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Confused about plug color and jetting

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So, I just rebuilt my motor and changed the jetting as per the manual and FMF recommendations. I have a FMF Gnarly on the bike. The old jetting was stock jetting. 178 main, 50 pilot, stock needle second clip. I changed it to 170 main, 48 pilot, stock needle second clip, 1.5 turns. I figured the bike would run great with this new jetting. Well, I fouled my first plug ever on this bike today! Was riding in 75-85 degree temps at about 4,000-5,000 feet.

I looked at the old plug that I removed from the bike before the rebuild and it looked good riding in the same conditions. I would have thought that leaning out the jetting would not have resulted in fouling a plug. Here is a pic of the old plug, it's light brown, and a pic of the second plug I ran today, it's black? I would just go leaner, but am confused on why the plug with the richer jetting looked good. Any thoughts??? Also, with this new jetting, I can't get the bike to idle like it did before. It dies about 10-15 seconds.

Old plug

DSCN1288_(Medium).JPG

New plug

DSCN1287_(Medium).JPG

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Were you always at the high altitude?

Yes, that is what has me so confused. I rode the same exact trails. Temps were consistent as well.

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Paul, I'm definitely no expert, but you have a new top end. Maybe you have now have higher compression, which could be sucking in more gas.

Its still hard to believe that you fouled a plug because of the main.

I would have thought that you would have dropped the pilot to a 45 at two turns out instead of a 48.

Keep us posted. Some of us (namely me...) want to know how you jet your bike.

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The black plug was pretty normal for my bike running at higher altitudes. Mo matter which jets were in place, the plug stayed black. Once I set the squish band and bumped compression a bit, the plugs started to become brown again.

I can give a thought, but its hard to know how accurate it is. This would be based on experience with only two yz 250's. The 97 and 06 responded similarly.

My thought is: I was not able to ride hare enough to create any heat. I could for moments, but not consistently enough to get a decent looking plug. By the time I had dropped down to real small pilot jets, lean needles and small mains. I was starting to feel lean symptoms from the bike.

When altitude goes up, compression and pressure drop. Eventually you reach a point where efficiency had dropped a substantial amount. Setting the squish and moderately increasing the compression brings things back in line.

Not that this was necessary. My bike ran decently. Jetting changes would have been plenty. I think the proof was the pinging issue. If the problem was rich jetting, why would the bike still ping in stock form? Thsi also stopped once the head was brought back in line.

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If it were mine I would drop the pilot to a 45. Then warm the bike up real good and set the air screw so that the bike idles If more than 2 1/2 turns out then drop to a 42.

I have found on my 2000 YZ250 that the pilot is very sensitive to change once you get in the proper range with the air screw. If it doesn't idle I know that the air screw needs adjusting. I'm running a 45 and operate at 3000 feet with a rad valve.

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Thanks for the comments guys. I've got a 45 pilot, so I'll give that one a shot.

On a positive note, I only had a small drip of spooge coming out of the pipe with the leaner jetting.

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Yes, that is what has me so confused. I rode the same exact trails. Temps were consistent as well.

Could the difference be that the worn rings create a lean affect and the fresh top end will be richer? If you were jetted properly before the top end, you will need to rejet for a rebuild.

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Could the difference be that the worn rings create a lean affect and the fresh top end will be richer? If you were jetted properly before the top end, you will need to rejet for a rebuild.

Just the opposite actually. As the top end becomes worn the lower compression burns the fuel less efficiently and to keep spooge away the jetting needs to be leaned.

So if any jetting change is made it would be to lean the jetting as the top end becomes worn out.

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My first thought would be why doesn't it idle? I don't own a 00' but would think a 48 pilot should give you a good idle. Did you try adjusting your air screw for best idle? How did it run in the rest of the throttle range?

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My first thought would be why doesn't it idle? I don't own a 00' but would think a 48 pilot should give you a good idle. Did you try adjusting your air screw for best idle? How did it run in the rest of the throttle range?

I wasn't able to get it to idle with the fuel screw either, but I haven't spent a whole lot of time on the idle yet. I dropped in the 45 last night and plan do play with the jetting this week. I ended up ordereing replacement reeds as well in case they are bad. They were in the bike when I bought it, so I'm not sure how old they are. They look fine, no chips or anything, but I figured it would be better to make sure everything else is good before I work too much on the jetting.

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