Small Puff of Black Smoke When I Blip Throttle

[Martinld123]

If it isn't one thing it is another, lol. My latest concern is when I blip the throttle from idle on my 100 BN2 there is a small puff of black smoke coming from the exhaust. This was not there until recently on my freshly rebuild engine with 3300 miles on it. It clears if I blip the throttle a few times but returns if car idles for just 10 seconds or so. No smoke at idle that I can see. All four plugs are gray to coffee color with just a small ring of carbon on outside thread edges. Can only be seen if standing at back of car and just started I believe? Thanks Marty

 

[Keoke]

If the smoke is not white, you are alright----Rich carbies

 

[bdcvg]

unless You are experiencing excessive oil usage , I would not worry much. Maybe somebody else will have a more educated cause.

 

[Bob_Spidell]

Here's my theory: Engine is 'loading up' at idle; i.e. with no load some unburned fuel remains in the cylinders. When you rev the engine--esp. if you start to move--the 'mean effective pressure' increases and the leftover gas gets burned resulting in a momentarily rich mixture; hence a puff of black smoke.

 

[John Turney]

Did you put heavier oil in the carb dashpots? Black smoke is a sign of a rich mixture.

 

[Martinld123]

John,
Yes I used straight 30W oil for sometime when I thought I was running lean on acceleration. I know it should be 20W and changed it back yesterday. I have turned jet mixture in 1 turn to see if that makes difference also. Plugs seem good though. Marty

 

[vette]

Marty, a little black smoke at idle just means the carbs are alittle rich. If it is running well and idles well then it is nothing to worry about. If you want to try to get rid of it then you can adjust the carbs alittle leaner which it sounds that you have done that. But I don't adjust any SUs unless I have done the mixture test as described in the book. Dave.