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Old 11-05-2008, 10:42 AM   #1
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Default Stumbling idle on cold start.

First off, on cold start what AF are you guys idling at and on what injectors?

The weather has just gotten colder and on a cold start my car has a stumbling idle for the first like 5 minutes of the warmup. It tries to hold 1500 rpm, but stumbles repeatedly. After 5 minutes, smooth as butter. When the weather was warmer, this was never a problem.

My AF is like 13.1 when warming up, maybe a bit leaner. Could a lean warm-up enrichment be the culprit?
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Old 11-05-2008, 11:00 AM   #2
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You gotta run your **** rich when it's cold. Fuel doesn't like to vaporize when it's cold. I run 10.5-11:1 until coolant is 100*C. And I complain cause mine doesn't idle dead smooth at 700RPMs at 30*C and I have to idle the damn thing up to 1K to get it to smooth out....
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Old 11-05-2008, 11:23 AM   #3
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I had no idea it had to be that rich. I will change the settings and post back.
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Old 11-05-2008, 11:35 AM   #4
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Also, running less timing and more throttle helps smooth out idle. But I hate engines that run like that. I like them advanced and throttled back. Makes them quieter at idle, but it ain't as smooth when cold. IE-my **** would idle at 700 at 30*C smoothly if I'd back the timing to 10* and adjust the throttle to reset the RPMs.
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Old 11-05-2008, 11:52 AM   #5
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100*C? Doesn't warmup stop at 160*F? 100*C is over 200*F. Did you mean *F?
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Old 11-05-2008, 12:14 PM   #6
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Define "cold"

I idle in the high 14s when warm, usually mid to low 13s when cold. (Cold in CA and FL being defined as < 70F.) Injectors are 440cc.

I idle at about 1,200 RPM when cold. I think my ignition advance in that region is somewhere around 16. And yes, it's a tad lumpy. Not "OMG, it's going to stall" lumpy, just "Hmmm- that car sounds like it needs a tune-up" lumpy.

More warmup fuel is certainly something to try, especially if you've not tuned warmup down in the temperature regions you're currently experiencing.
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Old 11-05-2008, 01:40 PM   #7
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Quote:
Originally Posted by Saml01 View Post
what AF are you guys idling at and on what injectors?
My cold start AF is quite fat at around 11.5-12 at 1200 RPM when ambient is <50F. It doesn't stumble per se, it just sounds like a cold engine. Once coolant is up to 100F, idle gradually reduces and AF gradually leans. Running 10g with 460cc. Warm idle is smooth at 14.5 AF and 1,000RPM. Suppose I could run a slower idle but it would certainly need to be richer to keep it smooth.
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Old 11-05-2008, 01:43 PM   #8
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Originally Posted by TrickerZ View Post
100*C? Doesn't warmup stop at 160*F? 100*C is over 200*F. Did you mean *F?
Yeah, all temperatures should have been in Fahrenheit, not Celsius. And I'll also add that idling 10.5-11:1 is not typical. But my engine likes fuel.
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Old 11-05-2008, 01:46 PM   #9
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needs more return line.
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Old 11-05-2008, 01:48 PM   #10
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needs more return line.
+1
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Old 11-05-2008, 03:08 PM   #11
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I warmup no richer then 12:1. Does great.
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Old 11-07-2008, 10:57 AM   #12
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Quote:
Originally Posted by patsmx5 View Post
Also, running less timing and more throttle helps smooth out idle. But I hate engines that run like that. I like them advanced and throttled back. Makes them quieter at idle, but it ain't as smooth when cold. IE-my **** would idle at 700 at 30*C smoothly if I'd back the timing to 10* and adjust the throttle to reset the RPMs.
Thanks for the tips guys, this timing stuff is confusing.

Last edited by Saml01; 11-07-2008 at 11:11 AM.
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Old 11-07-2008, 11:50 AM   #13
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Quote:
Originally Posted by Saml01 View Post
Thanks for the tips guys, this timing stuff is confusing.
Yes it is. I idle ~20*BTDC. I let it warm up and then set my idle to ~700RPMs. So quiet you seriously can NOT hear the engine running. You can feel it a little, but can't hear it. Reminds me of sitting in a camry.

