corvettedave383

whats the ideal a/f mixture at idle??

cheers

dave

Matt Gruber

ideal for what?
1. least pollution?
2. best power?(switch ac on without stalling)
3. best fuel economy?
4. cool lumpy sound?
5. smooth idle?
6. cold start, no choke, no stall?

capevettes

Not sure how you want to quantify it but generally it's the adjustment that gives you the highest/smoothest idle (notwithstanding a radical cam)

corvettedave383

best power, cold start/no choke would be ideal

thanks

dave

Gordonm

Ideal AF ratio is 14.7 to 1. Most of the time with a carb that is not going to work well. Start adding big cams, large heads and it goes down. I have been able to run about 13.5 at idle and that is about it. Full power 12 to 1 seems to provide the best power. At cruise if I can get it to 14 to 1 that is great. Most of the time at cruise I am around 13.7 to 1, any leaner and it starts to miss a little.

Cold start, good luck. I run no choke and have to play with the throttle for about 30 to 45 seconds depending on how cold it is before it will idle poorly on its own. After a few minutes of running it will idle good.

stingr69

I guess Stoichiometric is perfect. 14.7:1 is the theoretical bogey but you may find a little lower works better for you.

-Mark.

Matt Gruber

should be 12-13 depending on if u read the cyl that is the 1st to miss, causing it to stall.
in the summer i do 14, but if it stalls when cooler, i'll richen it up. Like gordon says, u have to know how to drive with no choke.

Mr. Beam

After tinkering with timing and A/F for two days, I ended up with a compromise at 13.5-13.9. That's about the leanest I could run without having a choke...

Don't forget that the idlemixture affects your cruise too.




Mr. Beam

corvettedave383

I have done some basic monitoring with my LM1, at idle it sits around 12.3, then when I rev it on driveway, reving upto around 4000 - 5000rpm, the afr will go upto 15.5, is this good or bad??

specs on engine

383 forged motor 9.1 comp
BRC 3.750" crank, bcr 5.7 H - beam, bcr 2618 pistons,hd pins
custom machined blower cam(258/646 112lsa)
custom roller lifters solid
T & D - shaft rockers 1.6 ratio
dart pro 1 platinum alum 215 heads
Weiand - team G Intake Manifold - single plain ( 2800-7200 RPM Range)
quickfuel(holley) 750cfm double pumper, mech sec

thanks

dave

lars

Optimum A/F at idle and low-load cruise is 14.7:1. Most Holley and BG carbs run best with idle mixture in the high 13's. Q-Jets will idle well at 14.5 - 15.0.

However, this is on straight gasoline - most parts of the country now use oxygenated fuels. If you have 10-15% ethanol in your fuel, stoichiometric A/F is around 13.5:1, and you need to set idle mixture to this level in order to idle correctly and to avoid off-idle stumbles.

MotorHead

With the LM-1 you can set your carb up to get the idle, cruise and WOT exactly where you want it. Problem is, where you want it and where your motor likes it are usually different.

Don't worry about 14.7:1 or trying to run lean to save gas. Our V8's like it rich so if you can get it idle at 12.5-13:1 and cruise a little higher then WOT a little lower then call it a day. I had mine idling at 14:1 and cruise near 15:1, fuel economy was good but the motor didn't run right

RIght now with my new motor everything is in the 12's, playing it safe for now

corvettedave383

how does timing affect air/fuel ratio, or does it?

cheers

dave

corvettedave383

so if its going into 15.5 when reving engine on driveway, does this mean its way to lean?, is reving quickly on driveway, not really showing true wot under load??

cheers

dave

Gordonm

Reving the engine in the driveway will not show much. You need to have the car under full load and WOT to see what it is running. You will never get WOT readings sitting in neutral.

lars

Give the engine what it wants: Listen to it, smell it, and feel it - it's a total sensory experience. Set it up for mid-14's as a starting point if you have pure gasoline, and mid-13's if you have 10-15% ethanol. Then tune it and tweak it to what it responds to from there. The engine will "tell" you what it wants. Don't be afraid to go on the rich side - most performance engines respond very well to a slight rich condition. If you want mileage, old Escorts sell for about $1200...

corvettedave383

ive had it at about high 12's, when you hit the pedal quickly, it produces a small flame out the top of the carb, lean condition?, if I dial it back to low 12's, it fine, no flame, so do I need to change jet sizes or change timing to suit??

also when adjusting a 4 corner screw carb, should you change all 4 screws or can you just play with the primary metering block screws for idle??

cheers

dave

Matt Gruber

i "cruising" map mine every 250 rpm up to 4000, in the driveway, turn up idle screw and write down reading. Is 15.5 too lean? Choke it slightly, if rpm goes up, it is too lean. 15.5 sounds good for cruise mpg. also u can vary the advance looking for max rpm. My 61 had too much vac advance, and was too lean, so i rejetted and cut back the advance. surge/miss cured in my driveway, along with test drives(some load needed for final cruise tuning).
.
No, u can't simulate WOT in the driveway; well maybe by smoking some old tires

corvettedave383

Ive had a good play with it today, what iam finding is that either iam loading up to much and ehgine slowly idles lower and lower or its to lean, getting a flame out the top of the carb, when you hit the throttle quickly on the driveway, but the a/f isnt showing lean.

what do you think?, maybe change to bigger jets in carb, ive got a hole box of holley jets sizes

cheers

dave

zwede

Actually, all O2 sensors, wideband and narrowband, are lambda meters, not air/fuel. The meter measures lambda and then multiplies it with 14.7 to show you air/fuel. Since lamdba 1 (stoich) is richer on E10 than straight gasoline, the meter will still show 14.7:1 but in reality it is a bit richer.

So regardless if you're running gas, E10 or E85, the meter will show 14.7 at stoich. It's a limitation of the meters. Maybe someone will release a wideband where you can tell it what fuel you're using but until then they will all assume straight gas.

morganjd

LARS or SWEDE, I have always run mine around 13.5-14.0 with no problems. I put my LM1 on a mopar with a carter and it was close to 20.0, we got it down to 18.0, but it would not run any lower?? I also had it on a AAR cuda with tripower, and it was at 16.0 to 1, Any thoughts? I am using a holley 3310 780 (run in the pure stock muscle car drags). I looked at my data from the last trip to the strip and found that the A/F ratio was right at 12.8-13.0 until i hit 4th gear and then it dropped to 12.0. What do you think? Should i drill out the high speed bleeds? If so how much bigger. Car runs 12.95 @110mph, 454 71 LS6 4 speed 4.11, wide ratio M20