Help with a high idle in Park and N
December 6th, 2017, 07:20 PM
#1
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Join Date: Sep 2014
Location: Central Mass
Posts: 174
Help with a high idle in Park and N
Hi folks,
Looking for help with an intermittent idling issue I'm having.
Car is a 68 442 with a 455 swap, th 400, power brakes/steering & ac. Older 600 CFM Holley carb with vacuum secondaries.
It starts just fine, chugs a bit until the fast idle kicks in. Once fast idle is off it runs fine but wants to idle at about 1000 or 1100 in Park and Neutral. Idles at 650 in gear, runs and drives fine but wants to stumble a bit with hard acceleration.
I recently had a no-starting issue which members here were VERY helpful with. I ended up swapping plugs, points, wires, condenser, and coil. I am almost certain the points were the primary issue, but plugs were fouled as well and, as the car is coming out of a long hibernation, I did everything. It fired right up. Then set dwell, timing, and idle.
Potential culprit: Also picked up a new vacuum advance canister as my old one was gummed up, stampings on these indicated they adjusted for vacuum across the same range (when new obviously). I don't remember what the range was, but I was pretty thorough at the time.
I did not notice this problem when I was doing the tune up, and the car idled at 850 in park. Since then it has become more and more common, and seems to be happening all the time now. At the tune up, initial timing was set to 8 @ 1100 rpm. Pulls about 16 psi in gear. Had to lean things out a lot (only about a half turn open on both idle screws) to get my highest vacuum.
Looking for help with an intermittent idling issue I'm having.
Car is a 68 442 with a 455 swap, th 400, power brakes/steering & ac. Older 600 CFM Holley carb with vacuum secondaries.
It starts just fine, chugs a bit until the fast idle kicks in. Once fast idle is off it runs fine but wants to idle at about 1000 or 1100 in Park and Neutral. Idles at 650 in gear, runs and drives fine but wants to stumble a bit with hard acceleration.
I recently had a no-starting issue which members here were VERY helpful with. I ended up swapping plugs, points, wires, condenser, and coil. I am almost certain the points were the primary issue, but plugs were fouled as well and, as the car is coming out of a long hibernation, I did everything. It fired right up. Then set dwell, timing, and idle.
Potential culprit: Also picked up a new vacuum advance canister as my old one was gummed up, stampings on these indicated they adjusted for vacuum across the same range (when new obviously). I don't remember what the range was, but I was pretty thorough at the time.
I did not notice this problem when I was doing the tune up, and the car idled at 850 in park. Since then it has become more and more common, and seems to be happening all the time now. At the tune up, initial timing was set to 8 @ 1100 rpm. Pulls about 16 psi in gear. Had to lean things out a lot (only about a half turn open on both idle screws) to get my highest vacuum.
December 6th, 2017, 08:46 PM
#2
CH3NO2 LEARN IT BURN IT
Join Date: Jul 2010
Location: Land of Taxes
Posts: 4,929
Ill take a stab as I have some time:
Sticking and or intermittent high idle problems are likely something has gone awry on the choke mechanism or the other linkages on and to the carb. Disconnect the throttle cable and move it. It should feel smooth. Actuate the carb throttle with the cable disconnected. The throttle shaft should feel smooth. Look at the TH400 kick down switch above the gas pedal for smooth movement.
"Potential culprit: Also picked up a new vacuum advance canister as my old one was gummed up, stampings on these indicated they adjusted for vacuum across the same range (when new obviously). I don't remember what the range was, but I was pretty thorough at the time".
You need to purchase an adjustable travel (not rate) vacuum advance canister. Try 8 or 10* of vacuum advance. Plug it into a manifold unported vacuum source.
I did not notice this problem when I was doing the tune-up, and the car idled at 850 in park.
Bring this down to 650-700RPMs, 850 is too high unless you have an aggressive cam?
"Since then it has become more and more common and seems to be happening all the time now. At the tune up, initial timing was set to 8 @ 1100 rpm".
Way too high RPMs to be setting the timing, the centrifugal advance could be starting to advance.
"Pulls about 16 psi (Hg or vacuum not PSI) in gear. Had to lean things out a lot (only about a half turn open on both idle screws) to get my highest vacuum".
Take the vacuum reading at idle in park. With a normal cam and zero vacuum leaks with a healthy engine, carb and distributor, you should be at 18-22 Hg of vacuum with the right timing and AF mixture.
To recap:
1. Set curb idle to 650 with a warm engine and choke fully disengaged.
2. Set base timing to 10-12*BTDC with vacuum advance blocked off.
3. Hook the vac advance back up. Here is where you want the vacuum advance to bring in another 8-10* at curb idle. This is where an adjustable can comes in handy.
