Varying spark, out of ideas

[Traxfish]

I've had for a while a problem with varying spark timing and it's been getting worse, varying about 3-4 deg each rotation. Now the engine runs pretty poorly at idle and does not get better with RPMs.

I've put on a new distributor, cap and rotor included, and just checked point gap again, setting it to .018.

I put on a new coil and new wires.

I thought I had diagnosed a timing chain with weak stretch, as it had all around low compression (106-122 psi) and varying spark timing. I read of a way to test chain stretch without removing the cover by counter rotating engine twice stopping at TDC, then rotating the engine until the distributor rotor moves and measure the degrees it took. Supposedly 7 is ideal, 10 is limit of acceptable, and I measured 12 degrees. So I then did a timing chain replacement and the engine is not running better. I've not checked compression or timing chain stretch since however.

What else could be the culprit?

 

[cudamark]

Any backlash in the drive gear? Pull the distributor and use a large screw driver the twist the gear back and forth to see if it has excessive play. don't get carried away twisting the gear, or, it might pop out and move a cog or two. Just take up the lash if it has any. You can start by just rotating the rotor back and forth just taking up the lash too. There will be some, but, there shouldn't be a lot.

 

[Scody21]

Timing chain. I had the same issue after about 25-30 hours on my motor. Changed out the Cloyes true roller to a Pro Gear from Hughes. The issue went away.

 

[4spdragtop]

Wouldn't timing chain slop show up if you moved crank back n forth and watching the rotor? If sloppy chain rotor would hardly move.

 

[Traxfish]

Wouldn't timing chain slop show up if you moved crank back n forth and watching the rotor? If sloppy chain rotor would hardly move.

That's how I tested and diagnosed a stretched timing chain. It did not fix my timing issue though.

 

[Traxfish]

Any backlash in the drive gear? Pull the distributor and use a large screw driver the twist the gear back and forth to see if it has excessive play. don't get carried away twisting the gear, or, it might pop out and move a cog or two. Just take up the lash if it has any. You can start by just rotating the rotor back and forth just taking up the lash too. There will be some, but, there shouldn't be a lot.

It had very little movement.

 

[cudamark]

Maybe you got a bad distributor. Wouldn't be the first time I've seen that. Any sort of point plate movement can cause it.

 

[Scody21]

Wouldn't timing chain slop show up if you moved crank back n forth and watching the rotor? If sloppy chain rotor would hardly move.

There’s a “drive” side of the chain and a “slack” side of the chain. So moving the crank back and forth is not going to really show you how much slack you have. Best is to pull the cover and look at it with a degree wheel and check the amount of movement. If the timing is jumping 3-5 or more degrees, I bet it’s a chain..

 

[halifaxhops]

OK typical bad distributor ground with points. Bet one surface is painted. The hold down will not do a good ground. Easy check ohm meter to a good ground. 1 ohm or less is what you want.

 

[Traxfish]

OK typical bad distributor ground with points. Bet one surface is painted. The hold down will not do a good ground. Easy check ohm meter to a good ground. 1 ohm or less is what you want.

I've had the thought of a bad ground but am unsure how exactly to check it. Could you go into more detail on how to check it? I am unsure of what should be grounding to what inside the distributor.

 

[Bewy]

Is it a big block?

 

[Desoto291hemi]

I've had the thought of a bad ground but am unsure how exactly to check it. Could you go into more detail on how to check it? I am unsure of what should be grounding to what inside the distributor.

The ground of the distributor will not have any effect on cylinder compression .
Your compression numbers are very low,,,,across the board .
I believe that is where you need to give your attention .

If the cylinders are sealing,,(,rings and valves,),,,,then it should be camshaft timing related .
If it ran good before,,,,and now it doesn’t,,,,check your cam timing components .

Tommy

 

[halifaxhops]

I've had the thought of a bad ground but am unsure how exactly to check it. Could you go into more detail on how to check it? I am unsure of what should be grounding to what inside the distributor.

OK the points and condenser are mounted to a metal advance plate which is screwed to the housing. They have to be grounded to work properly you have a classic symptom of a bad ground the timing is the spark "walking" Pretty easy check take a ohm meter and check if you have a good ground one ohm or less between the distributor and the block itself. Seen the block painted, just a fiber gasket, distributor painted. The hold down clamp is hit or miss, has to be grounded both to the block and the distributor. Main reason newer distributors use a o ring, older say dual points have to have a hard-to-find metal gasket to insure the ground. Only on points electronic distributors do not need a ground at all. Hope it helps

 

[Traxfish]

The ground of the distributor will not have any effect on cylinder compression .
Your compression numbers are very low,,,,across the board .
I believe that is where you need to give your attention .

If the cylinders are sealing,,(,rings and valves,),,,,then it should be camshaft timing related .
If it ran good before,,,,and now it doesn’t,,,,check your cam timing components .

