Is spark plug reading accurate enough for air/fuel ratio? If so, how should my plugs look and how much variation in spark plug reading is OK between my richest cylinder and my leanest cylinder? This will be for a 340 Six Pack in stock condition.
Thanks.

No, in my opinion plug reading is not good enough nowadays.

First your only getting an "overall" reading, so you have no indication if for instance your "rich" looking plugs are rich at idle, at full throttle, or at cruise rpm.

And, I think reading plugs is a lost art, so it may be difficult to get it dialed in.

I'm saving up to get a wideband 02 sensor and A/F gauge. I think that's the best way to go.

No.

Can any of these meters be used without drilling any holes in my exhaust. I don't mind if it reads (4) cylinders at a time if there is a probe that goes in the exhaust pipe. Also, does anyone rent these meters? Thanks.

Most are made to use a probe installed near header or manifold outlet... like a new car. Drill hole in the exhaust or collector and weld in a bung. Sorry.

The Innovate gauges have an available clamp to put the sensor in the end of the tailpipe. Not sure how accurate they are positioned there. I had mine installed on the driver's side at a muffler shop. They charged me a half hour of shop rate to weld it in. I am only seeing the odd number cylinders but I have an old Heathkit gauge to check to see if both sides match. I might someday install a second sensor on the other side if I have trouble.
No one around me rents the meters.
Craig

My innovate directions read at least 24" past the collector but before the X or H pipe.

The smartest guy i ever knew on carbs and engine combustion always said that the color of a plug is indicative of the combustion event. If your O2's are telling you its rich but produces a white plug what would you trust?

For me plug color 1st electronics 2nd.

Hysteric

I still do it the old way by reading the plug since I have been doing like that since the early 70's. I dont have an 02 sensor setup on my car as when I first built it they were not around and I have just never bought the setup and put it on my 63. I am very happy with the mixture and performance but I do agree I should be able to fine tune even more with the 02 sensor setup as it can tell you when at what throttle and what carb system is working when the mixture is off. Also my 850 DP is an older one and does not have all the jets in the metering blocks so I would have to update to modern billet metering blocks or do some drilling for sure. If you have the money for the setup it surely cant hurt any and it surely can help alot for many. Ron

Spark plug reading is an art and requires experience to get right. It is a good tool at the track, where you are only concerned with WOT mix but even then should be used with the time slip. Of course, then you need to have clean plugs in for the run and shut the engine off after the stripe so as not to dilute the plug's coloring with engine operation other than wide open.

If this is a street car, plug reading has little or no value. A/F ratios are changing constantly while driving so there cannot be any useful coloring on the plugs.

Fuel distribution among cylinders requires monitoring the individual cylinders in order to measure and then requires more expertise than most of us have to adjust.

If you do not want to drill your exhaust and do not plan to leave the monitor in place, have you thought about having a competent dyno shop tune your carbs? Most wide bands will cost at least a couple hundred dollars, plus there are the jets, bleeds and power valves necessary and then let's not forget about the learning curve.

It might be just as cheap and certainly easier to have a good carb shop do it.

two cents

I've been messing with a Innovate LM-2 for quite a while now, it really is the best way to set up your carb. I used the tail-pipe clamp fitting for a while, but it would give very erratic readings at idle. Guessing turbulence was affecting the readings - seemed OK at cruise and wide open. I didn't like guessing at the idle AFR setting so I had a bung welded in - much better!

1) No. I do look at them for signs of detonation and oil consumption though.

2) They do make clamp on bungs. Drill a hole and clamp on.

3) Location? I have seen them on a header tube right off the port and as far back as an h-pipe, they all worked fine.

4) Narrow band or wide band, either is better than just reading the plugs.

Nope.

It was explained to me once that what you are looking at on the plugs, is what the car was running at idle and coasting. I.E. by the time you roll to a stop: your WFO, or cruising state, whatever you are looking for, is gone from the plug.

Basically you would need to run the car at whatever throttle you are trying to get the reading, and then shut the ignition off, hit neutral, slam the brakes on, yank the hood open, and pull a plug.

I believe it, it makes sense, my opinion only.

I use a wideband instead.

Think about it: EFI fuel tables are not connected to the spark plug, they are connected to the exhaust pipe.