Lean idle. Rich cruise.

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aaronk785

aaronk785

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How do I lean out the cruise and richen the idle. Bought an afr gauge and the results surprised me. Reading 10.8 at WOT, 11.8 cruise and 15.0 at idle. Box stock proform 750 dp. 3 emulsion blocks all open. 74 an#80 jets. IABs .070 and hsab .028. So plugged the center emulsion other two are .028. Lowered primary jet to 70 and went for a spin. New readings were 11.1 WOT, 12.3 cruise and 14.5 idle after opening the idle mixture screw about a 1/4 turn. Idle was much better in gear. Engine is a 10 to 1 408. Home ported sm heads with 2.055 intakes. Howards solid 254 and 260 at .050, lift is .550 in. And .555 ex. On a 108 lsa. Ported rpm intake and 1.625 dougs headers and 3in. exhaust with x pipe. 8in. 4000 stall converter and 3.91 gears. Cruise rpm is 3000 to 3200. Car is 3550 with my fat *** in it.next move is to disconnect secondaries and tune primary only, the get WOT with sec. Jets.
 
I go up on the IFRs some. What's in it now?
 
Lean out the primary to bring the cruise into check. You will probably have to open up the PVCR’s once you do this as it will probably be lean at WOT.
 
Set the cruise afr with the main jet. Then add the fuel on the mains back in with power valve channel restrictors. Then set the timing of the power valve (what rpm/load it goes rich) with 1/2 cruise vacuum or close to it, Error on the earlier side. Then play with secondary jetting for wot afr.
 
If you can’t add more fuel to idle with the idle mixture needles, and assuming your transfer slot opening is set, then you’ll need to get in to the idle feed restrictions and open them up a little at time, trimming with the needles as you do.
 
Ifr are .032. yes the idle screws will richen the idle. I will work on the primary jets and pvcr. Idle will be easy. Thanks everone.
 
What is the idle timing?

Depending on what the cruise rpm is, you might be cruising on the T/slot.....or the main system.
Am I reading this right: best idle is 1/4 turn out from seated? That suggests too much T/slot is exposed at idle.
 
Sorry. I meantt an additional 1/4 turn. They are at about 7/8 out. Initial timing is 22 and 34 total with 50 at cruise. When I richen the idle the cruise goes richer also.
 
Get your timing sorted first. Then block the middle e bleed hole. The other two holes should be no bigger than .028 and .026 would be best.

Then LOWER the idle feed restrictors down in the metering block. It’s a simple drill and tap modification.

You don’t want a jet (the idle feed restricters) above fuel level. Why they almost always stick them up high I can’t say. It’s just stupid but they keep doing it.

You’re idle feed restriction should only be .026-.028 and the idle air bleed should start in the .068-.072 range to start.

Your .028 main air bleed isn’t bad but .026 would most likely be better.

Then you’ll need T slot restricters. They ALL need them. For a single 750 I start at .072ish and tune them from there.

Once you do all that, block the power valve and leave the primary main jets alone. Go drive the car. On flat level ground you want the SMALLEST main jet at a cruise you can get without getting a lean surge. That is cruising on the main jet, not on the T slot so depending on gearing and tire size you’ll want to be cruising at 3000 rpm or so. Any lower than that and you might still be getting fuel from the T slot and you don’t want that.

Once you have your cruise A/F ratio correct you don’t touch the primary main jets again unless you are changing your cruise A/F ratio.

Put your power valve back in and you can tune for WOT. If you are lean you add fuel to the primary side by increasing the size of the power valve channel restrictors and the secondary main jet.

It’s a bit of work but the end result is worth the effort.
 
Already blocked the middle e hole. The others are .028 but I have 26s I'll put in. I think my timing is ok. 22 at idle. 34 WOT and 50 cruise. I can make the t slot restrictors I have blanks to drill. Thanks for the info. I'm learning.
 
aaronk785 said:
How do I lean out the cruise and richen the idle. Bought an afr gauge and the results surprised me. Reading 10.8 at WOT, 11.8 cruise and 15.0 at idle. Box stock proform 750 dp. 3 emulsion blocks all open. 74 an#80 jets. IABs .070 and hsab .028. So plugged the center emulsion other two are .028. Lowered primary jet to 70 and went for a spin. New readings were 11.1 WOT, 12.3 cruise and 14.5 idle after opening the idle mixture screw about a 1/4 turn. Idle was much better in gear. Engine is a 10 to 1 408. Home ported sm heads with 2.055 intakes. Howards solid 254 and 260 at .050, lift is .550 in. And .555 ex. On a 108 lsa. Ported rpm intake and 1.625 dougs headers and 3in. exhaust with x pipe. 8in. 4000 stall converter and 3.91 gears. Cruise rpm is 3000 to 3200. Car is 3550 with my fat *** in it.next move is to disconnect secondaries and tune primary only, the get WOT with sec. Jets.
Click to expand...
Before changing bleeds and emulsions... did you simply try adjusting the 4 idle mix screws about 3/4 out?
Is the power valve correct??
One jet size up ?
Simple stuff first
 
Idle screws are at 7/8 out now. Turning them out makes cruise richen up. Jetted down 4 sizes in front and still super rich at cruise and wot.
 
