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Discussion Starter #1 (Edited)
Hi all,

I have a 68 Lemans/GTO clone with a 68 400 ys code motor. Compression is in the 125-145 range. It has a cam but I have no info on it. It also has an edelbrock intake and 1406 carb and MSD 6AL ignition box and MSD distributor. She has headers too and a 3.55 10 bolt posi rear end with a muncie 4 speed. (close ratio I think)

When I bought her she had been stored 10 years and run occasionally. the advance springs had rusted off and the "commutator"? was rusty which I cleaned. I replaced the advance springs with the closest size I could with an over the counter advance curve kit.

It has no vacuum advance. She idles fine at 750 and is a rocket. I had set the initial timing to stock 9 degrees BTDC but got a low speed surging or mild bucking that interferes with the pleasure of driving it. I read on the forum and at the MSD site that mentions initial to be about 12 degrees before TDC at 750rpm for a Chevy small block. THEN, they say to run your engine full advance and set your timing there at 35 degrees. Then check initial to see what the spread is?

Being a back yard mechanic and not really dealing with the advance curve and never having a hopped up car before, Can you give me step by step on how to time this beast? What would timing be with this setup? BTW, NO pinging at ALL even when trying to and I run premium gas.

I also tried (mentioned on this forum) setting intitial timing at about 22 degrees BTDC and fully advance about 44 degrees. It seemed a little better but is still annoying at lower speeds.:|
 

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Discussion Starter #2
Update: put heavier advance springs with initial timing at 14 BTDC, no change for the better. I then turned the distributer to about 30 degrees btdc and seemed worse but still drivable. No hard cranking either.
 

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Update: put heavier advance springs with initial timing at 14 BTDC, no change for the better. I then turned the distributer to about 30 degrees btdc and seemed worse but still drivable. No hard cranking either.

There is a Google Custom Search box in the upper right hand corner of the page. Timing has been covered in many previous posts and how to check and set it up. Here are just a couple you can click on and read over. 44 degrees advance on a Pontiac engine will melt pistons.

https://www.gtoforum.com/f170/timing-adjustment-455-pontiac-125953/
https://www.gtoforum.com/f170/total-timing-115153/
https://www.gtoforum.com/f170/1968-gto-set-base-timing-total-timing-130433/
https://www.gtoforum.com/f170/set-timing-before-engine-start-131713/
https://www.gtoforum.com/f170/1968-gto-timing-issue-132423/
 

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Discussion Starter #4 (Edited)
This may be a stupid question? If I set my initial timing first to 12 btdc at 750rpm and then my my total to 34 by setting the adjustable timing light to 34, rev up the engine to 3k while setting the balancer mark to zero, won't that change my initial timing reading?

Update: did as instructed.
Engine warm at 750 rpm idle, set timing to 12btdc.
Increase rpm till advance max’s
Have timing light set at 34 degrees and move distributor again till marker goes to zero
Reset idle back down to 750rpm (it increased a little)
Check base timing reads 22 btdc with timing light reset to zero
CORRECT? Thanks for all you help!
 

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This may be a stupid question? If I set my initial timing first to 12 btdc at 750rpm and then my my total to 34 by setting the adjustable timing light to 34, rev up the engine to 3k while setting the balancer mark to zero, won't that change my initial timing reading?

Update: did as instructed.
Engine warm at 750 rpm idle, set timing to 12btdc.
Increase rpm till advance max’s
Have timing light set at 34 degrees and move distributor again till marker goes to zero
Reset idle back down to 750rpm (it increased a little)
Check base timing reads 22 btdc with timing light reset to zero
CORRECT? Thanks for all you help!

Sounds like you got it and a learning lesson along the way. As long as you do not have any detonation when you lug or put a load on the engine, and it starts up easy, you should be OK. Detonation will kill an engine, so you would want to drop your timing back a couple degrees and see how that works.

Keep in mind that the vacuum advance serves 2 purposes - it'll help with gas mileage (who buys a GTO for gas mileage?) and keep your engine running cooler. So if you notice your engine running a higher temp than what you would like to see, most likely adding the vacuum advance will take care of that problem. Otherwise, leave it off and run it as it is. :thumbsup:
 

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Red, well maybe. The only fly in the ointment is you seem to stop revving at 3000. That is where you might be confusing what you want your springs to do with what your total centrifigal advance really is.

