We have all smelled the eye watering vapor smell of a rich condition, but how does it happen and how is it fixed? or in some case's is it a no win situation?

My A/F gauge strated to act stupid recently for unknown reasons, about 2 weeks after I thought I had everything dialed in and adjusted right. Then out of the blue it just would not read unless I was over 3k rpm. Sent it off the manufacturer who happily checked it out said there was noting wrong with it but replaced the circuit board anyway just to make sure and sent it back.

Reinstalled and at the suggestion of the manf. replaced the o2 sensor with a new one to go with it. well same crap. this time I could get readings at a more reasonable cruise speed but not a consistent reading and any let off of the gas pedal produced the same no read situation.

I am stumped on this one. One thing that happened while coming in from a cruise was the motor dieseling after shut down, this told me there was something off.way off.

So I broke out the timing light, vac pump, vac gauge, etc and re went over everything, timing was at 11 initial with 18 after vac adv kicked in. I have always heard give it more timing until it ings and back off, So I kicked it up to 15° initial with 20° with vac sec kicked in and 48°-49 all in. I backed out the idle air fuel screws a 1/4 turn or so to get the vac up to 14.5-15" from 12.5-13" of where it was. vac on distr is on timed ported on the carb. (carb is qft 780 vac sec)

went for a test drive and no ping, still no read and same situation when on the gas at cruise a/f wold read on the lean side. This gauge has the ability to adjust the setpoints of low and high from the factory settings so tried that just to see which made no difference.

Side note, when I had tuned it the first time I think I still had some 101 octane gas with 108 booster in the tank, now it just has 91. would this be so significant to have to re time and re adjust the mixtures?

Is it at all possible I am so rich that the o2 sensor just won't read until I get into a lean cruise condition?

Pretty much ready to toss this a/f gauge and spend $300+ on a dual system now that I have two o2 sensors and bungs on both sides. but just pisses me off, and I also want to be able to answer the question, is too rich a condition enough to make a a/f gauge not read? and if so at what point do you have to lean it out?

Narrow band O2 sensors I am guessing.

At 3k you are probably out of the circuit in the carb that is running rich and into a different circuit. Both your nose and the gauge are saying "too rich".

Sounds like the carb if afu.

Well no I am using wide band single on the driver's side. thinking I may fall into that one category of it's just not ever going to be right based on the build.

Welcome aboard! What I'd do is restore the settings to where they were & find/address whats making it diesel then see if your good on the gauge. Wasn't everything good in the 'hood until SOMETHING changed ((vac leak?) that showed up as dieseling & exactly at that point the gauge went off

Thanks, That's kind of what I have done already went back under there added some more timing, went from 11° initial to 15° backed out the 4 corner idle air mix 1/4 turn each with a vac gauge hooked up going from 12.5-13 vac to 14.5-15 vac. 15" of vac is about all I have ever been able to get out of this motor at idle. manifold vac over 2k is 20" wondering if backing out even more made it too rich but the issue came about before any of these adjustments.

Hadn't done anything different to the motor or parts. wondering if tuning with air cleaner off vs. on is messing things up? I use a full A12 air cleaner assembly(made for a 4 bbl) with a 3" K&N oval filter.

Not hearing or seeing any vac leaks, these last adjustments seem to have stopped the dieseling, which happened when the initial timing was at 11°

Or could just be crap product A/F gauge. it is a cheap one from ebay so maybe that's the issue, looking at the FAST dual set up just hard to drop $450 on it.

Look at Spartan 2 wideband O2 sensor. $125 ea.

A wideband should read ratios from like 10:1 to 20:1 so it should read anything remotely runable.

I have never had an A/F meter and cannot justify one now. I used to read the spark plugs (Old School) and that always got me in the ball park, then change the jetting in the carb from there and always set the idle by the vacuum gauge, higher is better.

Heh, heh, heh! just keep thinking that, it's comforting. On the other hand, a pretty decent wide-band setup costs no more than a few tanks of gasoline.

R.

You really can't read p[lugs with today's pump gas. It's just not the same as it was.

Plug reading certainly has it's place at the track. But on a street car, the mix is constantly changing. Particularly at early throttle openings where a street car spends a good deal of it's time and is the key to drivability. It is impossible to derive anything useable from the plugs during the transitions. Mere 'seat of the pants' would reveal more useable info than reading the plugs.

Adjusting the idle mixture screws alone without getting the IFRs and IABs where they need to be, is simply getting the best vacuum reading with what might still be a terrible A/F ratio at idle. You have no way to know without a way to measure.

Dialing in a carb with a wide band O2 is not for everyone. But there is no substitute for it on a street car.

69JB, What does the O2 read in fresh air? Does it read accurate?

And if you decide to try a different O2, you do not need to spend the coin on a dual system. Either side of the exhaust will usually tell you what you need to know. And if in doubt, just put a bung in both pipes. It would only take a couple of minutes to swap the sensor from one side to the other.

Do you know anyone with a carb you could borrow for a little while just to see what the readings with it would be?

Running many carbs (especially Holleys) without an air cleaner will cause them to run very rich as the car's speed increases. That is because the air flow to the carb will put extra pressure on the primary fuel bowl. You can avoid this on a Holley by putting a 90* spark plug boot on the primary bowl vent and face it towards the rear of the car. But it is much better to have the air cleaner on when testing.