Vacuum Advance info
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Vacuum Advance info
Found a thread yesterday that explained how to adjust the vacuum advance. The thread explained setting the base timing, and turning a screw in the vacuum advance to adjust sensitivity. Thought I added it to my favorites but guess I goofed.
Anyone able to help with locating that information again?
I'm having problems with the 400CID pinging at about 65 mph. Timing has been retarded and moved pinging problem from 45 to 65 mph. This thread sounded like it might be the answer and I can't find it anywhere.
Anyone able to help with locating that information again?
I'm having problems with the 400CID pinging at about 65 mph. Timing has been retarded and moved pinging problem from 45 to 65 mph. This thread sounded like it might be the answer and I can't find it anywhere.
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Found the allen screw, not sure it's gonna help, I think the vacuum advance is seriously malfunctioning. Next 2 questions are 1) can I get by with the vacuum disconnected, no vacuum advance? and 2) can I replace the vacuum advance or do it take a whole new distributor?
Beartracks, If I can get this beast to cooperate it's coming to your part of the country. My son lives in Albuquerque and I'm trying to get it in condition to bring out to him over Christmas.
Beartracks, If I can get this beast to cooperate it's coming to your part of the country. My son lives in Albuquerque and I'm trying to get it in condition to bring out to him over Christmas.
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
The engine will run without a vacuum advance, but you will notice a severe lack of power and drop in gas mileage. Have you tried running with zero vacuum advance, then slowly stepping it in until it accelerates smoothly with no pinging? And is your base timing set to the specification on the emissions sticker?
You can buy replacement vacuum advances. All that holds them on there are two long screws on the outside, and I beleive a C-clip holds the rod to a pin on the baseplate inside the distributor.
You can buy replacement vacuum advances. All that holds them on there are two long screws on the outside, and I beleive a C-clip holds the rod to a pin on the baseplate inside the distributor.
Lead Driver
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From: Albuquerque
If you have a timing light you should be able to see the timing retard when you pull the vacuum hose off the vacuum can. If not you can tell it slows down a little. Also check that the centrifugal advance is free and not stuck.
ABQ is about a mile above sea level so if you are comming from lower altitude you would normally increase the timing a little.
ABQ is about a mile above sea level so if you are comming from lower altitude you would normally increase the timing a little.
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From: MI
Do you still have your EGR hooked up? With out EGR you will have a difficult time setting up a vacuum advance and not have it detonate. Is the engine stock. With out vacuum advance you will notice no lack of power or acceleration some loss in fuel economy might be noticed. Vacuum advance only works during part throttle steady state conditions. The vacuum advance should not be working at idle. If it is you have it hooked to manifold vacuum it shoud be hooked up to a ported vacuum. You probably need your distributor recurved to run best with the new style gas they have today.
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From: spartanburg, SC
"The engine will run without a vacuum advance, but you will notice a severe lack of power and drop in gas mileage."
I didn't lose any power when i unhooked it from two different trucks. I think its just there to work with EGR.
I didn't lose any power when i unhooked it from two different trucks. I think its just there to work with EGR.
Last edited by F150daniel; Dec 14, 2004 at 11:22 AM.
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MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
Ignition timing is advanced in two ways: centrifugal advance and vacuum advance. Centrifugal advance advances the timing as RPM increases and vacuum advance advances the timing as the engine comes under load and ported vacuum increases. Both advance mechanisms give the fuel\air a little more time to burn since the combustion takes the same amount of time no matter what the engine conditions are.
While different engines respond differently to the absence of vacuum advance, the idea is that they make the distributor throw the spark in time so that the combustion hits to give the right amount of power under a load. And the less you have to keep on the gas to get the same amount of power, the less gas you are using.
While different engines respond differently to the absence of vacuum advance, the idea is that they make the distributor throw the spark in time so that the combustion hits to give the right amount of power under a load. And the less you have to keep on the gas to get the same amount of power, the less gas you are using.
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Senior User
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From: MI
Originally Posted by F150daniel
"The engine will run without a vacuum advance, but you will notice a severe lack of power and drop in gas mileage."
I didn't lose any power when i unhooked it from two different trucks. I think its just there to work with EGR.
