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 Idle, off-idle, and cruise Air-Fuel Ratio
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Sitting Bull

139 Posts

Posted - 30 Mar 2017 :  2:52:21 PM  Show Profile  Reply with Quote
(This is a continuation of the Wideband AFR Gauge thread. Since I'm focusing on tuning and this question now relates to the carb modification I thought it best to create a Subject with a more accurate heading.)

Background: my Idle, off-idle, and cruise Air-Fuel-Ratio is about 16.3:1 based on my AFR gauge. I would like to richen the mix to about 15:1. this should make my car stop being stinky, reduce risk of knock, and possibly have engine run cooler in the summer.

I'm thinking step one is to slightly richen my idle to about 15. After setting idle correctly I can then see if any further changes are needed for cruise which I will change via primary rods. The other method would be to change primary rods to richen cruise in the hopes that the idle goes richer but with the risk that it doesn't.

So, to simply richen idle I think I need to open up my idle tube orifices. (note: I've already backed out the idle screws plenty and there is no more change seen on AFR gauge.) I'll have to re-read the two Q-jet books I have to confirm things. Doug Roe's book mentions that the primary metering jet/rod is too large to have any effect on, i.e. it won't restrict, the idle circuit. That tells me that I have no choice but to pull out my idle tubes, measure them somehow, then open them up with a correctly selected drill bit.


71 LeMans Sport Convertible with 340,000 miles driven year round since 1994.

Blued and Painted

3505 Posts

Posted - 31 Mar 2017 :  01:19:12 AM  Show Profile  Reply with Quote
If it's lean everywhere, i would check for a low float level.
Also check the fuel pressure.

Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.
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Sitting Bull

139 Posts

Posted - 31 Mar 2017 :  11:54:36 AM  Show Profile  Reply with Quote
Let me qualify my previous statement.

It's not lean everywhere.

Idle, off-idle, and cruise is lean at 16:1.
Medium throttle (power piston up, low vacuum) is rich at 12:1.
WOT is rich at 11:1 - 13:1.

I have determined that I must drill out my Idle tubes from (estimate) 0.033" to 0.036", drill out my idle down-tubes from (estimate) 0.040" to 0.046" and drill out my idle port from (estimate) 0.070" to 0.090".

It's gonna snow up here today and tomorrow. That might be a great time to do this work. I hope my carb gasket arrives today (unlikely) so that I can stop the weeping. Most likely I will need to do things twice, which is ok since I will probably want to adjust my primary jets once I get the idle circuit set correctly.

71 LeMans Sport Convertible with 340,000 miles driven year round since 1994.
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Sitting Bull

139 Posts

Posted - 31 Mar 2017 :  11:55:28 AM  Show Profile  Reply with Quote
Some useful information


?6 AFR - Rich Burn Limit (engine fully warm)
?9 AFR - Black Smoke | Low Power
?11.5 AFR - Best Rich Torque at Wide Open Throttle (WOT)
?12.2 AFR - Safe Best Power at Wide Open Throttle (WOT)
?13.3 AFR - Lean Best Torque
?14.6 AFR - Stoichimometirc Air/Fuel Ratio Value (Stoich)
?15.5 AFR - Lean Cruise
?16.5 AFR - Usual Best Economy
?18 AFR - Carbureted Lean Burn Limit
?22+ AFR - EEC / EFI Lean Burn Limit

71 LeMans Sport Convertible with 340,000 miles driven year round since 1994.
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Sitting Bull

160 Posts

Posted - 02 Apr 2017 :  8:20:06 PM  Show Profile  Reply with Quote
I'm glad they have the footnote that says based on fuel type.

You really don't know what you are getting at the pump because there may or may not be 10% alcohol.

I've seen some 100% gasoline pumps. Its expensive. Is it still legal?

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5448 Posts

Posted - 03 Apr 2017 :  09:34:34 AM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
It's about 2.89 for 91 octane no ethanol here (perfectly legal to run on the street.) Ethanol 93 would be about 2.49 when i saw the no ethanol at 2.89, for reference. 87 was about 2.10.
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