Rich Idle is Good?
05-07-2006, 08:56 PM
#1
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Rich Idle is Good?
Ok my new 76 has a the aluminum small block 427 and single plane manifold and a holley 850 carb.
The original owner did change out the cam to these specs
Part Number: 119661 Grind Number: HR-230/359-2S-12.90 IG
Engine Identification:
Start Yr. End Yr. Make Cyl Description
1957 1987 CHEVROLET 8 FAIR IDLE, PERFORMANCE USAGE, GOOD MID-RANGE TORQUE AND HP, 3600-4400 CRUISE RPM, 10.0 TO 11.5 COMPRESSION RATIO ADVISED, .900" BASE CIRCLE FOR LONG STROKE CLEARANCE. BASIC RPM 3000-6500
Engine Size Configuration
262-400 C.I. V
Valve Setting: Intake .000 Exhaust .000 HOT
Lift: Intake @Cam 359 @Valve 539 All Lifts are based
on zero lash and theoretical rocker arm ratios.
Exhaust @ Cam 372 @Valve 558
Rocker Arm Ratio 1.50
Cam Timing: TAPPET @.004
Lift: Opens Closes ADV Duration
Intake 35.0 BTDC 77.0 ABDC 292 °
Exhaust 83.0 BBDC 37.0 ATDC 300 °
Spring Requirements: Triple Dual Outer Inner
Part Number 99838
Loads Closed 112 LBS @ 1.650 or 1 21/32
Open 327 LBS @ 1.120
Recommended RPM range with matching components
Minimum RPM 2600
Maximum RPM 6600
Valve Float 7000
Cam Timing: TAPPET @.050
Lift: Opens Closes Max Lift Duration
Intake 8.0 BTDC 42.0 ABDC 107 230 °
Exhaust 56.0 BBDC 2.0 ATDC 117 238 °
Car runs and sounds great with a very mild loop at idle. But the smell at idle was pretty bad. At first I was thinking rich, but then I remembered that unless you see black smoke, smelly eye burning exhaust was usually indicating a lean condition. There is also a wide band O2 sensor indicating aprox 14 air fuel ratio. It was pulling aprox 12to 14 inches of vacum at idle.
After making sure the bowls were set correctly, it started to idle a bit low and rough (not sure why they did not seem of by much) so I bumped the idle to around 900 (smooth but two fast?) and was getting about 12-13 inches of vacum. I took the idle mixture screws out 1/4 turn and the vacum went to about 14-15 inches and the smell seemed to go away. Normally I would be happy, but here is the wierd part, the O2 sensor went to about 12 for the air fuel ratio which would be considered rich. Any further turns of the idle mixture screws either in or out caused a drop in vacum. Any thoughts or comments? I am pretty much an amature at carbs (love the injection!). Do engines that get their power in higher RPMs need the extra fuel at idle or should that not make any difference? Thanks!
The original owner did change out the cam to these specs
Part Number: 119661 Grind Number: HR-230/359-2S-12.90 IG
Engine Identification:
Start Yr. End Yr. Make Cyl Description
1957 1987 CHEVROLET 8 FAIR IDLE, PERFORMANCE USAGE, GOOD MID-RANGE TORQUE AND HP, 3600-4400 CRUISE RPM, 10.0 TO 11.5 COMPRESSION RATIO ADVISED, .900" BASE CIRCLE FOR LONG STROKE CLEARANCE. BASIC RPM 3000-6500
Engine Size Configuration
262-400 C.I. V
Valve Setting: Intake .000 Exhaust .000 HOT
Lift: Intake @Cam 359 @Valve 539 All Lifts are based
on zero lash and theoretical rocker arm ratios.
