Optimize the spark curve for a SlantSix.

[timk225]

I'm tuning up the 225 in my 1973 Duster, and I feel that the distributors' centrifugal weight / advance curve could be better. I took the distributor apart last winter and fully cleaned and regreased it, but made no changes. It had the one light and one heavy spring.

I bought a timing light for the first time in over 20 years, just for this car. But I cheaped out and got a basic one, not a dial back type.

I'm running around 3-4 degrees initial advance of timing, but even with 93 octane I think I hear it pinging at higher rpms under heavy load. I have to wonder if it is getting too much advance from the weights and slots.

I already reduced the base timing several degrees, which helped, but it is still there at high rpms and wide open throttle.

Has anyone developed a better slot length / spring combo / base timing number that will work better on a mild performance build?

So far my /6 is all stock except for a Holley 350 2 bbl on a Super Six iron intake. I have a TTI 2.5" exhaust to put on the stock exhaust manifold but haven't done it yet.

I have a few other build items to install, like an RV10-RDP cam, new double roller timing chain, and it'll get the head ported if and when I go through the engine. It isn't leaking oil but it is using a quart every 600-700 miles, and I don't think it was ever apart, so I'm sure the valve seals and ring seal is not optimal.

 

[WV64Signet]

@RustyRatRod
@slantsixdan

 

[halfafish]

Also @halifaxhops !

 

[cudamark]

Have you verified that the TDC timing mark on the balancer is correct by using a piston stop?

 

[Mattax]

I bought a timing light for the first time in over 20 years, just for this car.

You don't need the dialback. Its just convenient.

I'm running around 3-4 degrees initial advance of timing,

At what rpm?
Under 750 rpm it shouldn't matter, but it might depending on the distributor.

I think I hear it pinging at higher rpms under heavy load.

You will need to check timing at at those higher rpms.
Did I mention that a tach is needed?
Also you will have to mark the damper or make (or buy) timing tape for the damper. There is a "how to" article called Hillbilly timing tape. ;)

I have to wonder if it is getting too much advance from the weights and slots.

Not the slots or the weights. The heavy spring combined with a the light spring control the mid to high rpm advance. Unless your racing this 6 and running it higher than most sane people. LOL.

 

[Mattax]

It's got about 4 degrees advance with the distributor vacuum advance hose unplugged, and around 16-20 with it plugged in

getting it to idle right is a problem. It will only idle around 1500 rpms

I want to replace the vacuum advance module

Did you replace the vacuum advance?

Did you get the slow idle down to normal. Or at least normal for '73?

If this is the original distributor for the engine, what's the tag number?

Here's an example of the acceptable timing advance for a '73 225.
The initial can be set plus or minus 2.5 degrees, but it has to be at 750 rpm or less. By 800 rpm its advancing.

This only holds true for that distributor.
A distributor with a different curve will need a different initial, and end up with roughly the similar timing from 1800 rpm on up. For example a non-smog '67 distributor was set at 5* BTC at 550 rpm.

 

[timk225]

The distributor tag number is 3755042. There's also a 50 and a 2 on the tag. Vacuum advance was ok. I've since changed the carburetor to a 350 Holley 2 bbl, it'll idle slowly now. I have a tach in the car. I'll have to check the timing and see what the numbers I get are.

No I didn't use a piston stop to verify the timing mark is accurate.

 

[Mattax]

The distributor tag number is 3755042.

That shows in the shop manual as the original distributor for a 225 with automatic that year.
The above graph shows the acceptable advance based on the specs in the FSM.
1 bbl carb was all that was offered that year. A carb with more capacity should allow the engine denser air at mid to high rpm with the throttles fully or nearly fully open. This combination should burn faster and therefore need a little less timing under those conditions.

I have a tach in the car. I'll have to check the timing and see what the numbers I get are.

As you can see from the graph, the advance is very quick above 750 or 800 rpm.
This is why rpm is so important.

You can make small adjustments to the spring tension by turning the spring perches. The perches are mounted as eccentrics. More tension on the primary spring will delay the advance so it starts at a higher rpm. Earlier tension on the secondary spring slows the advance sooner.

Two other quirks that may be cause pinging.
What is the status of the EGR system?
The EGR valve should be closed when the air temps are cool, and when the throttle is nearly wide open, and IIRC at idle.
The slant six did not use ported vacuum for determining the throttle at idle or near wide open.
Instead it used venturi vacuum. Doubt your new carb has a way to connect to that.

