Spark Plug / Heat Ranges?

4 replies to this topic
  • Fastest1

Posted November 25, 2001 - 09:27 PM


I was curious about a statement another user about using a CR7E instead of a CR8E sparkplug. Does the CR7E run hotter as to burn off the carbon? I am presently running a CR8E (stock rec.)and it seems to foul on a regular basis. I have run many single cylinder 4 strokes and not had anyting like this. The bike runs excellent. The electrode after doing a plug chop is tan. I just did the BK Mod so I dont know if that will help, but it has fouled a plug since doing it. The pilot checks out to be in range and the bike doesnt run rich. It just gets a sooty plug and becomes a bitch to start. Maybe I dont rev it high enough during riding to blow the carbon off! LOL though it is the truth! Let me know what you think. Thanks :)

  • yzernie

Posted November 25, 2001 - 09:38 PM


What is your current jetting?


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  • Fastest1

Posted November 25, 2001 - 10:19 PM


170MJ,48 pilot, Needle 4th clip from top. Sea level. It does run a little richer than my road bikes if you judge by the pipe! I might experiment with the jetting some. I dont think it is going to pull much harder than it does. I just want it to be consistent and not foul plugs. Thanks

  • yzernie

Posted November 26, 2001 - 07:33 AM


BK mod?? Race fuel?? If not, you are probably jetted a bit rich even for sea level. I might try a 45 pilot, needle stock and a 165-168 main, fuel screw starting at 1 turn out.

If you have an aftermarket pipe, it may run a tad lean with stock jetting. The above jetting should work with an aftermarket pipe too.

Let me know, Ernie

  • mikeolichney

Posted November 26, 2001 - 03:32 PM


I am one of the guys having good luck with the CR7 plug in cold weather. If I run a CR8 below 40 degrees F I am risking a fouled plug. My 01YZ426 is stock except for the BK mod. I don't run race gas because my YZF is silly fast and carburates cleanly (barring plug problems of course), I don't need the expense or hassle of race gas. I originally had a bad CDI on this bike, but even after the CDI swap, I had the fouling in really cold weather, just not as bad. It was driving me crazy all last winter, (I race a winter HS series in Colorado, I DNFed two races with fouled plugs, it would die usually in the first 15 minutes). When spring came the bike ran much better, and ran great all summer with the CR8. The last couple months the problem resurfaced. So I am convinced its outside temperature related. No, its not the winter gas because we still get warm days and the bike runs fine then with the same gas.

I noticed that it I let the bike idle on the stand until it boiled over before I rode it, I usually would not foul a plug in cold weather. I remember a guy in NM said his had a similar problem, if it did not foul in the first 15 minutes, it would be okay. Just like mine, his would fire up easily with a fresh plug. These things and the black sooty ceramic on my CR8 led me to the CR7 solution. I had not got to do a race in cold weather yet, so I had not yet posted this theory of mine. But yesterday I ran a 2 hour HS and my bike ran perfectly! Right after the race it started snowing, it was so cold. So I think I got it now. The engine runs cooler in cold weather, and at elevation, like NM or CO, the bikes don't make as much power or heat (less oxygen to burn with). So the plug stays too cool, especially during warmup. The carbon builds up on the ceramic and gives a path to ground (fouls). The hotter plug burns the carbon off the ceramic. Try a CR7 if you think you are jetted well but still have a black ceramic insulator.

Consider this: If I run a CR8, I get a black ceramic insulator, if temps are below 40F. If I run a CR7 with the same jetting, I get a white to tan insulator. I always get black sooty threads, maybe thats the pump gas I use.

Notice that I did not change the jetting. This would lead me to conclude that I am too rich in one case, or too lean in the other (if one jets by plugs). BUT THE JETTING DIDNT CHANGE! Thats why I don't trust the plug reading (other than to look for little balls of aluminum to tell me I am roasting a piston). I think it was SUnruh that says he cuts the plug and looks down at the ceramic base to tell, but thats too much trouble for me. I jet by feel now, and a temp/altitude chart (email me if you want a copy, its an excel speadsheet based on air density, many TT people have got it from me and seem to think it works pretty well. BTW, I think a 48 pilot is too rich even at sea level, search for posts on setting idle circuit). I should be running the same jetting at 30F as you run at 70F. I run a 168 main and a 42 pilot at 30F.

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