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Huffa 2

Spark plug reading -plug chop method ?

9 posts in this topic

94 L model, 6000 miles, original plug, I think.

Right now I have a stock carb with dynojet kit with a 160 main, needle supplied by them with a E series muffler at 5 disc. ON the E, the less disc the richer, more leaner, so instead of changing jets this would be the easiest to do first. Now I'm at about the richest disc setting the E will go, so of course if I get a reading of too lean, then it's jet time.

1) to do a SP chop reading should my plug be brand new ?

2) should it at least be cleaned 1st ?

3) so at idle, I simply leave it idle for a few minutes and check it?

4) what throttle postion would give me the needle reading, if I should raise,lower or leave where it's at ? Mid throttle ?

5) main jet I floor the thing in say 3rd gear, kill it, coast to stop and check ? Do I do top rpm in 3rd and kill it?

6) when it's real hot and humid out, would jetting be richer ?

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1) Start with A clean plug, dosen't have to be new.

3) don't use plug readings for idle, turn your air/fuel screw to adjust for best idle. If you have to go in or out too far with screw then change idle jet.

4) about half to 3/4 thottle, If your too lean here the bike will surge.

5) yes use 3 rd and hold it there for a little bit, need some time for plug to color.

6) hot and dry make it richer, cold and humid make lean out.

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Plug readings are a mixed bag. I've found that--in my opinion--part of the problem is the subject is commonly misunderstood. Reading plugs with an accuracy that is actually useful is not as straight forward as we're sometimes led to believe. First thing to do is ignore the plug reading charts that have a whole bunch of plug tips of different conditions. Those charts are best applied by commercial mechanics trying to figure out why a tractor isn't running right--NOT a performance enthusiast trying to decide between a 160 or 165 main jet.

The old "tip of the ceramic insulator is a paper bag color" comparison applied to a plug with some hours of general run-time might be useful for determining if your overall jetting is in the ballpark. But I wouldn't rely on that for any measure of fine-tuning. There's just too many variables unaccounted for with a plug that looks like this:

after2hours.jpg

Yeah, that looks nice--but I have no idea exactly what running conditions/settings where actually responsible for that coloring. Plugs don't just "remember" the last the last thing that happened to them. You could run a plug rich and then run the plug lean and it might come out looking great.

1) to do a SP chop reading should my plug be brand new ?

Yes. Brand new. To discern small jetting changes accurately you need to use a new plug for each test. An old plug will be a confused reading of all different throttle positions, engine loads, engine temps, etc. It doesn't really help that much to have an "average" because you may be arriving at that "average" by being jetted too far in opposite directions on different circuits.

I should also note that there is so much variation in composition of the pump fuels supplied today that reading a plug with you're particular fuel takes experience. There won't be (or at least may not be) an exact color or "look" to the plug that will indicate a precise a/f ratio that will hold across many different fuels. Some tuners choose to ignore what plugs look like these days for that reason.

So anyway, here's a good method for reading plugs:

-Warm up the engine on an old plug.

-Stop and install the new plug.

-Start it up and immediately take off.

At this point the best results may not be obtained exactly the same way for every application and fuel. Generally a single pass is sufficient; other times making a couple passes on the same plug will yield better results. By "better results" I mean results that are easier to interpret. But the basics are:

-Keep the throttle wide open as much as possible. We're testing the main jet. Any time spent idling or at partial throttle openings can confuse the results because the other carb circuits are at play.

-Accelerate at WOT through the gears until you get to 4th. Then hold that gear all the way to max rpm.

-At max rpm hit the kill switch and pull in the clutch.

-Coast to a stop and pull the plug for examination.

If multiple passes seem to yield better results for you then just remember to spend minimum time at partial throttle openings between the runs. After you've competed a pass, killed the engine, and oriented yourself to make another run; start it up and TAKE OFF staying in full throttle as best you can. This will help ensure that the only thing the plug tells you is information about the main jet size.

The place to read a plug is WAY DOWN inside at the BASE of the ceramic insulator. You'll never be able to fine tune by taking a casual looksy at a plug that's been run at all different throttle positions/temps/rpms/etc. As I mentioned, you may get a general indication of how things are but you'll never get "dialed" by looking at the end of the insulator on an old plug.

The reason they're called "plug chops" is because in many cases you've got to actually cut away the threads of the plug to get a good look at the appropriate area. There are also special plug reading tools that look a lot like the thing your doctor uses to see inside your ear.