Finally, what is the status of the CTO? That's the coolant temperature switch which was in some (all?) OSAC setups. If the coolant was getting over 225*F, it switched the vacuum advance signal from ported to manifold vacuum. If coolant temps are staying reasonable I wouldn't worry about it, but is another little gimick that was needed with the retard timing for the early emissions systems.

 

[Mattax]

In this thread I show an example of pre-smog distributor timing for a 225, and then we look a much later distributor for a 2 bbl 225 in the catalytic converter era. That seems to be what the OP had.

Post in thread 'Distributor recurve'

Jul 20, 2021

Plotted out the one for a '78 2 bbl.
Specs looks like potentially decent starting point for cammed engine that needs more intial.

• Mattax

 

[slantsixdan]

I'm tuning up the 225 in my 1973 Duster, and I feel that the distributors' centrifugal weight / advance curve could be better. I took the distributor apart last winter and fully cleaned and regreased it, but made no changes. It had the one light and one heavy spring.

That's how they were built.

I'm running around 3-4 degrees initial advance of timing, but even with 93 octane I think I hear it pinging at higher rpms under heavy load.

High-test gasoline is not necessary or beneficial for a stockish Slant-6 tuned properly, so finding and fixing the cause of this ping should be near the top of your list.

I have to wonder if it is getting too much advance from the weights and slots.

Maybe, but there are other possible causes, too. Restricted exhaust, improper or sub-optimal spark plug selection and/or plugs installed without removing their metal ring washers first, lean mixture, carbon buildup in combustion chambers, mud buildup in cooling passages, poor oil control (oil getting into combustion chamber) — even if it's not enough to cause much or any visible smoke, it'll take cause the effective antiknock performance of the gasoline to nosedive, and it'll carbon up the combustion chambers, aggravating the ping further.

I already reduced the base timing several degrees, which helped

These engines with those distributors tend to run optimally with base timing around 5° BTDC. The factory setting of TDC gives mushy acceleration and driveability and poor MPG.

It isn't leaking oil but it is using a quart every 600-700 miles, and I don't think it was ever apart, so I'm sure the valve seals and ring seal is not optimal.

There y'go. That right there will easily cause your pinging, or at least contribute to it.

For distributor dial-in, see Doug Dutra's article here. For better ignition, do the HEI upgrade . Spark plugs: as described in this post.

 

[timk225]

I did replace the spark plugs when I got the car, they have about 1500 miles on them now. I'd have to check what type and if the ring washers are still on or not, I think I took them off.

The exhaust is the stock 1-7/8" OD pipe, I replaced the muffler with a Walker turbo style last autumn. I have a TTI 2-1/2" system waiting to go on. I did port the exhaust manifold outlet some when I put the Holley 350 on a few weeks ago.

The carburetor has #63 jets in it, mileage has been in the 14-15 range. The Holley 1920 was about the same for mileage.

I flushed the cooling system and it has never had an overheating problem. It has a new radiator, hoses, and water pump. I'm running a new 180 thermostat.

For the first several tanks of gas I ran Seafoam and / or Marvel Mystery Oil in the fuel, to try and get the combustion chambers cleaned out, if they were carboned up in the first place.

I will check the plugs and a few other things. I will get a piston stop tool from Summit Racing and check for the TDC mark accuracy.

The temperature control valve and the EGR are not connected to vacuum lines. The new carburetor has only 2 vacuum lines. One to the distributor and one to the PCV valve. Manifold vacuum goes to the power brake booster and the heater / AC control air door assembly. I thought about leaving the PCV just open to the air, as it is probably letting an oil mist into the intake, further hurting my problems.

I have tried in the past to test for vacuum leaks in the various hoses to minimize problems.

 

[timk225]

I pulled the spark plugs this evening. They are Champion 3405's. I did have the sealing rings still on them, so I took them off. One step towards running better!

What's the effect of taking the sealing rings off, I can't see how it'd make much difference.

The spark plugs all looked pretty much the same. Clean center electrode, white side electrode. Although on #6, the side electrode was more of a gray color.

Analysis?

