Should this cam be a "stinker"
Thread Starter
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From: Check this riff........
Should this cam be a "stinker"
The cam in my bill mitchell 427 small block was replaced by the preious owner with this one.
Crane Cams Part number 119661
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 °
At idle the exhaust fumes are unbearable. I set the Fuel air mixture using a vacum gauge and it seems dead on. Engine sounds great, idles great, but the fumes are REALLY nasty at idle. It smells lean to me, but is it unburned gas from this cams overlap?? If its fine, I am going to lose the sidepipes and make someone else breath that chit in.
Its a holly carb, but I believe bill mitchell does extensive mods to the carb so specs are unknown.
Crane Cams Part number 119661
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 °
At idle the exhaust fumes are unbearable. I set the Fuel air mixture using a vacum gauge and it seems dead on. Engine sounds great, idles great, but the fumes are REALLY nasty at idle. It smells lean to me, but is it unburned gas from this cams overlap?? If its fine, I am going to lose the sidepipes and make someone else breath that chit in.
Its a holly carb, but I believe bill mitchell does extensive mods to the carb so specs are unknown.
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
You need to look at how to lean out the idle. This is not caused by a mild cam. Most extensively modded carbs include things like I did of drilling the primary throttle blades starting with a 1/8th inch hole.
Safety Car
Joined: Jul 2002
Posts: 3,770
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From: St Louis MO
Sounds like your carb needs some work done on the idle circuit. If it has the adjustable metering blocks, you can decrease the size of the idle feed restrictors. If the carb has the classic holley metering blocks you can put some thin wires in the idle fuel passage.
Brett
EDIT: I have a picture I can email you that shows the passage the wires need to go into.
Brett
EDIT: I have a picture I can email you that shows the passage the wires need to go into.
Safety Car
Joined: Oct 1999
Posts: 4,101
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From: NorCal
Have someone stick a 5-gas emissions analyzer in one or both side exhaust before you start changing stuff. My experience is that rich mixtures, while mildly annoying, are nothing like an excessively lean mixture. Very high hydrocarbons (ppm) tend to burn the eyes, cause headaches and generally smell terrible to the senses. I'd guess that at this point ...
Hans
Hans
Thread Starter
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From: Check this riff........
cool, so it sounds fixable. 
GKULL, do you mean that even using the 4 idle circuit screws, and vacume readings that I could be missing something?
Brett could you pm me those pictures?
thanks,
--marc
ps I understand how open ended my question is with a unknown modified carb and really appreciate your input.
GKULL, do you mean that even using the 4 idle circuit screws, and vacume readings that I could be missing something?Brett could you pm me those pictures?
thanks,
--marc
ps I understand how open ended my question is with a unknown modified carb and really appreciate your input.
Thread Starter
Race Director
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From: Check this riff........
I will try and recheck it tonight, I last tried adjusting it a couple of months ago, so I will recheck the vacum settings. I know it had decent vacum, and thats why I used vacum readings and the idle mixture screws to try and adjust the idle.
Is there anytime that an engine will idle worse and pull lower vacum, but be correct (a/f ratio), then using the idle screws, vacum guage, and sound of idle?
Is there anytime that an engine will idle worse and pull lower vacum, but be correct (a/f ratio), then using the idle screws, vacum guage, and sound of idle?
Last edited by BigBadRed; Jan 29, 2007 at 02:14 PM.
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Like Brettmc said: restricting the idle feed circuits. I have Demon carbs so they are manufactured way rich on the older style. The problem is that the four corner idle screws are not able to lean the idle mixture all the way.
I drilled holes in all four throttle blades to induce something like a vacuum leak to lean the A/F ratio. Then I was able to dial in the four corners using a vacuum gauge.
I drilled holes in all four throttle blades to induce something like a vacuum leak to lean the A/F ratio. Then I was able to dial in the four corners using a vacuum gauge.
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From: St Louis MO
I don't have that picture posted so PM me your email address and I'll send her your way.
Brett
EDIT: Drill the throttle blades only as a last resort. I drilled mine early in my tuning efforts only to discover it caused a problem later. I did a nono and filled the holes with jb weld.
