Carb Jetting and Spark Plug Reading

[allanrps]

HI all,

I got my bike running about a month ago and I got the carbs synced up and the idle dialed in. If I remember correctly I'm 2 7/8 out on the left idle mix screw and 3 1/4 on the right. Before I even started it for the firsts time, I went up one size in jets to 142.5 and raised the needle 1 notch because I replaced the stock airbox with a K&N air filter. Recently (I think) I've been having a problem where after coming to a stop, my engine revs fall below idle range and eventually stalls unless I give it a little throttle to stabilize the idle. It idles great and starts on the first kick usually, half choke.The power seems pretty consistent. From idle to about 2.5k it's pretty week, as I would expect. It makes most power around 4k and 6.5k I think, and I don't take it up much more than that since my tach is currently out of commission. I went through a tank of gas and only got 140 miles out of it, which seems like an entirely unreasonable figure. I can superficially see that as denoting a overly rich fuel mixture, but I have a hard time believing that such a small adjustment could increase fuel consumption by 33%. Perhaps I'm running too rich/lean, causing my engine to output less power, causing me to use more throttle? I pulled the right plug and it's dry, blackened, and brown at the tip. I guess that means I'm running too rich, but I just wanted to make sure, since I read that replacing the stock air box would require a richer jetting, and I thought I made a modest adjustment. Any thoughts are much appreciated.

Thanks

 

[allanrps]

I pulled the left plug. It is black and dry from carbon fouling, with a slight brown at the tip of the electrodes. On the back of the ground electrode the carbon was flaking away. The right plug was all black except for the porcelain, which was perfectly white except for the portion which was nearest to the ground electrode, and the tip of the electrodes, which were brown. The carbon on the left plug flaked off relatively easily while that on the right plug was much more obstinate. I don't have the slightest idea what all that could mean, except that I'm running too cool somewhere in the rev range.

 

[16VGTIDave]

Put the needles back to stock and try that. You are obviously running rich. Make sure to go for a good run at highway speeds for 20 min to get the engine up to operating temp before attempting any further diagnosis or tuning.

Attempting to set the idle mixture based on turns out is NOT correct. The turns out is a starting point to get the bike running. Only. After you have the bike at operating temp (oil temp above 80C or 180F), then you can adjust idle mixture and sync.

 

[allanrps]

Ok, I'll do that and see. Is there a reason why you would suspect the needles rather than the jets? And why is it that the porcelain was white on one spark but black on the other?

If you mean setting the idle based on a specific number of turns out of the idle mix screws, such as the number in the manual, that was not what I did. I tuned the idle and then recorded the number of turns out the screws were at.