But the idle is very "weak" cause the engine is barely using any air/fuel at all. So initial tip in sucks. IE-if I stomp the pedal, it has a delay/hickup before it revs up. But I like it idling this way.

If I were to put my timing back to 10*BTDC, the engine wouldn't even idle because it would slow down and not spin fast enough to run smoothly. So I'd have to compensate by adjusting the idle at the TB to get it back up. The net result is the same RPM, but now the engine is pumping more air/fuel through it. It's easier to light more air/fuel. Plus it's easier to maintain target A/F ratios when your moving more air/fuel.

Only thing that sucks is that if I idle at 700 it doesn't do good when I turn on the A/C. I either gotta retard timing some and idle it back up a bit, or, hook up a fast idle valve to automatically idle the engine up some.
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Old 11-07-2008, 03:15 PM   #14
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I think im gonna need a flash animation to understand this.

More timing = less air\fuel used = less throttle adjust?

Less timing = more air\fuel used = more throttle adjust?

More timing idles worse when cold, less timing idles better.

Just break it down for me.
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Old 11-07-2008, 05:05 PM   #15
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Quote:
Originally Posted by Saml01 View Post
I think im gonna need a flash animation to understand this.

More timing = less air\fuel used = less throttle adjust?

Less timing = more air\fuel used = more throttle adjust?

More timing idles worse when cold, less timing idles better.

Just break it down for me.
I thought I did break it down...

I'll try again I guess.

You add timing, the engine speeds up. ( to a point of course, you can't add a million degrees of timing)

You open up the throttle by adjusting the little set screw and the engine speeds up.

You retard timing and the engine slows down.

You close the throttle by adjusting the little set screw and the engine slows down.

So you got two ways to play with idle. Timing and throttle. And fuel is a third, but it makes much smaller differences.

If you want a smooth, strong idle, and great tip in (what 99% of normal people want), then keep base timing conservative (like 10* or something) and then adjust throttle to get RPMs where you want.

If you want a quiet idle, not as smooth, shitty *** weak engine that bogs when you turn on the headlights and dies when you turn on A/C, then advance the timing a lot and back off the throttle. That's the way mine is. It uses a bit less fuel at idle, sounds different (better IMO), and quieter.
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Old 11-09-2008, 12:08 PM   #16
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Awesome, thank you.

Last night I backed off my timing a bit, and richened up the warmup and it was night and day.
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Old 11-18-2008, 08:08 PM   #17
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Why is the tip in better when the timing is around 10* vs 18*?

and.

Why is the engine stronger when the timing is retarded to 10* vs 18*?
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Old 11-18-2008, 08:19 PM   #18
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Quote:
Originally Posted by Saml01 View Post
Why is the tip in better when the timing is around 10* vs 18*?

and.

Why is the engine stronger when the timing is retarded to 10* vs 18*?
The assumption with either of these is that you have the same RPM and AFR at idle. IE-say you have two identical engines at an 800 RPM idle. Only difference is one is idling at 10* BTDC and the other at 18* BTDC. The one that is at 18* will have the throttle adjusted to be less open than the engine at 10*. This is because the motor running 18* is more efficient and, given the same air/fuel as the 10*motor, it would idle much higher. So you have to choke the engine back a bit more to get it to idle at the same RPM.

Difference of course is one engine is pumping more air/fuel through it. That would be the 10* motor. This makes it more stable and generally smoother. It's not really "stronger" as it's actually less efficient. But if you try to idle away from a red light without applying throttle, the 10* motor (the one that has the throttle opened more) will make more power and not stumble or hickup as much as the other engine.

I dunno what else to tell ya. The lowest cell in my timing map right now is 20*. Fired it up this morning. It was 33*F out. Idled at 800 smooth as could be from a cold start. At 20* BTDC. Amazing what you can do at 10.5:1 AFR's.
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Old 11-18-2008, 09:19 PM   #19
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It seems you arent the only one that believes in a lot of advance for idle. I have spark tables of about 15 people, and the majority idle at more then 15.

Can you do me a favor and post your warm up enrichment table please.
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Old 11-18-2008, 10:00 PM   #20
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