Plugs after a good highway run and carb settings.

[Courtney]

So went for a good 30 mile run today in varied traffic, mostly 60mph stuff, but some all in the 3-3500rpm range. Got home and pulled the plugs for a quick look and found 1,2,3 a bit sooty, blackish with some tan edges on the shielded side behind the electrode. No. 4 was a nice tan. So here is my question on this bone stock engine with bone stock carbs.

1) What rpm do the mixture screws affect? They are all set at 2 full turns out, is this why the plugs might be sooty? Don't really know if they affect it at 3-3500rpm?? No.4 obviously is running right for it's cylinder conditions??

2) I'm getting 37mpg US gallon, is that in the ballpark? I'm wondering what else could make the plugs sooty as there are no other adjustments on my bone stock system. New aircleaner, spotless carbs, all stock. Thoughts? Not interested in hearing about plug chops etc etc for true perfect measure of air/fuel ratios, I'm interested in average across the board readings like I usually do/see in references looking for a nice light tan color.

[Rasputin]

No two mixture screws will be exactly the same. Where one is 2 turns out the next may be 1 3/4 turns out for the best setting. The engine has differing health in any cylinder so each has it's own setting.
Note I did not say anything about chops or checking the plugs after a ride and it idled a few minutes. To each his own but that 37 MPG might improve a bit if you do them.
Damn now I went and mentioned them....silly me!

[donebysunday]

plugs

Soot is most likely a little oil or the carbs running rich.
What's the compression and what if any oil used ?

What about float settings ?

[BA80]

So went for a good 30 mile run today in varied traffic, mostly 60mph stuff, but some all in the 3-3500rpm range. Got home and pulled the plugs for a quick look and found 1,2,3 a bit sooty, blackish with some tan edges on the shielded side behind the electrode. No. 4 was a nice tan. So here is my question on this bone stock engine with bone stock carbs.

1) What rpm do the mixture screws affect? They are all set at 2 full turns out, is this why the plugs might be sooty? Don't really know if they affect it at 3-3500rpm?? No.4 obviously is running right for it's cylinder conditions??

2) I'm getting 37mpg US gallon, is that in the ballpark? I'm wondering what else could make the plugs sooty as there are no other adjustments on my bone stock system. New aircleaner, spotless carbs, all stock. Thoughts? Not interested in hearing about plug chops etc etc for true perfect measure of air/fuel ratios, I'm interested in average across the board readings like I usually do/see in references looking for a nice light tan color.

The RPM you were running is a bit low for these engines, they love 4 to 6K. If you getting 37MPG and it's running good don't mess with it.

[Courtney]

No two mixture screws will be exactly the same. Where one is 2 turns out the next may be 1 3/4 turns out for the best setting. The engine has differing health in any cylinder so each has it's own setting.
Note I did not say anything about chops or checking the plugs after a ride and it idled a few minutes. To each his own but that 37 MPG might improve a bit if you do them.
Damn now I went and mentioned them....silly me!

Always appreciate your input Ras, especially the comment about the mixture screws. I might lean out the 3 sooty ones by a quarter turn and see what happens. My Clymer says the factory setting is 1-1/4 out, so I'm above that. No worries about the plug chop, I knew SOMEBODY would call me to task. on it.

[Courtney]

The RPM you were running is a bit low for these engines, they love 4 to 6K. If you getting 37MPG and it's running good don't mess with it.

It is running good BA, but I also appreciate what Ras is saying as well. So I'm assuming then that the mixture screw WILL affect these bikes beyond the idle? That would make sense if I'm out 2 full turns and getting plugs. As I mentioned above, I'll turn the sooty plug carbs in a 1/4 turn and see what happens.

[Courtney]

Soot is most likely a little oil or the carbs running rich.
What's the compression and what if any oil used ?

What about float settings ?

I'm pretty certain the carbs are rich with 2 full turns out. No 4. likes it there though, it's a nice tan. The other 3 will get turned in a 1/4 turn and then I'll check it after a good hiway run. Compression is great.

[BA80]

The circuits being used in the carb have very little to do with RPM and everything to do with throttle position. At cruising speeds you at 1/4 to 1/2 throttle. The idle mixture screws have very little to do with the fuel there.

A good basic understanding......... http://home.everestkc.net/malsin/Mot...theory_101.htm

[Courtney]

So BA, these carbs and plugs were fine before I cleaned them. That's all I did, as they'd been sitting for 3 yrs. Why would all of a sudden they be rich on 3 cylinders when nothing else changed? Any thoughts on this? Surely they can't soot up by idling for the minute it took me to shut it down? Oh yeah, ONE MORE SYMPTOM. On full choke when I start it, it starts right up, but then chugs as if it's way too rich on that circuit. Does that ring any bells??

[BA80]

When it starts just click the lever back to the middle position and it should smooth out until it gets warmed up then it'll get rich running again and you can take off the choke. I don't even use the enricher on my bikes unless that's the only way they'll start.


As I said before, if it's running good and your getting 37 MPH just ride the thing. If those plugs are 3 years old change them. New plugs can make a huge difference

[MPittma100]

Idle Mix Screws

These screws are what they say they are. They affect idle mixture. Once the throttle plates are open, the carbs begin to include another circuit (main metering).

Some sooty appearance will be on any plug in any engine that is running unleaded fuel. Also will be in/on exhaust. Nature of the fuel. The brownish color is what you are looking for. Yours is a fine example as to how a plug should look with everything stock. Your MPG is your verification.

MP

[Old Red]

While cursing down the road at 5,000 RPMs, what happens to the fuel mixture in the idle circuit if you close the throttle? Manifold vacuum goes way up and the idle circuit goes way rich. Just thinking the idle circuit is more active than we may think. If we run a constant 5000 RPM, then the idle circuit is not important, but if we are riding in traffic or riding Deals Gap it becomes a bigger player.

The Other Earl

[BA80]

Earl, even at that there would be more air than fuel getting in the cylinder. The idle circuit only allows so much fuel in through that tiny oraface and the minute amount is controlled by the mixture screw, a valve if you will.

The idle circuit is virtually insignificant above idle rpm. It only makes a small difference on decal as far as the decal popping due to a lean condition at closed throttle.

[Rasputin]

I find that pictures in some of the links I have to be very informative. I will post a couple of pics from the links to show what I mean and then follow it with the links so you can explore further.





Below are 2 of the links I favour.

http://home.everestkc.net/malsin/Mot...theory_101.htm

http://home.everestkc.net/malsin/Mot...theory_101.htm

[TopCatGr58]

Hey Ras,

Thanks for posting these diagrams. I remember some time back someone posting similar type diagrams that showed that the Pilot jets DO contribute fuel input throughout the entire throttle position range, albeit in an ever decreasing rate from Max at idle downward to WOT, but it's still providing fuel flow.

The pilot jets are just a little smaller than 1/2 the size of the mains, but they are also metered for even less fuel flow with the pilot screw adjustment. But these are CV carbs, and fuel flow is actuated by vacuum, and even though much more of the vacuum affect is directed on the mains and needle jet during the higher throttle positions, there's still air flow/vac. that's still pulling fuel thru the pilot circuit as well.

I point out the K&L pilot jet fiascos that we have seen. Aside from folks having difficulty in being able to get their bikes/engines to idle properly, they also have problems throughout a big part of the throttle range and response. If the pilots ONLY contributed to just idle, then they shouldn't cause the throttle response problems above idle when they are not genuine Mikuni jets, but alas they do.

Good luck Courtney in adjusting your pilot jets to get them all to match the #4 plug color.

T.C.