robmack wrote:My understanding is that the TB sync procedure with butterfly valves closed sets the correct idle speed of the engine.  Adjusting the brass coloured screws varies the amount of air allowed into the cylinders through bypassing the closed butterflies, making sure it is uniform across all 3 or 4 cylinders.  Idle speed is not set by adjusting the stop screw (common misconception).

Once the butterfly valves are cracked open by twisting the throttle, the amount of air introduced through the main throttle tubes overwhelms any air that may be flowing through the bypass jets.

At small, mid and full throttle openings, the synchronization of the air across all intakes is guaranteed by the adjustments that were made to the TB assembly at the factory (via the interlinkage screws with the blue paint)

RicK G wrote:The ECU cant adjust the idle speed all it does is react to what is fed to it from the sensors and adjusts the amount of fuel injected.

robmack wrote: Adjusting the brass coloured screws varies the amount of air allowed into the cylinders through bypassing the closed butterflies, making sure it is uniform across all 3 or 4 cylinders. Idle speed is not set by adjusting the stop screw (common misconception).

If BMW intended the idle mixture to be adjustable the AFM flap would need to be set partly open so as to have some adjustment either way... lean or rich.

The FICU is an analogue system, therefore the amount of fuel is not programable.

Adjustment of the TB air bypass screws will not be valid unless the butterflys are fully closed as an open butterfly valve will render the effect of the bypass virtually ineffective.

beamer: "... Which is the primary restriction to airflow..."
It has to be the TB air bypass adjustment imo

• L-Jetronic is an open-loop EFI system.  It was designed long before closed loop was an itch in some engineer's groins.  There is nothing in L-Jetronic to feedback combustion completeness information to the ECU to cause it to correct fueling.
  
• The ECU is not an analogue computer.  It has a digital computer in its heart but depends on analogue sensors.
  
• The TPS informs the ECU when the TB is at idle, with fully closed butterfly valves. This causes the ECU to select a fuel map that pre-determines the injector open time at idle
  
• Combustion air is drawn in from the air box to support ignition.  The sync screws in each TB determine the volume of air supplied to each cylinder for such ignition. They are adjusted to ensure balance in the volume delivered to ensure a smooth idle.
  
• The MAF air bypass provides just the right amount of additional air needed to ensure complete combustion (i.e. low CO2 levels)
  
• Seating the TB sync screws fully will deny air for combustion and the engine will stall.  They are opened to 1.5 turns as a baseline for the correct amount of air to support combustion at proper idle given the injector open time.  Closing the screws will deny air and lower the idle speed; opening the screws will permit more air and increase idle.
  
• Twisting the throttle will (1) move the TPS off-idle changing injector on-time because a different part of the fuel map is selected, (2) increase RPM and change spark timing, (3) introduce a need for more air which will open the MAF barndoor, overwhelming the MAF air bypass effect and (4) cause more air flow through the TB throat, overwhelming the effect of the TB sync screw air bypass
  

robmack wrote:

• The MAF air bypass provides just the right amount of additional air needed to ensure complete combustion (i.e. low CO2 levels)
  

Beamer wrote:@indian036 , this is not a microcontroller system but what you say seems to be equally valid for a purely anologue  electronic system.


@all, I'm not discussing the setting of the idle speed.

The question I am trying to clarify is whether the warning about NOT using the throttle cable ( twistgrip ) when syncing the TB is valid.

One thing that is important to know when setting the butterflys is dont use the choke lever to set the position because it pulls on the #4 body where the main throttle cable pulls between #2 and #3 body and the slack built into the adjusters sets different openings from one to the other. It can be a real trap.

It seems to me that it should be done under true running conditions, ie. using the throttle cable with any play in the linkage which it may or may not have.

Kaptain Holister wrote:+1 all that except

No matter what adjustment you make to the AFM air bypass, it will not affect the amount of air entering the cylinders for combustion. Adjustment here will only alter the position of the barndoor which as far as I understand, should be closed at Idle.

robmack wrote:
[*]L-Jetronic is an open-loop EFI system.  It was designed long before closed loop was an itch in some engineer's groins.  There is nothing in L-Jetronic to feedback combustion completeness information to the ECU to cause it to correct fueling.


[*]The ECU is not an analogue computer.  It has a digital computer in its heart but depends on analogue sensors.

Rick wrote:dont use the choke lever

robmack wrote:
KH, so that I'm not misunderstanding what you've written, I'll replay it in my words.  You're saying that the adjustment screw on the side of the MAF physically alters the position of the barn door - opening it more when adjusted inwards, closing it when adjusted outwards.  There is a mechanical linkage to the door.  Is this what you meant? (apologies ahead of time if I misinterpreted).

1. If the bypass is adjusted for idle mix here, the barndoor and wiper will adjust accordingly. How do you find the correct setting for this anyway unless you are looking for idle speed. I know bugger all about carburettors but I thought idle adjustment involved throttle and mixture.
   
