911 air/fuel ratio's.

Porsche 2 · 09-02-2018, 10:26 AM

Have a good condition stock 1977 911 2.7L engine, with exceptions including: 40IDA Webber carbs and street/carb friendly "S" cams. Engine runs very well with smooth even idle at 800rpm and smooth acceleration all the way up to plus 6000rpm. Have Clewett's crank fire ignition, set from 8 degrees at idle, 28 at 3000rpm, and 34 at 6000rpm.

I have just installed an INNOVATIVE "double" Air/Fuel gauge with a sensor in both the driver and passenger side headers just after the collection points. While readings may not be absolute correct (I note from other threads here), it is probably close and usefully shows/helps balancing the left and right carbs, and shows change in A/F ratio at different rpm's and load on engine.

My carbs are fitted with 34 primary venturies, and tall secondary venturies. Jets are: 140 mains with 180 air corrections, and 55 idles.

I am attempting to make jet size changes to get close to 14.7 A/F. The 55 idles cause very slight hesitation on acceleration with cold engine, compared with no hesitation with 60's. A/F's with 55's are 10.5, while 60' are 9.5. - thus sticking with 55's. A/F's in transition zone range from 11.5 to 13.5 with average around 12.5.

A/F's under power above 3000rpm range from 13.5 to occasionally mid 14's. Using 135 main jets increases these numbers by 1 - thus too lean.

So, acceptable A/F's while carbs are operating on mains, but too rich while at low power and rpm's, especially at idle. Is this as good as it gets with carbs?

juanbenae · juanbenae's Garage

that's about what I see with a 3.oL, weber 40's, webcam 120/104 cam grind (bad cam choice on my part in hindsight). I run 60 idles, 160 mains, 175 AC, 36mm venturi & F8 emulsion tubes. the biggest improvement in my differing set ups over a period of a year was moving to the F8's from F3 ET and moving to a 2 in 2 out exhaust from a 2 in 1out. per a weber data sheet the F8 will help promote a leaner mixture across main & idle circuts and I did notice such

at idle im anywhere from low 10's to 11, and on excel im about 12.5 to high 13's and mid 13's on freeway. have a ever so brief lean spike when at low RPM and then burying throttle. not enough to have any sort of hole in the power band.

today's fuels also play havoc and I see changes in AF from summer to winter blend gas here in CA. ive been told we just have to live with it, although I suspect a more streetable cam choice might have helped my case.

Trackrash · Trackrash's Garage

Under power above 4K rpm you should be around 12.5 to 1. 14 to 1 is way too lean.

If you can get it to idle and run leaner than that under cruise conditions great. I can't.

If you are 12.5 -13.5 to 1 idle and light cruise you are probably as good as it gets with Webers.

FWIW, I have been fighting a rich transition for ever with my carbs and can't get it to run with anything less than 65 idle jets.

I think you should stick with the 60 idles and check your plugs after a long drive.

I guessing you have the '77 S cam? The early S cam would require a change of pistons.

juanbenae · juanbenae's Garage

found it.

Porsche 2 · 09-04-2018, 09:21 AM

Thanks fellows for your responses. I am considering trying F8 emulsion tubes instead of my F3' to increase the A/F's from idle through lower rpm ranges (?)

Trackrash · Trackrash's Garage

Let us know how the F8s work out. I have not heard of anyone using them on a 911.

I'm running F3s. I tried changing to F26s, but did not notice any difference during transition. For some reason, my carbs have always been a little rich in the upper transition range. IOW On the freeway at a fast cruise I see around 11 to 11.5 - 1 AFR.

franklinstower · franklinstower's Garage

i had similar issues on my BMW 2002 with dual sidedrafts and a tisa ground cam. No jetting would correct the issues until I started changing the E-tubes. I would say it is a worthwhile endeavor to try different ones. Just make sure you don't go lean! Aircooled + Lean + Advance = No Bueno!

Porsche 2 · 09-04-2018, 11:41 AM

I have a set of 145 main jets and will replace the 140's with these to check WOT above 3000 A/F's as next step to make safe the top end rpm A/F's stay below low 14/1's.

Porsche 2 · 09-04-2018, 11:47 AM

I have a set of 145 main jets and will replace the 140's with these to check WOT above 3000 rpm A/F's, as next step to make safe the top end rpm A/F's stay below low 14/1's.

Trackrash · Trackrash's Garage

Quote:

Originally Posted by Porsche 2

I have a set of 145 main jets and will replace the 140's with these to check WOT above 3000 rpm A/F's, as next step to make safe the top end rpm A/F's stay below low 14/1's.

That is a good strategy. You really want to be below 13 at WOT above 4K rpm, BTW.

Good luck and let us know how it works out.

Porsche 2 · 09-05-2018, 11:35 AM

Replacing the 140's with 145's brought the A/F down one - so, at rpm's above 3000 and open throttle powering long uphill A/F is averaging 13.5 with no spikes into the 15 range. Will stay with the 145's.

Challenge remains to tune idle A/F from current 10.5 up, while not increasing frequency of acceleration from low rpm position. I will stay with the 55 idle jets which give the highest A/F at 10.5. Using 60's eliminates hesitation, but A/F drops to 9.5

Porsche 2 · 09-05-2018, 11:46 AM

Above word in third line should be hesitation not acceleration!