How to time my Lemans HP 400 - Pontiac GTO Forum
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post #1 of 27 (permalink) Old 07-31-2019, 03:21 PM Thread Starter
 
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How to time my Lemans HP 400

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.

Last edited by Red Skeleton; 07-31-2019 at 08:31 PM.
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post #2 of 27 (permalink) Old 07-31-2019, 06:10 PM Thread Starter
 
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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.
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post #3 of 27 (permalink) Old 07-31-2019, 08:33 PM
 
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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...ontiac-125953/
https://www.gtoforum.com/f170/total-timing-115153/
https://www.gtoforum.com/f170/1968-g...timing-130433/
https://www.gtoforum.com/f170/set-ti...-start-131713/
https://www.gtoforum.com/f170/1968-g...-issue-132423/
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post #4 of 27 (permalink) Old 08-01-2019, 01:13 AM Thread Starter
 
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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!

Last edited by Red Skeleton; 08-01-2019 at 05:22 PM. Reason: update
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post #5 of 27 (permalink) Old 08-01-2019, 06:06 PM
 
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Originally Posted by Red Skeleton View Post
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.
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post #6 of 27 (permalink) Old 08-01-2019, 06:11 PM
 
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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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post #7 of 27 (permalink) Old 08-01-2019, 06:14 PM
 
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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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post #8 of 27 (permalink) Old 08-01-2019, 06:26 PM Thread Starter
 
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Originally Posted by PontiacJim View Post
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.
So you think that 22 degrees base timing is ok instead of 12?
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post #9 of 27 (permalink) Old 08-01-2019, 06:59 PM
 
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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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post #10 of 27 (permalink) Old 08-01-2019, 07:00 PM Thread Starter
 
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Originally Posted by Lemans guy View Post
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-g...-issue-132423/
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, LeMans & GTO Engine Tuning and High Performance

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