Last edited by Brettmc; Jan 29, 2007 at 05:21 PM.
Thread Starter
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From: Check this riff........
I recently sold a car and had an a/f meter and 02 sensor that I had removed before selling and had forgotten about. I installed it last night and after letting the engine warm up got idle readings around high 11s to low 12s. Adjusted idle screws and got the readings high 12s to low 13s. Can I lean this further to high 13s low 14s, or will this make the transition circuit to lean. Also, it has a pressure gauge on the fuel line reading 15psi. 
I am sure that is causing some problems, but I also noticed an adjustable pressure regulator installed on the firewall and nothing connected to the return side. Is there ANY reason NOT to have the return line connected??? I thought this was very bizzare.
I am sure that is causing some problems, but I also noticed an adjustable pressure regulator installed on the firewall and nothing connected to the return side. Is there ANY reason NOT to have the return line connected??? I thought this was very bizzare.
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Joined: Jul 2002
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From: St Louis MO
Sometimes good idle readings in the garage translate to a horrible transition on the road. Road test your adjustments with a data logger (wife/SO) 
You're right, 15 is too much. It should be 7-9. A return line is not necessary but your fuel pump will love you if you do have one.
You're right, 15 is too much. It should be 7-9. A return line is not necessary but your fuel pump will love you if you do have one.
Last edited by Brettmc; Jan 30, 2007 at 03:01 PM.
Thread Starter
Race Director
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From: Check this riff........
Sometimes good idle readings in the garage translate to a horrible transition on the road. Road test your adjustments with a data logger (wife/SO)
You're right, 15 is too much. It should be 7-9. A return line is not necessary but your fuel pump with love you if you do have one.
You're right, 15 is too much. It should be 7-9. A return line is not necessary but your fuel pump with love you if you do have one.
A return line is not necessary I didn't know that, I guess i really did grow up with fuel injection!
Le Mans Master
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From: Butler Pa
Safety Car
Joined: Jul 2002
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From: St Louis MO
Ill do the road test, but do you think its safe to lean out to high 13s/low 14s at idle to start with? Seriously don't want to create a lean condition further down the carb circuits.
A return line is not necessary I didn't know that, I guess i really did grow up with fuel injection!
A return line is not necessary I didn't know that, I guess i really did grow up with fuel injection!
You should inject that 427 then! I grew up on carbs and I'm trying hard to resist injection.
Safety Car
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From: St Louis MO
Thread Starter
Race Director
Joined: May 2002
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From: Check this riff........
Le Mans Master
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From: League City Tx
If the fuel pressure is too much higher then 7-9psi, the fuel will bleed past the needle and seat and over flow the float bowls. Sometimes it may not spew out the vent, sometimes it might but either way that extra fuel will get into the engine through the boosters.
Get your wide open throttle AFR in the 12.8-13.0 range, leave your cruise/part throttle in the 13.2-13.6 range and the idle AFR can also be in the mid 13s when HOT. Keep in mind though if you set the carb up to deliver a 13.5 AFR at idle when hot, when you start it cold its not going to want to idle well because the AFR will be in the 14.0-14.5 range. UNLESS you have a functioning choke. If you set the AFR to say, 13.0-13.2 or so at idle when its cold so it will idle well without having to throttle it, its going to be rich when hot, like in the 12.5-1 range, HENCE the smell, burning eyes etc.
LM1 dataloggers rule. Its like the toy you can never put down when you start tuning.
Get your wide open throttle AFR in the 12.8-13.0 range, leave your cruise/part throttle in the 13.2-13.6 range and the idle AFR can also be in the mid 13s when HOT. Keep in mind though if you set the carb up to deliver a 13.5 AFR at idle when hot, when you start it cold its not going to want to idle well because the AFR will be in the 14.0-14.5 range. UNLESS you have a functioning choke. If you set the AFR to say, 13.0-13.2 or so at idle when its cold so it will idle well without having to throttle it, its going to be rich when hot, like in the 12.5-1 range, HENCE the smell, burning eyes etc.
LM1 dataloggers rule. Its like the toy you can never put down when you start tuning.
