a check after driving for a few miles. The racers, years ago always did a plug check on full throttle, driving hard and immediately de-clutching and killing the engine simultaneously to get a reading relevant to full power (Where, of course, mixture setting is very critical)
I would stick with the recommended plug and check both running temperature and engine tune. You may set timing to the book but that is not necessarily the best setting as the book was written years ago and fuel has since changed.


06-04-2005, 11:51 AM
Timing was set based on having too much advance and getting some pinging, so I came back to stock. Carbs are set close to as lean as they will go. Chokes are not stuck.

Engine running temp is just a hair over midline (about 185 degrees.) It's nice to have a TR3 that doesn't overheat. (I have the TR4 radiator.)

The whole point is that I somehow didn't get the recommended plug--at least not after the conversion to Autolite...I don't know how much cooler the plugs are in my car. The plugs I took out when I first got the car were also fouled, but I don't recall the part numbers on those.

06-04-2005, 12:50 PM

You stated that the carbs are in tune... What's the condition of the carbs? Is it possible that there is any wear (jets, needles, throttle shafts, butterflies)? What do you mean when you state that the carbs are set as close to lean as they will go? Do you mean you've hit the stops on the adjustment nuts, or that you've leaned them out as far as you can and still maintain a decent idle? If the carbs are in good condition, and you have the correct needles, you should be able to lean them out to the point that the engine won't run.

I had a heck of a time with fouled plugs a while back. Based on the old adage that 95% of carburetor problems are electrical, I avoided the temptation to start tearing into my carburetors. However, I eventually succumbed, and rebuilt my carburetors Ė no more fouled plugs. 5% of the time, it really is the carburetors. /ubbthreads/images/graemlins/smile.gif

06-05-2005, 12:43 AM
The carbs were rebuilt. The jets, grosse jets, needles are all stock and in good shape. The front carb is dang near as lean as the nut will take it. The rear could go more on the nut, but the car actually runs very well -- just a wee bit of fouling on the plugs. I do think I have the wrong plugs in there. Does anyone know how the heat range of the Autolite 425 compares to the Autolite 273, Champion L87Y or NGK BP6H5 which are evidently spec-ed for the TR3?

06-05-2005, 12:57 AM
According to Autolite's website they spec the 275 for a 60 TR3. I found the folowing at https://www.garbee.net/~cabell/SparkPlugData.htm

Autolite Part Numbering Scheme

Every spark plug company has a unique numbering system that may or may not have some built in meaning. Autoliteģ spark plug numbers indicate the heat range with the last number of the part number. Everything previous to that, regardless of the number of digits, can only be used to indicate a similar design (spark plug family). For example, a 23 is one heat range colder than a 24. 3924 is one heat range hotter than the 3923. The 23 and 24 will look exactly alike, but they will not look like the 3923 or 3924.

Some part numbers also use prefixes to indicate special features. AP as in AP24 indicates a platinum center wire on a 24. APP indicates a platinum center wire and a platinum ground (or side) wire. MP is for motorcycle platinum and WC is especially for personal watercraft.

[/ QUOTE ]

06-05-2005, 04:54 AM
Hello Sam,
there is something wrong if your jets are adjusted so far up, they should be two full turns down, near enough, if all else is correct. Does the engine burn oil as this can mislead setting the mixture?
I can't advise on your plug numbers as even the Champion number you quote is not one I know of. If you have an old original manual, does it specify N5 as the standard plug?


06-05-2005, 10:04 AM
I agree with Alec regarding the mixture settings. If everything is set correctly, you should be somewhere in the neighborhood of 12 flats (or 2 full turns) down.

Now regarding spark plugs...

All of these different Autolite part numbers being thrown around got me curious, so I've spent the past half hour at www.sparkplugs.com (https://www.sparkplugs.com) doing some cross reference checks. Here's what I've discovered:

Sparkplugs.com lists the Autolite 275 and Champion RL87YC as the proper plug for a TR3/4. The extra "R" on the Champion part number simply designates it as a resistor type plug. So far, so good; however, look what happens when I use their cross reference guide...

When cross referencing Autolite to Champion, here's what I found:

4123 crosses to RL82YC (a colder plug than the standard L87YC)
425 crosses to RL86C
273 crosses to L82YC
275 crosses to RL95YC (a hotter plug than the standard L87YC)

When cross referencing Champion to Autolite, hereís what I found:

RL87YC crosses to 4123
L87YC crosses to 4114 & 275
RL95YC crosses to 275
RL82LC crosses to 4123
L82YC crosses to 4123
RL86C crosses to 414

Obviously, new numbers keep popping up, and I could have kept going.

My conclusion: The next time I change my plugs, Iíll probably go with the Champion L87YC and if I have any problems, Iíll go back to the Autolite 4123 plugs that are currently in my car.

06-05-2005, 11:03 AM

Comparing your results to the chart below from the page I previously posted it's not all that confusing. The two numbers in the middle of the champion plug number designate heat range, same as the last digit of the Autolite. Looking at the crossreference pattern I would guess that Champion may have larger overlapping heat ranges as well as smaller more narrow ranges. From what I see, the following occurs:

Autolite range Champion Range
3 82 or 87
4 86 or 87
5 86 or 95

So, there could be some overlapping especially since your champion numbers are a mix of temperatures, resistor type, and firing tip design.


Geo Hahn
06-05-2005, 01:29 PM
...Carbs are set close to as lean as they will go.

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I always use the stock plug or NGK equivalent and do the final mixture setting based on plug color. I have never had a problem getting a toasty brown with this approach... except when something else was amiss.

06-10-2005, 06:09 PM
Have you checked the float level in the bowls? If they are too high then you will run rich. Since you have Gross jets which are not stock then the float level may need some adjusting.

06-10-2005, 06:41 PM
Have you checked the float level in the bowls? If they are too high then you will run rich. Since you have Gross jets which are not stock then the float level may need some adjusting.

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Over the years I've heard a number of stories of over-rich problems caused by using Grose Jets in place of the needle valves. Might be worth swapping out to see if it makes a difference.


06-10-2005, 10:28 PM
HI Geo, Thats a proper way to go,get the proper plug color.Those charts do not know that the car is about 40 years old or how you drive your car and that five star gasoline is no more. I bet she climbs Yarnell Hill too.---Keoke /ubbthreads/images/graemlins/laugh.gif