Bad Spark Plug?


6 replies to this topic
  • BASSic

Posted May 29, 2008 - 07:09 AM

#1

I installed a fresh spark plug not too long ago and rode for an hour or two. I decided to clean my carb, and after reinstalling it, I had difficulty getting my bike to start; I pulled the plug and sure enough it was wet. I wiped it off and let it dry, then reinstalled it but prior to torquing it down, I kicked the bike over a few times and then went to take the plug out to check again and felt a snap. The threads of the plug were stuck in the head, and the rest of the plug pulled out. I thought this was odd because I never torqued the plug down. See the middle spark plug in this picture:

Posted Image

I took another spark plug and cut the threads off to find out what size extractor I should use. It ended up being a #4 extractor. Since the hole is so deep in the head, you can't get a tap wrench or adjustable wrench on the extractor to turn it. I ended up using a 1/4" drive 5/16" 12-point socket with a 3" extension to turn the screw/bolt extractor. I was able to remove the stuck threads by hand - no need to use anything for leverage. I guess I was lucky. If the hole wasn't so deep I probably could have used my finger to take it out.

When I was taking the plug out to check it that second time, I did have some difficulty getting the plug remover tool straight over the plug so it may be my fault that I snapped it off, but what I'm wondering about is the color of the insulator. If you look in the picture, the spark plug in the middle is the one that broke off inside the head. The insulator (which is normally covered by the threaded part) is black from carbon. The plug on the bottom, which I put in a vice and hacksawed the threads off, has a clean white insulator. This plug has been run for 40 hours or more yet the insulator is cleaner than the one above it which has only been run for an hour or two.

Another thing I noticed is that all the old plugs I have have a nice coating of carbon on the plug that can be scraped off with a fingernail. I looked down into the engine and noticed the top of my piston is also coated with carbon and it doesn't look clean. How much is too much? Is it normal for the plugs to be this dirty in a 426? NGK's website doesn't seem to agree. All the old plugs I have look like the second picture "DRY FOULING." I know spark plugs can't be 'read' in the same sense as they can be when burning leaded gasoline, but it does seem like a lot of carbon buildup on my plugs.

Could this be jetting related?

  • matt4x4

Posted May 29, 2008 - 07:34 AM

#2

your piston can easily look black, probably more likely that it does.
black plug means rich, since you cleaned your carb, you likely reset everything to pre removal?
you might be rich on the pilot circuit, especially if a partial blockage a while back was hidden by opening the pilot screw up more, now that the carb is clean, you should readjust your pilot circuit as per instructions (you can search for those) to get it set right.

  • BASSic

Posted May 29, 2008 - 08:02 AM

#3

All these black plugs were prior to cleaning the carb. I haven't had a chance to ride much since I cleaned it, maybe an hour or two.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • grayracer513

Posted May 29, 2008 - 08:27 AM

#4

Black plugs can be caused by a number of things, among them, poor ignition, low combustion temperatures, oil fouling, or low plug temperature caused by not running the engine long enough to warm the insulator up, as well as a rich mixture. Carbon in excess on the plug is usually from oil, really cheap fuel, or inefficient ignition.

It's virtually impossible to get any meaningful information from "reading" a spark plug that hasn't run for at least 20 minutes, except to see that it's wet, and what that means as far as getting the engine to run again will depend a good deal on the strength of the ignition system. Really strong ones can start a fouled plug, but the 426 doesn't occupy this category, unfortunately.

I haven't seen a spark plug broken like that in a long time, nevermind two. Torque on the plug should be no more than 15 ft/lb, and it may also be that you are using a plug wrench that does not fit straight down on the plug.

  • matt4x4

Posted May 29, 2008 - 09:01 AM

#5

Grey - he broke the second on purpose (while it was off the bike) to compare...
and I just had this thought - would like your opinion on it...
if the plug was cracked for a period of time, could that have led to the darker insulator color since it essentially creates a leak there where exhaust gasses can travel. One would think it would also make it harder to start the bike etc...

  • BASSic

Posted May 29, 2008 - 09:38 AM

#6

Grey - he broke the second on purpose (while it was off the bike) to compare...
and I just had this thought - would like your opinion on it...
if the plug was cracked for a period of time, could that have led to the darker insulator color since it essentially creates a leak there where exhaust gasses can travel. One would think it would also make it harder to start the bike etc...


That's exactly what I was thinking. While putting in that fresh plug the first time, I may have cracked it, causing a leak. The bike was also a pain to start with that plug before it cracked off, but that was partly because I flooded it (accel pump rod was not moving freely, after cleaning it was shooting too much fuel into the engine). As soon as I put the old plug in, it fired right up.


and it may also be that you are using a plug wrench that does not fit straight down on the plug


I admitted that's part of the problem, if not the sole cause; I was using a spark plug socket with an extension and a universal joint but it was too close to the frame. I dug through a box of parts the previous owner gave me with the bike and found the Yamaha spark plug socket which is much easier.

  • grayracer513

Posted May 29, 2008 - 09:45 AM

#7

he broke the second on purpose (while it was off the bike) to compare...

I should take my own advice and read through the whole post before responding, eh?

if the plug was cracked for a period of time, could that have led to the darker insulator color since it essentially creates a leak there where exhaust gasses can travel. One would think it would also make it harder to start the bike etc...

That's a possibility, but from experience with plugs leaking from actually being loose in the thread, and leaking between the insulator and jacket, I believe that if the compression loss had been significant, it would have been audible (maybe even loud), and a small leak that was there for a period of time should have stained the exterior of the plug shell near the hex black.





Related Content

Forums
Photo

Is it worth it? (Cam and High compression piston) by macgi77


Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies
Forums
Photo

Megabomb Fitment by 288yz450


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply
Forums
Photo

YZ450F 03 Sparks driving me crazy by SirAttard


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 5 replies
Forums
Photo

James Stewart back on a YZ450F by YamaLink


Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  47 replies
Forums
Photo

yz250x vs 450 4t comparisons (preferably desert riding) needed by JakeNorthrupYZ450F


Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 9 replies
 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.