Read my spark plug...

[theholycow]

I'm pretty sure that ported vacuum never goes higher than manifold, right?

My spark knock only comes when my vacuum advance is behind a check valve, causing advance even when there shouldn't be vacuum. If I use manifold or ported I get the skipping/bucking instead (or maybe "before" is more appropriate than "instead", theoretically).

I like to think that somewhere between those two is just the right amount of advance to avoid skipping/bucking and avoid spark knock.

However, maybe it's just the difference between manual and auto, and I need the extra advance to avoid skipping/bucking plus major redesign or workarounds (CAI, etc) to avoid spark knock.

[GasSavers_Erik]

True- ported will never be higher than manifold, but manifold will give you more advance and efficiency at idle (like if you were running your AC at a stop light).

Maybe the skipping/bucking lean condition is caused by a stuck power valve in the carb- not richening it up like it should when you open the throttle under load. Maybe with no vacuum advance and under load, the slow burning extra lean mix pushes on the pistons well after TDC which doesn't give you much power. Maybe it continues to burn through the exhaust stroke and into the next intake cycle- where the lingering flame lights the incoming charge and causes the intake backfire.

Another possibility- are you sure your plug wires and dist cap and coil are all in good shape? Are your spark plug gaps set too wide? Is the 12v wire/connections going to your HEI ignition module in good shape?

It takes more voltage to fire a spark plug under high compression and maybe some of the current trying to fire a plug on the compression stroke is finding an easier path and jumping to a wire/terminal of a cylinder on the intake stroke. But it seems that this arcing would be occurring all the time, not just when you have less vacuum advance...

[Jay2TheRescue]

Originally Posted by Erik

Is there any way to tell if the manual trans model of your car also used the DS-VMV?

All cars built on that platform were automatic.

[theholycow]

I want to take a moment to thank Erik and everybody else who is still reading this thread. Your expertise, thought, and time means a lot to me. I know it must get frustrating when the thread goes in circles or when I stubbornly disagree with something, but your help is bringing me towards my goals.

Originally Posted by Erik

True- ported will never be higher than manifold, but manifold will give you more advance and efficiency at idle (like if you were running your AC at a stop light).

By "efficiency" in this context, do you mean I will use less fuel usage at idle or better combustion? I find it hard to believe that idling at 1500 can use less fuel than 400-900 with a carburetor, even if the combustion is more efficient.

Maybe the skipping/bucking lean condition is caused by a stuck power valve in the carb- not richening it up like it should when you open the throttle under load.

That's another thing to add to my list of reasons to rebuild the carburetor.

Another possibility- are you sure your plug wires and dist cap and coil are all in good shape? Are your spark plug gaps set too wide? Is the 12v wire/connections going to your HEI ignition module in good shape?

My cap and wires are in good shape on visual inspection. I really ought to try getting better quality parts.

The coil tested good according to the procedure in Haynes/Chiltons, but that was a year and a half ago. Time to test it again.

Gaps should still be good but I could check them.

12v to HEI module...no idea. I'll have to look into that.

So, I had a bunch of thoughts while driving home...

1. Distributor advance behind that check valve is essentially the same as no advance, isn't it? The only difference is base timing. I could get exactly the same result (good driveability, high idle, spark knock in heavy load) by removing advance entirely and retarding initial timing a little.

2. Assuming it was done on purpose, why did someone hook it up like that? What problem were they trying to solve by doing something non-OEM on an otherwise OEM car? In the past 15 years it has been serviced by 3 or 4 different mechanics of varying professionality. In the 15 years that preceded that, I don't know, but there's evidence that Grandpa wasn't concerned about doing things right in his old age (household lamp wire used to repair dome light).

3. Is the spark knock I'm experiencing (in this configuration) really abnormal? If this was a 3 speed auto it probably wouldn't knock...it would downshift. If it's (for lack of better word) normal, that indicates that I need to retard timing a little for overdrive manual compared to 3 speed auto.

4. When I pulled plug wires at idle to test for misfire it was 1500RPM vacuum-advanced idle. What if there is a misfire but all that vacuum advance (or the extra RPM) masks it? I need to try it with no vacuum at the distributor.

5. I agree that an intermittent vacuum leak is the likely cause of my varied idle when using ported vacuum, but why wouldn't it affect anything when on manifold or check valve vacuum advance? Or does the extra advance mask it?

I think these options for vacuum advance might all give me good driveability:
1. Check valve vacuum with retarded initial timing
2. Manifold vacuum with advanced initial timing
3. Ported vacuum with advanced initial timing

As an experiment, if nothing else, I should try the OEM configuration, which I have not tried yet: Ported and non-check-valve manifold vacuum into the SAVM.

