New Lambda gauge - Lean Best Idle

[matrophy]

I have been struggling to get Lean Best Idle on my new Weber 32/36 and went up 4 sizes of idle jets before I could stay under the recommended 2 turns out on the mixture screw. I installed a wideband AFR gauge (Lambda actually) and in the process of tuning the carb, it seems that tuning for Lean Best Idle by ear and tuning by the closest to stoich on the Lambda gauge are 2 different things. The engine runs smoother by ear but has a reading of about 0.85 on the gauge - so running rich. A slight tweak of the mixture screw - maybe 1/8 of a turn gives a Lambda reading of 0.95-ish but not as smooth of an idle.

 

Is this to be expected? Is it safe to run it when it is set for closest to stoich vs what idles the smoothest? If not, I'm questioning the value of an AFR gauge - at least for idling.

Edited May 24, 2021 by matrophy

[datzenmike]

If you were at full throttle then running lambda might be a problem. The extra fuel cools the intake air and it is safer for the engine, but at idle you don't make enough heat to damage anything. While 14.7 is perfect, the graph is very rounded at the peak so there isn't much difference on either side.  My idle is low 15s and there's not much difference (running or mileage wise) between 13.7 and 15.7 air/fuel ratios.

 

A 0.85 lambda is 12.5 a/f ratio. That's way too rich for idle and not only wastes gas but will foul the plugs. 

 

Just for fun check that both intake and exhaust side plugs are firing. Then check the timing, 3 BTDC.

[matrophy]

I saw a spec somewhere of 0.92 to 1.03 as being an acceptable 'stoichiometric' Lambda reading. Never thought I would ever use the word stoichiometric after Chem 1A in college.

 

When I was having trouble getting the carb to idle at less than 2 turns out, I replaced the plugs and checked that all of them were firing, checked that the coil is in spec electrically and checked the timing (which was 3 degrees BTDC) today. Side note - I also checked compression and I'm proud to say that it was 175 lb in all 4 cylinders.

 

So, it sounds like setting the idle closest to stoich is the way to go even though it's not the smoothest idle?

 

I do kinda wonder why it ran the best when it was so rich.

[datzenmike]

Molecular musical chairs. If there is a chair for everyone when the music stops it still takes a little jockeying for position and get everyone seated.  But if there are twice as many chairs as before it's much easier to find one open.

 

How about hyper-eutectic?

[Charlie69]

matrophy do not be so fixated on the 1.5 or 2 turns suggested.  It is a starting point.  I have always tuned my carbs by ear.  I have to emission test every 2 years on any vehicle that is 67 and newer.  My 86 goes through emissions just fine tuned by ear.  Here they put the sniffer in the tail pipe test at idle and also test under load at 40 MPH.  It would be nice if the emission testing places would hire people that knew how to drive standards.  I have done written complaints about this but it does no good.

 

Also is your engine running perfect fresh rebuild?

[matrophy]

4 hours ago, Charlie69 said:

matrophy do not be so fixated on the 1.5 or 2 turns suggested.  It is a starting point.  I have always tuned my carbs by ear.  I have to emission test every 2 years on any vehicle that is 67 and newer.  My 86 goes through emissions just fine tuned by ear.  Here they put the sniffer in the tail pipe test at idle and also test under load at 40 MPH.  It would be nice if the emission testing places would hire people that knew how to drive standards.  I have done written complaints about this but it does no good.

 

Also is your engine running perfect fresh rebuild?

The engine only has 68000 miles on it and compression is 175 all the way around so it's probably pretty close to being fresh.

[Charlie69]

I hope to get my 86 carb tore down this afternoon.  I will post all the jets and tubes.