1979 GM V8 350 Timing Marks Must be Off

Unfortunately, there is only one tried and proven method to verify that your TDC markings are accurate, and that would be using a PPS ..... aka positive piston stop procedure.

Here's a very simple version of a spark plug port "piston stop" for use with an assembled engine.



This PPS is adjustable so that the piston can be gently stopped at approx 30* before top of stroke, and in either direction.

NOTE: 30* is a suggestion only.... this can very some, but try to stay near 30* +/- for better accuracy.
Some will suggest only 10* ....... but I find better accuracy when 30* +/- is used.


You'll be using a degree wheel on the crankshaft, and a temporary but adjustable pointer aiming towards the degree wheel markings.

You'll roll the crankshaft gently in one direction, until the #1 piston reaches the stop.
Move the pointer and/or wheel, so that they align at near 30* or so, leaving the remaining degrees closing to a "would-be" ZERO if the stop was removed.

Now roll the engine in the other direction until the piston again gently reaches the stop.
Adjust the wheel or pointer to read close to the same degrees, but again leaving the remaining degrees closing to a "would-be" ZERO if the stop was removed.

NOTE: you may need to extend or shorten the PPS shaft so that near 30* before top of stroke can be achieved.


Repeat unit the reading is identical in either direction. This may take up to 5 or 6 or even more attempts.

Once your degree reading is identical in either direction, the stop can be removed.

Now with the stop removed, you'll roll the crankshaft to ZERO pointer/wheel location.

This is your true TDC for #1 cylinder, and you can now re-mark the harmonic balancer.


However, you'll want to find out why the markings shifted in the first place.


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Thanks! I'll give this a try.

Any common causes of timing problems like these and/or ways of diagnosing them?

RicardoMarine said:

Unfortunately, there is only one tried and proven method to verify that your TDC markings are accurate, and that would be using a PPS ..... aka positive piston stop procedure.

Here's a very simple version of a spark plug port "piston stop" for use with an assembled engine.

This PPS is adjustable so that the piston can be gently stopped at approx 30* before top of stroke, and in either direction.

NOTE: 30* is a suggestion only.... this can very some, but try to stay near 30* +/- for better accuracy.
Some will suggest only 10* ....... but I find better accuracy when 30* +/- is used.


You'll be using a degree wheel on the crankshaft, and a temporary but adjustable pointer aiming towards the degree wheel markings.

You'll roll the crankshaft gently in one direction, until the #1 piston reaches the stop.
Move the pointer and/or wheel, so that they align at near 30* or so, leaving the remaining degrees closing to a "would-be" ZERO if the stop was removed.

Now roll the engine in the other direction until the piston again gently reaches the stop.
Adjust the wheel or pointer to read close to the same degrees, but again leaving the remaining degrees closing to a "would-be" ZERO if the stop was removed.

NOTE: you may need to extend or shorten the PPS shaft so that near 30* BTDC can be achieved.


Repeat unit the reading is identical in either direction. This may take up to 5 or 6 or even more attempts.

Once your degree reading is identical in either direction, the stop can be removed.

Now with the stop removed, you'll roll the crankshaft to ZERO pointer/wheel location.

This is your true TDC for #1 cylinder, and you can now re-mark the harmonic balancer.


However, you'll want to find out why the markings shifted in the first place.


.

Click to expand...

Abrego said:

Thanks! I'll give this a try.

1..... Any common causes of timing problems like these

2.... and/or ways of diagnosing them?

Click to expand...

1.... I don't see Marine harmonic balancer trouble very often, but when/if we do have trouble, one issue can be that balancer's outer wheel may have shifted on the inner hub.
I would suspect that is what happened to your balancer.

2.... I think that with your spark timing being so far off according to the balancer...... yet you can get this engine to run OK when you deviate from the specs, you've diagnosed the problem fairly well.



BTW.... and I doubt that I even need to say this..... but you will be gently rolling your crankshaft over by hand... not with the starter motor.

This guy must have used the starter motor.




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Ouch! I feel a little bit guilty laughing at that pic!

So would the outer wheel shifting like this have any effect on the motor's balance?

And would there be any reason to suspect a timing chain issue? ... or is that part of a completely different system?

What would it mean if the timing continues to diverge from my new settings.

I just got a tune up before storage last fall and the motor gradually got rougher over about 12 hours of operation.

timing chain,not if your 36 degrees is correct. You need to determine the true TDC of #1 piston first, then compare the balancers mark with the index plate .
When facing the engine, #1 is the first cylinder on your right.

Also timing is set at 650 -750 rpms........not 950


Is this a thunderbolt ignition or points ignition?

If it was tuned up and ran good for a while then it is most likely not a timing issue unless the hold down clamp on the distributor loosened.

Need some more info..................

Abrego said:

Ouch! I feel a little bit guilty laughing at that pic!
I had to snicker also. (Poor guy!)

So would the outer wheel shifting like this have any effect on the motor's balance?
If the outer ring had been drilled to remove material for balancing, it may cause a very slight affect.

And would there be any reason to suspect a timing chain issue?
As per Kim's post, do the test. A degree or two would be rather constant.... but certainly not to the amount of error that you've mentioned.

... or is that part of a completely different system?
The chain and sprockets drive the camshaft, of which also drives the ignition distributor.

Baring no chain/sprockets slack..... a 1 or 2 degree error at the crankshaft shows up as a 1/2 or 1 degree error at the distributor.... or visa-versa, depending on how you look at it.


