Does My Distributor Need to Be Recurved?
08-18-2006, 09:47 AM
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Does My Distributor Need to Be Recurved?
I have been trying to improve sluggish performance with this car for some time and I appreciate the advice I have previously received from this forum.
Here is the background information on this car: The car is a 67 small block that was originally sold in California, but the K-19 smog pump has been removed. The car is a powerglide with AC. The car also has had the distributor modifed with a Petronix sytem.
When I purchased the car, the vacuum advance did not work, the 3814 carb leaked profusely and the initial timing was set at about 10 degrees ATDC. I have since installed a new vacuum advance diaphragm and sent the carb out for a rebuild. The carb has a 6.5 power valve and #70 jets installed. The stamp on the bracket of the vacuum advance bracket that I removed was B1. The replacement was a Borg Warner from advance Auto and the vaccum advance now functions.
After the above changes to the carb and vacuum advance, the car still needs to have the initial timing set at 10 degrees ATDC.
When I bring back the timing to the factory specs of 4 ATDC and adjust the fuel air mixture secrews to optimize the vacuum at 20" hg @ 850 rpms and then engage the transmission, the vacuum drops to 15 " of hg and the rpm falls to 550 rpms. When I add the AC the vacuum drops to 13" hg and the rpms falls to 450. I can get the car to move away from a stop, but I have to carefully manipulate the accelerator to keep from stalling the car. I might add that the mixture screws are approximately 1 turn out from bottom.
In order to drive the car with some degree of confidence that it will not die at a stop sign, I keep the initial timing at 10 degrees ADTC and back out the idle mixture screws to about 2 turns from bottom and increase the idle rpms to 1000, but I am compensating for a deeper problem. I also get about 10 miles to the gallon with these settings.
So after a long explanation, do I need to consider having the distributor recurved? Are there other things that I need to be investigating first?
Mark
Here is the background information on this car: The car is a 67 small block that was originally sold in California, but the K-19 smog pump has been removed. The car is a powerglide with AC. The car also has had the distributor modifed with a Petronix sytem.
When I purchased the car, the vacuum advance did not work, the 3814 carb leaked profusely and the initial timing was set at about 10 degrees ATDC. I have since installed a new vacuum advance diaphragm and sent the carb out for a rebuild. The carb has a 6.5 power valve and #70 jets installed. The stamp on the bracket of the vacuum advance bracket that I removed was B1. The replacement was a Borg Warner from advance Auto and the vaccum advance now functions.
After the above changes to the carb and vacuum advance, the car still needs to have the initial timing set at 10 degrees ATDC.
When I bring back the timing to the factory specs of 4 ATDC and adjust the fuel air mixture secrews to optimize the vacuum at 20" hg @ 850 rpms and then engage the transmission, the vacuum drops to 15 " of hg and the rpm falls to 550 rpms. When I add the AC the vacuum drops to 13" hg and the rpms falls to 450. I can get the car to move away from a stop, but I have to carefully manipulate the accelerator to keep from stalling the car. I might add that the mixture screws are approximately 1 turn out from bottom.
In order to drive the car with some degree of confidence that it will not die at a stop sign, I keep the initial timing at 10 degrees ADTC and back out the idle mixture screws to about 2 turns from bottom and increase the idle rpms to 1000, but I am compensating for a deeper problem. I also get about 10 miles to the gallon with these settings.
So after a long explanation, do I need to consider having the distributor recurved? Are there other things that I need to be investigating first?
Mark
08-18-2006, 10:01 AM
#2
Melting Slicks
08-18-2006, 10:31 AM
#3
Melting Slicks
Mark,
I am not an expert in this field but I would question the fact that the car runs so well with the timing set at 10 degrees ATDC. All of these engines are supposed to be set so the timing is BTDC not ATDC. Are you sure that the distributor isn't installed one or two cogs off? I would set the engine at TDC on the No. 1 cylinder and pull the cap on the distributor to see where it lines up. Just to make sure that the distributor is indexed correctly.
Steve
I am not an expert in this field but I would question the fact that the car runs so well with the timing set at 10 degrees ATDC. All of these engines are supposed to be set so the timing is BTDC not ATDC. Are you sure that the distributor isn't installed one or two cogs off? I would set the engine at TDC on the No. 1 cylinder and pull the cap on the distributor to see where it lines up. Just to make sure that the distributor is indexed correctly.
Steve
Last edited by 66BlkBB; 08-18-2006 at 10:33 AM.
