Jump to content

Is my yz125 spark plug the right colour?

Recommended Posts

Hi, I have a 2000 yz125, I was previously having problems with fouling plugs so I've played around with the carb and now I've got it to where I think it's running good, just wondering if the spark plug is the right colour and its not too lean. Any input would be greatly appreciated

 

20170915_184923.jpg20170915_184911.jpg20170915_184829.jpg

Share this post


Link to post
Share on other sites

At first glance the insulator would say lean but...

for an accurate reading you must look deeper, link: http://www.braigasen.com/howtoread.htm

 

Just riding around isn't a good indicator, you have to do a WOT blast then plug chop as described in the above link.

Pilot and needle circuits are tuned by 'feel', only the main jet can be judged by plug coloration.

 

Unlike race fuel, using pump gasoline also adds another variable as different formulations or batches might have different coloration / deposits.

 

Depends on application as well, for WOT motocross that may be too lean, but for trail / off-roading that could be near perfect.

Edited by mlatour
  • Like 1

Share this post


Link to post
Share on other sites
At first glance the insulator would say lean but...
for an accurate reading you must look deeper, link: http://www.braigasen.com/howtoread.htm
 
Just riding around isn't a good indicator, you have to do a WOT blast then plug chop as described in the above link.
Pilot and needle circuits are tuned by 'feel', only the main jet can be judged by plug coloration.
 
Unlike race fuel, using pump gasoline also adds another variable as different formulations or batches might have different coloration / deposits.
 
Depends on application as well, for WOT motocross that may be too lean, but for trail / off-roading that could be near perfect.



Thanks! That website is very helpful, hopefully I can get er tuned perfectly
  • Like 1

Share this post


Link to post
Share on other sites
At first glance the insulator would say lean but...
for an accurate reading you must look deeper, link: http://www.braigasen.com/howtoread.htm
 
Just riding around isn't a good indicator, you have to do a WOT blast then plug chop as described in the above link.
Pilot and needle circuits are tuned by 'feel', only the main jet can be judged by plug coloration.
 
Unlike race fuel, using pump gasoline also adds another variable as different formulations or batches might have different coloration / deposits.
 
Depends on application as well, for WOT motocross that may be too lean, but for trail / off-roading that could be near perfect.



So i have now done a plug chop and it looks lean, there is a 2mm ring of very light brown, currently i am running a 350 main, needle position clip is at 3 and air screw is 3/4 turn out, where do i go from here? Get a larger main jet?

Share this post


Link to post
Share on other sites

Yes, main jet affects 3/4 to full throttle.

If you want darker, go richer!

 

There are other tuning variables like

throttle slide cut-out, different needle diameters and tapers etc.

but in general to get you in the ballpark :

 

Pilot jet & air screw :  idle/off-idle transition 

needle: 1/4 to 3/4 throttle

main jet : 3/4 to full throttle

 

start tuning from the top down: main jet, needle, pilot jet

the air screw can require daily fine tuning according to ambiant temps and humidity

 

 

Edited by mlatour

Share this post


Link to post
Share on other sites
Yes, main jet affects 3/4 to full throttle.
If you want darker, go richer!
 
There are other tuning variables like
throttle slide cut-out, different needle diameters and tapers etc.
but in general to get you in the ballpark :
 
Pilot jet & air screw :  idle/off-idle transition 
needle: 1/4 to 3/4 throttle
 
 



On the weekend on the track when im mostly wide open than yes a richer main jet would make it less lean but when im trail riding (1/4-1/2) throttle than the plug looks alright. do i maybe have to turn out the air screw and or move the needle clip position to make it leaner and then go up on a main?? If that makes any sense..

Share this post


Link to post
Share on other sites

The air screw fine-tunes the pilot jet circuit,

say commonly available jet sizes are: #37.5, #40, #42.5 

by fine tuning the air screw you get the equivalent to an unavailable 'in between' size, say like a #38.5 pilot jet

The air screw only works within a certain range, beyond about 1 turn in/out from it's nominal starting point

you have to go to the next leaner or richer size.

 

It has no effect on the other metering circuits.

Edited by mlatour

Share this post


Link to post
Share on other sites

On a track + trail bike, 

you could easily run a much leaner than stock pilot jet to prevent fouling at lower speeds/rpms and,

keep your richer MX settings for the needle and main jet circuits.

 

By turning in the air screw when at the track, that would perhaps richen the circuit enough not to require a pilot jet change.

 

Myself run 2 steps leaner pilot, and 1/2 step leaner needle settings on an MX only ridden YZ125.

