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Perfect Idle for a 302/306?
The 71 needs the idle adjusted. It's runing at 700 rpm and sounds like it's really loading up and almost wanting to stall. What is the ideal rpm for a mild 306? It might be running a bit rich too...every now and then there is a bit of black smoke and some carbon from the pipes.

[Image: 1gq8uo.png]
1971 Mach 1 - 306cid/C4 Bright Yellow
"Just relax, I've got a friend named Felix who can fix anything!" ~James Bond
Well, "mild" can be subjective.

Probably, the best (easiest) solution would be to determine what cam it has. Once that is determined, it will be a lot easier to find out what the desired idle speed should be based on the "grind."

That having been said,, the stock idle for a 1971 - 1973 Mustang 302 with a dual diaphragm distributor is 800 RPM. If it has a single diaphragm distributor, the desired idle speed is 500 RPM. I would think that a "mild" cam should not depart too much from those numbers.

Hope this helps!

I did some tinkering already and found that 800 rpm in gear seems to be a good number. 700 was too low but if I go to 900, then it idles at like 1700 rpm in park. Test drive around town last night was a success!

[Image: 1gq8uo.png]
1971 Mach 1 - 306cid/C4 Bright Yellow
"Just relax, I've got a friend named Felix who can fix anything!" ~James Bond
Again, sounds like you are "on it", Barry.


It depends on cam and carb. Setting the idle mixture with a vacuum gauge and then finding the engines "happy place" as far as idle speed goes. It shouldn't drop more than 150 - 200 rpm between park/neutral and drive. sounds like you found a good spot @ 800 rpm.

Jeff T.

Low buck, touring style, '73 Convertible "rolling restoration", 351c, 2v heads with a shave and a haircut, Performer intake, Holley 650(ish), roller rockers, screw in studs, guideplates, stainless valves, Duraspark / Motorsports MSD, T-5 conversion. 1-1/8" front, 3/4" rear swaybars KYB shocks and some home brewed subframe connectors. Future plans; JGC steering box, Cobra brakes and... paint, interior, etc.

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passenger.

[Image: 1_12_09_14_10_15_11.png]
idle and air/fuel ratio are two different beasts.black smoke from the tailpipes tells you you are running rich.that being said, idle will change as you bring A/F back to the magic 14.7:1 .idle can be set with a vacuum Gauge for a good starting point.timing will also effect idle quality and A/F to a point also.start out with 10 degrees BTC on timing,play with idle till you get to the point where you only lose about 200 RPM between park and drive. A/F can be set by seat of the pants feel, but a gauge works a lot better,especially if you want to get the most out of your motor.after you get the base settings you can bump timing up to 14 degrees BTC,small block fords really like that for performance.that has worked for me on stock 289s to full built 5.0s hope that can help you
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