If you stop revving at 3000 then all you have determined is that is all the advance you get until 3000. your may or may not have more advance.

Here is how to tell, forget 3000. remove one spring from your vac advance and just have one medium weight spring on it. set the base at 10 or 12 and start it up

now rev it up until the timing no longer will advance no matter the number once it stops and you use your light or timing tape to tell. Stop, replace the missing spring. use a medium one for now.

what is the reading? say it is 38......subtract whatever base timing you had say 12.....38 -12 = 24.......24 is your centrifigal timing. Now you want 36 or 34, say 36.

then use the centrifigal number in this example 24 and add to get 36.....add 12.....24 + 12 = 36....set your base at 12.

I think it is unlikely that you have 12 degrees of centrifigal timing, likely you have more.... and if you have say 24 centrifigal and set base at 22....that is 46 and you will destroy your engine.

so you have to make sure what your centrifigal is, once that is known subtraction gets you the base. Now don't despair that it idles nice with 22 degrees and not as good with it at 12 degrees. a properly st up vac advance can will give you 10 more degrees above idle and can go above the 36 number because vac advance drope at at WOT and does not count against total.

Make sure, I think you may have too much because you are confusing the springs and at what RPM they bring in the total centrigigal to what the total is.

The total centrifigal is limited by the weights and bushings where they reach their mechanical limits, springs have no effect on the limit,...only how fast that limit is achieved,....once you get all the things I said right then mess with the springs and change weights and bushings if required.

you will get it let us know how you do
 

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typo sorry.....remove one spring from Centrifigal advance weights...not vac!...Vac has no weights

you remove a spring just to achieve the total fast and at a lower rpm for the test only
 

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Discussion Starter #8
Sounds like you got it and a learning lesson along the way. As long as you do not have any detonation when you lug or put a load on the engine, and it starts up easy, you should be OK. Detonation will kill an engine, so you would want to drop your timing back a couple degrees and see how that works.

Keep in mind that the vacuum advance serves 2 purposes - it'll help with gas mileage (who buys a GTO for gas mileage?) and keep your engine running cooler. So if you notice your engine running a higher temp than what you would like to see, most likely adding the vacuum advance will take care of that problem. Otherwise, leave it off and run it as it is. :thumbsup:
So you think that 22 degrees base timing is ok instead of 12?
 

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Red nobody can tell what is OK at base unless you know what Centrifigal is. If Centrifigal is 12 or 14 then 22 at base will work.

So you have to verify Centrifigal for sure. The usual springs are stiff and at 3000 rpm will not bring all the advance in. So when you stop at 3000 you may not have measure all the advance...

Use the one spring method make sure the other spring is not too heavy and retest, the advance will come in fast that way at lower rpm and you will know that it reached it limit.

Then you can determine base timing and add vac as needed and mess with springs etc
 

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Discussion Starter #10
Red, well maybe. The only fly in the ointment is you seem to stop revving at 3000. That is where you might be confusing what you want your springs to do with what your total centrifigal advance really is.

If you stop revving at 3000 then all you have determined is that is all the advance you get until 3000. your may or may not have more advance.

Here is how to tell, forget 3000. remove one spring from your vac advance and just have one medium weight spring on it. set the base at 10 or 12 and start it up

now rev it up until the timing no longer will advance no matter the number once it stops and you use your light or timing tape to tell. Stop, replace the missing spring. use a medium one for now.

what is the reading? say it is 38......subtract whatever base timing you had say 12.....38 -12 = 24.......24 is your centrifigal timing. Now you want 36 or 34, say 36.

then use the centrifigal number in this example 24 and add to get 36.....add 12.....24 + 12 = 36....set your base at 12.

I think it is unlikely that you have 12 degrees of centrifigal timing, likely you have more.... and if you have say 24 centrifigal and set base at 22....that is 46 and you will destroy your engine.

so you have to make sure what your centrifigal is, once that is known subtraction gets you the base. Now don't despair that it idles nice with 22 degrees and not as good with it at 12 degrees. a properly st up vac advance can will give you 10 more degrees above idle and can go above the 36 number because vac advance drope at at WOT and does not count against total.

