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Air fuel ratio gauge vs carb test


Iain T
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I've just installed an afr gauge and although I haven't been out for a run yet to clear it's throat at tick over the gauge is around the 14 but when I raise, just a tad, the air piston on each carb hiccups and stalls. The gauge is saying very slightly rich from ideal 14.7 and the carb test is telling me they are lean. 

Any suggestions as to the anomaly? 

The gauge is an AEM X series which according to the blurb doesn't require calibration. 

Cheers 

Iain 

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Be very careful bandying words like "ideal" around. The 14.7 AFR (a.k.a. Lambda 1) is the chemically correct "stoichiometric" ratio. This is what you need to run at (or, actually, wobble a bit either side of) for a 3-way catalyst to operate optimally. It is not the perfect AFR for the engine, especially a 1950s technology engine like ours. Getting close to 14.7 at idle would be lean for a Vitesse mk2, very lean for a Mk1 or 1600

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Thanks, I do realise I am using new tech on an old engine so I'll go old skool for the idle and see what the gauge says. The engine did seem happier at around 12 to 13 at idle. What sort of values at wot and cruise is acceptable for our engines? 

Iain 

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Max power is created at AFR 12.5 or lower (richer) so is what you want at WOT, but you can only really control that with damper oil and maybe piston spring strength?

At idle my modern engine struggles at anything leaner than early 13 something. So don't be too disappointed. As to light cruise, you should be able to head towards 14.7 and be ok but it may mean the car feels lean, or not like to pull away from the light cruise. You will need to experiment.

 

Lastly, I think you may be wise to remove the sensor once you are happy with what you have achieved. They are not keen on a lot of rich running (which is what you get with carbs)

 

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I realise it's at cruise and wot that I need to get the mixture as best as I can. At the moment I have 6AC needles but also have a pair of 7B's which are 506 thou at the bottom and slightly narrower along the entire length than the 561 thou 6AC's. Another option is the B5BU which is 540 thou but actually wider than the 6AC's in the middle and also available. Time for a blast up the A13!

I will certainly remove the sensor after I have finished tuning.

Thanks

Iain

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Pete, correct I was quoting the smallest diameter. 

After a quick charge down the A13 the idle is hovering around 13-13.5, at wot it's about 12.5-12.7 and cruising at 3000rpm about 14.5. At lower speed cruising 2000-2500rpm it's the same as idle. The engine picks up fine throughout the rev range. On backing off after cruising it is initially as low as 10 then up to 15-15.5. I will check the plugs and just out of curiosity the jet heights with a depth vernier. I set them the same prior to adjusting so they should be fine. I did notice the front carb needle had been filed and then forgot to measure it! 

I would say it's not far off using the standard 6AC needles? The 7B's will just make it richer and use more fuel.... 

Some needle variant comparisons. 

Iain 

IMG_20200715_125112.jpg

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just remember from  days running the factory smoke emission dept  , in those days all were a fixed jet downdrafts either zenith or holley the only cars we messed with were either directors H120 on webbers or roll cars  hunters and sceptres on strombergs ,  we did bag and sniff tests with a snazzy rolling road and route mapper , lots  of heat to soak motors overnight and best place on a cold winters day .  all gone in the skip and history book.

happy days when things were simple 

Pete

 

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How very true, using new tech on our cars should be an aid not the rule. You need to know your car especially with non standard engine set ups and take note of all the tell tale signs. 

In our foundry we used kirksite for our press tools, although there was a temp gauges on the ovens the guy could tell by looking at the surface of the molten metal if it was ready to pour. It takes experience and getting it wrong a few times to get "the knowledge". 

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  • 1 month later...

Update

After several runs and many tweaks it seems the 6AC needles are not right for my engine as they go far too lean on moderate acceleration and high revs plus the engine seems hesitant. I also have pairs of 6J and 7B needles and installed the 7B's first. Big difference mucho more torque and no hesitation but at cruise the afr was around 11-12. Is this too rich for cruising? Will it result in coking up? I then used the 6J's a sort of half way house between 6AC and 7B but about the same diameter at the bottom of the taper at max revs and standard on Mk1 Vitesse. The engine still pulled ok and afr showed slightly leaner mixture up to 12-13. I have kept the 6J's in and confirm what NonMember (do you have a name?) said that the 'magic' 14.7 number is way too lean for an old engine, well certainly my engine.

Tickover on both needles is fine, any advice re 6J or 7B?

Iain

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24 minutes ago, Iain T said:

 NonMember (do you have a name?)

It's in my sig because I never remember to sign forum posts so I let the sig do it for me 😀 Of course, some people don't see the sig blocks...

I think 11-12 is a bit rich for cruise so if the engine seems happy on the 6J then I'd stick with them.

Rob

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Cheers Rob, just confirms what I think. With the 6J's at WOT afr goes down to 11.5'ish so plenty of fuel getting in. To effectively use the 7B's you must have to have a hairy cam and sorted head. My cam is a hybrid 0.26 inlet and outlet 260 degree and even with my head they are just too rich.

Cheers

Iain

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6 hours ago, Iain T said:

14.7 number is way too lean for an old engine,

The Triumph 2L will happily run 14.7:1 (or even into the 15s) at light cruise (65 - 70 on the flat) provided the fuel distribution between cylinders is even.  Lean hitch sets in in the upper 15s - early 16s.   11 - 12 is way too rich for cruise - mine doesn't go below 12 even at full chat. 

Nick

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Nick, I'm still doing trial runs but have the rad out now to fit an electric fan. 

My biggest issue with the 6AC needles was pinking under acceleration and going lean (16+) at high revs. I use 20/50 engine oil in the carbs which most people seem to prefer. I have no idea what the springs are. 

Once I've fitted the fan I'll give it a longer run. 

Iain 

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