Q: Timing Adjustment to Fix Overheating Problems
What are the steps in adjusting my timing to fix the overheating problem.
A: Here’s the deal on timing, back to basics.
First, disconnect and block off your vacuum source to the vacuum advance. Its ONLY purpose is to increase gas milage. Next, if you keep your engine at or below 800 RPM you will have no mechanical advance. So now the only timing is due to INITIAL timing. Let’s say you set the spark right at TDC, the flame takes so time to form at “explode” so the force of the explosion happens AFTER TDC, piston is already on its way down, you don’t get much power. So you ADVANCE the timing to whatever gives you the best performance, which is usually between 7-12 degrees BTDC. If you have too much timing and the spark happens too far before the piston reaches TDC, then the force is pushing against the direction the piston is moving and you get detonation or knock. This is BAAAAAD.
As your engine RPM’s increase, the amount of TIME it takes for the engine to turn say 30 degrees is much faster than at idle, but the TIME is take the flame to burst hasn’t changed. So the timing is increased further by the MECHANICAL advance. Usually adding up to 20-25 degrees of advance to the INTIAL timing, for a TOTAL timing of 32-38 degrees.
Finally, vacuum advance was added for the purpose of further increasing timing at cruise. Most times it is connected to a ported vacuum sorce. This source has NO vacuum at idle, the most slightly off idle and then decreases to zero again at Wide Open Trottle (WOT). When cruising you can add an additional 15 degrees or more of vacuum advance, bringing your timing up to 55-60 degrees BTDC. This is the one to be careful of, too much and you can get detonation. Always stick to the low side to avoid detonation. Remember, it’s just for gas milage.
One more thing to touch on. Manifold vacuum is maximum AT IDLE. All other times it acts just like Ported. If you hook your vacuum advance to manifold vacuum you will have 8-12 intial plus 15 or so vacuum for 23-27 degrees BTDC of timing AT IDLE. This will affect temp, though I’m not sure which way. I DO know, that most 1st gen birds came with a TEMP activated switch which actually switched the vacuum advance soure depending on the engine temp in order to keep it cool! The was specifically for long periods at idle.
Now, lets say your car is set up this way but you SET the timing to 12 degrees WITH the vacuum advance hooked up to manifold vacuum. That vacuum advance can is giving you 15 degrees or more of advance. That would mean your REAL INTIAL timing (without the vacuum advance) would be 3 degrees AFTER TDC, which is way to far retarded.
Bottom line, turn off the vacuum advance until you figure out the heating problem. Its ONLY purpose is to improve gas milage. Set the INITIAL timing to 8-14 degrees. See if that helps.
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Content last modified: January 15, 2014 at 9:50 pm