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#92444 09/03/17 01:08 PM
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Some people trim their weights, change their springs to change the timing characteristics. I've got a distributor from Tom Langdon, think it's from the Atlas-6 engine, so was curious how my timing came out. I'd like to compare it to others. Engine L250 with flat-top pistons and a medium CAM. I've got a really nice Bosch timing light with digital readout for advance and RPMs and used grease pencil to make the "0" marking really clear.

First thing that really surprised me was that the timing advance dropped from 700 RPM to 1000 RPM. With vacuum on, the idle works well at 970-1000 RPM. Goes down a bit when the fan goes on, relay is on the same circuit.

Mechanical advance kicks in at 1650 RPM.

Vacuum off. % is (adv change)/(rpm change)

6.3 @ 720
5.5 @ 1000, -0.8, -0.29%
8.1 @ 1800, +2.6, +0.33%
11.0 @ 2000, +2.9, +1.45%
12.6 @ 2500, +1.6, +0.32%
15.0 @ 3000, +2.4, +0.48%

Vacuum on.

30.5 @ 970. 24.5 timing & 250 RPM increase by adding vacuum
30.5 @ 1600
32.6 @ 1800, +2.1, +1.05%
34.6 @ 2000, +2.0, +1.00%
37.2 @ 2500, +2.6, +0.52%
39.2 @ 3000, +2.0, +0.40%

I was afraid the neighbors would call me in if I did higher RPM. :-)


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Tom Langdons distributors are not from Atlas engines. They did not use dist.
I think they are from early 80"s chevy citations and such with a 2.8 V6, when they still had vacuum advance. Hard to find now.

Anyway. Can you take a picture of the weights in the distributor?

I think your advance should come in faster.
It should start moving at around 1200 and have full mech timing at the top of your cruising rpm on highway.

So you need to know tire height, trans and rear ratio.
You also need to know the timing characteristics of your engine.

These engines tend to like timing in general.

Idle with vacuum around 25-26

WOT with no vacuum 34-38

Cruise on HWY with vacuum 44-52ish

I'd suggest buying a spring kit for the weights. Use one of the lightest springs and a medium spring on your weights. See how that changes it and report back.


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It will have sharper response and better part-throttle acceleration with a faster advance curve as suggested, just be sure to listen for knock! An automatic with a higher stall converter is much more tolerant of early & high spark than a stick. Lower (numerical) axle is always worse.
There may be several different shape advance weights for that distributor (there are for the V8) which make the advance rate curve non-linear (each .010" of weight motion away from relaxed does not have the same effect on advance degrees). I haven't experimented with this, but in any event if you're assembling a distributor from parts make sure the 2 weights are an exact match.

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Have you been working on your timing? Here is where you can increase your power.


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Hi Tom, I originally had my timing at 8+BTDC, but I was worried about knock and such, so I dropped it back to about 6.5, but you can see that when I did the test, at 1,000 RPM it's really 5.5, lower than I intended.

I started a youtube channel with a video that shows the distributor, weights, springs. Be the first to subscribe! Haha. Video at the end.

Here's the video. 250 Video with Distributor

I've got a set of the springs on order, will see what the lightest one yields.

It could be the carb still isn't set right, but given the fact that the timing isn't where it should be, that could be making a difference.

I've got the spare electronic ignition distributor from the spare 250 engine. But the connectors are all different, so I'm not going to put that in to see how it does. I have the original points distributor, I could always try that. :-)

Of course, it could just be my expectations are set too high, but she really kicks it when the RPMs are high.


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I tried the spring kit out today. The springs are dramatically lighter than the original. I tried the middle, but swapped right away to the heaviest. It was still way too light. The advance started around 900 RPM and advanced dramatically higher than the other. With vaccuum installed, at 1800 RPM it was starting to knock with 42.9 advance where it was 32.6 previously. Ten additional degrees advance!

I immidiately went back to the original spring and bumped up the advance. At 970 RPM where I like the idle, it's 8.6. At 720, where it runs without vacuum, it's 9.3, still the mystery of more advance at lower RPMs.

The car felt to run lighter, better than previously. I'll leave it here for a while and run it.

I've got a full video production in the works, will post when I eventually get it online.

Last edited by mshaw230; 09/16/17 08:14 PM. Reason: fixed a sentence

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I have to admit, and it may very well may be me being dense here, but I am not sure you have this correct.

