Lean Burn Test..? Or similar...

[Buffalo]

I heard reference on here a few weeks ago, of someone doing a lean burn test, or something like that from the FSM. I think it was referring to retuning the engine to make sure the carb was running as lean as possible...

Does anyone know what I am talking about? I can't see much in the archives.

I'd be very interested if there was such a thing as although reasonably tuned, my truck is running far too rich to me (just the exhaust smell at idle, really)

 

[inkpot]

Howdy! Try turning the idle mix screw clockwise 1/4 turn at a time until you can hear the rpm drop by 100-200 rpm. In the good old days, that was the general starting spot for idle mix. Leave the idle mix there, and adljust the idle speed screw back up a bit if needed. You may need to fine tune plus or minus a 1/4 turn at a time till you are happy with it. John

 

[Marc311]

Do a search for "Lean Drop." IIRC, you set the RPM to 690 and turn the mixture screw either way to maximize the rpm. Reset to 690, and adjust the mixture to the max rpm. Keep repeating until you can no longer increase your rpm via the mixture.

Do a search to get the particulars since I am going off memory. You may need to plug some vacuum lines as well.

 

[2mbb]

Here it is from the emissions manual

 

[silicon_toad2000]

If you wanted to get it spot on, you used to be able to buy a device called a colour tune. It was effectively a spark plug with a glass window and a sighting tube.
You could see the colour of the flame in the cylinder. if the flame was white it was too lean, if the flame was orange or red (bad memory) it was too rich. You adjusted the idle mixture until it was blue.

 

[rchalmers3]

If you wanted to get it spot on, you used to be able to buy a device called a colour tune. It was effectively a spark plug with a glass window and a sighting tube.
You could see the colour of the flame in the cylinder. if the flame was white it was too lean, if the flame was orange or red (bad memory) it was too rich. You adjusted the idle mixture until it was blue.

Wow, thats a blast from the past. I had one of those back in the early seventies. Kinda worked but it was only good for idle and better for spotting variation in fuel distribution among the cylinders.

The lean drop is a more effective method for controlling idle emissions. The blue flame in the combustion chamber would indicate something close to best idle, a mixture too rich for todays requirements.

Thanks for the memories!

Rick