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Black plugs: Plan of attack!

  • Thread starter Thread starter ty998
  • Start date Start date
T

ty998

Guest
Bike: 81 GS650 GX
Carbs taken apart and cleaned
K&N air filter in airbox

Problem: Plugs have a dry black powdery substance on them (carbon I presume) that is easily cleaned off with an emory cloth. After a very short ride they are black again. All 4 plugs are consistently black. No difference in color between them.

Fuel/Air screw is 2 1/2 turns out.

Spark is blue after cleaning plugs.

Boots connected to carbs are in good shape.

Upon start up (with choke on initially then closing it after about 5 seconds) bike revs to 3000 and stays there for about 15 seconds then slowly settles and idles at 1000. On each consecutive start up (without choke) it starts up and idles perfectly.

Plan of attack:
1. Turn air/fuel screw in 1 turn and see what happens/check plugs afterwards
2. Another thread suggested replacing the boots on the plugs. If the spark is good i don't think it would need that.

Bike doesn't have any noticeable performance issues.

Any other ideas?
 
Your idle mixture screws are 1 full turn out from factory, but they were notoriously lean as such.

You are running too rich, and your startup hints at that. Yes, a poor spark could also account for sooty plugs. Keep in mind that it is possible for the leads to give you a good spark grounded against the head, and the bending of the lead wires with the spark plugs installed could lead to a poor spark. It's happened to me...probably not what's going on though.

Without knowing what is going on inside the carbs (larger mains, shimmed needles, etc.) I would suggest you start off with a highest RPM method for setting the idle mixture, reset the idle, take it for a nice long ride, and see what the plugs look like.

Other things that could cause running rich that would affect all four cylinders:

overly oiled and/or dirty air filter element
larger than normal pilot and/or main fuel jets
maladjusted float levels

I'm sure there are more, but that's what I can think of, off the top of my head.
 
BigD_83 said:
Your idle mixture screws are 1 full turn out from factory, but they were notoriously lean as such.

You are running too rich, and your startup hints at that. Yes, a poor spark could also account for sooty plugs. Keep in mind that it is possible for the leads to give you a good spark grounded against the head, and the bending of the lead wires with the spark plugs installed could lead to a poor spark. It's happened to me...probably not what's going on though.

Without knowing what is going on inside the carbs (larger mains, shimmed needles, etc.) I would suggest you start off with a highest RPM method for setting the idle mixture, reset the idle, take it for a nice long ride, and see what the plugs look like.

Other things that could cause running rich that would affect all four cylinders:

overly oiled and/or dirty air filter element
larger than normal pilot and/or main fuel jets
maladjusted float levels

I'm sure there are more, but that's what I can think of, off the top of my head.
Click to expand...


Thanks for the feedback.
I will reset the mixture screw to 1 1/2 and see how it does.
Also when rebuilding carbs i validated the float bowl heights were correct (according to Clymer). Of course Clymer didn't help me out much when correcting valve clearance (thx BassCliff). :lol:
Air filter is brand new.
 
ty998 said:
Thanks for the feedback.
I will reset the mixture screw to 1 1/2 and see how it does.
Also when rebuilding carbs i validated the float bowl heights were correct (according to Clymer). Of course Clymer didn't help me out much when correcting valve clearance (thx BassCliff). :lol:
Air filter is brand new.
Click to expand...

Did you oil the filter before putting in in the airbox?
 
When warm, does turning idle stop screw effect idle ? you're running rich, so I'm wondering about choke circuit supplying excess mixture (not turning off)
 
tom203 said:
When warm, does turning idle stop screw effect idle ? you're running rich, so I'm wondering about choke circuit supplying excess mixture (not turning off)
Click to expand...
Good point, Tom.

The choke plungers may not be fully seated if there is a bit of grunge in their housing, or if their seals are swollen or cracked. I wouldn't bet on the latter though if it's all four cylinders, however, even one seal that is swollen up could be enough to let the choke hang up as they all move together
 
tom203 said:
When warm, does turning idle stop screw effect idle ? you're running rich, so I'm wondering about choke circuit supplying excess mixture (not turning off)
Click to expand...

Here is what I have experienced:
I will start up the bike for first time and it will rev high and hang there. As soon as I choke the bike a little (choke was off) it settles back down. So from this I assume there is an airflow issue, but I haven't been able to define a problematic area.

Also I don't have this problem when restarting the bike 5 minutes later.

Oh, I have also put in new plugs. Just for info sake here.
 
Don't lose sight of fact that "choke" circuit supplies mixture (air+ fuel)- your bike seems to not need this circuit very long- mine needs it for a good 30 seconds (maybe halfway) or it will be displeased.
 
ty998 said:
Here is what I have experienced:
I will start up the bike for first time and it will rev high and hang there. As soon as I choke the bike a little (choke was off) it settles back down. So from this I assume there is an airflow issue, but I haven't been able to define a problematic area.

Also I don't have this problem when restarting the bike 5 minutes later.

Oh, I have also put in new plugs. Just for info sake here.
Click to expand...
Interesting...

revving high without choke screams LEAN, but you have blackened plugs. If you leave the choke on for some time after you apply it, you could definitely be getting some low rpm fouling. What do the plugs look like if you take it on a great tear (i.e. after you get back from a ride)?

TheCafeKid referenced a Mikuni (EDIT- not Mikuni specific) carb tuning site that I am going to try to find for you. There was plenty of good information on that website.

EDIT- http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html
 
Last edited:
BigD_83 said:
Interesting...

revving high without choke screams LEAN, but you have blackened plugs. If you leave the choke on for some time after you apply it, you could definitely be getting some low rpm fouling. What do the plugs look like if you take it on a great tear (i.e. after you get back from a ride)?

TheCafeKid referenced a Mikuni carb tuning site that I am going to try to find for you. There was plenty of good information on that website.

EDIT- http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html
Click to expand...

I will be sure to check out that site. I unfortunately have not yet taken it on a "great tear" as I am still building the tail section. However, I do look forward to that. :D

I will try turning the fuel/air mixture screen in 1 turn and see what happens. I will update this thread once I get the opportunity to work later this week. Also I've just recently put everything back together and it does smell a little rich from the exhaust as well.
 
um factory are usually 2 turns out from seated closed, try 2.5, cause you need a lil more air, so it will not be carbon/black
 
platinum2 said:
um factory are usually 2 turns out from seated closed, try 2.5, cause you need a lil more air, so it will not be carbon/black
Click to expand...
Factory settings were usually in the 1.25 to 1.5 range, keeping them very lean to meet EPA standards.

Opening up the "idle MIXTURE adjustment screws" adds more fuel than it does air, because it controls the amount of a preset MIXTURE of fuel and air. The only way to adjust that preset MIXTURE is to change the pilot air jet (in the intake of the carb) or the pilot fuel jet (in the bowl, next to the main jet).

.
 
if its revving , it might have a air leak, bad rubber boot, closing the air screw .05 will add fuel, making it run richer not leaner
 
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