08-18-2006, 10:45 AM
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Originally Posted by Donald #31176
Duke
08-18-2006, 01:27 PM
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Originally Posted by mpate
After the above changes to the carb and vacuum advance, the car still needs to have the initial timing set at 10 degrees ATDC. Mark
The factory initial timing setting for your combination was 4* ATDC, but the distributor had 40* of centrifugal (in by 5100 rpm) to attempt to compensate for the retarded idle timing (which raised EGT to make the A.I.R. system more efficient at idle).
After mechanically verifying (and altering if necessary) your TDC marks, I'd set up the distributor to the same centrifugal advance specs as the #1111194 non-A.I.R. 300hp distributor (30* total centrifugal advance), set initial timing at 6*-8* BEFORE TDC, and work from there as a baseline to optimize the setup.
08-19-2006, 06:44 AM
#6
Melting Slicks
Originally Posted by JohnZ
If it REALLY shows 10* AFTER TDC to run decently, I'd seriously question the accuracy of the marks on the timing tab. Step #1 is to pull the plugs and use a piston stop tool to accurately determine true mechanical TDC and compare it to the existing marks on the timing tab. Then you have a correct baseline to work from and can diagnose it from there.
The factory initial timing setting for your combination was 4* ATDC, but the distributor had 40* of centrifugal (in by 5100 rpm) to attempt to compensate for the retarded idle timing (which raised EGT to make the A.I.R. system more efficient at idle).
After mechanically verifying (and altering if necessary) your TDC marks, I'd set up the distributor to the same centrifugal advance specs as the #1111194 non-A.I.R. 300hp distributor (30* total centrifugal advance), set initial timing at 6*-8* BEFORE TDC, and work from there as a baseline to optimize the setup.
The factory initial timing setting for your combination was 4* ATDC, but the distributor had 40* of centrifugal (in by 5100 rpm) to attempt to compensate for the retarded idle timing (which raised EGT to make the A.I.R. system more efficient at idle).
After mechanically verifying (and altering if necessary) your TDC marks, I'd set up the distributor to the same centrifugal advance specs as the #1111194 non-A.I.R. 300hp distributor (30* total centrifugal advance), set initial timing at 6*-8* BEFORE TDC, and work from there as a baseline to optimize the setup.
JohnZ
You enlightened me again with this. I did not know that the CA smoggers were set up to fire after TDC. What did they hope to accomplish by doing that? In everything that I have read, a well tuned properly set up engine is more efficient in burning fuel than any smog engine. It would seem to me that firing the cylinder after top dead center, coming off of compression, would rob you of the natural power of the engine and would not completely burn the fuel in the cylinder as efficiently as it would when firing under compression. I realize that there would be little difference in the actual amount of compression here but that little bit, it would seem to me, would be a cause for more exhaust emissions rather than fewer. It would also seem to me that if both engines, smogger and non, were out of tune/adjustment that the rate at which each engine put out exhaust emissions would be parallel and the smog engine would still put out more of the bad gases. But I've been known to be wrong before.
Steve
08-20-2006, 01:11 PM
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Originally Posted by 66BlkBB
#############################
JohnZ
You enlightened me again with this. I did not know that the CA smoggers were set up to fire after TDC. What did they hope to accomplish by doing that? In everything that I have read, a well tuned properly set up engine is more efficient in burning fuel than any smog engine. It would seem to me that firing the cylinder after top dead center, coming off of compression, would rob you of the natural power of the engine and would not completely burn the fuel in the cylinder as efficiently as it would when firing under compression. I realize that there would be little difference in the actual amount of compression here but that little bit, it would seem to me, would be a cause for more exhaust emissions rather than fewer. It would also seem to me that if both engines, smogger and non, were out of tune/adjustment that the rate at which each engine put out exhaust emissions would be parallel and the smog engine would still put out more of the bad gases. But I've been known to be wrong before.
Steve
JohnZ
You enlightened me again with this. I did not know that the CA smoggers were set up to fire after TDC. What did they hope to accomplish by doing that? In everything that I have read, a well tuned properly set up engine is more efficient in burning fuel than any smog engine. It would seem to me that firing the cylinder after top dead center, coming off of compression, would rob you of the natural power of the engine and would not completely burn the fuel in the cylinder as efficiently as it would when firing under compression. I realize that there would be little difference in the actual amount of compression here but that little bit, it would seem to me, would be a cause for more exhaust emissions rather than fewer. It would also seem to me that if both engines, smogger and non, were out of tune/adjustment that the rate at which each engine put out exhaust emissions would be parallel and the smog engine would still put out more of the bad gases. But I've been known to be wrong before.
Steve
08-21-2006, 12:27 PM
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Timing at 4 degrees After ( Response to John)
I was able to to verify that with the timing marks aligned and that number 1 is TDC on this former K-19 car. The timing is currently set at 4 degrees After Top Dead Center. I did note that the Vacuum Advance Can is stamped B20. Is this the correct can?