The distributor itself will have so many distributor degrees of advance built in. Different distributors can have different amounts of advance. So for example, a later 235 distributor generally has like 12 distributor degrees of advance built in (24 degrees at the crank). So you are starting out with 24 degrees of total mechanical advance. If you need more advance, say you want 32 of total advance, you then set the distributor at 8 degrees btdc. This is the initial lead and is added to the 24 distributor degrees. You can keep adding initial until the motor pings to get more, or you can modify the internals to get more advance and less initial (a more involved process.)

The next piece to the puzzle is just how fast or slow you want the distributor degrees (the mechanical advance) to come in. This is a function of the weights and springs. Find the specification for the distributor. It should tell you when the timing will start to come in. It's common to see the advance start at like 1500 rpm and have it all in by sometimes as much as 3000 rpm. A lot of cars like it to start in a little earlier, like 1000 or 1200 and have it all in by 2500 rpm. This is accomplished by swapping the heavier springs for some lighter springs. Sometimes one light and one medium, other times both medium or both light. It's trial and error to see what the motor likes, but in any event the total timing is mechanical plus initial. The one issue you will need to be cognizant of is that the light springs wear out and stretch out faster then the heavier springs so if you use them, keep an eye on the timing curve to confirm that this has not happened.

The vacuum advance is only operative at idle and when the motor is running at highway no load speed and is determined by the advance can that is installed (gm had many different part numbers for different vacuum advances) or you engage in some trial and error using an adjustable vacuum advance. But the formula is not initial plus mechanical plus vacuum.

Last edited by mdonohue05; 09/17/17 04:09 PM.
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The formula for total advance under heavy throttle (below the vacuum setting) is initial plus mechanical.
The formula for total advance under light throttle (above the vacuum setting) is initial plus mechanical plus vacuum.

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Mdonahue05 and panic,

Great info, directly to the point. I was wondering if anybody knew the timing profile for a stock distributor. I wanted to know if my distributor was running a different profile that might affect things.

Where might I find the specs for the distributor I'm using and for the original?

Thanks!


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Originally Posted By: mshaw230
Mdonahue05 and panic,
Where might I find the specs for the distributor I'm using and for the original?


I've never seen a reference book with such data compiled for comparison. Not even the shop manual gets into this level of detail.

Based on what I have seen online and at swap meets - there were the occasional publication of supplements to shop manuals when extensive changes were made during milestone years - Bendix brakes, power-glide etc.

6 to 12 volt was a major move affecting dizzies and wiring harness, tuning specs/procedure - 55 Caddy Shop Manual Supplement.

In this case they went to the trouble to share the advance curve data. Why is somewhat puzzling because this information is not needed for a flat-rate tune up. It's internal engineering.

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Interestingly enough, I am pretty sure its in my old light blue Chilton's, also some older delco remy books. It is in section 6y of the 55 chevy shop manual under specifications. I will have to check but I am certain its also in the 57 shop manual. It must be in the later shop manuals. If its not, I would be really surprised, How else would you diagnose an advance problem.

keep in mind, it was not uncommon for a good shop to have a sun distributor machine to test the distributors. The technical information was also supplied by sun either in a card box or on a scroll on top of the machine. For some distributors, it was the easiest way to get a steady dwell reading was to set the points up on a machine.

Last edited by mdonohue05; 09/18/17 06:16 PM.
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If you can find the old MITCHEL manuals, they had a lot of the minute technical information. They were WAAAAYY better than Chilton for the professional shops!


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I still need to curve my '78 HEI. A lot of other things are apart right now that need to be put back together before I can tune again. I could throw my stock 22* vac adv on, and turn it back to the factory initial advance, and curve the stock springs and weights to get a baseline. I can say from experience that it is definitely wakes the engine up. laugh

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Okay, back to the Camaro. I've taken the past 9 months off buying a small farm, gutting the house and rebuilding it from the ground up. Whew. Still not done, but my portion is nearly done.

Now pictures. Smugmug works! At least for the 30 day trial...

A friend stopped by with his '67


I had the garage sealed and colored...


Now, back to the distributor. Ok, I don't know why I keep thinking inline six Atlas engine. Maybe I just can't stand the thought of a Chevy Citation part in my car... Thanks so much for the great information! I need to get my ignition timing spread better, lower at idle with vacuum, starting faster, higher at the high end. If I can't get this to work, I guess I go with a different distributor, but hey, no fun in that!

I got a good picture of the weights and springs.


The weights have 37 stamped on them.
The plate were the springs are attached: 83
The plate underneath: 847

Do you know what the numbers mean? The springs are much heavier than those I got from an advance kit.

Looking at the parts for a small block v8, they're not expensive, maybe I should just buy a set of weights and springs and see if they fit?


