DCH340 Fast Idle Adjustment FSM Procedure Clarification

[Cardinal Grammeter]

The instructions basically say, "with the fast idle screw on the 2nd step, set the throttle plate to the gage dimension."

 

This has 2 problems:

 

1)  Which one is the 2nd step, and

2)  Where is the choke setting.

 

From other sources I found a statement that "step 1" is the "high" step.  So the "2nd step" would be the "2nd from the highest."

 

Then from a carb kit, it appears that the above 2nd step should be when the choke is closed.  Without the relationship from the choke to the fast idle cam, you can't adjust anything.

 

NOTE:  Technically, the fast idle adjustment is "adjusting that screw."  There is no mention of "Fast Idle Cam Adjustment" which would involve bending the link between cam and choke.  Technically, this should not need adjustment, but after almost 50 years of exposure to the Forces of Chaos, this adjustment should be checked.

 

So here are what I think are the Bench Test steps:

 

1) With choke closed, bend link so fast idle screw is on the 2nd step,

2) Adjust fast idle screw until you get the gage dimension on the throttle plate.

 

For the 1974 620 L18 DCH340-12man, -13auto, it is

Manual:  .95mm  (.0374") - #63 drill

Auto:  1.17mm  (.0461")

 

EDIT:   DONE & ROAD TEST:

 

The stock Choke to Fast Idle Cam Link puts the Fast Idle Screw on the 2nd step of the Cam when the choke is closed.

 

I had to significantly reduce the fast idle screw to match the #63 gage point, however I was also planning on shortening the Link to put the Screw on the 1st Step of the Cam - I needed Longer fast idle time.  I figured the higher step might have possibly made up for the lower rpm setting of the Fast Idle Screw.

 

It wasn't!   Fast Idle was way too slow.  So I gave it about 1 full turn higher which is a real PITA to do on the car.

 

Result was just about perfect as far as fast idle goes and this was in 20*F weather.

 

AND carb idle was down where it was supposed to be.  (When the carb was acting up and popping all the time when fed throttle too quickly, the idle was much higher.  Then the carb seemed to get a little better - and the idle dropped some but was still high.)

 

There is still a "flat spot" at lower mid rpm where I can feel massive lean but not enough to pop most of the time.  If I drive it conservatively, it won't pop, but it is clear that torque is down during this flat spot.

 

I'm wondering if I've got some BCDD issues - I seem to recall just pushing the BCDD adjusting screw in should cause a change in rpm, but it the screw felt "bottomed" and nothing happened.  I also know the BCDD makes a very loud hissing noise - I can't remember if this is proper operation or a malfunction.  I remember posting it on some other thread here.  

Edited February 19, 2022 by Cardinal Grammeter

[crumb831]

tell me more about the fast idle spring?

 

[Cardinal Grammeter]

3 minutes ago, crumb831 said:

tell me more about the fast idle spring?

 

There isn't a Fast Idle Spring.  

The electric choke black plastic cover has a bi-metal spring in it that deforms as it heats up.

As the spring moves, it opens the choke.

As the choke opens a link moves the fast idle cam that has steps in it.

The fast idle adjusting screw is on touches those steps and holds the throttle plate at a higher opening until the choke opens fully, the fast idle screw comes off the cam, and the throttle plate goes to the opening set by the idle screw.

 

This is very basic, universal operation.

 

The issue is the timing:  as the choke opens, the idle is reduced.   

If the choke opens too quickly, the fast idle will drop too quickly.

If this happens you wind the choke spring tighter but this could result in too much choke.

OR you could turn up the fast idle screw to deal with the choke opening too soon.

 

It's a PITA since the fast idle screw is very hard to get to.

[crumb831]

there's a thermal spring that pulls the fast  idle cam back to kick the choke off. if i knew how to upload a pic from url  i would show you what i mean

[ggzilla]

The thermal spring moves the cam ... but only if you blip the throttle. On its on it won't move the cam. That's why the owner's manual says to "depress the pedal slightly and release to reduce engine speed"

[datzenmike]

It's the second highest step. 

 

This is only for setting the throttle plate opening. This is done at the factory and no way this will change but you can check and verify it. On a manual transmission place the cam on the second step, turn the carburetor upside down and the throttle plate should be 1.33mm to 1.47mm open. Impossible to fit a feeler gauge so use a drill bit. A #55 1.32mm or 0.0520" should just barely fit, and a 1.5mm 0.059 should not fit.  

[Cardinal Grammeter]

Is there a fine hair clock spring on the fast idle cam?  If there is, it isn't a thermal spring.  The only bimetal (thermal) spring is in the black electric choke housing.

 

[crumb831]

OK WELL YEA THATS THE ONE