Exhaust @ Cam 372 @Valve 558
Rocker Arm Ratio 1.50
Cam Timing: TAPPET @.004
Lift: Opens Closes ADV Duration
Intake 35.0 BTDC 77.0 ABDC 292 °
Exhaust 83.0 BBDC 37.0 ATDC 300 °
Spring Requirements: Triple Dual Outer Inner
Part Number 99838
Loads Closed 112 LBS @ 1.650 or 1 21/32
Open 327 LBS @ 1.120
Recommended RPM range with matching components
Minimum RPM 2600
Maximum RPM 6600
Valve Float 7000
Cam Timing: TAPPET @.050
Lift: Opens Closes Max Lift Duration
Intake 8.0 BTDC 42.0 ABDC 107 230 °
Exhaust 56.0 BBDC 2.0 ATDC 117 238 °
Car runs and sounds great with a very mild loop at idle. But the smell at idle was pretty bad. At first I was thinking rich, but then I remembered that unless you see black smoke, smelly eye burning exhaust was usually indicating a lean condition. There is also a wide band O2 sensor indicating aprox 14 air fuel ratio. It was pulling aprox 12to 14 inches of vacum at idle.
After making sure the bowls were set correctly, it started to idle a bit low and rough (not sure why they did not seem of by much) so I bumped the idle to around 900 (smooth but two fast?) and was getting about 12-13 inches of vacum. I took the idle mixture screws out 1/4 turn and the vacum went to about 14-15 inches and the smell seemed to go away. Normally I would be happy, but here is the wierd part, the O2 sensor went to about 12 for the air fuel ratio which would be considered rich. Any further turns of the idle mixture screws either in or out caused a drop in vacum. Any thoughts or comments? I am pretty much an amature at carbs (love the injection!). Do engines that get their power in higher RPMs need the extra fuel at idle or should that not make any difference? Thanks!
05-07-2006, 10:37 PM
#4
Racer
a cam such as you have would want to be richer at idle and also cruise/light load driving conditions, also the cam would like 16 to 18 degrees of initial timing with 36 total (without any vacuum advance counted in) at 2800-3200 rpm.
the smell you discribe is most likely high HC (hydrocarbons/unburnt fuel/misfire) due to misfire from too lean of a mixture at idle that was made less by the richer mixture. a rich mixture (high CO or carbon monxide) has no smell but carbon monxide is a deadly gas while high HC will burn your eyes, smell and make it difficult to breathe.
my target air/fuel mixture would be
at idle 12.5-13.5 to 1 a/f
at light load cruise speeds 13.2-13.5 to 1 mixture
at wide open or acceleration 12.5 to 1 a/f mixture
just a note a wide band based o2 sensor can give a false lean reading at idle and low engine speeds when you have a high overlap cam such as yours so the way you set the idle mixture makes sense.
the smell you discribe is most likely high HC (hydrocarbons/unburnt fuel/misfire) due to misfire from too lean of a mixture at idle that was made less by the richer mixture. a rich mixture (high CO or carbon monxide) has no smell but carbon monxide is a deadly gas while high HC will burn your eyes, smell and make it difficult to breathe.
my target air/fuel mixture would be
at idle 12.5-13.5 to 1 a/f
at light load cruise speeds 13.2-13.5 to 1 mixture
at wide open or acceleration 12.5 to 1 a/f mixture
just a note a wide band based o2 sensor can give a false lean reading at idle and low engine speeds when you have a high overlap cam such as yours so the way you set the idle mixture makes sense.
05-08-2006, 11:37 AM
#5
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Thanks olescarb, sounds like I should be about where I am then right? Maybe just a tad rich? The little bit I know about idle mixture and carbs is that at any given idle rpm, you want maximum vacum. And that is what I got. Any richer, vacum would drop, any leaner, vacum would drop. I was just suprised by the o2 sensor reading. But if its off a bit, I could very well be in the range you are talking about. 
05-08-2006, 02:12 PM
#6
Race Director
isn't a single plane intake less efficient at idle and low RPMs? If so i would think that your lower RPM settings would at best be compromises to get the better performance at the higher RPMs
05-08-2006, 02:17 PM
#7
Race Director
That is a very mild cam for a 427 ci motor. Those cam numbers and rpm numbers are based on a 350 motor. A 427 will need a lot more duration, the lift is not bad but you should be up in the 245 @ .050 range or even more. I'm running 252/260 @ .050 with a .550 lift in a 385 SB and it is not to wild at all. 