I didn't lose any power when i unhooked it from two different trucks. I think its just there to work with EGR.
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From: MI
Originally Posted by fmc400
Ignition timing is advanced in two ways: centrifugal advance and vacuum advance. Centrifugal advance advances the timing as RPM increases and vacuum advance advances the timing as the engine comes under load and ported vacuum increases. Both advance mechanisms give the fuel\air a little more time to burn since the combustion takes the same amount of time no matter what the engine conditions are.
While different engines respond differently to the absence of vacuum advance, the idea is that they make the distributor throw the spark in time so that the combustion hits to give the right amount of power under a load. And the less you have to keep on the gas to get the same amount of power, the less gas you are using.
While different engines respond differently to the absence of vacuum advance, the idea is that they make the distributor throw the spark in time so that the combustion hits to give the right amount of power under a load. And the less you have to keep on the gas to get the same amount of power, the less gas you are using.
Vacuum advance increases as the engine load decreases. Under load if it increased to much it would burn up the engine. under load engine vacuum decreases as the load decreases the vacuum increases.
MSEE
Joined: Apr 2004
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From: Austin, TX
Manifold vacuum indeed decreases with load, but vacuum advance should be hooked up to ported vacuum right? And as the engine goes under load, ported vacuum increases a bit from idle and then drops off as you approach WOT. Isn't that true?
Last edited by fmc400; Dec 14, 2004 at 04:10 PM.
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That is true of manifold vacuum, but vacuum advance should be hooked up to ported vacuum right? And as the engine goes under load, ported vacuum increases from idle and then drops off. Is that true?
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
Yes that sounds right, ported and manifold vacuum come close. And as OffOrWFO said, the gauge jumps from zero inches to manifold vacuum. And during that short period, when ported vacuum increases from zero to nearly manifold vacuum, is where you want vacuum advance to take part.
Sitting at idle, the vacuum advance is not doing anything and your timing is at its initial setting. You step on the throttle, the vacuum advance pulls the baseplate a tad and your timing advances. You push it to WOT and pull hard, the vacuum advance lets it back. That's because the vacuum advance is hooked up to ported vacuum. It starts at nothing, goes up some as you get on the gas, then drops back down.
Sitting at idle, the vacuum advance is not doing anything and your timing is at its initial setting. You step on the throttle, the vacuum advance pulls the baseplate a tad and your timing advances. You push it to WOT and pull hard, the vacuum advance lets it back. That's because the vacuum advance is hooked up to ported vacuum. It starts at nothing, goes up some as you get on the gas, then drops back down.
Last edited by fmc400; Dec 14, 2004 at 04:49 PM.
Lead Driver
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From: Albuquerque
Think of it this way. Vacuum advance does just what it says. When it sees vacuum it increases the timing. Under load the vacuum reduces which reduces the timing to prevent knock. Ported/full is only diferent at idle although you need to set up the intial and centrifigal curve to match wichever you use.
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Okay, back to this issue a little after the fact. The engine has NO pollution control stuff on it. I've been running with the vacuum line disconnected from the advance. Had some problems with the new carburetor and took it to the mechanic today. He hooked the vacuum line back up. ABSOLUTELY MAJOR KNOCKING. Pulled the vacuum line and the problem stopped. Decided to stick the allen wrench in and crank it out a couple of turns as recommended. OOOOOPPPPS. The wrench goes all the way to the back of the module and doesn't engage the screw. Obviously the advance module is history.
My question now becomes, since there is no pollution stuff, including the EGR, on the motor, is there any point in messing with replacing the module?
Haven't really noticed that there has been any change in available power or performance. Just got the new carb set up so haven't had a chance to check my mileage (was getting about 11 mpg - highway - before the carb decided to call it quits, 8 after. Didn't notice any change in mileage with the advance unhooked.)
My question now becomes, since there is no pollution stuff, including the EGR, on the motor, is there any point in messing with replacing the module?
Haven't really noticed that there has been any change in available power or performance. Just got the new carb set up so haven't had a chance to check my mileage (was getting about 11 mpg - highway - before the carb decided to call it quits, 8 after. Didn't notice any change in mileage with the advance unhooked.)