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Author Topic: Reading Plugs for a 79 gs550 (VM22 carbs)  (Read 185 times)

Offline k.graham

  • Posts: 15
Reading Plugs for a 79 gs550 (VM22 carbs)
« on: Aug 04, 2017, 01:17:09 »
Hey all. I've searched around the internet, and many forums regarding plug reading and have found that there are quite a few different ideas on what a good looking plug is and what is not. I know the obvious white is lean, black is rich and toasty brown is just right. What I'm a bit confused by is where exactly on the plug I should be seeing that colour? I did a couple plug chops today at 1/4 throttle, 1/2 throttle and WOT. I didn't get time today to grab brand new plugs to do the testing, but I figured it was still worth checking and seeing to if nothing else, get a general idea. I did notice some subtle differences at the different intervals, but for the most part they all looked fairly similar.

Here are the clearest photos of the most extreme spread I had in the runs that I took. My hope is that someone here with more experience can tell me what these plugs would indicate to them about the conditions it was pulled at.


Photo 1


Photo 2


Photo 3



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Offline 1969 Honda_cb750

  • Posts: 1
Re: Reading Plugs for a 79 gs550 (VM22 carbs)
« Reply #1 on: Aug 04, 2017, 09:37:46 »
They look sooty to me. I would take it for a long, slightly hard ride, like cruise at legal speed for a half hour or so, get it nice and hot, then park it and let it cool off, then read them . I don't like pulling the plugs when hot, I think it could hurt the threads. And I also don't (as many suggest), shut down instantly while at high speed. It doesn't make a noticeable difference to me. If they are still sooty after a nice run, then start looking at leaning it out. Have fun!

Offline teazer

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Re: Reading Plugs for a 79 gs550 (VM22 carbs)
« Reply #2 on: Aug 04, 2017, 13:50:49 »
First hing to know about reading plugs is that what we are really looking at is the temperature the plug reached.  Rich mixtures lower the temperature and the plug does not self clean.  Lean mixtures tend to burn hotter and burn the plug clean.  In theory, there should be no color on the plug at all.  The center should be white, and the only place to read mixture is the so called "mixture ring" around the base of the insulator.

The thing is that plug temperature changes even more with operating load that it does with changes in mixture.  At idle for example plug temps are low and there is no way for the plug to clean itself and it looks sooty - which we read as rich - but it isn't.  The same applies to a slightly lesser extent and cruising speeds and moderate loads.  The plug is barely hot enough to self clean - and that's only if the timing is spot on and the plug is an appropriate heat range.

Manufacturers fitted plugs that would be OK at high speed on hot days and for most of our day to day riding, they tend to be a little colder than the conditions require and consequently look rich but they are not.

So in essence, the only jet we can reasonably use a plug reading to determine is the main jet and only when we run the motor hard enough to get the plugs up to peak temperature.   If we were to use a plug reading at low load to jet down, there is a very high probability we would melt something if we ever ran it flat out.

Next thin is that you only need to run at WOT under load for 10-20 seconds to color the plug and the motor must be cut off cleanly to read the plug under load. Slowing down or returning to the pits/garage/side of the road) colors the plugs with different conditions. 

So the process is flat out running and kill the motor and then check the electrodes for signs of overheating from ignition timing or too hot a plug.  Set timing and replace plugs as required and repeat to read fueling on the main jet. Your plugs for example show no signs of heat on the electrodes that I can see, indicating that the plugs were not hot enough because of load or heat rating.

To get fueling right book some time on a dyno with exhaust sniffer and an experienced operator who knows how to set it up.  You are only looking for the magical 14.7: a under cruise conditions.  Idle has to be richer and so does full power running - closer to 13:1 for power or richer and richer again for idle.

Low speed and part throttle are best done by feel i.e. responsiveness on the road.

Offline jpmobius

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Re: Reading Plugs for a 79 gs550 (VM22 carbs)
« Reply #3 on: Aug 04, 2017, 14:33:56 »
First hing to know about reading plugs is that what we are really looking at is the temperature the plug reached.  Rich mixtures lower the temperature and the plug does not self clean.  Lean mixtures tend to burn hotter and burn the plug clean.  In theory, there should be no color on the plug at all.  The center should be white, and the only place to read mixture is the so called "mixture ring" around the base of the insulator.