This isn't from an XR but here's an example. This plug was colored with 3 WOT passes of about 7 seconds each. That was the only run time on the plug. Main jet = 148.

plug148mainmediumrh3.jpg

Same story but with a 152, just to show the difference.

plug152mainmediumoi2.jpg

When performing a plug chop test it's important to warm up the engine on an old plug, insert a new plug, and then immediately perform the WOT pass while spending as little time as possible at any throttle opening other than WOT. 4th usually works well for the pass. Then remove the plug and examine the "mixture ring" at the base of the insulator.

I've heard of reading a/f ratios off the top edge of the plug's threaded surface--the metal part that faces the piston. But I've no experience with this method. I believe it's the same idea as actually looking at the piston face to evaluate a/f ratios. Reading the "mixture ring" at the base of the insulator is tried and true--I do know that. Whatever method is used, a new plug is still required to get good info.

Here's another pic of a couple of test plugs:

Plugs.JPG

As you can see, you don't necessarily have to cut all the threads away. Whatever it takes to get a good look at the bottom of the insulator is what it is. If your lucky, you may be able to see the mixture ring well enough without cutting anything. This is more likely to be the case on a colder plug (high numbers, i.e. 9 vs 8) because the bottom of the insulator isn't as deep into the plug. A magnifying glass can help too.

Here's some more. This one looks a little lean:

10-29-05pass1-05.jpg

This one looks too rich:

AR3933-terry-wise-plug2.jpg

3) so at idle, I simply leave it idle for a few minutes and check it?

There are better ways to confirm your pilot circuit jetting than using the plug.

4) what throttle postion would give me the needle reading, if I should raise,lower or leave where it's at ? Mid throttle ?

Typically the main circuit (main jet) is the one associated with plug readings. I'm not sure if there's any reason that you couldn't read a plug for the needle circuit though. Just do the same as you would for the main except instead of WOT use 1/2 throttle.

6) when it's real hot and humid out, would jetting be richer ?

Yes. Hot air is less dense. Humid air is also less dense. Anything that reduces the density of the air will push your jetting rich. Said another way, when it's hot and humid, you'd need to lean out your jetting to compensate.

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Thank you VERY VERY MUCH for such in depth info, Hawk :thumbsup: :thumbsup:

You make sure you :naughty: never leave this site, ya hear!

Well I did what you said (part of it) before I read your post. Warmed it up with old plug so it would not be on choke circut with new plug, then idled it for a few minutes with new plug and got no reading :lol: .....just came out looking like new again.

Then I got to thinking I should check the valves 1st, being that I just bought this bike not too long ago and who knows if they were ever checked, then I'll get back to the plug test.

I'm sure I'll have more questions along the way and will be happy just to have it in ballpark and more a hair on the rich side then lean.

Great pictures/ fine fine info !!! :worthy:

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1) Start with A clean plug, dosen't have to be new.

3) don't use plug readings for idle, turn your air/fuel screw to adjust for best idle. If you have to go in or out too far with screw then change idle jet.

4) about half to 3/4 thottle, If your too lean here the bike will surge.

5) yes use 3 rd and hold it there for a little bit, need some time for plug to color.

6) hot and dry make it richer, cold and humid make lean out.

You too gotwheels but am sure you learned quite a bit from reading Hawkmans post ?

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nice test! i really like the night sky pic with the plug in the foreground!:thumbsup:

I didn't even notice that ! Does look cool :thumbsup:

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Yes. Hot air is less dense. Humid air is also less dense. Anything that reduces the density of the air will push your jetting rich. Said another way, when it's hot and humid, you'd need to lean out your jetting to compensate.

Hawk... cant thank you enough. I'm learning all about jetting for my kx500. It blew a base gasket, and i purchased the bike thinking it needed a new top end. It turned over freely, so i knew it couldnt be too bad, and it wasnt. The piston looked nearly new. No markings on the cyl wall, yet the base gasket was missing pieces. The exhaust power valve was NOT hooked up at all. No wonder.

At any rate, I had a question about the statement above. I'm learning about the fact that there is rich and lean for air/fuel, but there is also a different type of "rich/lean" for fuel mixtures. When you said above "when its hot, you need to lean out your jetting to compensate" you're stating for 2 strokers... keep your oil at the same mixture (32:1) for example... but for hot air, use a smaller jet? Perhaps the next size down? 165 instead of a 170?

I'm going riding this weekend, and typically FMF calls for a 170 main. Its going to be COLD. 32 degrees and sunny. WOuld I want to be go buy a 175 for this weekend? Thanks so much.

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damn...impressive post!

Just a stupid question...what do you use to cut the threads? dremel? saw?

I did not know that it was down way down on the insulator you where supposed to look...goes to show...you defenitiley learn something ever day!

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