1976 Bus - Test for AFM? ** It's still bucking!!**

TrollFromDownBelow · Post by **TrollFromDownBelow** » Tue Jul 07, 2009 5:32 am

Is there a test I can do to know if my AFM is working properly? I have (what appears to be) a mixture problem. I've got a high level of confidence that I've tracked down and fixed any vacuum leaks (although haven't done a leak down test on the engine yet). Going to do a fuel pressure check and compression check tonight.

Gypsie · Post by **Gypsie** » Tue Jul 07, 2009 10:11 am

Bentley gives resistance specs for the contact points on the afm connection port. I believe that should let you know if the they have porper resistance.

I think there is another process to test if the swiper is making proper contact with the 'circuit board' at the various points.

Have you tried Colin's AFM adjustment pocedure? I would start there, as it is just a process to narrow the lean/rich range done with a screwdriver and a tach if you are not comfortable guaging RPMs by ear.

make notes about your starting point and any changes for reference.

airkooledchris · Post by **airkooledchris** » Tue Jul 07, 2009 10:19 am

sorry for the bad scan, its the only one I could find online at the moment.

chitwnvw · Post by **chitwnvw** » Tue Jul 07, 2009 10:33 am

Gypsie wrote: Have you tried Colin's AFM adjustment pocedure? I would start there, as it is just a process to narrow the lean/rich range done with a screwdriver and a tach if you are not comfortable guaging RPMs by ear.

What is this procedure?

Gypsie · Post by **Gypsie** » Tue Jul 07, 2009 11:03 am

http://itinerant-air-cooled.com/viewtop ... hlight=afm

There are other threads that have it in them but this is one.

Search AFM in title only and there are several

chitwnvw · Post by **chitwnvw** » Tue Jul 07, 2009 11:15 am

Maybe we should take what you quoted and put it in tech articles. Sluggo's short piece on "Testing the Brake Proportioning Valve" looks awfully lonely in there.

Gypsie · Post by **Gypsie** » Tue Jul 07, 2009 10:30 pm

I wonder if Colin would have a better turn of phrase than my cut and paste abomination?...

Yes, I wonder.?.?.

chitwnvw · Post by **chitwnvw** » Wed Jul 08, 2009 11:35 am

Your best field test for mixture, once it is running decently enough to not require babysitting at the AFM, is to simply nudge the wiper infinitesimally counterclockwise or clockwise with your chubby finger and listen to what happens:

if the idle goes UP counterclockwise, it is still too lean.
if the idle goes down counterclockwise, it is too rich.
if the idle goes UP clockwise it is too rich.
if the idle goes down clockwise it could be OK if it recovers the instant you let go.

I am running too lean.

Your dynamic adjustment (B) changes the slope of the map. Imagine a nice curve up towards "richer sooner", or imagine a curve towards horizontal which is "leaner later."

Is CCW richer sooner?

Your dynamic adjustment (B) changes the slope of the map. Imagine a nice curve up towards "richer sooner", or imagine a curve towards horizontal which is "leaner later." If you rotate the black plastic cog (by prying the chrome lockwire away from a tooth with your regular screwdriver tip and rotate the cog CCW (richer) or CW (leaner) you either unwind the spring coiled in inside, or wind it up still further.
This adjustment primarily affects the higher rpm/load.

It seems to be affecting my engine differently. The dynamic dial, 'seems' to be enrichening the idle/low rpm/ no load, feels leaner than before adjustment of dynamic dial at higher rpm. By that, I mean that there is a subtle feeling of loss of power at higher rpm/under load. Feels like my carbs did before they were dialed in.

So now we have to check the high speed rpm. Bring engine to 2500 to 3000 rpm and hold it there with whtever mechanical means you can. A screwdriver expertly wedged is good. Sit and watch the engine hold that speed consistently. Now gently nudge the wiper barely 1/16" of an inch (maybe 1/4" if no noticeable change) CCW. Revs go up? Or do they go down? Now nudge CW 1/16" (or 1/4" if no change was noted). Revs go up? We want to know if any of your nudges brought revs up. . .

Are we looking for a best compromise between adjusting at idle and at rpms? Or do we adjust for idle and then do the same at RPMs?, and that's where we leave it?

We really could use some graphs...

TrollFromDownBelow · Post by **TrollFromDownBelow** » Wed Jul 08, 2009 12:57 pm

just curious ... Colin always gives the warning abt no being 'ham handed' with the wiper when adjusting the mixture. I notice when at idle, that if I am ham handed (move wiper tip more than 1/4 inch) in the CCW (richer) direction) it has a significant increase in idle speed (several hundred rpm) before bogging/decreasing.

Is this normal, or is this indictive of a bigger problem that I need to address?

chitwnvw · Post by **chitwnvw** » Wed Jul 08, 2009 1:06 pm

TrollFromDownBelow wrote:just curious ... Colin always gives the warning abt no being 'ham handed' with the wiper when adjusting the mixture. I notice when at idle, that if I am ham handed (move wiper tip more than 1/4 inch) in the CCW (richer) direction) it has a significant increase in idle speed (several hundred rpm) before bogging/decreasing.

Is this normal, or is this indictive of a bigger problem that I need to address?

Pretty sure that is normal. He stated in the quote of Gypsie that what you want it 1/4" of movement w/o affecting rpms.

TrollFromDownBelow · Post by **TrollFromDownBelow** » Wed Jul 08, 2009 7:30 pm

OK - trying to eliminate all potential issues:

Compression test
# 1 115
# 2 ?? Has an timecert so can't check (plug is married to it)
#3 110
#4 110

That's with all plugs in and cold ..

manifold vacuum @ idle aprox 15

Fuel pressure is a steady 33-34 psi at idle 35 at high rpms.
Based on the operational symptoms, and low vacuum with decent compression, I swear it has vac leak...but I'm having difficulty finding it. Maybe the AFM, haven't checked per Bentley .. yet

vdubyah73 · Post by **vdubyah73** » Thu Jul 09, 2009 5:33 am

Did you look for the sneaky vacuum leak through the valve cover gaskets. Does your idle change noticeably if you remove the oil fill cap while running? Does one or both of your valve covers drip oil only when the engine is not running? do your valve cover gaskets look like they've been sucked inward?

TrollFromDownBelow · Post by **TrollFromDownBelow** » Thu Jul 09, 2009 6:23 am

Sure did check the valve covers ... based on posts I found here =). Driver's side did appear to be sucked in at the top, so replaced it.

Idle does change if I remove oil filler cap ... what does this indicate?

vdubyah73 · Post by **vdubyah73** » Thu Jul 09, 2009 6:44 am

TrollFromDownBelow wrote:Sure did check the valve covers ... based on posts I found here =). Driver's side did appear to be sucked in at the top, so replaced it.

Idle does change if I remove oil filler cap ... what does this indicate?

Indicates strong vacuum in the crank case through the crankcase ventilation hose. Disconnect the hose and plug it, see if that changes anything. If vacuum in case is strong enough and the valve covers are warped or the bails are old and tired, replacing the gasket may not be enough.

TrollFromDownBelow · Post by **TrollFromDownBelow** » Thu Jul 09, 2009 7:07 am

Which hose is considered the crankcase ventilation hose? I don't recall seeing a pcv valve ... if it's got one, where would it be located?

Itinerant Air-Cooled

Itinerant Air-Cooled

1976 Bus - Test for AFM? It's still bucking!!

1976 Bus - Test for AFM? It's still bucking!!