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I think you want to consider taking a breath here and first sort out your baseline timing, what the mechanical advance actually is on the distributor you have in hand before you start swapping parts. Is there a delco number or some other identifier on the distributor? If so, that is the place to start. You can figure out what year and application, then the stock specifications. From there, you can decide where to go from that point, i.e., advance starting earlier, or start at stock but all in earlier then stock, lighter springs, modified profile on the weights, combination of both. Frankly, this is best done on a distributor machine to really get it right.

Give some consideration to removing the distributor and sending it out for testing and recurving. Pat Mason, Mason Racing Ignitions in Ottsville, PA worked at Vertex magnetos for a lot of years. He builds and rebuilds all kinds of ignitions, including vintage Mallory ignitions. He is excellent and reasonable. I was just at his shop last weekend dropping off my Vertex for a little check up. You might consider sending your distributor to him and tell him what you are looking for, and then let him set this thing up for you correctly. I think you will be way ahead if you do. Just a thought anyhow. If you are considering sending your distributor to him, call as he is old school, extremely knowledgeable, and a decent guy to boot.

http://masonracingignitions.com/

Last edited by mdonohue05; 06/27/18 12:25 PM.
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I talked to Tom Langdon, who shed some light on this. The distributor is from a 1985 Chevy S10 pickup. I gave Tom all my information on the car, except that I once again gave the wrong CAM, shucks. That led to the discussion again of the CAM being too big. I really have the Compcams 260H installed, which should be much better for what I want.

Tom said I should start with a much higher timing, 18. When I told him I was around 7 or 8, he said in a very funny way to add another 10. I believe he said I wasn't even on first base and to add 10 and and call him in a week. Of course, he said to run high octane with this advanced. I'm going to run an experiment now, I think, see if it knocks and if it runs better.

Tom said that the distributor curve would help a few %, but a smaller CAM would make a huge difference. I'll call him in a week and let him know how it goes and that I actually had the smaller CAM and see what he suggests.

mdonahue05, that's a great suggestion. I've heard about people who have the machinery to set up distributors. If it's really that I can just start with a higher timing to start with, I'll go that route.


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Earlier this year I purchased a 1969 sun 404 machine. Rebuilt all the mechanicals and electric. Learned how to use the machine and then stock piled tools and parts. Then I rebuilt a couple of 235 distributors. What a difference having the machine to sort out the advance curve and the modifications to change the curve. I am still learning but it’s clear, to me at least, what a real pro can do with a distributor and a distributor machine. Which is why I thought you might benefit from having someone knowledgeable set your timing curve up.

Last edited by mdonohue05; 06/30/18 12:15 AM.
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Wow, I'm like a high school kid again, spinning tires off the light, squealing around corners. Not having enough advance in the lower RPM was the base problem.

Here's my timing curve:

Mechanical only.
1100 17.5
2000 22.9
2500 24.8
3000 27
3500 28.9
4500 28.9

With vacuum on.
1300 41.9
3000 50.9
3500 53.3

Now, just think what it would do if the distributor were trimmed as suggested by mdonahue05!

Next thing to do is replace that leaky oil pan gasket.

Thanks for all your help and suggestions!


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More cam usually means more overlap. This leaves exhaust gas in the chamber which is slower to ignite - adding some initial advance frequently helps. Since the intake valve closing is also frequently later, cylinder pressure is down - so the extra advance may not cause knocking (or at least less than you might expect).
Pay careful attention to the complex shapes of both the center rotor and inner surface of the advance weights shown. As engine speed increases, the spark increase is NEVER linear, but has periods where the speed (degrees per 100 rpm) changes. 2 different weight sets will give different characteristics even with the same curve. This is why a Sun machine and an experienced operator really help.

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This is a great explanation.

My friend has a stock cam in his Camaro 250. Given this information,the timing would be more conventional, 8BTDC. Is that what people do?


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I have always been a fan of timing based on vacuum readings - as succinctly described here at
automotivemileposts.com

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Not convinced.
"Timing chains can slip over the years".
Chains don't slip. They develop slack and slightly retard the cam (which reduces pinging), or skip a tooth.

"4. Keep in mind when adjusting the timing, you have to turn the distributor clockwise to advance and counter-clockwise to retard."
Or, turn the distributor clockwise to retard and counter-clockwise to advance. See - there is more than one engine?

"5. ...look for a maximum reading on the gauge. For instance, if the reading on your gauge peaks at 18 inches of vacuum, that is the maximum for your engine."
Never, it will always be higher at 1,500 RPM than idle.