Make sure, I think you may have too much because you are confusing the springs and at what RPM they bring in the total centrigigal to what the total is.

The total centrifigal is limited by the weights and bushings where they reach their mechanical limits, springs have no effect on the limit,...only how fast that limit is achieved,....once you get all the things I said right then mess with the springs and change weights and bushings if required.

you will get it let us know how you do
Thanks Lemans guy, any comment on this thread? difference is mine has a cam. https://www.gtoforum.com/f170/1968-gto-timing-issue-132423/
 

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Right Red that is a good discussion on timing, I recall it.

but a hot cam does not change the need to have the correct "total" timing so your engine does not detonate. A hot cam cam usually take more idle timing. That can be achieved thru base timing only, like you did with your hand at 22. Or thru a combination of base timing and vacumn advance set on full manifold vacumn.

i usually set em at 20 to 28 degrees of idle timing with 10 degrees from vac advance....so ten to 18 of base timing and then the vac advance on top of that.

but that base is dependent on what the centrifigal is to achieve 36. so I set the base at 14 if I have 22 degrees centrifigal...that gives 36.

a hot cam can allow more idle timing but not more total timing. as a rule a most cams, 12 to 14 degrees base,with 10 degrees vac on top for 22 o 24 idle timing. a radical cam can take a bit more, 16, 18 + 10 degrees vac. as long as you don"t get hard starts, starter kickback or detonation on acceleration. Smart to be a little conservative, I know Pontiac Jim likes his at 34.

as far as the total, it is truly a range say 32 degrees to 40 degrees. with 36 the dead on center. 34 is a tad safer, 38 is a tad hotter.....so it can be adjusted, but I always start at 36. if detonating turn down to 34 or 32 if needed.' If you want to experiment you can push 38, just make sure it does not detonate.

with timing we all want to start everywhere but at the begining.., springs, RPM levels base timing etc.

it is like a house you can start with the second floor bedroom or the roof, without the foundation everything will be wrong and the foundation of timing is .......

What is your "total centrifigal advance", ...it is one number unless you change it with different weights or bushings.......regardless of RPM's it will not exceed that number

now you can st the proper base and add vac advance and adjust as needed. you cannot just add base timing to an unknown centrifigal number without risking damage.

so Just verify your centrifigal number, then once known you can begin. With that test I described it will come on fast since the springs are not holding it back....

and you will know the total.

I do strongly recommend a proper vac advance setup,....if you are street driving the car at all.

you are doing great, and your car sounds cool too........the right timing will smooth it out and make it run even better!:nerd::nerd::nerd:
 

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Discussion Starter #13
Please tell me again this is correct? Sorry!

This may be a stupid question? If I set my initial timing first to 12 btdc at 750rpm and then my my total to 34 by setting the adjustable timing light to 34, rev up the engine to 3k while setting the balancer mark to zero, won't that change my initial timing reading?

Update: did as instructed.
Engine warm at 750 rpm idle, set timing to 12btdc.
Increase rpm till advance max’s
Have timing light set at 34 degrees and move distributor again till marker goes to zero
Reset idle back down to 750rpm (it increased a little)
Check base timing reads 22 btdc with timing light reset to zero
CORRECT? Thanks for all you help!
Please read the above and tell me I did it correctly, just making sure it was read carefully as I'm still confused if it doesn't ping and doesn't have a tough time starting like the starter is straining I'm ok (It doesn't). Then if it still is lurching a bit at slower speeds I should ad a vacuum advance to advance timing MORE to eliminate that?

I've gotten a wealth of information from you guys and I appreciate but some posts use terminology that confuses me. Base and initial timing mean the same thing right? Stuff like that.

One other thing is since I've had this taken compression which is good but I haven't pulled a valve cover to see if this has adjustable valves. Do most other than stock cams have adjustable valves?
 

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Still lurching at lower speeds could be anything. Lean carb, rich carb, bad carb, bad PCV valve, vacuum leak, more/less timing, camshaft selection, fuel pressure, valves, etc.. This is where you come into play and start playing around to figure out what it is.

initial/base timing : this is your timing at idle with the vacuum advance disconnected.

total timing : this is your high RPM timing with full mechanical advance but with the vacuum advance disconnected.

maximum timing : this is your high RPM timing with the vacuum advance connected, also about what your timing is during cruise conditions.

idle timing : this is your initial plus any vacuum advance at idle.