Thanks--Mark
Thanks--Mark
08-21-2006, 01:02 PM
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The "B20" can (VC-1765) is fully-deployed at 12" Hg. vacuum, and is generally recommended for the L-79 350hp engines which have idle vacuum of 13"-15" Hg.; the can generally recommended for the 300hp engines, which have idle vacuum of 16"-18" Hg. is the one stamped "B1" (VC-680), which is fully-deployed at 16" Hg. The "B20" can you have installed will work, but may be a bit aggressive for your combination.
There's no reason your combination shouldn't run a LOT better with initial timing set at 6*-8* BEFORE TDC (set with the vacuum advance line disconnected and plugged), and with the vacuum advance line connected to a full manifold vacuum source (not a "ported" source).
You need to determine how much centrifugal advance your distributor is providing first, as its original specs provided 40* of advance, which is way excessive when combined with 6*-8* BTDC initial timing (would be 46*-48* total timing vs. the desired 36*-38*). The only difference between a K-19 engine and the standard engine was the distributor centrifugal advance curve and the carburetor calibration - with those two things set back to the stock non-A.I.R. calibrations, it should run like a top.
There's no reason your combination shouldn't run a LOT better with initial timing set at 6*-8* BEFORE TDC (set with the vacuum advance line disconnected and plugged), and with the vacuum advance line connected to a full manifold vacuum source (not a "ported" source).
You need to determine how much centrifugal advance your distributor is providing first, as its original specs provided 40* of advance, which is way excessive when combined with 6*-8* BTDC initial timing (would be 46*-48* total timing vs. the desired 36*-38*). The only difference between a K-19 engine and the standard engine was the distributor centrifugal advance curve and the carburetor calibration - with those two things set back to the stock non-A.I.R. calibrations, it should run like a top.
08-21-2006, 01:27 PM
#10
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Originally Posted by 66BlkBB
What did they hope to accomplish by doing that? In everything that I have read, a well tuned properly set up engine is more efficient in burning fuel than any smog engine. It would seem to me that firing the cylinder after top dead center, coming off of compression, would rob you of the natural power of the engine and would not completely burn the fuel in the cylinder as efficiently as it would when firing under compression.
Lower compression=lower NOX emissions, cool down combustion with EGR=lower NOX emissions, the fact that these increased fuel consumption and robbed power were of no consequence, they just figured burn the unburned hydrocarbons and CO in the exhuast manifold and cat converter.
Doug
08-21-2006, 07:56 PM
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[QUOTE=JohnZ]The "B20" can (VC-1765) is fully-deployed at 12" Hg. vacuum, and is generally recommended for the L-79 350hp engines which have idle vacuum of 13"-15" Hg.; the can generally recommended for the 300hp engines, which have idle vacuum of 16"-18" Hg. is the one stamped "B1" (VC-680), which is fully-deployed at 16" Hg. The "B20" can you have installed will work, but may be a bit aggressive for your combination.
/[QUOTE]
John-
Neither Lars not I recommend the B1 VAC any longer. Many have tested to require 18" to fully deploy. They're boat anchors. I recommend the 15" B22 (VC1802) for 300HP w/MT. For 300 HP P/G I recommend the 12" B20, also for all BB, and consider it optional for 350 HP even though they were OE with a 8" VAC as they pull 14-15" at typical idle speed of about 750-800.
The 12" B-20 is also optional for 300 HP w/MT as all 300 HP engines in '66 and '67 had a 12" VAC, probably to accomodate PG idling in Drive.
Duke
/[QUOTE]
John-
Neither Lars not I recommend the B1 VAC any longer. Many have tested to require 18" to fully deploy. They're boat anchors. I recommend the 15" B22 (VC1802) for 300HP w/MT. For 300 HP P/G I recommend the 12" B20, also for all BB, and consider it optional for 350 HP even though they were OE with a 8" VAC as they pull 14-15" at typical idle speed of about 750-800.
The 12" B-20 is also optional for 300 HP w/MT as all 300 HP engines in '66 and '67 had a 12" VAC, probably to accomodate PG idling in Drive.
Duke
08-22-2006, 12:39 PM
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[QUOTE=SWCDuke]
Good point - I hadn't thought about the Powerglide application, as I've never owned an automatic; when it's placed in Drive, rpm drops, and manifold vacuum will also, requiring a more aggressive can (relative to manifold vacuum in neutral) to maintain full idle timing.
Originally Posted by JohnZ
The 12" B-20 is also optional for 300 HP w/MT as all 300 HP engines in '66 and '67 had a 12" VAC, probably to accomodate PG idling in Drive.
Duke
Duke