 

[Newbomb Turk]

I pulled the spark plugs this evening. They are Champion 3405's. I did have the sealing rings still on them, so I took them off. One step towards running better!

What's the effect of taking the sealing rings off, I can't see how it'd make much difference.

The spark plugs all looked pretty much the same. Clean center electrode, white side electrode. Although on #6, the side electrode was more of a gray color.

Analysis?

Pictures?

 

[timk225]

Here's the plugs I have.

 

[Newbomb Turk]

Here's the plugs I have.

View attachment 1716453440

View attachment 1716453441

We need pictures of the business end of the plugs. The ground wire and the nose.

 

[timk225]

Yes, I figured that's what you meant. It'll be a day or two before I can get those. I should have thought of taking photos today.

 

[cudamark]

I pulled the spark plugs this evening. They are Champion 3405's. I did have the sealing rings still on them, so I took them off. One step towards running better!

What's the effect of taking the sealing rings off, I can't see how it'd make much difference.

The spark plugs all looked pretty much the same. Clean center electrode, white side electrode. Although on #6, the side electrode was more of a gray color.

Analysis?

IMO, it would probably make the situation worse if anything. It will slightly RAISE the compression ratio, not reduce it. Not only that, but, if has always had the sealing ring, the threads on the end of the plug are now being jammed into some deposit build up which could possibly make removing the plugs next time a bit harder.

 

[timk225]

I did a bunch of work today. Changed the intake valve stem seals and tested the timing mark with a positive stop tool. I had ordered a piston stop and these valve seals: 1973 PLYMOUTH DUSTER MAHLE Original SS45298 Mahle Original Valve Stem Seals | Summit Racing from Summit. I think I should have got these instead: 1973 PLYMOUTH DUSTER MAHLE Original SS45403 Mahle Original Valve Stem Seals | Summit Racing . I am assuming these with the wires actually clamp onto the valve guide and stay down, as the other umbrella type seals had an .810" inside diameter while the guide OD itself is only .630". How are these umbrella seals supposed to seal anything when they don't even touch the guide? The engine had the same kind already in it, they were rock hard and 3 were cracked down the side. The #6 cylinder had the biggest cracks, which might have contributed to its cylinder's side electrode being a little darker.

I did the piston stop test while the plugs and rocker shaft assembly was off the car, for minimal interference in feeling the stops. First test was at 9 degrees advanced then turned backwards to 7.5 degrees ATDC. Then I turned the stop adjuster out a turn to get 13 degrees advanced and backwards to 11 degrees ATDC. So if anything, the timing mark might be advanced by a degree or so. Pretty close.

The engine itself was pretty clean inside. The spark plugs are laid out in order with #6 on the left and #1 on the right. I also had a new set of O rings for the spark plug tubes, I put those on too. And Amazon is sending me a new valve cover gasket in a few days. The old one was too hard and brittle to reuse.

Are there certain use cases for the positive stop vs the umbrella style seals?

 

[timk225]

Any thoughts on the 2 types of valve seals?

 

[cudamark]

I always liked P.C. seals myself....like the 2nd version.....especially on the intake valves.

 

[timk225]

I did more reassembly today, and am ready for the valve cover gasket to arrive tomorrow, and put it together and start it up.

I invested a whole $20 in a Pittsburgh torque wrench at Harbor Freight. I read its instructions, gave its spring the pre-use "exercise" as specified, and torqued the rocker shaft bolts back to 25 ft lbs. I marked the pulleys to turn the crank 1/3 turn at a time, and went through the valves to check their lash settings. I was planning for 13 / 23 cold lash. As it turned out, it was already pretty close to that. I tightened up a few intakes very slightly, but no exhausts needed to be touched.

 

[Killer6]

IMO, it would probably make the situation worse if anything. It will slightly RAISE the compression ratio, not reduce it. Not only that, but, if has always had the sealing ring, the threads on the end of the plug are now being jammed into some deposit build up which could possibly make removing the plugs next time a bit harder.

The heat path for the plug is shorter & more efficient thru' the Al-U-minimum tube w/o the washer, which insulates the plug from the head. The difference in compression is almost -0-, & so is it's effect on detonation, compared with the quicker heat dissipation of running sans washer. It sounds like serious oil contamination/carbon build-up in the combustion zone could be root cause here,..yes, modern PC-style seals will perform much better, unless the guides are complete sh*t.