The thing is that plug temperature changes even more with operating load that it does with changes in mixture.  At idle for example plug temps are low and there is no way for the plug to clean itself and it looks sooty - which we read as rich - but it isn't.  The same applies to a slightly lesser extent and cruising speeds and moderate loads.  The plug is barely hot enough to self clean - and that's only if the timing is spot on and the plug is an appropriate heat range.

Manufacturers fitted plugs that would be OK at high speed on hot days and for most of our day to day riding, they tend to be a little colder than the conditions require and consequently look rich but they are not.

So in essence, the only jet we can reasonably use a plug reading to determine is the main jet and only when we run the motor hard enough to get the plugs up to peak temperature.   If we were to use a plug reading at low load to jet down, there is a very high probability we would melt something if we ever ran it flat out.

Next thin is that you only need to run at WOT under load for 10-20 seconds to color the plug and the motor must be cut off cleanly to read the plug under load. Slowing down or returning to the pits/garage/side of the road) colors the plugs with different conditions. 

So the process is flat out running and kill the motor and then check the electrodes for signs of overheating from ignition timing or too hot a plug.  Set timing and replace plugs as required and repeat to read fueling on the main jet. Your plugs for example show no signs of heat on the electrodes that I can see, indicating that the plugs were not hot enough because of load or heat rating.

To get fueling right book some time on a dyno with exhaust sniffer and an experienced operator who knows how to set it up.  You are only looking for the magical 14.7: a under cruise conditions.  Idle has to be richer and so does full power running - closer to 13:1 for power or richer and richer again for idle.

Low speed and part throttle are best done by feel i.e. responsiveness on the road.

Pay close attention lads.  You'll have to travel a very long way indeed to get better advice.
Mobius


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Offline k.graham

  • Posts: 15
Re: Reading Plugs for a 79 gs550 (VM22 carbs)
« Reply #4 on: Aug 05, 2017, 01:04:55 »
First hing to know about reading plugs is that what we are really looking at is the temperature the plug reached.  Rich mixtures lower the temperature and the plug does not self clean.  Lean mixtures tend to burn hotter and burn the plug clean.  In theory, there should be no color on the plug at all.  The center should be white, and the only place to read mixture is the so called "mixture ring" around the base of the insulator.

The thing is that plug temperature changes even more with operating load that it does with changes in mixture.  At idle for example plug temps are low and there is no way for the plug to clean itself and it looks sooty - which we read as rich - but it isn't.  The same applies to a slightly lesser extent and cruising speeds and moderate loads.  The plug is barely hot enough to self clean - and that's only if the timing is spot on and the plug is an appropriate heat range.

Manufacturers fitted plugs that would be OK at high speed on hot days and for most of our day to day riding, they tend to be a little colder than the conditions require and consequently look rich but they are not.

So in essence, the only jet we can reasonably use a plug reading to determine is the main jet and only when we run the motor hard enough to get the plugs up to peak temperature.   If we were to use a plug reading at low load to jet down, there is a very high probability we would melt something if we ever ran it flat out.

Next thin is that you only need to run at WOT under load for 10-20 seconds to color the plug and the motor must be cut off cleanly to read the plug under load. Slowing down or returning to the pits/garage/side of the road) colors the plugs with different conditions. 

So the process is flat out running and kill the motor and then check the electrodes for signs of overheating from ignition timing or too hot a plug.  Set timing and replace plugs as required and repeat to read fueling on the main jet. Your plugs for example show no signs of heat on the electrodes that I can see, indicating that the plugs were not hot enough because of load or heat rating.

To get fueling right book some time on a dyno with exhaust sniffer and an experienced operator who knows how to set it up.  You are only looking for the magical 14.7: a under cruise conditions.  Idle has to be richer and so does full power running - closer to 13:1 for power or richer and richer again for idle.

Low speed and part throttle are best done by feel i.e. responsiveness on the road.


Wow, thanks Teazer! Very clear and comprehensive. I've seen so many differing ideas in this area of reading plugs, and I'm guessing that a good many of them are from people who maybe don't have the knowledge behind them to back it up. What you say makes perfect sense and is explained in terms that are plain and logical. Thanks so much for your help and input!



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