The V8 Owners Forum -

Hi All, had a quick mess about last week with my mates exhaust gas analyser, it's a 3.5 with a hurricane cam, slightly ported heads, edelbrock 500 and manifold, i had initial reading of CO 0.5 and HC 400ppm, i took it out for a run and it changed to CO 2.5 and HC 280ppm, all done at idle, i was thinking the run cleared all the unburnt fuel out of the system? anyone else got figures from there mot? john

HC most likely came down as a result of getting the internals fully up to running temp.
Typical figures I'd expect would be as follows:-
CO - 1.0 to 2.5%
HC - 650ppm maximum.

Ignition timing has a big effect on HC, weaker mixtures will reduce CO and HC (to a point where combustion becomes inefficient then HC will rise rapidly), richer mixtures give high CO and HC.

Anything less than 1.0% CO will give rise to very high exhaust valve temps and most likely a rough idle (remember, the fuelling with carbs is nowhere as precise as EFI and the rover V8 engine will become less tolerant of leaner mixtures when on carbs), anything over 3% CO is a waste of petrol, anything over 6% is risking "bore wash"! HC really ought to be anywhere below 600, the lower the better really!
I find that a smooth idle is easily achievable with a CO of around 1.75 to 2%.

mine a 4.8 runs best at 2.6 co at idle that is air/fuel ratio of 13.5 and 3.8 co at full throttle that is 13.0 air/fuel ratio
hope it helps

During my MOT, My megasquirted chevy was about 2.5% co and about HC 250 PPM. I just told the ecu to do closed loop 14.7 afr at idle - didn't do any tweaking as the mot wasn't actually that bothered because the car is 1974.
It actually idles much better at richer mixtures on mine though.

Cheers guys, food for thought, at least i know it will pass the mot !! john

Modern cars with fancy controls can idle very well at 14.7:1, but generally, most prefer to run richer than that for a smooth idle.

Especially once you start tinkering.