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Old 09-07-2004, 15:36   #1 (permalink)
Jim Mock You Rock
 
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Do fuel mixtures change when driving or freerevving?

I was just wondering if fuel mixtures change say if your checking them in park and takethe revs up compared to if they were under load?
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Old 09-07-2004, 16:18   #2 (permalink)
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Re: Do fuel mixtures change when driving or freerevving?

I wouldn't tune my engine using free-revving to determine the correct mixture/jet air fuel ratio. It's under true load that the check needs to be done (ie: on the road or a dyno). The carby has several fuel circuits inside that won't work unless fully loaded up therefore giving a false reading for the regime you're after (car acceleration).

The engine will accelerate free-revving in park quite easily, throttle plates barely open and therefore pulling in less fuel. The mixure or air fuel RATIO could be close, "Less fuel + less air" vs "more fuel + more air". I'd say it would be slightly leaner though.

Air fuel mixture could be quite different at WOT and having FULL fuel flow through your jets/main metering system and ALL throttle plates fully open. Free revving will also have a higher manifold vacuum than a loaded situation, where it will be close to or on zero when loaded up. This has implications with power valves opening up etc and the secondaries opening up if you're running a vac sec carby. What will be markedly different is the cyclinder temperatures at a loaded WOT. Cruise AFR is about 14.7:1 and full power should be richer at about 13.2 to 13.5:1 (engine dependant).

As long as you're not blowing grey smoke (rich) or pinging its head off (lean) you're somewhere in the middle. No substitue for the sensor gear on a dyno for EXACT numbers. Can also check your spark plugs after a run.
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