Timing / Idle Questions - 1978 Bay Window T4 2l FI

[chrmcc]

Hello, I recently noticed my bus running a bit rough and the exhaust smelling rich.

Some pertinent history - about two years ago the points burned up during a long trip, at that time I swapped the distributor w/a Pertronix electronic w/vacuum advance and a Pertronix coil. I also replaced plugs and cables. At the time I thought I set the timing right, but I am questioning that at this point, I think I focused on the idle timing of 7.5 @875, not the all-in w/o vacuum @3500+

Today, I looked at the plugs, lots of black soot, definitely running rich. I cleaned them up and reinstalled. Then I warmed it up good. I noticed idle was at ~950 RPM with timing of ~18 degrees.

I set the adjustment on the AFM to about the mid-point which brought the idle down just a tad, it had been at 3/4 open.

I adjusted the fuel at the screw near the throttle linkage, I could damn near screw it all the way down before it brought the idle down (but also got rougher). I opened it about 1-1 1/2 turns and it stabilized.

With the timing light connected and vacuum advance disconnected, I loosened and adjusted the distributor. I adjusted the idle via the distro to 875 which brought the timing down to ~4 degrees. However, now revved to 3500 the timing was around 28 degrees, which I believe is what we want.

I decided initial timing be dammed and locked the distro down.

When I reconnected the vacuum and revved it to 3500 the timing was ~40. Is that expected, and ok? Again, I verified w/o vacuum it was 28 at 3500.

It drove well afterwards and seemed to have better power but I am wondering why I cannot get the both the initial timing and all-in within spec. I am questioning my choice of distro/electronic ignition.

I plan to add an AFR, head cylinder temp, and oil temp gauge to get better insight.
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Thanks in advance!

Chris McC

78 VW Sunroof Bus - mostly stock 2000cc FI
99 Porsche 996 Cab - stock 3.4 flat six

[heimlich]

Look at that. A fellow Houstonian.

18 degrees at 950 sounds like too much. Is this with the vacuum hose connected? If so it sounds like something is pulling too much vacuum.

If you have something pulling too much vacuum changing the distributor won't fix it unless the vacuum canister is the problem.

Bring it over to me and I can take a look at it for you.
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[busdaddy]

[chrmcc]

[heimlich]

[chrmcc]

[heimlich]

[raygreenwood]

If it WAS running well at some point with the distributor you have....the LAST thing you do is tweak the AFM adjustment.

So you have a Pertronix Single vacuum/dual advance distributor right? Tis one?

https://www.jbugs.com/product/186504.html?abalt=1&...DgEALw_wcB

Becuase it has mechanical advance as well as vacuum, as idle speed increases, if it begines mechanical advance you will see extra timing when you put the light on it at idle. While the may not be correct...the function is NORMAL>

The first thing you should be looking for is WHY the idle speed has increased which is most probably what is causing the advance.

Typically when you have a vacuum leak in L-jet, it bypasses the AFM flap so you run lean. Since you are not running lean (the black plugs)....the first things to check are fuel pressure and a properly functioning fuel pressure regulator.

At idle, the vacuum should be at about its highest. This causes the rising rate fuel pressure regulator to drop fuel pressure.
If the regulator is shot or teh vacuum line is leaking or disconnected, you get maximum fuel pressure at idle.

This will run you rich. It can also run idle a little bit high as its extra fuel. As long as there is enough air coming in to support combustion....instead of stalling/flooding, it may idle higher which is what can give you higher idle timing.

You have a lot of things to check before you start tweaking the AFM. Ray

[chrmcc]

[wcfvw69]

Chris,

As an FYI-

If your bus is going to be a long term keeper for you, consider replacing that Chinese distributor. I've autopsied the Pertronix and most of the other Chinese distributors and to be frank, they are junk like most poorly made VW parts from China.

The original distributors are vastly superior to the Chinese clones. Most of the Chinese distributors I've autopsied were sent in from customers who couldn't get their VW to run right with them. The customers wanted to know what was wrong with them. So, I autopsied them, shared my findings and threw the parts in my metal recycle bin.

I'm not trying to sell you anything but rather, give you and others a heads up.

