Secrets of the AFM

[vwlover77]

Ever wonder what that little AFM wiper is doing while you drive your Bus? Well, Colin is trying to help me troubleshoot a "pogo-ing" under acceleration so I shot a short "undercover" video. You may enjoy it if you're an L-Jet geek!

http://www.youtube.com/watch?v=ZkoqQdeym14

[Bleyseng]

ok, cool! Why does it flutter?

[dtrumbo]

The dog was happy you came home!

[ruckman101]

Funky.


neal

[vwlover77]

Yes, Ellie the golden retriver was happy to see me back!

Colin's current thought is that I've got some sort of ignition problem - missing under load. I'm checking out the ignition system now.

[Bleyseng]

Are you running the triple tip spark plugs? Those really help and so does new plug wires after a new cap, rotor and using a pertronix.

[vwlover77]

No triple tip plugs. But, I do have a Bosch cap, rotor, plugs (W8CC), and plug wires that are basically new. The Pertronix has been in place for quite some time.

This is not a high-compression engine.... Getting a reliable spark should be easy.... Key word: Should!!!

[Amskeptic]

No triple tip plugs. But, I do have a Bosch cap, rotor, plugs (W8CC), and plug wires that are basically new. The Pertronix has been in place for quite some time.

This is not a high-compression engine.... Getting a reliable spark should be easy.... Key word: Should!!!

P.S.
I couldn't be happier with my fine breaker points and factory-original condensor.
Explain why my distributor cap wire pull cylinder dropout test is NOW dead-perfect consistent amongst all cylinders, whereas the Pertronix always threatened that #3 was dying because it barely dropped in rpms when I pulled #3 wire terminal off the cap? I thought it was injectors, swapped #3 with a known strong cylinder, no change, I thought it was hydraulic lifters going flat, jtauxe obtained new ones but thankfully the Pertronix melted just outside his house ... I think the Pertronix had a poorly moulded trigger magnet inside the plastic wheel deal.

[vwlover77]

I can take a hint ....

I pulled out the Pertronix and put the points and condenser back in. Timing set to factory spec, points reading 46 degrees of dwell.

Verdict: No difference.....

I was thinking about what could be messing with the airflow after shifting into 3rd, and decided maybe the decel valve could be the culprit. Blocked it off and drove the Bus again.

Verdict: No difference.....

Blargh....

[Amskeptic]

I can take a hint ....

I pulled out the Pertronix

No hint there, Don. I just had a loafing cylinder at idle with the Pertronix, and I waited until it committed suicide in Los Alamos before putting the points back in.

and put the points and condenser back in. Timing set to factory spec, points reading 46 degrees of dwell.

Verdict: No difference.....

At least showing that points do work as intended ...

I was thinking about what could be messing with the airflow after shifting into 3rd, and decided maybe the decel valve could be the culprit. Blocked it off and drove the Bus again.

Verdict: No difference.....

Blargh....

I cudda tole you THAT, Skippy.
The decel valve is closed at all times but at overrun throttle release.

I got rid of the minor bucka-bucka I was telling you about today, outside of Fort Worth. The heat wave broke, from 102* to 81*, so I leaned the mixture to heat things up and save a little fuel. No more bucka bucka. Only three cog teeth lean did it.

It is a tricky little sweet spot from rich bucka bucka to lean bucka bucka.

Go three teeth lean, then another three teeth lean, if symptoms do not go away. If they diminish a little, try another three teeth lean. If your idle goes to hell, get that mixture screw turned in a couple of turns rich.

If everything goes badder still, reverse the above.

What did you do about the tires, even though your video did not show the dancing engine like I have seen in bad porpoise cases?
I will be in your area .... some time. Maybe with a pokey old Type1 bus ...
Colin

[vwlover77]

So here is the latest update:

Tried a new distributor, with both points and Pertronix, no difference.
Tried going really loose and really tight on the AFM cog wheel spring with appropriate wiper position adjustments to maintain a reasonable overall mixture, no difference.
Tried leaning the mixture and enriching the mixture, no difference.
(I've done all these tests with an LM-1 attached.)

My current thinking:
The AFM flap responds to the airflow into the engine. Something is causing the airflow to be slightly erratic, and apparently it's not mixture related or ignition related. My guess is that I've got an intake valve (or more than one) that is not fully closing or not sealing properly. The leaky valve would allow air to be pushed back into the intake tract during the compression stroke, interrupting the smooth flow.

I think I need to do a leakdown test to find test this theory.

Another strange thought occurred to me. Is it possible that the valve seals fine but a hydraulic lifter has pumped up too far and the valve is being held open? Hydraulic lifters bleed off oil down the center of the pushrod as they operate, correct? What if one doesn't bleed down at the right rate? Could it hold a valve open?

[RSorak 71Westy]

My understanding of hydraulic lifters is that they dont change height rapidly. I.E. they just move fast enough to take up the overall slack and then generally attempt to stay in this position. Have you ever tried to compress a pumped up lifter and see how slowly it bleeds down?

[Amskeptic]

So here is the latest update:

Tried a new distributor, with both points and Pertronix, no difference.
Tried going really loose and really tight on the AFM cog wheel spring with appropriate wiper position adjustments to maintain a reasonable overall mixture, no difference.
Tried leaning the mixture and enriching the mixture, no difference.
(I've done all these tests with an LM-1 attached.)

My current thinking:
The AFM flap responds to the airflow into the engine. Something is causing the airflow to be slightly erratic, and apparently it's not mixture related or ignition related. My guess is that I've got an intake valve (or more than one) that is not fully closing or not sealing properly. The leaky valve would allow air to be pushed back into the intake tract during the compression stroke, interrupting the smooth flow.

I think I need to do a leakdown test to find test this theory.

Another strange thought occurred to me. Is it possible that the valve seals fine but a hydraulic lifter has pumped up too far and the valve is being held open? Hydraulic lifters bleed off oil down the center of the pushrod as they operate, correct? What if one doesn't bleed down at the right rate? Could it hold a valve open?

Any of your valve train musings would wreak havoc on the rest of the running parameters, particularly the idle. I need to get up there.
Colin

[vwlover77]

Latest LATEST update:

It was not the ignition system.
It was not the fuel-injection.
It was not the valves or lifters.


What was it????

IT WAS THE TIRES!

Got new Hankook RA08 tires installed on all 4 wheels and the hesitation / surge / porpoising is GONE!

As Colin had surmised earlier, an out of balance or out of round or "squriming" tire must have been "exciting" the AFM flap and causing the flutter. And apparently all 4 of my tires were bad because rotating them front to rear never improved things.

Colin is a GENIUS!! (As if we didn't already know that) =D>

[steve74baywin]

IT WAS THE TIRES!

out of round or "squriming" tire must have been "exciting" the AFM flap and causing the flutter. And apparently all 4 of my tires were bad because rotating them front to rear never improved things.

That is a trip.
In fact, check it again and make sure it isn't still doing it.