Optimizing 246 carb jetting with O2 meter

This past weekend a friend of mine loaned me a really cool tool and I used it to good effect in order to perfect the carburation jetting on our 308 and 246. The tool is the Innovate Motorsports LM-2 handheld oxygen sensor which has an adapter that allows you to temporarily place a Bosch oxygen sensor in the exhaust and analyse your air/fuel (AF) ratios while driving. Here is a link to the tool in case anyone is interested in learning more: https://www.innovatemotorsports.com/products/lm1.php

While much has been written about balancing carbs and setting the correct idle mixture there is not a lot out there discussing getting the jets correct for actual driving. Of course jetting will vary a lot depending on fuel, atmospheric conditions, and altitude but regardless of where you are the principles are the same. Reading spark plugs will get you in the ballpark but has a lot of shortcomings. O2 values are instantaneous and can be immediately co-related to conditions such as load, throttle opening, etc.

To this end we fitted the LM-2 to the exhaust, calibrated it by the instructions, and used on road testing to optimize the mixture. In order to get as accurate results as possible for each drive test we re-fitted the full airbox to replicate the natural ram air effect of the intake rather than run on open carbs. Our O2 tool has the capability of data logging and mapping but I preferred to drive with a passenger who could call out the values in real time. Doing multiple runs at various throttle openings and conditions allowed me to understand the full picture better when making jet selections. This combined with feeling how the car behaved with different jets allowed me to choose what I thought felt best but that still was within our target AF ranges.

As a point of reference AF ranges in the mid 12:1 range will generally make the most power while the best fuel economy is in the mid 13-14:1 range. Our target was a range in the high 12's-mid 13's to trade off a hair of power for a clean burn and sharp throttle response. Playing with an assortment of Idle jets, Air correctors, and high speed jets we were able to achieve that target bang on. We even played just outside of that range on some runs in order to feel how the car behaved differently. In the end our target range resulted in the best feel anyways so we went with that as our final setup.

In all the whole procedure took maybe 4-5 hours and was a great excuse for repeated high speed runs on the motorway

Here are some photos of the setup and of 'Dino' overlooking our work

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We did not do any dyno tuning however we got to the point where making a singe jet change in either direction could both be felt and predicted and verified by the O2 readings. The goal was not to make the most power at wide open throttle: WOT (as is the case with dyno tuning) but to achieve the best balance of power, driveability, throttle response, and clean burning. Most time was spent figuring out part throttle cruise settings as well as transient between cruise and WOT. One thing we did find is that the settings did vary a little between having the airbox on and off and I suspect that the ram air effect of the side scoop (not possible to replicate on a dyno even with big fans) plays the biggest role in these differences.

 

O2 sensors are best placed at the base of the header before the muffler. In this position they are far enough from the open atmosphere that they are not affected at low engine speeds. The clamp on sensor is not accurate at idle and gives erratic numbers because air is sucked back onto the sensor through the exhaust tips. As soon as the engine speed picks up just a little then this problem goes away entirely and the numbers become very stable and predictable. I would not recommend this setup for setting up carbs at idle but would not hesitate at all to use this method for anything past say 1500 rpm.

 

Peter on the idle circuit stock is a 50 jet and I had been driving around with a 55 but found it a little rich for my liking. The engine had just a bit too much fuel smell and the O2 readings turned out to show mixtures in the 10:1 range with that jet. Conversely the stock 50 jets were just a little too lean where the engine would hesitate on acceleration and the O2 readings were in the high 14:1 range. The real sweet spot came in the harder to find 52 idle jets were the O2 numbers were in the mid-high 12:1 range and just cleaned up the exhaust without sacrificing throttle response or initial acceleration.

The high speed circuit was less sensitive and I chose to go with a setting that was a little leaner. I experimented with a number of different high speed jets and air correctors. I bounced back and forth between performance at wide open throttle to redline and part throttle to redline ensuring that I remained at a limit of 13.9:1 AF ratio. In the end it came down to using a 230 air corrector and either a 125 or a 135 main jet. Both offered great acceleration, and throttle response. The 135 would hover around a 12.2-12.9:1 AF while the 125 would be in the 12.6-13.7:1 ratio and I just liked the slightly leaner engine note better so I decided on the 125 main. Both a great ratios so it came down to personal preference.

 

Tony are you sure about the numbers as you wrote them? I ask because a change of air corrector to a smaller one will yield more fuel and therefore a richer mixture. 13.5 is a LEANER mixture than 11.7. Is it possible that you inverted those numbers by mistake?

My experience is that a mixture lower than 12:1 is a little on the too rich side while a 13.5 is actually pretty good especially at the top end. Here is a decent little video that talks about air fuel mixtures and their pluses and minuses:

 

Wow hard to believe its been 5 years since my original post. Honestly since setting the carbs back then I have not touched them. The engine pulls beautifully from idle and almost behaves like it has fuel injection. The only change was going to slightly hotter spark plugs as the combined driving I do really responded well to NGK BP5ES conventional plugs. I did the same on my 308 with equally good results.

 

I ran the exact same BPR7EIX for years and they worked well but actually found better results with the hotter non-iridium plugs. Also I learned that the irridium plugs are rated one heat level cooler than their number. That is to say that a 7 plug behaves more like a 6 plug so the difference is small but I found to have cleaner plugs with the BP5ES . These are not to be mistaken for BPR5ES plugs which loose some spark energy because they are resistor plugs. The BP5ES are a little harder to find but really work well. Also they are 1/3 the money of an iridium plug