Cheers-
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[skills@eurocarsplus]

[Wildthings]

The stock distributor has two return springs for the centrifugal weights, one uses thin wire and a tight coil and is under tension at all times. The second one uses larger wire and has a loose coil and is not under tension until the centrifugal timing has already advanced by 10° or so. Your symptom could be because the spring with the smaller diameter wire either broke or came off it's pegs.

I agree with others that the Pertronix distributors are trash. I thought nearly twenty years ago that this might be a good way to get around the problem with low quality Bosch points so bought one, it pretty much had problems right out of the box so I got the vendor to send me another one which also had problems right out of the box, so I sent them both back and installed a Pertronix ignition module in my old distributor, but it failed after a few months so some non Bosch points went in anf I have had no problems since.

[heimlich]

[Bill Schwimmer]

I am curious if you have one of the aluminum aftermarket timing scales. I chased my tail for a bit timing my bus. I matched up an OE plastic scale to it. And they are wayyyy off to the retarded side.

[Wildthings]

[mikedjames]

With ported vacuum if the throttle plate covering the hole is slighly uncovering it at " idle, " the ported pressure drops from atmospheric as the hole is uncovered down to about half atmospheric.
So messing with the throttle endstop screw to fix a different problem causes too much advance at idle, made worse by the engine running faster and winding on centrifugal advance.

The $15 China distributors are made to have more maximum advance than a Bosch. Because that makes them seem more exciting.

My China distributor is sawn off and used as a No1 indicator for setting valves alongside crank fire system. Best use for a Chinese part...
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[skills@eurocarsplus]

^^^
this guy gets it
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[raygreenwood]

In my last comment I said that a vacuum leak would bypass air past the AFM flap and cause a "lean condition" and not the rich condition you are gettintg.

That is both correct and incorrect. Let me clarify please.

On this crude diagram, IF....you have a vacuum leak that is leaking anywhere BETWEEN the AFM aand the OUTSIDE of the throttle body (TB)....that air bypasses the AFM and gerts no fuel assigned to it so it WILL cause a lean condition.

This would NOT cause your black spark plug or your excessive idle timing issue.

HOWEVER:

If you have a vacuum leak into the intake system DOWNSTREAM/inboard of the TB....while this will still bypass air around the AFM...causing a lean condition of some level (could be big or small depending on the vacuum leak size).....because the air leak is actually on the HIGH VACUMM side of the TB....it can cause several other issues.

1. The leak I first described (between the AFM and TB) only affects fuel mixture. It does not affect VACUUM level.

2. The second type of air leak (downstream of the TB)...simultaneusly affects fueling by bypassing the AFM (lean input) but it also reduces vacuum.

This mimics a "throttle open" position. You are getting air past the throttle flap. While one might see that still as lean....the actual vacuum leak CAN or WILL affect the vacuum operated fuel pressure regulator.

At idle with no vacuum leaks, the rising rate fuel pressure regulator should give you about 28 psi. When you open the throttle and vacuum bleeds off you can get up to 36 psi of fuel pressure.

If you are idling and 29 psi of fuel pressure is what you should have...that is considered "100%" at idle. This means that every 1 = 3.517% of your fuel mixture.

So if you have a vacuum leak at idle on the inboard side of the TB....lets say you are at 36 psi when you should be at 29 psi. ...thats 7 psi too high ...with each 1 psi worth 3.517%....you could easily be 24.6% too rich across the board at idle.

So....with a small amount of extra air leaking in AND extra fuel being added in because of that vacuum loss....you would easily see a rich burn, higher idle speed and higher idle timing...IF...centrifugal advance is kicking in due to the high idle.

A quick check. At idle, pull the vacuum hose off of the advance unit. If the idle timing drops down again...and/or you feel suction at that small vacuum hose....then either your throttle is not closing all the way or the throttle plate is worn out. With the throttle plate fully closed there should be no vacuum advance signal.

Next check. Put a gage on the fuel rail. If yoyur fuel pressure is higher than 28-29 psi at idle....well...you have found the issue. To find out why....Take a hand vacuum pump and apply vacuum to the fuel pressure regulator while idling. If the pressure drops to 28-29 you have verified that the regulator is working.

Start checking for vacuum leaks now. Check the 0-ring between TB and plenum, runner boots, injector seals, and vacuum line or gasket to the intake plenum all the way to the gaskets between runner and cylinder head.

Ray

[skills@eurocarsplus]

[raygreenwood]