[GasSavers_Erik]

Efficiency at idle- If the throttle butterflys are completely closed (so there is a set port size that fuel and air is coming through) and the vacuum is the same amount for the 600 rpm idle and the 1200 rpm idle, then in my mind the only reason for higher idle rpm with a different ignition timing setting would be better efficiency. So- the same amount of fuel is being used, but you are getting more rpm (kinetic energy) out of the heat released from combustion


Check to see that you have good corrosion free connections on the fat wire that plugs into the HEI dist.


1. I am a little confused on where the check valve is and which way the one way valve is facing. Depending on which way way the valve is facing, it is possible that the check valve can store vacuum (like a brake booster check valve does) so the vacuum to the dist never is vented.

So you did still have spark knock when you left the vacuum advance disconnected?

2. Since there were so many of these cars out there, maybe mechanics figured out a bandaid fix and used it to improve drivability or idle- whichever the person was complaining about.

3. Good point- maybe it is unavoidable under certain conditions and you will just need to change your shift points

4. Yes- check for misfire at the lowest rpm possible. I also think it would be good to look at your vac gauge while you pull each spark plug wire and see that it responds the same way.

5. Maybe the idle is so high that the miss is masked

[theholycow]

Originally Posted by Erik

1. I am a little confused on where the check valve is and which way the one way valve is facing. Depending on which way way the valve is facing, it is possible that the check valve can store vacuum (like a brake booster check valve does) so the vacuum to the dist never is vented.

I don't blame you for being confused. I may have never clearly posted about it.

Condition of the car before I began monkeying around: Manifold vacuum from the carburetor -> {not sure, maybe a check valve or maybe straight hose} -> charcoal canister -> TVS in manifold -> SAVM's "mani" port. When I connected the vacuum gauge to that line (where it went from the TVS to the SAVM) it always read 22+ no matter what. It took an hour to bleed off.

Current condition: That same line, without the SAVM involved, is connected directly to the distributor. This effectively stores vacuum in the line and keeps the vacuum advance fully engaged at all times. I get no skipping/bucking, but with heavy throttle I can sometimes get knock.

Why it's hooked up like that: At first I accidentally misidentified it as manifold vacuum.

So you did still have spark knock when you left the vacuum advance disconnected?

I haven't tried that yet. However, I predict that I will have skipping/bucking, which happens at less load than the knock does so I can't say if it would knock or not.

[theholycow]

Weather, time, and money have kept me from working on this more. However, I had a thought...

Chilton has this to say about firing orders:

If a distributor is not keyed for installation with only one orientation, it could have been removed previously and rewired. The resultant wiring would hold the correct firing order, but could change the relative placement of the plug towers in relation to the engine.

What are the chances that something like that happened? In 2009 I made a mistake and had to wire the spark plugs based on the firing order documented in the Chilton book. Could I have the wrong firing order because of a hack someone else did in the past? If so, how would I determine the correct firing order?

[Jay2TheRescue]

If this is the case, Take the #1 cylinder to TDC. Then remove the distributor cap. The rotor should be pointing to the #1 cylinder. If that wire is going elsewhere, then you're miswired.

[GasSavers_Erik]

Jay's method is a good way to tell- but you must already have cylinder #1 right in order for you to be able to use the timing light.

I think they were meaning that say someone had their plugs wire hooked up so that the #1 plug wire was on the side of the cap closest to cylinder #1, and then they pulled the dist and put it back in clocked differently. They they wouldn't be able to get it restarted until they rearranged the plug wires using Jay's method. As long as the plug wires are in the firing order (also consider the direction of rotation of the dist), if one cylinder is timed right, they all are timed right.

Back to my example- a person might take their car to a mechanic and when it comes back, even though it runs fine, they might notice that all of the spark plug wires appear to be stretched really tight. Well, if the mechanic pulled the dist and reclocked it, it is possible that more of the cap terminals might be farther from their respective cylinder than the stock configuration.

[theholycow]

I still haven't worked on it (same reasons - mainly weather and time). However, I'm halfway through my second tank of 93 octane fuel. On 93 my spark knock certainly seems reduced.

I've also noticed that the dieseling when I shut off is very much improved. It shuts down pretty darn well for 1500RPM with a 53 pound flywheel. On 87 it would run-on and detonate and the vacuum gauge would jump around between vacuum and pressure...now it quits reasonably smoothly, reasonably quickly, and never goes into pressure.