Stock engine except for reduced squish band height and 15% race fuel. (total of 94 octane)

In my application, leaner than stock has always worked better, 80+ hours on the engine and no problems.

 

Edited by mlatour

Share this post


Link to post
Share on other sites
On a track + trail bike, 
you could easily run a much leaner than stock pilot jet to prevent fouling at lower speeds/rpms and,
keep your richer MX settings for the needle and main jet circuits.
 
By turning in the air screw when at the track, that would perhaps richen the circuit enough not to require a pilot jet change.
 
Myself run 2 steps leaner pilot, and 1/2 step leaner needle settings on an MX only ridden YZ125.
Stock engine except for reduced squish band height and 15% race fuel. (total of 94 octane)
In my application, leaner than stock has always worked better, 80+ hours on the engine and no problems.
 



I have an fmf fatty pipe and a cutout in the rear side plastics if that makes a difference , so i should go leaner on the pilot jet, and then richen the needle and main jet?

Share this post


Link to post
Share on other sites

Screenshot_20170904-133121.png

This is a jetting chart for running an fmf pipe i got online, i am running a 350 main and 35 pilot, im on clip position 3 for the needle and at 1/2-3/4 on the air screw, doing exactly what that chart shows would only make it leaner, would it not?

Screenshot_20170904-133121.thumb.png.c1c67f221610f303e3dc36302b9c9a26.png

This is a jetting chart for running an fmf pipe i got online, i am running a 350 main and 35 pilot, im on clip position 3 for the needle and at 1/2-3/4 on the air screw, doing exactly what that chart shows would only make it leaner, would it not?



Im looking at year 2000

Share this post


Link to post
Share on other sites

Charts can only generalize, each specific application benefits from fine tuning to it's exact requirements,

no one but you can experiment as to what works best for your particular engine and riding style.

 

Start with the stock baseline specs, richen the main jet to get good coloration, lean the pilot jet to obtain crisper low rpm operation and prevent fouling.

For the needle settings, too rich will feel like a stutter/hitting a rev limiter, too lean will bog and not accelerate hard.

 

But before you attempt any fine-tuning, the engine must be in proper operating condition :

good reeds, fresh top end, proper fuel/float level, fresh silencer packing, fresh gas, cleaned/oiled air filter, good crank seals, no air leaks etc.

If any of these are questionable, that will alter the readings and add more variables to complicate things.

(example, low compression makes the engine 'act' rich)

 

As mentioned, my own settings would be considered lean by common standards and charts but, that's what works best in my application.

Edited by mlatour

Share this post


Link to post
Share on other sites

Yea man if that was ran hard( wide open) for 20-30 minutes, it does look lean, if in and out of throttle I'm unsure!

Share this post


Link to post
Share on other sites
1 hour ago, Herman Bueckert said:

 and at 1/2-3/4 on the air screw, doing exactly what that chart shows would only make it leaner, would it not?

 

 


Im looking at year 2000

 

 

When you set the air screw, is it for fastest idle and best response . Did you set the idle screw as low as you can get it to eliminate it from the equation?

google spanky's jetting guide, it has a really easy to understand explanation on the correct setting.

Share this post


Link to post
Share on other sites
Charts can only generalize, each specific application benefits from fine tuning to it's exact requirements,
no one but you can experiment as to what works best for your particular engine and riding style.
 
Start with the stock baseline specs, richen the main jet to get good coloration, lean the pilot jet to obtain crisper low rpm operation and prevent fouling.
For the needle settings, too rich will feel like a stutter/hitting a rev limiter, too lean will bog and not accelerate hard.
 
But before you attempt any fine-tuning, the engine must be in proper operating condition :
good reeds, fresh top end, proper fuel/float level, fresh silencer packing, fresh gas, cleaned/oiled air filter, good crank seals, no air leaks etc.
If any of these are questionable, that will alter the readings and add more variables to complicate things.
(example, low compression makes the engine 'act' rich)
 
As mentioned, my own settings would be considered lean by common standards and charts but, that's what works best in my application.


Thanks, that helps a lot!
Yea man if that was ran hard( wide open) for 20-30 minutes, it does look lean, if in and out of throttle I'm unsure!


It was run in and out not constantly wide open

Share this post


Link to post
Share on other sites
When you set the air screw, is it for fastest idle and best response . Did you set the idle screw as low as you can get it to eliminate it from the equation?
google spanky's jetting guide, it has a really easy to understand explanation on the correct setting.


I actually had it tuned at a shop and i was told it was set good at 1/2 out but to get the highest idle i have to go out about 3 turns.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×