If your cam was a little smaller, I could tell you exactly what to do for the low-speed circuit; but the biggest I have tuned is the 292/108 Mopar, that measures around 248/249 @050.
On that cam, I ran it at 12/14* idle timing, which required the throttle to be opened too far up the transfers . I fixed that by drilling the Throttle valves to allow closing them some. When I got it right, the 750DP would idle on the mixture screws set near or a lil less than 3/4 turn. Once the T-slot was synced up, She'd idle down to 600 and less by retarding the timing.
To get the bottom end torque back, I ran a two-stage timing curve, giving 28*@2800, then slowing to 34* by 3400. This allowed running 87E10 at 11.3 Scr with alloy heads no problem. To get some Part-Throttle torque, I ran a modified V-can of 22* Thus, at 2800 the timing was 50 which was not too much. To explore what the engine wanted, I bought/installed a dash-mounted, dial-back timing module with a range of 15 degrees. She wanted from 60 to 63 degrees. I needed some Idle retard to smooth out the idle at 550, for idling around the parking lot (@5*), with 3.55s and an A833, so that left 6/7* for cruising and so, 56* is all I could give her. But my ancient carb did not have a 4-corner idle, so drilling was my best option.
AFRs?
IDK, the Edelbrock tool couldn't figure it out and finally crapped out. To their credit, Edelbrock warrantied it. But it crapped out again later, and a NEW Sensor didn't fix it. So I put it on my wall of shame.
AFR's?
I tuned it by ear and it ran fantastic. It only takes a lil longer without an AFR gauge.
Well, to tell the whole truth, I yanked that cam out before the summer was over. It did not like 3.55s, and that was all that I was willing to run. I replaced it with a 223@050, and I was in heaven for the next four years, until the cam dropped lobes right after an oilchange, in around 2004.
 
22* at idle is about half of what you would need with that cam. T/blades are open too far at idle; which is why backing out the mixture screws richens the cruise. You can use an adj vac adv unit to give another 18-20* at idle [ connect to man vacuum]. A 440 I tuned recently idles at 42* with very similar cam specs. Your engine is smaller so the cam will look 'bigger'.

img307.jpg
 
The AFR gauge might not be accurate at idle. Confirm the readings before you spend a ton of time chasing your tail. Wide band sensors are very easy to fool at idle. Performance cams suck air into the exhaust which fools the wide band into a lean reading. Small header leaks will also give a lean reading. Moral of the story is that wide bands often don't read correctly at idle on "hot rods".
 
With a cam that big you should probably just tune it by ear at idle while also watching the vacuum gauge. Play with the idle mixture screws and the initial timing to get the best quality idle with the highest vacuum reading. If the wide band tells you that it is 15:1 then you know it is lying to you. With a big cam the engine will need something around 13.5 at idle but the wide band might read 15 or 16 due to air sucked into the exhaust during overlap.
 
mopowers said:
Is this for 100% gasoline, or E15 mix? Or does it even matter?
Click to expand...

Good question. I see E10 or even E15 not make a difference in AFR readings.

Sometimes I use Lambda instead, as it doesn't care what the fuel is. But it's not as popular as A/F ratio but it does eliminate fuel choice issues.

To add to what Andy said above, not every gauge reads the same. So tuning to a specific number isn't always a good choice.
 
If you are cruzing at 3k rpm I would guess you are activating the boosters unless you have blead off a lot of signal. Try crusing at at lower speed (~2K RPM) and see how your AFR reacts. You need to figure out if you are tuning the Idle circuit or main circuit or a combination.
Other than the AFR gauge showing lean/rich- what drivability issue are you trying to address? (Stumble, plug fouling, just don't like how it smells etc?) Like a lot of other people said, the AFR gauge can be erroneous at idle due to the overlap.
 
Id be tuning to somewhere 12.5 - 13.5 at WOT

closer to 14 at cruise
looking for 14 , 14.1 14.2 if i lift off the throttle, and its running down on the idle circuits with the throttle closed on the stop. i.e higher RPM than idle, with higher vacuum and at that "raised" RPM way less chance of sucking nice oxygen up your tail pipe to mess with the reading.
The constant blow down due to the motor turning over at speed see's to that.

idle in neutral will then be what it is, and if you can get off cleanly so be it

if its making your eyes water and blackening the tail pipe, as suggested above, look to your initial advance, a bit more will clean up any just off idle problems.

The stoichiometric number for e10 is 14 to 14.1 and non ethanol fuel like 1970s leaded super or proper race stuff would be 14.7.

I have no clue about holley carbs and v8 motors with cam like yours but post above do

just don't kill yourself trying to tune to 14.7 as a baseline aim for 14 with your foot off the gas and 12.5 13 at WOT for bit of safety. do the go faster tweaking from there.

I went down a rabbit hole for a long time trying to achieve a setting that is impossible

wobbling about between 12.5 and 13.5 foot to the floor and no worse than 14.2 with it off or just a crack open

job done in my view. my plugs are not that bad Honestly :)

Dave
 
Modern gauges should all read the same to within a tolerance that is small. A modern wideband sensor comes paired with a calibration module in the plug connector. they etch off a chunk of some resistor or chip in the plug connection section to get it calibrated
that connector and sensor are now paired for life

usually a warning sticker saying do not disconnect this bit disconnect the wire at the other end of this part with the flip up clip weather proof connector

what happens is someone then takes 1 sensor ignores the warning sticker or it comes off and then swaps the connector bit along with the wire to another sensor and you end up with something that is way way out
i.e you have 3 parts
the wire harness
the plug bit
and the sensor that bolts into your headers

The majority of the plug bit is married to the sensor and is there to provide calibration BUT can be removed if you are persistent enough

most of the screw in wideband sensors will be bosch or similar these days and they do put effort into getting it right. when they make the sensor they don't know if its going into a aftermarket kit or its going into a box of 5000 to be delivered to Peugeot/FIAT :) for the challenger production line, they all need to be calibrated correctly because the ECU they usually connect to, expects them to be.

AEM and similar all use a chip and LED based gauge, again the tolerance stack up in the circuit will be tiny and inconsequential.

in the past or with unknown history, or cheap knock-off/fake ..... ????

new stuff as in last 10-15 years should be pretty accurate

Dave
 
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