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Ignition timing is set when the engine is at idle, and is advanced by changes in a control system, which system depends on which car you have.
Your timing at idle should be 12 degrees before TDC. Your check with the timing light should have shown that. The ICM computer sytem will cause the timing to advance according to engine speed and manifold vacuum (indicates engine load).
The manifold vacuum gets to the ICM via a white nylon tube that begins at the intake manifold with a 90 degree rubber elbow, runs down under the front of the engine along with the wiring harness, and connects to the ICM. The ICM has a little device on it that looks like the same things that control the flaps in the cabin heater - a/c air directions.
Check that you are getting vacuum at the ICM connection. If you pull the tube off the ICM, you may 1) kill the engine from idleing, and/or 2) have a hard time getting the tube back on. I've never done it. Blow through the tube to be sure it's clear, or see if there is vacuum at the ICM end of the tube when at idle.
A word about spark advance. It's a matter of being able to fire the mixture in the combustion chamber at the right time relative to the position of the piston.
The mixture in there does not explode, as many think. It burns, it has a flame front, and takes time from ignition to reach its maximum expansion.
The engine gets best power when that maximum expansion is reached when ALL of the expansion can be utilized to push the piston downwards. (Note that if that maximum expansion point is reached too soon, you get a knock because the expansion is trying to push the piston down the backwards way. Thus the knock sensor momentarily reduces the spark advance.)
To get the best use of the expansion, the ignition must begin before the piston up to TDC. How much before? Well, the flame front speed is pretty much constant, but piston speed is not. At idle speed, (750 rpm +/-) the 12 degree TDC setting is OK. As engine (and therefore piston speed) goes up, the ignition must begin farther ahead of TDC.
Thus the need for a dynamic spark advance system. Our LH Volvos use a computer to do that, and believe me it's a lot simpler and more accurate than what was before.
Use your timing light to (1) check that timing is at 12 degrees and steady at idle, and (2) check for amount of advance on acceleratin when you are NOT having the problem.
Long post, I know. Hope it helps with the timing understanding.
Good Luck,
Bob
:>)
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