Mudstud
1/2 ton status
sbc 350. Hit it with the light last night and noticed the mark on the balancer was moving around while it was running. What causes this? The distributer is tight and cannot be turned.
CK5
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timing fluctuates while running
- Thread starter Mudstud
- Start date
HEI ?..Points ?..
If it computer controlled by an ECM,it could be the ECM or the wiring causing this..or the timing was not properly set,you have to unplug a wire first in order to set the static timing properly..
On a points distributor the breaker plate can wobble around on its bushing,or the distributor shaft bushings in the housing can let the shaft wobble around and cause erratic timing on both points and HEI..
A failing pick up coil or reluctor on HEI can cause this too.
A loose timing chain may show up as a wandering timing mark also..
A long shot is the harmonic balancer may be loose on its rubber mounting and letting it slip around and show the timing mark appearing to move,while the engine is actually firing at the right spot consistently..
If it computer controlled by an ECM,it could be the ECM or the wiring causing this..or the timing was not properly set,you have to unplug a wire first in order to set the static timing properly..
On a points distributor the breaker plate can wobble around on its bushing,or the distributor shaft bushings in the housing can let the shaft wobble around and cause erratic timing on both points and HEI..
A failing pick up coil or reluctor on HEI can cause this too.
A loose timing chain may show up as a wandering timing mark also..
A long shot is the harmonic balancer may be loose on its rubber mounting and letting it slip around and show the timing mark appearing to move,while the engine is actually firing at the right spot consistently..
If the vacuum advance or ECM advance disable wire (for a computer controlled vehicle) is still connected, then the timing will fluctuate back and forth.
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Mudstud
1/2 ton status
It's HEI. Based on what 1 ton said, this sounds like it's normal.
diesel4me said "If it computer controlled by an ECM,it could be the ECM or the wiring causing this.." is that all okay? "or the timing was not properly set," I don't doubt this at all. "you have to unplug a wire first in order to set the static timing properly.." I have no idea what you mean here, more specifics please.
It is a 1969 model 350 out of a Camaro SS. It has GM HEI. It also has an ancient Holley TBI system on it. The brain has dials to tune it in; idle, mid range, power and then choke and accelerator pump. When I did the test I did have the vacuum disconnected and I plugged the hose coming out of the TBI (on the tbi side).
So how should i set timing on this?
Thanks
diesel4me said "If it computer controlled by an ECM,it could be the ECM or the wiring causing this.." is that all okay? "or the timing was not properly set," I don't doubt this at all. "you have to unplug a wire first in order to set the static timing properly.." I have no idea what you mean here, more specifics please.
It is a 1969 model 350 out of a Camaro SS. It has GM HEI. It also has an ancient Holley TBI system on it. The brain has dials to tune it in; idle, mid range, power and then choke and accelerator pump. When I did the test I did have the vacuum disconnected and I plugged the hose coming out of the TBI (on the tbi side).
So how should i set timing on this?
Thanks
Probably by disconnecting the wires, fuel line, hoses, and junking it. Lol
Someone may be more help on that specific item than I, but the early aftermarket fi stuff was generally problematic.
How does it actually run? If it just needs to run ok, and it does, I'd not worry about it.
Someone may be more help on that specific item than I, but the early aftermarket fi stuff was generally problematic.
How does it actually run? If it just needs to run ok, and it does, I'd not worry about it.
There is a wire , I can't remember the color, but I am pretty sure someone will chime in on that, but to time it you need to unplug it, it's on the distributor, and it's the advance from the ecm.
Your timing chain is getting sloppy.
They last a long time but near the end of their life, they get a lot of play in them.
This causes the timing mark to bounce around randomly at idle.
Here is a test;
Remove a valve cover.
Either one.
Remove the spark plugs (makes it easier).
Turn the engine over backwards and forwards while watching any one of the rocker arms that happens to be partially depressed.
You'll know right away if it is a worn out timing chain.
Engine will still usually run well enough.
Could go many more years that way.
But I'd replace the timing set and crank seal and water pump while in there.
