Why is my spark plug all black and sooty?

[ullert]

https://postimg.org/image/hfvohs121/

This is the spark plug to the #2 cylinder when I pull it from while it's running in idle up to maybe 3/4 throttle. If I pull the plug running at WOT the plug looks OK.

This cylinder has given me problems since I bought the bike. Even now that the essential maintenance is done (carb synch, adjustment to spec and valve clearance) the plug is black
and when cold ambient or raining the cylinder wont fire. The bike runs better than ever but you feel something is not right.

Should I do a compression test? might it be bad piston rings or burnt valves? could it just be a bad spark plug wire?

Hoping to learn something from this. Thanks.

[hampshirehog]

https://postimg.org/image/hfvohs121/

This is the spark plug to the #2 cylinder when I pull it from while it's running in idle up to maybe 3/4 throttle. If I pull the plug running at WOT the plug looks OK.

This cylinder has given me problems since I bought the bike. Even now that the essential maintenance is done (carb synch, adjustment to spec and valve clearance) the plug is black
and when cold ambient or raining the cylinder wont fire. The bike runs better than ever but you feel something is not right.

Should I do a compression test? might it be bad piston rings or burnt valves? could it just be a bad spark plug wire?

Hoping to learn something from this. Thanks.

What bike and what year?

[Steve]

What bike and what year?

Looking at his other post, it's an '80 or '81 2-valve four-cylinder that is not an 850.

[Guest]

This is the spark plug to the #2 cylinder when I pull it from while it's running in idle up to maybe 3/4 throttle. If I pull the plug running at WOT the plug looks OK.

Sounds like the vacuum diaphragm in your petcock is leaking.


Mark

[jprice90]

Are those DR8ES plugs?

[Steve]

Are those DR8ES plugs?

Yeah, I guess that would narrow it down to a 650, wouldn't it?

Now the question is "G, GL or E?"

.

[JJ]

Are those DR8ES plugs?

DR8ES-L, to be exact.

[Bortasqu]

If only the original thread creator said which bike he has.

[ullert]

Gs500l 1981

[ullert]

DR8ES-L, to be exact.

Yes, they are

[tkent02]

GS500 sold in europe is our GS650, only made smaller for some reason.

Too bad about the L thing.

[JTGS850GL]

2nd the possibility of a leaking petcock vacuum diaphragm. Check the vacuum line for the smell of gas. Replace the petcock if so. DON'T try to rebuild. Very poor success rate on those rebuild kits.

[JJ]

I agree that the probability of a leaking petcock diaphragm is very high.

[ullert]

I agree that the probability of a leaking petcock diaphragm is very high.

Why would that affect only one cylinder?

[Steve]

Because the vacuum line that pulls the petcock open only goes to #2 cylinder.

If the diaphragm leaks, the gas that makes it through the leak goes directly to the intake of #2, after the throttle butterfly, so it is subjected to full engine vacuum all the time.

.