CO and HC figures ?

[JSF55]

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

[badger]

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%.

[v8man]

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

[Eliot]

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.

[JSF55]

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

[stevieturbo]

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.