Timing jumps around when using timing light

[OkieFishMan]

Alright, so I broke out the timing light to verify the timing on my engine. It is kinda jumping around and next to impossible to adjust or set timing. Could this be a symptom of a worn distributor? Or something else?

 

[1987 GMC Jimmy]

Did you unplug the vacuum advance to set the timing? If so, it very could be a worn distributor bushing/gear or a timing chain stretching out if the motor has higher miles, but the "next to impossible to adjust" suggests that the advance is connected.

 

[OkieFishMan]

Yeah, I unplugged the vacuum advance. I don't think it is the timing chain. I had a distributor wear out on a k20 with a small block, and the weights in the distributor had worn through the posts that they hand on, so I was not getting my full advance at higher rpm. This is behaving differently than that, but you never know.

I found a cardone remanufactured dizzy for $47 at autozone, so I went ahead and ordered it. I figure it can't hurt. And online it didn't even charge me a core, so I will hang on to the old one too.

 

[74 Shortbed]

What's the condition of the plug wires???.

 

[MrMarty51]

Might be one of them "Delayed Saprk Timing" units. BWAHAAAAAHAAAAHAAAAAHAAAAaaaaaa. LOL

 

[74 Shortbed]

Might be one of them "Delayed Saprk Timing" units. BWAHAAAAAHAAAAHAAAAAHAAAAaaaaaa. LOL

 

[HotRodPC]

As 74shortbed, what's the condition of the plug wires and is this an inductive pick up timing light? What is the condition of the clamp of the timing light and wire to the clamp? If the light is not reading good you can get an erratic reading. If that's all known good, then yes it can be a worn distributor and do make sure the vac advance is disconnected. The plate maybe fluctuating.

 

[HotRodPC]

BTW, you can use another plug wire just for testing of the #1 plug wire to see if the fluctuation stops. If it's smoother on another wire, then the #1 plug wire needs replaced. Of course you'll need to find where the timing mark it is hitting and use some other marker. All you're doing here is trying to see if it smoothes out and is not erratic. You can't tune or time off of another cylinder and be accurate. This is just a atest to eliminate #1 plug wire at being defective or not.

 

[OkieFishMan]

Plug wires are all brand new. Timing light is a couple years old, but on my other pickup it worked great, no fluctuation at all.

I bet I do have one of those "delayed spark units"...........lol

Hopefully the reman dizzy I ordered will fix the prob.

Thanks guys!!