KyleF

So who wants to play the what if game. On a TPI, 88 to be exact, say you disconnect the EST wire and you are setting the timing. Distributor clamp slightly lose, idle around 600RPMs and you see the timing mark jumping around a few degrees. What would you assume is going on? Bad timing chain, bad distributor gear, bad ICM or is there something else that could be in play?

BlowerWorks

Perhaps normal for a V8 ?

383vett

Your pickup on #1 might be contacting an adjacent wire.

aklim

Cheap inductive clamp? BTDT. It probably picked up signal from #3.

3D-Aircrew

You can check the play on the timing chain by putting a socket on the crank and rotating it clockwise until the engine turns to the zero mark on you timing gauge. With the distributor cap removed then turn the engine counterclockwise until the rotor just begins to move. Read the degrees on the gauge and there is a spec for that that I don't remember but higher is bad.

KyleF

Is it? I have never seen it. Maybe I should be more specific when I say jumping around. Maybe +/- 2°.

Yes, not the most expensive timing light

Yes, I plan on checking the timing chain, but... the engine has only 60K miles on it and nothing on the car says it lived a hard life before I got it. Just creating a list of things that could potentially be going on in the ignition system. Have a WTF going on with the idle/starting. I don't expect to find a bad timing chain, but who knows. Have to check it to know for sure.

I know I have good fuel pressure (as posted in a previous thread about the idle)
The scan of the MAF values at the ECM through the ALDL shows expected g/s
IAC counts are doing their thing so It would appear the IAC is doing it's job (and its fairly new... yes car ran OK with it for a time before this issue)

So now onto questions about the ignition system.

Not that I have fully ruled out the injectors (Non original), but just going through the checks as I have time to spend with the car. I just remember last year while doing an IAC re-learn procedure and giving it a basic health check, I checked the timing and the mark would not stay where expected on the timing tab. I do not remember my older 86 trans Am with a TPI doing this when I checked it's timing, but that has been years.

Hot Rod Roy

Regarding the timing "jumping around": Since the hold-down clamp for the distrib. is loose, the distrib. can move a little bit in the mounting bore of the intake manifold. This "wiggle" will cause the timing to jump around a little bit. Tighten the hold-down clamp, so the distrib. is snug in the clamp.

KyleF

Maybe a bit redneck, but I have always left it loose enough to move by hand so I could watch the mark while moving it. Loose may be overstating it a bit, but I can leave it tight and confirm similar behavior or lack there of.

SLVRSHRK

Could be play in the distributor shaft. Due to the nature of the mesh between the gears on the distributor shaft and the cam, vertical play in the shaft will translate into a change in rotation, or timing shift. (Good example of this is how you have to rotate the distributor shaft as you insert ot into the block) Due to the pulses in the engine from the cylinders firing, the speed of the valvetrain is not constant, rather it varies with these pulses of power, and when it pulses, it will pulse the distributor shaft, and the shaft will try and bounce. If you pull the distributor out of the car their should be about .015" of spacing between the gear and the housing. Too much space and you will see the timing jump around due to the distributor shaft moving up and down. Pretty common issue, I have seen it on both my 78 and 86. Correctly shimming the distributor is just one of those things that should be done to ensure your engine is running as good as it can. Timing was always rock solid after doing this.

You can buy shim packs at the major suppliers like Summit, Jegs, etc.

It's an easy thing to correct, just remember to mark your distributor gear and shaft when you remove the gear to install shims. While the roll pin holding it on is centered, it is not centered between the gear teeth, and putting the gear on 180 degrees out change your timing. So pay attention when you disassemble and it will go back together exactly how you took it apart.

As I said above, this may or may not be your issue, but I've always considered it best practice, so it wouldn't be wasted effort to check and correct if there is an issue.

Best of luck

84 4+3

Mine was mostly endplay from it sliding up and down because of the slight chop. Even a smooth idle bounces around 50 degrees and that's more than enough to make it jump a bit at idle. What's it look like at 800. Bet it holds steady. Mine does anyway...