Timing retards on its own but distributor stays put, WTF ???

83 C20 with Goodwrench 350.
Have had engine apart recently for new bearings, gaskets and timing chain.
Runs great, but every now and then I can hear the engine laboring, gas mileage goes to crap and my performance drops to struggling.
Check timing and it's at TDC instead of the usual 12 degrees BTDC or so where it likes to run.
I used a quality dual row chain/gear set, so I know it isn't chain stretch, besides the engine runs too well for a stretched timing chain.
Distributor feels smooth spinning it by hand when not installed, shaft and bushings are not sloppy or grinding, so I am sure it's fine.
I have had to reset the timing now FIVE times on the engine and am rapidly running out of space to rotate the distributor to advance the timing back to where it should be.
I can probably advance it one more time then I won't have space to rotate the dizzy.
At first I had a bolt just a tad too long on the hold down clamp thinking that was the problem, but replaced that with a shorter bolt and thought the issue was cured.
Nope, it keeps happening and following the mechanics and theory in my head starting at the crankshaft and ending up at the plug wires, nothing seems logical as to why the timing could change like that.
I may be forced to blame this one on the space aliens if it happens again.

EDIT: I just looked at the engine and the vacuum advance chamber is almost touching the carb/intake area and there is NO room to rotate it any more if needed to advance the timing.

Had this happen once to me when installed a new timing chain, sometime down the road, the woodruff key on the crank gear rolled and wiped out the keyway on the crank until I had a bad backfire one day and then it rolled 180°. Not really sure how that happened, but sure confused me trying to troubleshoot what was going on. Another way that can happen is break the gear pin on the distributor, sometimes will leave enough of the pin to keep it mostly in place, but starts to wear a slot through the gear as it turns.

I am seriously hoping for an easy fix, but I dread yanking the distributor on this engine.
In all my years of working on cars, this engine is a ROYAL pain trying to get the distributor to drop into place with the oil pump drive after meshing the cam gear.
I am not comfortable meshing the gear and then bumping the engine through or turning it through by hand for the distributor to drop into position, it refuses to cooperate and somehow the rotor never ends up pointing at the number one position where it should be and I end up yanking the distributor and start all over.
When I went through the engine recently, I put the distributor in first, THEN installed the oil pump and oil pan just so i would not have to fight the drive not lining up.

The pin shearing sounds extremely plausible and makes sense, but can the drive gear shift FIVE times and the thing still work ?

I'd just pop the dizzy and check it over full since that's the easiest.

As for the oil pump gear, I took an old distributor and gutted it, removed the gear and made it a pre-oiler for new motors using a drill. For lining up the pump drive, I just mark the position of where my distributor slot lines up to where I want it to drop on my primer drive, drop the primer drive in the hole and rotate the shaft to line up with my mark, slightly back from it for the gear rotation as it slides in, works great.

75Monza said:

I'd just pop the dizzy and check it over full since that's the easiest.

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And you are more than welcome to come here and fight with the re-installation ...

I also suspect the goodwrench engine plant in Mexico did not grind the distributor drive gear on the cam in the proper location from the troubles I have had in the past trying to seat a dizzy.
Set harmonic balancer at 0 TDC (and I KNOW it has not slipped) drop distributor in to mesh with the gear and it's nowhere close, have had 4 distributors I had laying around in the engine, and all 4 gave me major grief trying to install.

75Monza said:

As for the oil pump gear, I took an old distributor and gutted it, removed the gear and made it a pre-oiler for new motors using a drill. For lining up the pump drive, I just mark the position of where my distributor slot lines up to where I want it to drop on my primer drive, drop the primer drive in the hole and rotate the shaft to line up with my mark, slightly back from it for the gear rotation as it slides in, works great.

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Thanks !! I will do that since I have at least three junk distributors here with bad bearings or whatever.

Check to make sure the top of the distributor shaft is not slipping on the shaft. They are pressed on and can get loose. Wandering timing is the symptom.

As far as getting the distributor to drop back in place, have you ever tried moving the pump a bit with a screwdriver until the distributor drops in easily? It's the same as what Gearhead is talking about, but without making a special tool.

Joe

I have a long screwdriver for just that purpose and tried it, the distributors still fight me all the way seating in the oil pump and meshing with the cam gear and end up with the rotor pointing at number 1 position where it belongs.
VERY frustrating.
The only thing that's different in the engine is the oil pump bushing: since the steel bushings I used to buy 20 years ago are no longer available, I had to buy a hardened steel shaft with bushing machined in.
That MIGHT be the culprit for my difficulty.
And I REFUSE to install a nylon bushing because the heat dries them out and they crack and fail.

Thanks for the tip on the shaft, I will investigate that !!!

I'm not following why it's difficult to install the dizzy. Put engine at TDC, drop dizzy in so it engages the pump. Where your rotor is pointing is your #1 position. Install #1 spark plug wire then follow the firing order. If you don't like it there, you can rotate it 180° to be on the other side. If you want #1 to be a specific spot, here's a video that should help.

No offense intended.

Camar068 said:

I'm not following why it's difficult to install the dizzy. Put engine at TDC, drop dizzy in so it engages the pump. Where your rotor is pointing is your #1 position. Install #1 spark plug wire then follow the firing order. If you don't like it there, you can rotate it 180° to be on the other side. If you want #1 to be a specific spot, here's a video that should help.

No offense intended.

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Don't get much easier than that. I got lucky by marking mine and the new one dropped right in place without having to bring up #1 TDC.

If you scribe all your parts @ TDC then let your timing move, put it back on TDC and check all the scribes you should be able to see what does not line up. helping you further narrow your problem.
You have to scribe everything.

I'm not claiming to be perfect, but I have been working on cars for the better part of 50 years and this engine is the first one to give me grief installing a distributor.
It's mind boggling.
I will do the scribe marks where things need to go and see what happens.
I bought a brand new distributor today since I am having so much trouble with the current one and have no more room to rotate it to advance the timing where it needs to be.

rotate your wire positions on the cap when u run out of room to rotate the dizzy.