4. Now set the air-fuel on the carb to obtain the highest vac reading possible.
5. Now figure out what the centrifugal advance is doing. At what RPM does it start to advance and at what RPM does the total advance happen? The cent adv should be "all in" by 2500-2700ish RPMs for a total timing of 36-40ish degrees of timing at that final RPM. You may have to go back n forth between the AF mix and timing to obtain the highest possible vacuum reading. This is called tuning by vacuum.
These numbers are a general rule of thumb for a 9-10.5:1 compression Olds small or big block in stock or slight performance mode at or near sea level on non-ethanol 93 or better octane gas. Back off on the base timing if you're in the mountains or like 87 octane or hear loaded detonation (ping under load).
This is called super tuning. Do small tweaks on each system keeping track of where you started.
There are services where you can send your distributor out to get it tuned. Highly recommended. Same for the carb.
Road test I guarantee it will respond. Make sure you eliminated all vacuum leaks and the carb is operating as intended. Read the plugs after a few 100 miles.
Sticking and or intermittent high idle problems are likely something has gone awry on the choke mechanism or the other linkages on and to the carb. Disconnect the throttle cable and move it. It should feel smooth. Actuate the carb throttle with the cable disconnected. The throttle shaft should feel smooth. Look at the TH400 kick down switch above the gas pedal for smooth movement.
"Potential culprit: Also picked up a new vacuum advance canister as my old one was gummed up, stampings on these indicated they adjusted for vacuum across the same range (when new obviously). I don't remember what the range was, but I was pretty thorough at the time".
You need to purchase an adjustable travel (not rate) vacuum advance canister. Try 8 or 10* of vacuum advance. Plug it into a manifold unported vacuum source.
I did not notice this problem when I was doing the tune-up, and the car idled at 850 in park.
Bring this down to 650-700RPMs, 850 is too high unless you have an aggressive cam?
"Since then it has become more and more common and seems to be happening all the time now. At the tune up, initial timing was set to 8 @ 1100 rpm".
Way too high RPMs to be setting the timing, the centrifugal advance could be starting to advance.
"Pulls about 16 psi (Hg or vacuum not PSI) in gear. Had to lean things out a lot (only about a half turn open on both idle screws) to get my highest vacuum".
Take the vacuum reading at idle in park. With a normal cam and zero vacuum leaks with a healthy engine, carb and distributor, you should be at 18-22 Hg of vacuum with the right timing and AF mixture.
To recap:
1. Set curb idle to 650 with a warm engine and choke fully disengaged.
2. Set base timing to 10-12*BTDC with vacuum advance blocked off.
3. Hook the vac advance back up. Here is where you want the vacuum advance to bring in another 8-10* at curb idle. This is where an adjustable can comes in handy.
4. Now set the air-fuel on the carb to obtain the highest vac reading possible.
5. Now figure out what the centrifugal advance is doing. At what RPM does it start to advance and at what RPM does the total advance happen? The cent adv should be "all in" by 2500-2700ish RPMs for a total timing of 36-40ish degrees of timing at that final RPM. You may have to go back n forth between the AF mix and timing to obtain the highest possible vacuum reading. This is called tuning by vacuum.
These numbers are a general rule of thumb for a 9-10.5:1 compression Olds small or big block in stock or slight performance mode at or near sea level on non-ethanol 93 or better octane gas. Back off on the base timing if you're in the mountains or like 87 octane or hear loaded detonation (ping under load).
This is called super tuning. Do small tweaks on each system keeping track of where you started.
There are services where you can send your distributor out to get it tuned. Highly recommended. Same for the carb.
Road test I guarantee it will respond. Make sure you eliminated all vacuum leaks and the carb is operating as intended. Read the plugs after a few 100 miles.
Last edited by droldsmorland; December 6th, 2017 at 08:50 PM.
August 11th, 2022, 06:39 PM
#4
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Thread Starter
Join Date: Sep 2014
Location: Central Mass
Posts: 174
Old post, but I did find a simple solution to this one: Holley Carbs have a little weight in the end of the fast idle cam that helps it to drop down as the motor warms up/choke comes off and you actuate the accelerator. The weight in mine must have fallen out when I rebuilt the carb. Without the weight it was sticking, bouncing around, and coming on and off, causing the racing engine in park/neutral.
Replaced it with a new one and the problems went away.
That was a cylinder head rebuild, a timing chain, a cam, some roller rockers, a water pump, and two carbs ago... whole different set of problems now!
Replaced it with a new one and the problems went away.
That was a cylinder head rebuild, a timing chain, a cam, some roller rockers, a water pump, and two carbs ago... whole different set of problems now!
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