Pontiac engines do not use an adjustable valve train - they are non-adjustable due to the fact that they use hydraulic lifers. The rocker arm nuts get torqued down. You can make a non-adjustable valve train adjustable by using poly locks. This can be done to "zero lash" the hydraulic lifters which will prevent the lifters from "pumping up" and limiting engine RPM's by design. To "zero lash" the lifters will allow the engine to rev slightly higher.

IF you decided to use a solid lifter cam, these require a specific clearance between the rocker arm and valve tip per manufacturer's specification. In this case, you would have to make the rocker arm adjustable so that you could achieve the correct clearance between the rocker arm and valve tip. You use a feeler gauge to do this and then lock down the poly lock to keep the adjustment.
 

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Thanks John and that is a solid assessment from Pontiac Jim, breaking down how the timing is set, spot on.

Yet I think the thing that is confusing you is you are somehow believing that at 3000 RPM if you saw no more advance at timing.....that is all you have.......

You could be right,....but you must be sure. The reason I say this is that the springs on the distributors even new ones are very strong, and it may only advance to 34 At 3000 RPM but at 4000 or 4500 it may have more timing that you don’t know about. I find this routinely on distributors on my Sun Distributor machine, I run them up to 7 or 8 thousand RPM on the machine to make sure I know the total.

So without a dist machine the way to do this is the one spring removed procedure that I described in this thread. You can leave one medium strength spring on. Then when you do the test you will not have to run you engine to 7000, which you did not do and don’t want to, but by 3500 or so you can be sure their is no more advance and probably sooner than that.

So if you do that and you only have 34 still you can set your base at 22. But if you find more, you just saved your engine. And then can dial it in accordingly.

You can put a little c clip above the weight where the spring was removed, but they mostly stay on anyway as you only do it once and the RPM speed is not that crazy.

Don’t feel bad it all does get confusing, but you are further ahead than you think and Re read Pontiac Jim’s description of the timing set up

You will get it! Stay with it!:nerd::nerd:
 

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Discussion Starter #16
Thanks John and that is a solid assessment from Pontiac Jim, breaking down how the timing is set, spot on.

Yet I think the thing that is confusing you is you are somehow believing that at 3000 RPM if you saw no more advance at timing.....that is all you have.......

You could be right,....but you must be sure. The reason I say this is that the springs on the distributors even new ones are very strong, and it may only advance to 34 At 3000 RPM but at 4000 or 4500 it may have more timing that you don’t know about. I find this routinely on distributors on my Sun Distributor machine, I run them up to 7 or 8 thousand RPM on the machine to make sure I know the total.

So without a dist machine the way to do this is the one spring removed procedure that I described in this thread. You can leave one medium strength spring on. Then when you do the test you will not have to run you engine to 7000, which you did not do and don’t want to, but by 3500 or so you can be sure their is no more advance and probably sooner than that.

So if you do that and you only have 34 still you can set your base at 22. But if you find more, you just saved your engine. And then can dial it in accordingly.

You can put a little c clip above the weight where the spring was removed, but they mostly stay on anyway as you only do it once and the RPM speed is not that crazy.

Don’t feel bad it all does get confusing, but you are further ahead than you think and Re read Pontiac Jim’s description of the timing set up

You will get it! Stay with it!:nerd::nerd:
First of all the original springs that were on it were super light and if I revved the motor a just a little it would go full advance. I did your check by removing a spring and got the same reading. I've since put on heavier springs to restrict the advance more. I'll drive it tomorrow.
Still no one has addressed that the full advance and the base timing are RELATIVE. So when I set the full advance at 34 that means the base timing is set too? 34 is the most important and the base just is what ever number it turns out to be, right? Because if you change the base that's going to move the 34 number too, right? Is the 22 number you gave base timing?
 

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Red, Great......Answers to your questions are yes,yes,yes, and yes.

