Box Stock

Autolite 3910X racing spark plugs in stage 1 Predator 212cc

Shorttrackthing

Shorttrackthing

Member
So I found out why my Autolites were hella black, it's because we are running Rec Fuel 90 octane, which is ok, just causes the engine to run on the richer side. Does that effect performance, should we switch to E87

Here's the build:

Autolite 3910X Racing spark plugs
ARC DJ-2225 CNC Bored racing carburator
OMB Warehouse Top plate kit
Helix fuel filter
ARC racing air filter
Breather/catch can on valve cover
10.8 lbs Racing valve springs
AKRA Big Pipe and Muffler
Governor removed
Everything stock internally except for what i mentioned
 
That plug seems to be equivalent to an NGK 9, so yeah it's probably a bit cold for the build you have. Usually you don't need to go that cold until you're up into 100HP/L territory with gas.

Questions:

What jet are you running?
What do you mean by "rec fuel causes engine to run on the richer side"
 
your not going to make enough compression to justify anything over 87 octane. those plugs go black when idling all the time some people recommend the 32x but from looking at it its colder than the 10x. i know its not good for those Al heads but just take it out an clean them often or just replace. the biggest advantage to that plug is that its so long it gives you more compression
 
RadialFin said:
That plug seems to be equivalent to an NGK 9, so yeah it's probably a bit cold for the build you have. Usually you don't need to go that cold until you're up into 100HP/L territory with gas.

Questions:

What jet are you running?
What do you mean by "rec fuel causes engine to run on the richer side"
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That's what my engine builder said
 
spincycle said:
Alot of that black comes from the time you sit in staging with the motor idling or when you're letting it idle in the pits, keep those two situations to a bare minimum and you should see an improvement in the plug color...and ditto on running anything more than 87....
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Yeah my engine builder was saying don't let it idle for 2 long, i agree
 
I don’t hardly ever let me engine idle. I mean I let it warm up but I don’t just have it sit there and idle. I am not a big fan of idling these motors. I change my oil and plugs a lot and clean plugs and run them again. I run a light oil so it doesn’t take long to get everything lubed in the motor. I sit on the grid and all these peoples motors are idling for the entire feature before us. The moment the first guy in the grid pulls out I start it and go. Now if it is cold out then I will start kart in pits and get temp in it and rev it some and then throw a new plug into it and then make sure it fires on that plug. Take it up to the grid and then run it when they call us out. My engine is super rich on the low side so I will wet plug fast.
 
Like I've said before run a hotter plug. The tiny bit of compression lost will be made up in a better performing plug for your stock piston and head setup.
 
I know this is an older post but I run the same plug on this build...see image minus the 28mm mikuni, still working the bugs out in it, I'm currently running a Nibbi pwk24mm, how do you "read" this type of plug?
 

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Boomanchoo27 said:
I know this is an older post but I run the same plug on this build...see image minus the 28mm mikuni, still working the bugs out in it, I'm currently running a Nibbi pwk24mm, how do you "read" this type of plug?
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I believe this is a source of confusion for some especially when or mentors have explained how to read a plug. Surface gap plugs will look like they are fouled compared to a J gap plug. and I think because of this people tend to fault the 3910x of being too cold. these plugs will never get that oxidized chocolate look to them they will look black. the spark takes the path of least resistance. on a typical plug that's where the electrode is closest to the ground and it will spark there over and over where the 3910x will jump around to wherever has the most conductivity.
 
J'-remy said:
I believe this is a source of confusion for some especially when or mentors have explained how to read a plug. Surface gap plugs will look like they are fouled compared to a J gap plug. and I think because of this people tend to fault the 3910x of being too cold. these plugs will never get that oxidized chocolate look to them they will look black. the spark takes the path of least resistance. on a typical plug that's where the electrode is closest to the ground and it will spark there over and over where the 3910x will jump around to wherever has the most conductivity.
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So would you say this plug looks good?
 
I'm not a pro engine builder so i don't want to comment ask a builder. like i stated before. that plug was installed in every top builder's clone i ever received. i trust them to put the correct parts on my engine. i do clean my plugs often FYI.
 
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