LM-1 reading vs. plug reading

[bliebler]

I just had my AFR evaluated at a local shop. Two yrs ago I had it check and adjusted and always found that my spark plugs and tail pipe pretty sooty. The readings then were at 13.0 at idle and 13.1 WOT. Since then(a few months ago) I realized that I had my control rod adjusted wrong. I have socket end caps with a flat spot on the ends vs a sphere or rounded end. Now I have a precise 114mm and also since then repaired my speed switch(rpm tranducer).

The readings since adjusting my control rod one turn longer(richer) are the same. The car runs great but plugs sooty, even with a plug check at high RPM with new plugs and after warm up.

I have turned down the main after warm up 4 clicks lean and the plugs look perhaps a little lean but no popping or bucking at 4 to 5000 RPM. By a little lean I mean it looks more like a modern car.

Could the shops reading be wrong? The sniffer he used for the test fits accross the tail pipe not inserted up the pipe. Could this be giving me a leaner reading as the fresh air is around the sniffer?

I believe it was Ed Mayo that said your can turn the main rack lead until popping occurs around the 4 to 5000 RPM then enrichen it 2 to 3 clicks. I'm at 4 clicks lean and no popping yet!

I do not what to run too lean and burn a hole in a piston. Does it seem normal to have the plugs that sooty at 13.1 and is 13.1 that much richer that 4 clicks lean on the main and still no popping or backfireing at 5000RPM?

[JohnT]

With unleaded gas your plugs and tailpipe will always look sooty (Black). It is the lead in the gasoline that went from light brownish (very lean) to gray to black (too rich). With today's gasolines you have to rely on electronic measurements.

johnt

[docrodg]

Plug reading is very inaccurate and prone to error. IF the engine is not cut just right on clean plugs it will still be wrong. Best is to go with the LM1.