Autolite "X" plugs Letting the cat out of the bag.

Chipg56

Member
Many understand that the Autolite AR3910X is actually to cold for most stock class applications.

Years ago when I first heard about the Autolite X PLUGS I immediately consulted a heat range chart. The plug is actually available in three different heat ranges. In addition to the 3910 there is an AR3922X, and a AR3933X there was a very limited AR3911x but I don't think you will ever see one. It has always thought it was laughable that so many promoted the 3910X and apparently never bothered to consult a heat range chart, even Briggs and Stratton failed to do so when writing the rules for the LO206. Tired of fouling out this plug? Try the hotter ones. They are readily available.
 
Good info -Chip. I have probably ran 100+ races with the AR3910x over the last 5 years. I only remember fouling a plug once and it was with an engine builder who ran the carb extremely rich on the low end and we sat on the grid for 5+ minutes with karts running.
Anymore regardless of plug I try to not fire the motor up until we are for sure about ready to pull onto the track as all these carbs are setup purposely rich.
 
thanks for sharing, Chip.

The decision to go to the 3910X plug for the 206 was based on several factors the way I was told:
1.) non-resistor plug with rigid ground addressed some of the previous tinkering with plugs that was going on.
2.) readily available - every trackside vendor and kart shop stocked the plug already.
3.) cold plug was a practical/conservative choice to help prevent building excess engine temps.
4.) somewhat affordable spec plug (as compared to iridium, etc.)

Is it the "best" performing plug for the 206? Absolutely not - as you said, it's not even the preferred heat range, but it fits the requirements and intentions of the class quite well.



-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
33 years of service to the karting industry ~ 1Cor 9:24
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
 
Is it the "best" performing plug for the 206? Absolutely not - as you said, it's not even the preferred heat range, but it fits the requirements and intentions of the class quite well.

I think its important to remember that maximum performance is not on the decision tree when thinking about how the 206 is/was developed. Longevity, consistency, affordability are things that are important.

If you want performance, Brian or Jimbo can hook you up with the parts and knowledge to build an awesome Animal.
 
Just to be clear, I had absolutely no intention of bashing Briggs or the 206. Just wanted to let others know these plugs exist. This knowledge probably helps those with the China motors most.
 
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we used to foul those like crazy at low speed on the animal, but so far good luck on the clone
 
I think its important to remember that maximum performance is not on the decision tree when thinking about how the 206 is/was developed. Longevity, consistency, affordability are things that are important.

If you want performance, Brian or Jimbo can hook you up with the parts and knowledge to build an awesome Animal.
206, china engine, midget football, freeze tag, and the list goes on are intended to be "for fun" unfortunately when you inject the human element second place IS the first looser and no one wants to be the first looser. Those that were around when "china will save karting" started saw this principal in full effect. The initial rules were more than enough to have a very fun class until billy could not beat johnny and looked at the rules and said will if it does not say I can't .........
 
Just to be clear, I had absolutely no intention of bashing Briggs or the 206. Just wanted to let others know these plugs exist. This knowledge probably helps those with the China motors most.
THANK YOU! for this info!!! Autolite would barley speak to me and told me they would not give me advice based on a non OEM application. The fellow also said these plugs were no longer being made i have no idea but he said he could not find the part number in the system and that was his conclusion.
BUT.... these engines seem to only foul on the grid sitting at idle for too long. Several Top engine builders (who all use the 3910x plug) told me a hotter plug will not fix a carburetor that is out of adjustment.
Thake it for what its worth.. all these guys have dynos. I have also been told enriching the pilot will also aid in fuel delivery at high rpm. clearly why you don't see clones idling on the grid
 
Several Top engine builders (who all use the 3910x plug) told me a hotter plug will not fix a carburetor that is out of adjustment.
Thake it for what its worth.. all these guys have dynos. I have also been told enriching the pilot will also aid in fuel delivery at high rpm. clearly why you don't see clones idling on the grid
Absolutely true. Tuning a carb encompasses compromises, especially with the carbs we are using. Generally we want to be fat on the low end and lean on the high end. I am not a clone guy but from what I see clown carbs are generally set up real rich on the low side, presumably to get more fuel at mid range and then leaning out on the high side. I do something similar with (methanol) animals. Although they do not foul my set up does lead to fuel in the crankcase (when idled) obviously necessitating more oil changes. A carb set up that is to rich at idle combined with a too cold plug will foul more easily then a hotter plug. A hotter plug has a better chance of cleaning itself off when it operating at racing rpm's.
 
Was in the habit of bringing extras of these plugs and a plug wrench to the grid back when I ran clone engines, I now use them in my open stroker animal (on alky) and have never once had a plug issue....🤔
 
thanks for sharing, Chip.

The decision to go to the 3910X plug for the 206 was based on several factors the way I was told:
1.) non-resistor plug with rigid ground addressed some of the previous tinkering with plugs that was going on.
2.) readily available - every trackside vendor and kart shop stocked the plug already.
3.) cold plug was a practical/conservative choice to help prevent building excess engine temps.
4.) somewhat affordable spec plug (as compared to iridium, etc.)

Is it the "best" performing plug for the 206? Absolutely not - as you said, it's not even the preferred heat range, but it fits the requirements and intentions of the class quite well.



-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
33 years of service to the karting industry ~ 1Cor 9:24
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
what would be your choice of a good plug and heat range
 
what would be your choice of a good plug and heat range
For which engine?
Keep in mind that the correct spark plug is chosen given a number of factors, compression, ignition timing, A/F ratio through carb set-up, even pipe can affect the plug choice. You can generally read gas plugs pretty easily by clean-cutting them. Alcohol engines tend to wash the plug clean and are a bit deceptive when it comes to reading plugs. I prefer to look at how far back on the ground (strap or base) the heat has transferred. 3/4 of the way back the strap is good. To the base of the threads is too lean/hot.
A handful of plugs and a handful of laps will help you know...or put it on the dyno.
 
We run S/A and small block open, we have been using the 3910X in a tillotson 72mm with 310 cam, Barry Young head, Rapp 990 header and in S/A we use a Barry Young carb, in SBO a 28mm flatslide, never had any prob with the 3910X, just wondering if I should use something hotter.
 
We run S/A and small block open, we have been using the 3910X in a tillotson 72mm with 310 cam, Barry Young head, Rapp 990 header and in S/A we use a Barry Young carb, in SBO a 28mm flatslide, never had any prob with the 3910X, just wondering if I should use something hotter.
With high compression you probably have in either engine, the 3910 should be fine.
 
All the info I found online shows the 3933x to be equivalent to a 9 heat range NGK just like the 3910x. I ordered a 3933x and it looks to be the non projected version of the 3910x, The electrode is the same length but the ground "x" is shorter.
 
I've researched this a bit too and found that the AR3910 is in fact a very cold plug BUT the alternates you listed according to Autolite are about the same heat range... so Ideally, for plate motors and stockers the AR3923 and AR3924 would be ideal according to the Autolite chart
 

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I've researched this a bit too and found that the AR3910 is in fact a very cold plug BUT the alternates you listed according to Autolite are about the same heat range... so Ideally, for plate motors and stockers the AR3923 and AR3924 would be ideal according to the Autolite chart
are those x plugs or just conventional?
 
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