WR450F'12 choke + idle in one knob?


2 replies to this topic
  • WRF-Rowdy

Posted June 07, 2015 - 11:01 AM

#1

The EFI WR has only on knob at the throttle housing. Here are my findings so far:

 

One can turn it forward (counter-clockwise when viewed from the side) to raise the idle speed , or back to lower it.

When pulling out that knob firmly, it notches and stays pulled, that is described as cold start "choke" position.

There is a mechanical stop at "max-idle", my idle setting is 34 clicks below that max.idle setting.

At 34 clicks before max. idle I can lightly pull out that knob (w/o it clicking into choke-setting) and idle rises.

When hot (and not in gear) slightly pulling the chock-idle knob as described, gives e-start at first button press, always.

 

 

How can one single knob act as coldstart-choke (more air + lots of more fuel)

and a hot-start knob (just more air) at the same time?

 

Does the CDI know about the knobs position?  I didn't find a choke-knob position sensor.

 

If not and if that knob does NOT affect the throttle's butterfly position, but simply opens an air bypass path,

then raising idle would effectively mean creating a lean mixture!

 

Can anybody describe in detail what the

- idle adjustment action (turning the knob) and respectively

- choke action (pulling it out)

are doing technically?

 



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  • Krannie McKranface

Posted June 07, 2015 - 01:49 PM

#2

It's not a choke when you pull it, it's a 'fast idle' setting.  It mechanically adds more air by opening an additional airway.

The ECU is not connected, but the MAP sensor recognizes the increased air flow, and tells the ECU to make changes as needed.

 

 

A choke technically is an obstuction in the venturi (hence, choking the airflow, and raising the mixture ratio).  It does not 'add more fuel'. 

It used to be a butterfly valve in the venturi, or a sliding plate covering the intake.

 

A typical FCR carb does not have a choke, it has an 'enrichment bypass' knob, with adds more air and more fuel by bypassing the pilot jet circuit and adding in the power jet circuit in it's place/in addition. 

That's why there is a 'power jet' you can replace, to properly tune this circuit in the carb. 



  • WRF-Rowdy

Posted June 07, 2015 - 11:17 PM

#3

It's not a choke when you pull it, it's a 'fast idle' setting.  It mechanically adds more air by opening an additional airway.

That's what I suspected, just more air.

 

 

The ECU is not connected, but the MAP sensor recognizes the increased air flow, and tells the ECU to make changes as needed.

So cold start works like this:

The ECU reads ambinent temp and pressure(!) and togehter

with water temp automatically concludes "cold-start or not", correct?

 

But if you are right (I'd assume so) then the MAP sensor's value is only usable while the intake valve has opened:

In that very short time span the ECU detects the very small change in air flow (caused by the more or less open idle-ajdustment/cold-start circuit's air passage)

and adds the required amount of fuel, so that high idle speed is not lean and a low idle is not rich a mixture. :jawdrop:

 

If that really is how it works, then the bike is selftuning :lol:  with regard to

intake path restriction (backfire screen, hi flo air filter, airbox snorkel, cloggedness of air filter),

feet climbed / ambient temp changed during the last hour and

change in charge throughput due to more or less restrictive header/exhaust.

 

That means when you dialed in the fuel mapping as you like it (for example: tuned it to "Lambda=1 +3%", in the course of long, tedious experiments)

switching header/pipe/snorkel/CAM/airfilter will change MAP-value and fuel will be added or subtracted as needed, right?

 

Where the change in exhaust cam would affect MAP pressure only indirectly via possibly(!) higher back pressure

from an exhaust that cannot handle the increased amount of exhaust gas, thereby causing less scavenging, causing less air intake at the next cycle.

And if one would be still within the amount of additional exhaust gas the exhaust can handle,

different "racings" cams are causing more fuel to be injected, thanks to the MAP sensor.

Wicked cool :p

 

Back in pre-EFI times changig some major intake air/exhaust gas relevant structure would have required to reconsider

everything from idle mixture screw setting, to needle clip pos., needle type(!) , main jet and whatnot. 


Edited by WRF-Rowdy, June 07, 2015 - 11:18 PM.





 
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