"(The normal range could be from a high of 20 inches to a low of 15 inches, depending on the internal condition of your motor)."
Or, between 5 and 25 inches, depending on the engine.

"Turning the screws in will cause a richer mixture, turning the screws out will cause a leaner mixture.".
Or, turning the screws out will cause a richer mixture, turning the screws in will cause a leaner mixture depending on the carburetor.

In summary: you use vacuum, then you test to see if it pings. Then you retard it as needed - exactly as the stock settings.

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I am with you on the timing chain comment - it's just wrong = same with assuming the direction of distributor rotation. And I agree that base timing using this method may result at pinging during a load test.

But I thought the author does a good job of describing the inter-play between vacuum, spark timing and idle-mixture. Granted that vacuum varies with RPM - but the whole point of idling is to dial RPM down as low as possible without stalling.

Absent a vacuum gauge it is quite easy to establish an idle that is both lean and late.

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It reminds me of the complex math formulas to determine the exact tuned length for a specific RPM.
And it ends: "then cut it off 6" longer, and shorten it by testing".

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No it's not formulaic or cook book at all . . .

This is what I find most intriguing about setting up a non-stock engine. Take for example this posting from Steve's Nova site :
"this is just another variation on the "turn the distributor until it pings and then back it off". Don't these people own a timing light?
The trick is:
First set your base timing with a timing light, then tune the idle mixture with a vacuum gauge to get the highest reading at the lowest steady idle."

This assumes that the base timing is somehow a fixed BTDC value . . . when in fact it is just one variable to be established by tuning and testing.

The point is that one needs a timing light, a tach and a vacuum gauge to do this right.

Moreover, the variables are gating:
1) Minimize idle RPM without stalling
2) Base Timing must not be advanced to the point of hard starting - especially when hot
3) Idle vacuum should be steady - not bouncing in a range (which is an indication of a lean mixture).
4) Maximize idle vacuum to insure crisp throttle response.

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mshaw230, you are still not there because you don't know what the factory mechanical advance for that distributor should be. For example, your mechanical advance may not be fully coming in, and you are making up for it with initial advance. Or your mechanical advance may be coming in late. You really do need to dig up the specifications for that distributor and see where you are at for a base line and then go from there (dialing in the initial, and tinkering with the vacuum can for optimal vacuum advance). BTW, I am with stock49 here, you need a timing light and a vacuum gauge to do this right.

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when I set mine, I used a timing light and vacuum gauge to check in 500 rpm increments. I locked my throttle at each increment and adjusted timing for highest rpm at that throttle position and confirmed with vacuum readings. of course this meant my baseline was now an rpm at or above the 500 rpm increment but non the less gave me a target advance curve to match. I prefer to ignore the advance at idle and set the 3000 rpm advance dead on, then work backwards to get closer at other rpm's. idle can be a few degrees off for easy starting and I could care less if off idle is fairly close and w.o.t. is dead on.

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Ok, I went with the vacuum method today and found best performance at 29 BTDC. That seems really early. The vacuum was highest, the needle smoothed out. I found that 33 BTDC at idle was best and some of the RPMs, but it seemed that it didn't make much difference down to 29. I think I saw 64 BTDC at high RPM with vacuum on.

I took it for a quick drive on some hills, and didn't hear any pinging.

Anyways, it seems to run pretty well.

Thanks for the suggestions!

Last edited by mshaw230; 07/15/18 02:20 AM.

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Mark,
Glad to hear it is running better. Have to tell you this. You are just shooting in the dark.

The numbers you gave a kinda crazy.
I would suggest finding TDC on #1 and get a proper mark to reference from. To find TDC, I like to use a positive stop in the sparkplug hole, not a rope.

Let me know if you need help to do this.


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The numbers you are getting are not really in the sphere of what you would normally see in terms of advance. Take a look here, this is an example of a Sun Distributor Specification card. It tells you what your mechanical advance is, when it comes in, how many degrees at what distributor rpm (double the degrees and rpm for what the motor sees). This is your starting point. You then add initial advance and/or change the curve with weights and springs to have the advance come in fully earlier or later (generally earlier)

http://paramountd.com/Images/Parts%20Large/SpecCardBox1-4L.jpg

Once you know what its supposed to be, you then can use a timing light and a tape on the balancer like Tom mentioned and see whether you are in specification. If yes, you can now start making changes knowing what those changes will actually be, and then testing to see the effect.

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You may have left your vacuum advance connected, and with full manifold (not ported) vacuum.