2. If the adjustment for idle mixture results in the barndoor slightly open (rich) then this will result in a slightly rich mix across the whole throttle range as you've effectively biased the wiper output towards the rich end.
   
3. There will not be a complete range of adjustment for idle at the AFM anyway because at idle the barndoor will likely be closed or close to it so probably the only adjustment will be to the rich side by closing the bypass screw which opens the barndoor slightly. On the lean side, opening the bypass any more than when the barndoor is closed will have no effect on either the AFM output (barndoor can't close any more than closed) OR the air mass thru the TBs as no more air will pass thru the TBs than they allow.
   

The amount of air drawn into the engine is solely determined by the TB valves and their individual air bypass screws. Nothing will change that at any throttle setting. As a result, the same amount of air will pass thru the air intake system whether the AFM bypass is fully open or fully closed. As air is diverted thru the AFM bypass instead of past the barndoor, the force of air on the barndoor is reduced and it closes accordingly. As less air is allowed thru the bypass more air forces past the barndoor and it opens accordingly.

It says that doing this setting allows the bike to be within a whisker of the actual CO2 percentage if you had a sucker up the bikes exhaust pipe to check its settings.

The throttle stop screw is I believe used to stop the butterflies from banging into the throttle bodies

i wrote:This IMO is the function of the 50g pre-set load on the vane spring.

Beamer wrote:

Kaptain Holister wrote:The amount of air drawn into the engine is solely determined by the TB valves and their individual air bypass screws. Nothing will change that at any throttle setting. As a result, the same amount of air will pass thru the air intake system whether the AFM bypass is fully open or fully closed. As air is diverted thru the AFM bypass instead of past the barndoor, the force of air on the barndoor is reduced and it closes accordingly. As less air is allowed thru the bypass more air forces past the barndoor and it opens accordingly.

I don't think this correct.

At a given rpm and a given setting of the TB screws, there will be depression in the inlet manifold which means there is a pressure drop across the AFM. There are two paths through that device. One is controlled by the bypass screw, the other is a reactive element with a progressive resistance against increasing air flow. ie a negative feedback. Agree. That's basically what I've described.

If you screw the bypass to one extreme or the other the tickover will drop, indicating that the assertion that overall flow is not affected is incorrect.  Not so. What's happening by adjusting the air bypass is you are simply diverting air into or away from the air vane; ergo... it moves correspondingly changing the fuel mix... rpm changes. At any throttle setting a finite amount of air mass passes thru the TBs and AFM in response to the vacuum created. With a set amount of air passing thru the AFM, the bypass screw determines what % is bypassed and what % is forced past the air vane. The laws of physics will apply.

I don't think that the vane ( hence resistor wiper )  will move off base at tickover. If it does I suspect the ECU will ignore it as long at the TPS is closed.  This could be tested in situ. Many on this forum have either disconnected their TPS or adjusted them so that 'throttle closed' switch doesn't function. This gives a smoother transition when backing-off as the FICU would normally cut fuel with 'throttle-off' above 2k rpm giving hard deceleration. Not heard of any issues of this affecting idle. I'd be very interested to see this tested all the same.
In any case, if the spring load was strong enough to hold the vane in place against idle air flow plus any bypass adjustments, the vane may not open at the exact point the throttle is opened. I would say that is crucial.

This IMO is the function of the 50g pre-set load on the vane spring. I think the function of the 50g load is to hold the air vane closed at idle diverting air thru the bypass where it can be adjusted to fine tune the balance of the pre-loaded vane spring. In my scenario it will be important for the vane to begin to open the moment more air is introduced thru the TBs (ie throttle opens). Even brand new I think the vane springs would have had some variation in tension and would need some setting up/adjustment against a set amount of air movement.

I have not seen any factory information about the dead zone. Whether this is a design feature used to fine tune in between step changes in the gear wheel tensioner settings or just a sign of wear, I can only guess. My guess is that it is a feature that allows continuous control when the device is initially calibrated and as I did with my test unit. That is only a guess though. All units which I have examined have had a small dead zone. I think you are correct. It makes sense to have a well defined starting/base point for the wiper.

BTW german literature on the TB, labels the screws as Umlaufschrauben, which means bypass screws, not tick-over adjustments. I agree with you that tick-over is not adjustable other than that it can be affected by incorrectly setting the idle mixture via the ATM bypass screw. Cool

I wrote:I don't think that the vane ( hence resistor wiper )  will move off base at tickover. If it does I suspect the ECU will ignore it as long at the TPS is closed.  This could be tested in situ.

KH wrote:
In any case, if the spring load was strong enough to hold the vane in place against idle air flow plus any bypass adjustments, the vane may not open at the exact point the throttle is opened. I would say that is crucial.

warmas wrote:I want the 45-50mpg others have reported.

It appears they all open and close simultaneously and at the same rate. I think they are quite finely sychronised.

The imbalance in a pair of throttle body bypasses  decreases rapidly with throttle opening.

My manometer is a vinyl tube with red automatic transmission fluid

Beamer wrote:Hard tyres have much less contact area and thus much less friction with the road.