Flapper Air Flow Meter

[maston]

Hi all,
Does the air flow meter (flapper) have to be perfectly horizontal in the engine bay ?
Due to the silicone afm to plenum hose that I have fitted mine is at an angle pointing down.
I have a missfire at high revs.
Many Thanks

[mrzigazaga]

Hi Ian...The AFM works predominantly on suction as i understand it..Air being pulled through it opens the flap which registers on the electronic tract inside the top of the unit which in turn reads the amount of air and relays it to the ECU so the correct amount of fuel can be supplied.

Not sure if being at a slight angle would impede its operation?..

You say you have a mis-fire at high revs?...Is this at the same point every time? and does it rev okay after this point?...This does sound like an electrical fault which could be the TPS or the AFM...Have you been able to see a difference when you tried the other AFM from Stu?

I take it you have a healthy spark from the coil?...And your timing is correct.....Ziga

Edited by mrzigazaga on Monday 10th February 11:55

[maston]

Hi Mark,

Cheers for your reply mate.
Yes, with Stu's afm the running is better (thanks Stu for lending it to me mate) but still not perfect.
The reason I asked about the angle of the afm is that I was wondering that if it wasn't perfectly horizontal then there would be a chance that the fuel pump contacts on pins 36 and 39 of the afm could perhaps open under hard acceleration if the afm was not situated perfectly level and therefore disconnect power from the fuel pump and cause a slight missfire ?
I have tested the tps, all good.
Really running out of ideas now

[hallsie]

Im pretty sure it needs to be flat as the flap may bind a bit.

It is sounding like the TPS may be a bit worn, is it the small or big type?

[V8 Fettler]

Disconnect the TPS and it should still reach max revs without misfiring. Won't the flap close or open with gravity rather than just air flow if the AFM is at an angle?

[350zwelgje]

AFM on my wedge is at a small angle, a few degrees and no issues (also due to the silicon inlet hose).
What about acceleration and braking? Will cause also an influence on the flap, so think not to worry.

Shame that after all you did, the misfire hasn't been cured.
Assume you have completed the full checklist available on honestjohn2 website, otherwise would advise that.
Other things that spring to mind: Is the camshaft ok? No loose inlet trumpets in the plenum? ...

Rob

[honestjohntoo]

maston said:

Does the air flow meter (flapper) have to be perfectly horizontal?

mine is at an angle pointing down

I have a misfire at high revs

I look at it this way.

The flaps as shown in this image



are IMMUNE to the vertical forces of gravity!

But, imagine, if you will, that the flap spindle becomes horizontal, then those two flaps are now NOT immune to gravity and would try to hang downwards, with each flap wanting to be approximately 45 degrees to the vertical (depending upon their respective mass).

They can't, of course, because any gravity induced rotation is prevented by the stop fixed to the side wall of the tunnel.

The Flap Return Spring (FR) is also exerting a return force that is supposed to deal only with the flaps' reaction to any incoming rush of air.

Thus the position of the AFM Potentiometer (AP) should be determined ONLY by the force of the inrushing air but NOT being combined with the rogue forces of gravity imposed by the 90 degree change in spindle position.

OK, OK! I concede, the above 90 degree example is deliberately described to illustrate an extreme wrong spindle location of 90 degrees.

However it serves to illustrate that any offset of the spindle away from it's natural vertical position will allow gravity to cumulate it's unwelcome force with that of the Flap Return Spring.

The more the offset the greater the force of gravity acting on the flap.

I'd expect those with a qualification in higher mathematics could calculate the change of force acting on the flap depending upon the degree of spindle offset and to work out at what point it becomes critical.

But in crude terms, my view is that the AFM is designed to operate ONLY in the horizontal plane.

To answer your question, therefore depending upon the amount of spindle offset:

• I would not be surprised at there being an induced misfire at low or idle speeds,

and likewise:

• I would not be surprised if the delicate function of the Fuel Pump Switch Contacts (FPC) was upset at low or idle speeds.

If you wish to learn more about the complete function of the Flapper AFM you can find it on www.vintagemodelairplane.com

Edited by honestjohntoo on Monday 10th February 20:22

[adam quantrill]

Please bear in mind that according to Einstein's theory of general relativity, to the AFM gravity is indistinguishable from acceleration.

So under hard acceleration (maybe 1/3 of a g?) the AFM will "appear" to be tilted backwards by maybe 25-30 degrees to the vertical.

Also bear in mind that the rear of the car will squat down, further tilting the car backwards by a few degrees.

To counteract this you might want to tilt the AFM forwards by 10-20 degrees?

[Wedg1e]

maston said:

I was wondering that if it wasn't perfectly horizontal then there would be a chance that the fuel pump contacts on pins 36 and 39 of the afm could perhaps open under hard acceleration if the afm was not situated perfectly level

Two chances of this: fat chance, and no chance. If it were possible then any other wedge (or other V8-powered car) would be able to accelerate hard enough to cause the same problem. Or, any car accelerating up or down a hill would have the same problem.

The AFM could certainly be mounted upside-down (there are pics about of such installations); the flap is spring-biased and shimmed to keep it more or less central in the AFM bore.
The flap return spring, similarly, has more than enough tension to overcome any tendency of the flap to swing wildly in the breeze - otherwise, again, rapid acceleration of any V8 would cause the fuelling to vary like a woman. And as there are some flapper-equipped V8s out there with seriously more horsepower than most of our cars it would seem unlikely that nobody's noticed it before.

If you've swapped AFMs and the symptoms persist it is extremely likely that the fault is elsewhere.

[Jack Valiant]

Ian

I think your issue if it's with the AFM is with the mounting and securing of the unit fella. Mount it properly and securely. Get rid of the zip ties and jubilee clip bodges! Then as others have mentioned take the throttle pot off after marking, and check carefully that the tracks are ok with a analogue meter across its range of movement then if the same problem persists move on to other checks

Chris

[maston]

Thanks you all for your replies.

The throttle pot is newish and the readings are bang on...0.3v closed throttle rising steadily to 4.3v at WOT so that is fine.
Tomorrow I will install the afm in it's original position again but that means I will have to use the original afm - plenum hose and remove the silicone one which will result in slightly slower throttle response etc.
But if it cures the slight miss fire at high revs then that would be good.
I will report back.

[mrzigazaga]

Hi Ian...As i said before you could use an alloy elbow or joiners if you want to keep the silicone mate...
http://www.google.co.uk/aclk?sa=l&ai=C31St8Ff6...

[Jack Valiant]

As already mentioned fella it only needs a new mounting hole.... Come on!

By checking with a metr I mean with the unit disconncted and in the ohms range carefully check that it is a smooth increaseacross the range of movement. DC chcks are ok but this will show any potential issues on the wiper tracks

Chris

Edited by Jack Valiant on Wednesday 12th February 06:57

[maston]

Jack Valiant said:

As already mentioned fella it only needs a new mounting hole.... Come on!

By checking with a metr I mean with the unit disconncted and in the ohms range carefully check that it is a smooth increaseacross the range of movement. DC chcks are ok but this will show any potential issues on the wiper tracks

Chris

Edited by Jack Valiant on Wednesday 12th February 06:57

lol
Right took advice and now remounted in the original proper place with the afm now being level in the engine bay. Also using the original afm to plenum hose. Seems better again, I'd say 99% Still a very slight missfire between 4k - 5k revs but as I vary rarely drive at high revs (who needs to with 254 ft/lbs of torque !) I'm just gonna live with it.
Happy days...........

Thsnk you everyone who kindly offered advice