Date:         Thu, 16 Apr 2020 09:28:06 -0700
Reply-To:     Alistair Bell <albell@SHAW.CA>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Alistair Bell <albell@SHAW.CA>
Subject:      AFM failure
Content-Type: text/plain; charset=utf-8

I’m still working an evening shift to help with distancing at work 

So I have the days to do chores and fool with van. 

Couple days ago I’d planned on checking timing and buying a new starting battery. 

Checked timing, adjusted slightly more advance ( seemed to have more of a digifant hesitation at throttle tip in than usual ), and headed off to get battery. 

Van ran terribly. No power at throttle tip in, then it’s rev, then bog. Got the battery and made it home. 

I must have knocked a wire off? Maybe dizzy not screwed down? 

Checked timing again, checked for loose connections. Checked plugs and wires. Swapped in a spare distributor. Checked for vacuum leaks. Checked a lot of things in a haphazard way. 

Spent quite a while very puzzled. I was sure it was something to do with my timing adjustment. I didn’t go to Bentley and start doing all the test measurements.

But, finally maybe a clue. Hand on tb linkage trying to rev, sometimes felt like an intake back fire . Not really , but you know. And a metallic clink , sharp, right there. 

Took filter off, checked inside tb. 

Then finally some brain cells decided to work. I tried running with AFM disconnected. Ran better. 

Running out of time ... I have two old spare AFM, both from inline fours. Hooked one of them, up sans filter . Van ran fine! 

Got that afm on the filter box etc and headed to work. It’s not perfect by any means but got me there and back. 

Next morning I looked at faulty AFM. Carbon track scored but not as bad as the old AFM I swapped in.
But the resistance reading was awry. I don’t mean the varying resistance , I mean actual drop outs, no cont in spots. And I suspected the pivot point contact was bad. The AFM does not have the additional wire to pivot point that the later models have.

Did the cleaning, tweaked wiper just slightly to avoid scoring path. Cleaned and polished pivot point etc . Added a wire to pivot point.

Connected 5 volts to it and measure output with wiper sweep. The voltage increased with no dropouts. The values very close to what I saw published by dan houg on the samba post about his measurements of aftermarket AFM boards.

I installed into van. Used volt meter on O2 sensor, idle valve disconnected, engine warmed up. AFM mix screw adjustment ... was able to get in range at idle backing out the AFM screw more ( 0.5-0.7) but was 0.9 v above 1500 rpm. I loosened the screw on the wiper arm , the little Phillips head one, and very slightly moved wiper towards start of carbon track. That let me screw in the AFM screw closer to where it was to start ( about two turns out from bottom). It only dropped the high rpm voltage by a little.
Then I tightened up the AFM clock spring by one notch. That did it , could get the high rpm voltage to 0.7 or so.

That was good enough for me for now. Went to work. Van drove very nicely. Better than it did before the AFM failed. Throttle tip in is nice, power is good. Part throttle at low speed has no jerky jerky . I used to get inconsistent part/trailing throttle smoothness which I had put down to bad throttle position switch and or setting. I’ve spent a lot of time trying to get that adjusted right. Sometimes it would be fine, other times not. I’ll see if the AFM fix has any long term effect on this. It has improved it so far.


I will retest the settings with the volt meter on o2 sensor, and re check timing. I find it’s a back and forth thing with the adjustments. Because I really don’t know what I’m doing :-)

Lesson for me in all this. Being obsessed that it was a timing or ignition problem, and not stepping back and rethinking. 

Alistair