They last a long time but near the end of their life, they get a lot of play in them.
This causes the timing mark to bounce around randomly at idle.
Here is a test;
Remove a valve cover.
Either one.
Remove the spark plugs (makes it easier).
Turn the engine over backwards and forwards while watching any one of the rocker arms that happens to be partially depressed.
You'll know right away if it is a worn out timing chain.
Engine will still usually run well enough.
Could go many more years that way.
But I'd replace the timing set and crank seal and water pump while in there.
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I've seen the roll pin holding the distributor drive gear get sloppy and cause the timing to fluctuate also..
You can usually tell when a timing chain is getting sloppy just by turning the crankshaft with the harmonic balancer bolt with a breaker bar--you can "feel" the slack in it ,the crank will turn with less resistance until the slack is taken up..you can also take the distributor cap off and watch the rotor and see how far you have to turn the crank before it moves..an inch or more means its too loose..
I've had engines with timing chains so loose they rubbed a hole in the timing cover and leaked oil!..and they still ran decent,but I replaced the chain & gears right away,I did not dare trust them that way..all of them had nylon coated aluminum cam gears and most of the nylon teeth came off and ended up in the oil pump screen and stuck to the bottom of the oil pan...
It is best to take the oil pan off and inspect for that--one pump had a nylon tooth make it past the screen and it was smooshed inside the gear housing..had I run it like that very long I bet the engine would have spun a bearing..
.I used steel gears when I replaced both and the chain..
You can usually tell when a timing chain is getting sloppy just by turning the crankshaft with the harmonic balancer bolt with a breaker bar--you can "feel" the slack in it ,the crank will turn with less resistance until the slack is taken up..you can also take the distributor cap off and watch the rotor and see how far you have to turn the crank before it moves..an inch or more means its too loose..
I've had engines with timing chains so loose they rubbed a hole in the timing cover and leaked oil!..and they still ran decent,but I replaced the chain & gears right away,I did not dare trust them that way..all of them had nylon coated aluminum cam gears and most of the nylon teeth came off and ended up in the oil pump screen and stuck to the bottom of the oil pan...
It is best to take the oil pan off and inspect for that--one pump had a nylon tooth make it past the screen and it was smooshed inside the gear housing..had I run it like that very long I bet the engine would have spun a bearing..
.I used steel gears when I replaced both and the chain..
handloader90
Building!
Is the distributor clamp nice and tight?
Mudstud
1/2 ton status
The clamp is tight. 1ton and Iceman44, I know what wire you're talking about.
So what I'm hearing is- it's normal for a tbi engine timing to fluctuate. But be sure to set timing properly with distributer wire and vac advance disconnected and plugged. Wouldn't hurt to do some testing to ensure timing set isn't too sloppy.
Thanks gents. When I get some time I'll get into it again.
So what I'm hearing is- it's normal for a tbi engine timing to fluctuate. But be sure to set timing properly with distributer wire and vac advance disconnected and plugged. Wouldn't hurt to do some testing to ensure timing set isn't too sloppy.
Thanks gents. When I get some time I'll get into it again.
I may be ignorant here, but wouldn't a vacuum advance and tbi/ecm advance work against each other? As in the vacuum advance moves and the ecm counters because it reads advance that it is not doing?
Never dealt with this, but seems odd.
Never dealt with this, but seems odd.
He doesn't have vacuum advance if it's factory tbi.
It uses the map sensor for fuel injection cues and ignition timing along with the o2, knock, temp sensors.
ECM sets timing.
Nevermind.
Just re-read this thread.
It's an old Holley Pro-jection system.
I think they still used the stock HEI dist with mechanical and vacuum advance.
If that is true, disconnecting the vac advance should leave the timing solid as a rock at idle.
Loose timing chain.
Nevermind.
Just re-read this thread.
It's an old Holley Pro-jection system.
I think they still used the stock HEI dist with mechanical and vacuum advance.
If that is true, disconnecting the vac advance should leave the timing solid as a rock at idle.
Loose timing chain.
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