Originally Posted by Michaelgoesrawr
I should say ahead of time that this is the second time I have used a timing gun and I only have the basic knowledge needed to understand this so I apologize if I am doing anything wrong or not explaining myself clearly. I'm doing my best lol.
At idle and the gun set to zero advance it bounces from 8-10/12 degrees a few rotations and then jump to 16 degrees every few times. And when the gun is set to 8 degree advance it bounces from 0-4 a few rotations ad then jumps to 8 degrees. I don't know the advance curve but at higher rpms the timing mark advances up the curve and only deviates from the degree mark by tiny increments. I'm talking half a raised timing mark back and fourth., so it's fairly steady in my mind. And then goes back to bouncing when it hits idle.
I cant comment on the timing because he didn't say anything about not being able to set it. Just that he couldn't retard it anymore because the vacuum advance was hitting the block. Wouldn't moving the number one plug to the number five cap stop and changing all of the plugs one stop change where tdc is and mess up the timing?
I rolled the Jeep at 35mph that the engine was in prior to my current Jeep. Could that much bounce cause problems?
I have seen the type of timing variation you describe before, and IIRC did not affect emissions testing or drivability. One thing I am curious about, however, is whether the rpm is varying when the timing bounces. Regardless, I am not sure I would worry about this too much.
You can't mess up TDC. Each piston hits TDC once every crankshaft revolution. Ignition timing is which spark plug lights up relative to where each piston is in terms of the four stroke cycle and the firing order.
It is completely arbitrary as to which post in the cap you select as #1. The only thing that can matter is if, as on yours, the post selected prevents the rotation of the distributor to set the timing where you want it because the vacuum advance hits the block.
When the distributor vacuum advance hit the block when the timing was being set, any mechanic worth his or her salt woould just have selected another post as #1. It takes all of 2 minutes. That he or she did not, to me, and without hearing the other side of the story, tells me your mechanic was ignorant or dishonest (wanted to sell you a new distributor).
Here is the absolute easiest way to plug spark plug wires into the cap. With the motor off, line up the line on the crankshaft damper to the mark on the timing cover. Then pull the dist cap. See which post the rotor is pointing to. Select that as #1. In the direction of rotor rotation, plug in the rest of the firing order: 1 5 3 6 2 4. Start the motor. If it starts, you are done. If it does not, then swap the wires one half of the firing order. IOW plug #1 where # 6 was, # 5 where # 2 was, and so on. Now your motor will start right up.
The reason for this is that it is a 4 stroke motor. The crank turns twice for the cam - or the rotor - to turn once. So, whenever the timing marks are lined up, you have a 50:50 chance that the motor is on the top of compression stroke (when the plug should fire) or the exhaust stroke.
Some folks say to remove #1 plug and stick your finger in the hole to feel the compression to find out if you are on the compression stroke. Some say pull the valve covers to see if both valves are closed. Methods like this are fine; they just are extra work. You cannot hurt the motor by trying to start it with the wires 180 degrees out - you are just firing on the exhaust stroke. Nothing will happen.
If your dizzy is installed way off (physical relationship of the vac advance to the block), you need to pull it out and reinstall it such that the vacuum advance is in a better spot. Usually you only have to move it a tooth. Then, follow the procedure above to rewire the cap.