Wideband AFR readings on RV8 3.9

[jchase]

I've just installed a wideband AFR meter made by NGK sparkplugs, who make the sensor, so it should be fairly accurate. The car is a 1987 TVR 390SE with 3.9 Rover V8 modified by NCK.

With fuel pressure at 2.50 bar the car idles at 12.5 AFR! cruising at 40mph its about 14.3 to 14.8 and under full load acceleration 6,000 rpm gets you about 15 - which seems dangerously lean to me. (fuel pressure also checked out ok at this load). Also when you come off throttle, the car occassionally dies, or at least goes to a very low RPM.

Can anyone tell me if I'm right in thinking the Idle/cruise should be close to 14.7 and under full load close to 12.5 ?

Oh, it has a flapper injection system with lucas 4cu.

-Jim

[chassis 33]

Jim, that does seem a little lean. My understanding is that A/F should be about lambda .8 to .85 so around 11.75 to 12.5 A/F. Is the fuel pressure taken at load or at idle? I've found on my car that fuel pressure can drop to the late 20's underload in the past, I now run at about 42psi to try and keep the pressure in the system up even under load.

Regards
Iain

[jchase]

Fuel pressure tested at idle and load with digital fuel pressure meter. TVR claim 268 BHP fo rthis engine - so I would assume the fuel pressure would have to be raised to get this sort of delivery.

[stevieturbo]

Some engines do like to be rich for a smooth idle, but 12.5 is a tad rich.

Cruise etc is good at 14.7:1

WOT on a n/a car, high 12's, maybe even low 13's would be good.

Sounds like something is messed up !!!

Where are you measuring it ?
Would there be any air leaks before the sensor anywhere ?

missfires also throw apparent lean moments

[jchase]

The Lambda sensor is in the left hand side down pipe, about 6 inches upstream of the O joint to the centre box everything appears gas tight, so no air should be being sucked into the exhaust.

The AFM and ECU look to be modified by NCK , and bear the NCK number on them, however, the engine has been rebuilt subsequntly. The air bypass is turned all the way out, so I am thinking the flapper needs a bit of TLC.