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I double checked the timing with my older analog timing light, same answer. I disconnected the vacuum and put my gauge on it. I use the non-ported vacuum, fyi.

I read up on checking for TDC. I see some people use piston stops with hitting the stop from either side. TDC is halfway inbetween.

I'm going to try something novel. Ran out of time, it'll be a few weeks before I get back to this. I've got an orthoscopic camera that is narrower than a spark plug. i should be able to put that in and watch for the piston to come up to center. We'll see how close I can get.

Eventually, I still need to replace my oil pan gasket. When the pan is off, I can get an exact TDC.

Mark


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I put a fiber optic camera into the cylinder this weekend. It's really hard to get it just right, but it looks like it's probably correct, at least within 4 degrees.

I think the distributor needs tuning to get the advance increasing more aggressively. Ah, this winter...


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You are still batting a little in the dark here. Getting TDC is a good start but you still need to know what advance is supposed to be in the distributor that you are currently running. This is the most important figure as everything else emanates from that. Without knowing TDC and the specs for the distributor, you have no way to know what you need or don't need, or for that matter, whether the numbers you are getting are even close to what you should be seeing. Once you know what the base line is supposed to be, and can actually measure that base line with the motor running, then you can sort out whether the distributor is bad to begin with, and if not, what modifications to make to get you where you want to be.

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Take an old spark plug, knock the center out of it so you just have the metal part - cut the gnd. tip off. Then tap it for a suitable size length of grade 8 ready rod. Thread the rod out about 1" past the lower end of the plug metal threads and lock it using a nut.

Grind the threads off of the end of the rod about 1/4" from the end, that will go into the cylinder.

remove ALL the spark plugs

rotate the engine until #1 piston is down the hole - check with a screw driver.

put the tool in #1

SLOWLY rotate the crank using a ratchet and socket (DON"T USE THE STARTER !!!) clockwise until the piston stop. Make a mark on the balancer.

SLOWLY rotate the crank using a ratchet and socket (DON"T USE THE STARTER !!!) counter-clockwise until the piston stops.
Make a mark.

REMOVE the tool - TDC is 1/2 way between the 2 marks.

If the engine rotates without contacting the rod - extend the rod further into the cylinder another 1/2"

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WYKP?
The steel rod should be as far down in the cylinder as practical. This is because the OEM and most aftermarket pistons have piston pin offset, and the piston movement in thousands of an inch per degree of crankshaft rotation will differ slightly between BTDC (piston rising) and ATDC (piston falling). This difference quickly becomes insignificant as the piston goes lower into the cylinder in either direction. 1/8" down: big error, 1" down: much smaller error.
The rod's motion may be suspected as a method of measuring the piston's motion in thousandths. It is, and it's linear (directly proportionate), but not equal (unless the spark plug thread is exactly vertical/parallel to the bore axis). The rod will always move faster than the piston, by the cosine of the spark plug thread angle in the head.
If someone knows what that is, I will write and post a chart of plug thread rod position vs. actual piston position.

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Curious observation panic . . . but why would one need to infer piston movement from plughole-rod movement using trigonometry?

Your initial point is spot-on - the further down the bore one can set the piston-stop the more accurate the bisecting calculation.

BTW . . . this technique will not work on a stovebolt:

the spark plug angle is so acute that a threaded rod will contact the opposite side of the combustion chamber.

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True, was re-cycling some stuff I wrote years ago for a "conventional" chamber.

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I ran an experiment, put my original points & condenser distributor back into the Camaro with interesting results. The points & condenser worked better than HEI!

Comparing the mechanical advance curves, the HEI I have is very flat versus the much steeper original. I can either get this HEI set back up or I can go with another HEI. Davis will set one up as you buy it. Pertronix has one.

I don't have a good way to post pictures anymore, so I put the pictures into a video off my youtube account.

I thought my timing was too far advanced, so i backed it off. It starts more difficult and has some flat spots, but still seems to work a little better than the HEI. With the original distributor, I can now idle at much lower 800 RPM than before.

My next step is thus to work on the distributor.

https://youtu.be/_qEEwLWeaVc


Mark
'67 Camaro L6-250
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Here's the table of advances.
Original HEI
800 12.6
900 12.7
1000 14.4
1100 15.7 17.5
1200 17.8
1300 20
1500 21.8
1600 24.3
1800 27.8
2000 30 22.9
2250 32
2500 32.5 24.8
3000 33.8 27
3500 37.4 28.9
3600 37.4
4500 28.9


Mark
'67 Camaro L6-250
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