You now know that you have “12 degrees of centrifigal advance”.....the beginning number, you then would set the base,
vac plugged (you have none).......The bass should be 24 or 22. .....24 base gives you 36 Total advance........22 Base gives you 34 total advance. Either should work 34 is just a tad more conservative.

Yes “Total advance is the most important number” , this is the 34 or 36 we are discussing. If you have no pinging, starter kickback or hard starts you will be good to go!

At 22 it will run a nice and cool idle and smooth two. A medium and light spring would probably be best, get you close to 3000 RPM or so all in.

So you are there, you can run it this way, it is a racers setup, vac advance can add ten more for light throttle cruise....so you could be cruising with 46 if you ever decide to change, much better, more efficient burn on the leaner cruise mixture, cooler and less fuel too.

But that idea may not be what you want or consider it another day, sounds like you are good!

Nice work!:nerd::nerd:
 

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Discussion Starter #18
Red, Great......Answers to your questions are yes,yes,yes, and yes.

You now know that you have “12 degrees of centrifigal advance”.....the beginning number, you then would set the base,
vac plugged (you have none).......The bass should be 24 or 22. .....24 base gives you 36 Total advance........22 Base gives you 34 total advance. Either should work 34 is just a tad more conservative.

Yes “Total advance is the most important number” , this is the 34 or 36 we are discussing. If you have no pinging, starter kickback or hard starts you will be good to go!

At 22 it will run a nice and cool idle and smooth two. A medium and light spring would probably be best, get you close to 3000 RPM or so all in.

So you are there, you can run it this way, it is a racers setup, vac advance can add ten more for light throttle cruise....so you could be cruising with 46 if you ever decide to change, much better, more efficient burn on the leaner cruise mixture, cooler and less fuel too.

But that idea may not be what you want or consider it another day, sounds like you are good!

Nice work!:nerd::nerd:
Thanks Lemans Guy, but at 61 years old I just want driveability. I'm not going to swap cams but I want the best driveability I can get. Tomorrow I having the really loud turbo mufflers replaced with the quietest ones they have that are still listed as "turbos".

When I was about 19 I had a 68 Lemans with the Sprint package. I paid $50. for it because it had jumped a tooth. I wish I had that car now. OHC 6, 4 speed cast iron Saginaw, 4 barrel Rochester, dual exhaust manifolds into 1, stock tach, console, and it was fast for what it was. I could give a Boss Mustang a run for his money in the 1/4. I remember a California Highway patrol officer having me pop the hood to see what I had when I was pulled over for speeding. The Mustang driver insisted I open the hood after almost beating him in the 1/4, lol after telling him I had a 6.
 

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Discussion Starter #19
Ok, I'm losing my mind! She was running fair but is still lurching at low speeds and not idling steady, it will change on it's own. Started suspecting carburation. It used to want to squeal the tires when just barely accelerating away from a stop sign. Way back before I pulled the distributor for the first time and had the rusted off light advance springs, I made a sharpie mark so I know where it was.
After timing it properly,and replacing the springs with like ones, that mark was a 1/2" away from where it was.

So I thought I'd play with the timing , advancing it little by little until it would ping or strained starting. When I got to the strained starting, I'd back off (retard the timing). It took quite a bit but it still doesn't idle well and changes a little. I noticed the timing strobe moving around a little. It does go like a bat of of hell and I've NEVER gotten it to ping. 3rd gear at 20mph and floor it and it just accelerates. No over heating or pinging.

I then checked my timing and it was 52 degrees btdc from the zero mark on the scale. There's no way I can check the distributor except take it out and feel the bearing which I did about 20 miles ago. This was rusty inside when I bought it, which I cleaned out, the iron star that rotates was all rusty from sitting in a moist area.

I know something is wrong, but WHAT? Probably not related, but I hear the valves tapping like solid lifters, so I need to see what been done to the rockers if anything.
 

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Red, I think you need a new distributor. It is rusted and sticks, the shaft, the center piece may be too far gone. When you checked and said it advanced only 12 degrees it was likely not giving you the correct reading, it may have been sticking.

You should take it out and examine it closely, but with all the rust you described how can you have confidence in it?

I would recommend a new one, if it is MSD and you like those, get one with vacumn advance. It will make your car run much better.

Does not sound like this one is worth messing with.
 
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