What would it mean if the timing continues to diverge from my new settings.
First, understand that BASE advance represents ZERO ignition system advance. (accomplished by where the dist. housing is set)
Above BASE RPM, the spark continues to progressively advance up to the "full in" RPM.

IMO, once the ignition timing has been adjusted/set, the mechanical portion should not change on you.

I just got a tune up before storage last fall and the motor gradually got rougher over about 12 hours of operation.
OK... this is new info from your first post.
When you say "got rougher over about 12 hours of operation" .... please explain!
Rough as in Un-Balanced...... or rough as in the ignition advance became erratic?

Click to expand...

I'd also like to know which ignition system.
Is this a 1979 Merc TB system (I.E., Thunderbolt of which is EST) .... or is this a more conventional 1979 ignition system (I.E., mechanical advance)?


I'll still suggest doing the PPS procedure so that you know that your TDC markings are correct.

Once that's done, go back through the BASE advance procedure (and lock the distributor down), and then increase RPM as you strobe your markings while you look at the progressive and TA (total advance) up to the "full in" RPM.
You should see a rather linear curve during the progressive advance.


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My bet is who ever tuned it up was one spark plug wire off on the cap.........or even a crossed wire....????

He needs to start from square one.........

I really doubt the timing marks are off unless the timing cover was changed or the harmonic ballancer shifted......

kghost said:

My bet is who ever tuned it up was one spark plug wire off on the cap.........or even a crossed wire....????
Abrego, the OP, says; "but it runs like a champ at about say 36 BTC."
This suggests to me that the original TDC marking on the balancer, or that the tab on the cover, are not accurate.

He needs to start from square one.........
Yes, I fully agree!

I really doubt the timing marks are off unless the timing cover was changed or the harmonic ballancer shifted......
IMO...... that's a likely scenario given his comments and findings.

Click to expand...

Abrego also says; "All measurements at about idle speed 950 RPM or so with a digital timing light."

Abrego, I'd suggest either using this strobe light in Standard mode, or use a Standard timing light.
A standard strobe light will give you "real degrees" in "real time".


The part of this that I'm not quite understanding is where Abrego says; "the motor gradually got rougher over about 12 hours of operation."
If the outer balancer ring slipped on the inner hub, it would not have an affect on the ignition advance that had already been set.

IOW, had the outer ring already slipped (prior to the tune up), his problem would have been consistant from the get-go.... yes/no?


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Abrego, see if this helps you.

The same things are puzzling me as well.

I'll get an analogue timing light and compare the readings with the digital.

I'll also drive the boat around after the PPS and see if the reading (marks vs PPS) change.

The distributor is a manual advance original points model, I think. It has a metal spring inside on the base and a rotor/points around the cap. This Thunderbird thing sounds more digital and is probably NOT what I have.

Yes you have the conventional points ignition.

There are two counterwieghts under the point plate and two springs.
As the distributor shaft spins the wieghts spin outward, the springs control the when the counterwieghts move outward.
This is what controls the timing advance.

When you set initial to say 8 BTDC as the motor increases in speed the wieghts spin out advancing the point plate (rotationaly) advancing the timing.

If the springs are broken or the weights are frozen this could be an issue.

I would take the point plate off (two screws) and see if everything is free moving and the springs are intact.

If the other post is yours about moisture in the cap then this could all be part of your issue...........if the other post is not yours then disreguard.

If it is an original rotor/points ignition Distributor, it has 2 springs, one on each centrifugal weight, that controls them.

Sorry guys. I originally thought that they were two unrelated issues and decided to make two separate posts but I wish I could combine them now.

See my other post "Water is 1979 Mercruiser 260 Distributor" for the other half of the evidence.

MikDee said:

If it is an original rotor/points ignition Distributor, it has 2 springs, one on each centrifugal weight, that controls them.

Click to expand...

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Abrego said:

The same things are puzzling me as well.

I'll get an analogue timing light and compare the readings with the digital.
Your digital timing light should offer a Standard Mode. If so, I'd use it in the Std Mode.
However, when we use the Std Mode, the balancer must be marked off up to approx 35 degrees in order to read "real degrees" in "real time" once above BASE advance.

Doing this removes any potential user error or potential equipment error.

I'll also drive the boat around after the PPS and see if the reading (marks vs PPS) change.
After this has been done, take a punch and mark both the outer ring and the inner hub so that the markings align.
If the markings shift, then you'll know that the balancer is bad.

The distributor is a manual advance original points model, I think. It has a metal spring inside on the base and a rotor/points around the cap.
OK... that would be an old Kettering system with mechanical advance.
This could be a Mallory or it could be a Prestolite.
Prestolite uses a screw down cap method, whereas the Mallory may use a screw/clip down cap method.

This Thunderbird thing sounds more digital and is probably NOT what I have.
The Merc ThunderBolt is an EST.... "electronic spark timing" ignition system.
No mechanical advance.
This style distributor drives the triggering unit at a constant indexing with the driven gear.
The progressive advance is predicated by the ignition module via a spark event delay.

Click to expand...

So you guys think I should replace the spring/weight advancer things in the distributor and that may make my motor run OK at 8 BTDC?

NO

Look and make sure they are OK.free moving and the springs are functional and not broken or too loose...

No need to replace them if they are ok

Can I upgrade my mechanical distributor to a Thunderbolt. It sounds really cool! Would it be more resistant to water corrosion?

(Sorry, I'm posting to post threads now...)

OK... now you're asking similar questions in both of your threads. This is going to get confusing at best.

See my response in your other thread.


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