Tommy

I do need to run the test again, and use the oil check to verify if it's the pistons rings. The heads were rebuilt in 2021 by a engine building shop including the valves and installing hardened seats so I am pretty confident they are good.

I read that low compression numbers across the board are likely a symptom of bad valve timing via a stretched chain, as bad rings would likely yield more inconsistent numbers. I replace the gears and chain myself and have yet to run a compression test again.

 

[Traxfish]

Is it a big block?

Stock 318

 

[RustyRatRod]

Stock 318

Finally after 15 posts. Thanks. Have you checked the intermediate shaft bushing for wear?

 

[TrailBeast]

Stock 318

Is it possible that vacuum advance is coming in early enough to cause this?
I assume you are disconnecting it when checking timing.

Mechanical advance can also do this if you have a broken advance spring or two.

 

[67Dart273]

Is it possible that vacuum advance is coming in early enough to cause this?
I assume you are disconnecting it when checking timing.

Mechanical advance can also do this if you have a broken advance spring or two.

DAMN TB!!! I just sitting hear with breakfast and WAS JUST GONNA ASK if maybe the OP had the vacuum connected to manifold vacuum.

 

[Traxfish]

DAMN TB!!! I just sitting hear with breakfast and WAS JUST GONNA ASK if maybe the OP had the vacuum connected to manifold vacuum.

I did not. It's set per stock on the BBD, which is ported, and I disconnect and cap the vacuum advance when setting timing.

 

[Traxfish]

Finally after 15 posts. Thanks. Have you checked the intermediate shaft bushing for wear?

I have not. If I am understanding correctly, this is the distributor drive shaft bushing inside the block by the back of the cam, correct? I check the teeth for excessive wear by using a screwdriver to rotate that gear as described by another poster and found little rotational movement. The gear did rock back and fourth a bit which I am now thinking could indicate wear on the bushing.

 

[TrailBeast]

I have not. If I am understanding correctly, this is the distributor drive shaft bushing inside the block by the back of the cam, correct? I check the teeth for excessive wear by using a screwdriver to rotate that gear as described by another poster and found little rotational movement. The gear did rock back and fourth a bit which I am now thinking could indicate wear on the bushing.

I also mentioned the possibility of mechanical advance springs being damaged and letting the flyweights bounce around.

 

[Inertia]

Isolate your inductive timing lite pick-up away from any other plug leads.
The new hi-volt coils/wires can interfere with "pulses" .

 

[Traxfish]

I also mentioned the possibility of mechanical advance springs being damaged and letting the flyweights bounce around.

It's a new distributor, and I haven't messed with any of the mechanical advance inside of it, but not impossible.

 

[AJ/FormS]

Pop the cap, and check to see that the rotor snaps back when you turn it and let go.
If the rotor does not snap back, or does not snap to the same parking spot each time, then you gotta fix that first.
But if it does;
Defeat the Vcan, loosen the Distributor hold-down, get the thing running, then reach over to the distributor and grab the V-can and pull on it until the rpm don't go no higher. Reduce the curb idle screw to get the engine down to about 500/600 rpm.
NOW, check the timing.
If
the balancer has not slipped and
the cam-timing is close enough to correct, and
the stinking low-speed circuit is working;
Then
I would expect to see 25>maybe 30 degrees.
If this is not the case, My money is on a slipped ring on the balancer. Verify the TDC mark is actually synchronized to TDC #1piston. I don't care what stroke it's on, the engine runs.

However, if your compression is truly as low as you say it is (106>122), that engine is toast anyway. At your elevation, the pressure should be closer to say 140psi. I assume you have verified it with a second gauge.

Of course NONE of this addresses the varying timing. While 3>4 degrees at idle sounds bad, it's not that big deal on your engine.
Instead, defeat the Vcan, and rev it up until it stops advancing, it should stop around 3500; then see what it is doing. But if this takes more than 4000 rpm, and/or your rod bearings are ancient, IDK; maybe you don't want to go there.
If it levels out at some number close to 35 degrees, great. If not, then change the distributor until it does. If it runs there with little to no variance, then just drive it.
Bu if it is still jumping around, IDK. Cuz by 3500, the valve springs are dragging on the cam, and all slack in the distributor drive system should have been eliminated. My only thought, in this situation, is that the oil-pump drive is jumping up and down. That's not supposed to be happening at 3500. But it could at idle.

 

[Traxfish]

OK the points and condenser are mounted to a metal advance plate which is screwed to the housing. They have to be grounded to work properly you have a classic symptom of a bad ground the timing is the spark "walking" Pretty easy check take a ohm meter and check if you have a good ground one ohm or less between the distributor and the block itself. Seen the block painted, just a fiber gasket, distributor painted. The hold down clamp is hit or miss, has to be grounded both to the block and the distributor. Main reason newer distributors use a o ring, older say dual points have to have a hard-to-find metal gasket to insure the ground. Only on points electronic distributors do not need a ground at all. Hope it helps

Tested between the contact plate in the distributor and bare metal on the block and got 0.8 ohm.