IDLE AFR QUESTION

[rgalajda]

[kgb]

I leaned out the IFR's in my Mighty Demon 750 to .035 then to .032, drove a square mile course with lights at each corner, and recorded AFR, vacuum and RPM via an Innovate setup. The leaning is discernible between the two settings, got a fair number of spans in the 15:1 range and I'm not sure I want to go any leaner. I understand the motor isn't putting out much in the way of HP under these cruising and light acceleration conditions and doesn't need a particularly rich mixture, I just haven't seen details on how lean is too far and it's a personal comfort thing at this point. I'm also using an MSD distributor with only mechanical advance, no vacuum advance to help keep the burn clean when needed. That's another thing I've not seen put to the test, an A-B comparison of A/F and emissions with and without vac hooked up. I'd like to have a TPS input to the recordings as well, for a better idea of when the main circuit might be cutting in.

Would be interesting to somehow disable the main circuit to tune the whole idle circuit range without interference. I got a taste of that with the Mighty Demon as it came with a set of big HSAB's and the delayed activation was very clear in the car's function when conditions were just so. Made it very clear when the idle circuit was being deactivated.

[GTO Geoff]

Hi.
I am surprised the usual suspects have not jumped on & told you that you are wrong: that large [er] HSABs will start the system earlier....not later!!

[cat herder]

[kgb]

[Right hand drive]

[GTO Geoff]

Post dated Oct 25.

Big set of HSABs delayed the activation....

[Right hand drive]

[GTO Geoff]

RHD,
I don't how you keep missing it. It is in the last paragraph of the Oct 25 post. He says: on another carb, a Demon [ the brand makes no difference, they all work on the same principles ], he found bigger HSABs delayed activation & it was 'very clear'.

[rgalajda]

I was working on a 1971 383 4 speed Cuda yesterday. Way to much camshaft in my opinion for a stock engine. Idle was rough with 10” vacuum at 1100 rpm.
It had the original Holley carb on it , all restored. I removed the idle mixture screws ( blunt short tipped screws ) and replaced them with standard Holley pointed mixture screws.
We were then able to reduce idle rpm to 950 and vacuum went from 10” to 12” at a lower idle.
The factory idle mixture screw was the limiter.
This was a factory OEM 3 circuit Holley carb which had a separate idle jet and transfer slot jet.
IFR/passage measured .039” . Transfer slot jet/tube measured .024”.

In an aftermarket Holley carb we generally adjust idle mixture screws for best idle and vacuum gauge reading. If you then lean the idle mixture screw you will see your cruise AFR go leaner.
The old 3310-1 780 Holley carbs had large idle air bleeds usually measured .078” with the traditional .028” emulsion . The IFR usually measured .035”. ( using pin gauges )
These carbs ran rich at cruise speed, but idled well even with tight LSA cams and .225 @.050” duration in a big block.
I have reduced the IFR to .032” on my car which resulted in better cruise AFR. But the price paid was too lean idle until the car was warmed up and the intake manifold heat soaked, even with a choke.
I don’t like to have to feather the gas pedal when the engine is cold. For this reason I run a larger IFR with a TSR .
All I’m saying is that sometimes a TSR can be useful depending on the circumstances .