Reading spark plugs | GTAMotorcycle.com

Reading spark plugs

Brian84

Well-known member
I recently installed a small M4 street slayer exhaust from a gsxr600 onto my '00 SV650. The bike seems to be running fine, but I figured I would check my spark plugs just to see if it's running a bit rich or lean after doing the swap. I'm assuming the exhaust flow is a bit worse since the exhaust isn't made for my bike, and the muffler has a slightly smaller inlet diameter than the old one.

So I pulled the rear plug, and it looks like this:
3XeRQSoL8rROxx-S4tI3S70mKNDp-QWqfYUfwqYZOXE=w1079-h809-no


I'm guessing it's a bit rich, due to the colour of the base ring, but this is the first time I've done this. Also, I pulled the plug after the bike had cooled down from a previous ride, and the bike was shut off from idle. I'd like to eventually de-snorkle my air intake, and do a proper carb tune, but for now does this look pretty normal?
 
It looks reasonably normal to me, but I doubt that the exhaust would affect the mixture much at idle which is all you are reading. If you want to know if it is affecting the mixture under power, you have to shut it down under power (ie. full throttle (preferably in a high gear to allow you to run for a bit at constant throttle/rpm), clutch in, kill switch, pull the plug).

I would expect the 600 muffler to comfortably flow more air than the 650 can pump out (lower displacement but way more rpm).
 
Looks good to me. When my bike was running rich, the whole thing was soot black, including the white porcelain area around the center electrode.
 
It looks reasonably normal to me, but I doubt that the exhaust would affect the mixture much at idle which is all you are reading. If you want to know if it is affecting the mixture under power, you have to shut it down under power (ie. full throttle (preferably in a high gear to allow you to run for a bit at constant throttle/rpm), clutch in, kill switch, pull the plug).

I would expect the 600 muffler to comfortably flow more air than the 650 can pump out (lower displacement but way more rpm).

Alright thanks! I'll try checking the plugs under power as you say.
 
Read up on how to do a proper plug chop. From you pic I would say it is running fine however.
 
Read up on how to do a proper plug chop. From you pic I would say it is running fine however.

This and if it's an NGK plug, there's a chart that'll tell you what it means but you'll need to do more research on how to accurately get the right reading on the plugs. They do look fine tho :D
 
It looks reasonably normal to me, but I doubt that the exhaust would affect the mixture much at idle which is all you are reading. If you want to know if it is affecting the mixture under power, you have to shut it down under power (ie. full throttle (preferably in a high gear to allow you to run for a bit at constant throttle/rpm), clutch in, kill switch, pull the plug).

I would expect the 600 muffler to comfortably flow more air than the 650 can pump out (lower displacement but way more rpm).

I read somewhere that you should never pull and replace plugs hot. It may cause the plug to seize.
 
I read somewhere that you should never pull and replace plugs hot. It may cause the plug to seize.

No idea if that's true or not. I know I have done it hundreds of times on many different engines and never had a problem (other than getting burnt because the plugs are mf hot).
 
Different expansion rates for different metals is all. That and everything is tighter when hot :)
 
For me more important is engine behaviour, is there engine hesitation? black smoke? engine easy to flood? smooth revs in all RPM range?
 
Heads are alloy, plug is steel, if anything, they get looser.

Alloy does expand more then steel, but then steel will heat and cool slower. Also remember that you are expanding in every direction not just out. Personally I have never had an issue with plugs in a hot engine other then burning my fingers. Even with old iron heads.
 
Heads are alloy, plug is steel, if anything, they get looser.

colortunes remove any guesswork.

http://www.carbtune.com/colortune.html

Those things are useless for anything but setting the mixture at idle.
If you can't set the pilot circuit, you shouldn't be messing with carbs.

You want to tune... but you don't know how? Get an air/fuel meter or an exhaust gas temp gauge.

IF you know what you are doing you can still tune by the plug, but fuel has changed and you can't use the charts found in every manual any more. (When you're close the insulator will be as white as rice with a flame ring, a dark ring around the insulator. You want that ring near the bottom of the insulator).

As an aside: Did you know that you can tell if your ignition timing is right by reading the plug?

If you REALLY want to learn how to read plugs; Buy fuel from PRO Race Fuels and Mitch, the owner of Pro Race Fuels, will give you lessons on how to read a plug. (Mitch is a nice guy and would probably teach you everything he knows even if you don't buy his fuel.)
 
Those things are useless for anything but setting the mixture at idle.
If you can't set the pilot circuit, you shouldn't be messing with carbs.

You want to tune... but you don't know how? Get an air/fuel meter or an exhaust gas temp gauge.

IF you know what you are doing you can still tune by the plug, but fuel has changed and you can't use the charts found in every manual any more. (When you're close the insulator will be as white as rice with a flame ring, a dark ring around the insulator. You want that ring near the bottom of the insulator).

As an aside: Did you know that you can tell if your ignition timing is right by reading the plug?

If you REALLY want to learn how to read plugs; Buy fuel from PRO Race Fuels and Mitch, the owner of Pro Race Fuels, will give you lessons on how to read a plug. (Mitch is a nice guy and would probably teach you everything he knows even if you don't buy his fuel.)

Seems like I'm trying to run before learning how to walk.

In the end, I just want to make sure my engine is running reasonably well with a desnorkeled intake and an aftermarket exhaust. Yes, this will be my first time opening up carbs to rejet. I have a good theoretical understanding of this stuff, I just need to actually do it and start learning, which I understand might be frustrating/take a lot of effort, but I'm pretty determined to do this myself. As RockerGuy said, maybe it's most important to see just get a feel for how the engine runs as I change the jets without worrying about the plugs.
 
Seems like I'm trying to run before learning how to walk.

In the end, I just want to make sure my engine is running reasonably well with a desnorkeled intake and an aftermarket exhaust. Yes, this will be my first time opening up carbs to rejet. I have a good theoretical understanding of this stuff, I just need to actually do it and start learning, which I understand might be frustrating/take a lot of effort, but I'm pretty determined to do this myself. As RockerGuy said, maybe it's most important to see just get a feel for how the engine runs as I change the jets without worrying about the plugs.

Honestly with just basically a slip on you rarely have to rejet. If it was a full system it would be a different matter. See how it rides. If you don't have hesitation or loss of power you should be fine.
 

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