Question RE: Fast Idle Adjustment (1 Viewer)

[rstoddard]

Can anyone help explain the the Fast Idle Adjustment to me?

Yes, I have read the FSM and Haynes, and researched the subject but I’m still a bit confused.

Does the engine need to be cold when setting?

I understand you pull the choke pull all the way out, which should close the butterfly flap. However the procedure says “Open the choke valve with a screwdriver” which show’s opening the butterfly flap open; “ start the engine; Set the fast idle to 1800 rpm.”

So my question is, why open the butterfly flap after pulling the choke open? What is the point of that while settling the fast idle?

Appreciate the help in understanding this procedure !!!

 

[flx]

When you pull the "choke button" it closes the choke flap but it also push the accelerator on the carb.
The fast idle screw set how much the "choke button" pushes the accelerator.

So by pulling the choke and then forcing the choke butterfly open you get the acceleration part of the "choke button" without the choke part.

 

[mattressking]

Set choke when cold, set to 1800rpms, once it begins to warm, idle goes up and you push choke knob in until its warm enough to idle on its own at 650rpms.

The butterfly flap is the actual choke, the knob rotates the butterfly shaft of the choke flap and opens/closes so like @flx said, you set the choke 'kick-up' by adjusting the screw where it contacts the primary butterfly.

 

[rstoddard]

When you pull the "choke button" it closes the choke flap but it also push the accelerator on the carb.
The fast idle screw set how much the "choke button" pushes the accelerator.

So by pulling the choke and then forcing the choke butterfly open you get the acceleration part of the "choke button" without the choke part.

So why do they have you remove the choke part when setting the fast idle? Doesn’t the Fast Idle start with Choke closed?

I’m just curious why they want the choke open to set the rpm.

Does the vacuum subsequently pull the closed Choke open or does that have to be done manually by pushing the knob in?

 

[mattressking]

choke is manual, it is linkage driven for these trucks, vacuum does not play a role in opening/closing.

You set the fast idle technically with the carb off the truck. Close the choke completely then set the primary butterfly kick up clearance to 1.3mm. Then verify when cold that fast idle with full choke application is 1800rpms when engine is completely cold. That’s it that’s all.

Otherwise when cold, apply full choke when ice cold then adjust fast idle screw to 1800rpms if you aren’t removing the carb to set butterfly gap.

Image tells you everything you need to know.

 

[devo]

Carb Adjustment Screws

Where/What are the Carb Adjustment Screws on the FJ40 Carb? Every time I go to make adjustments I spend most of the time trying to figure out what screws to adjust. The picture in the Engine FSM is mostly useless as is the same diagram in the Haynes Manual. According to the Manual, there...

forum.ih8mud.com

 

[Pighead]

Set the fast idle to 1800 rpm.”

set to 1800rpms

1800 RPM?! That sounds awfully high to be running a freezing-cold engine for any length of time.
My top fast idle speed is 1200 RPM, but i drop it below 1000 almost immediately.

 

[rstoddard]

That’s the Spec in the FSM. Seems fast to me too.

 

[mattressking]

1800 RPM?! That sounds awfully high to be running a freezing-cold engine for any length of time.
My top fast idle speed is 1200 RPM, but i drop it below 1000 almost immediately.

So you bring up a good point, Toyota doesn’t truly designate the difference between 1800 rpm’s with or without fan clutch. I agree, most cars these days cold start at 1200, so one could assume it’s set high to assume fan load - but again - with or without fan clutch.

Personally I could see 1800rpms be according to a fan clutch truck with clutch disengaged (say 1min after warm up or so).

I’ll dig in some earlier manuals and see what I find

 

[rstoddard]

I wonder if it actually runs that fast with the choke closed (verses setting it with the butterfly open)?

The whole mechanism is confusing to me. I know there is a vacuum mechanism that pulls back the choke but I’m not sure exactly how it works.

 

[mattressking]

I wonder if it actually runs that fast with the choke closed (verses setting it with the butterfly open)?

The whole mechanism is confusing to me. I know there is a vacuum mechanism that pulls back the choke but I’m not sure exactly how it works.

There’s the choke breaker and opener - very simple systems. BVSV controls/restricts vacuum from diaphragm(s) and pulls linkage open/closed to prevent negligent drivers from leaving choke on/off. Not necessary, just a fancy feature.

All of your questions are answered in the 2F engine service manual. How to setup and diagnose, I recommend reading it all.

 

[Pighead]

I don't think anybody has mentioned year of carb, i don't have any kind of automatic choke breaker or opener. Just the cable that i pull and push. Nor a BVSV I'm pretty sure.

 

[flx]

I wonder if it actually runs that fast with the choke closed (verses setting it with the butterfly open)?

Yes !!
My guess is they ask you to set fast idle with the choke forcibly open because first, the engine would not run (or not fast) with the choke fully closed or almost fully closed. Second, if partially closed it is hard (in the field) to measure how much it is, to have a meaningful measurement of the fast-idle.
So they ask you to set the fast-idle with choke fully open at a speed they know works well but then in practice when you the fast-idle will be activated you will also have some choke slowing down the engine.

A very easy way to be convinced of this is just after starting your truck with the choke-button fully pulled, when you will start pushing a bit the choke-button the engine will accelerate while mechanically you are reducing the fast-idle! The choke is a big break on the engine.

So by setting your fast-idle, engine hot and choke fully opened, you have a repetitive way to set this value but your engine will never actually get to that speed during starting.


The choke breaker is a device that use vacuum to very slightly re-open the choke, it avoids the engine to die just after starting because it can't get air. But it's a 1 stop device, it will not re-open your choke enough for your engine to run smooth, particularly once hot.
Most US-carb should have one, some very old card did not have it and some other markets (Middle East) did not have it up to the end. The new OEM carb that can still be purchased (and all the copy) is a Middle East market carb and doesn't have the choke breaker.


The choke opener is a device that will reopen the choke as much as needed while the engine is getting hot. Only a few years in "rich" markets got this so it's not very common.

 

[brooklyn]

Going through this process as well and not getting a visual on this process, which normally means I am over thinking it.

If you’re setting the fast idle hot, choke open what rpm are you shootings for?

 

[rstoddard]

So by setting your fast-idle, engine hot and choke fully opened, you have a repetitive way to set this value but your engine will never actually get to that speed during starting.

As I understand what @flx is saying, the Fast Idle won’t get properly set (you won’t get to fast idle speed) if you try to do it when the engine is hot. Set it while the engine is cold.

 

[flx]

The doc calls for 1800rpm choke open and engine hot.

In practice the starting/heating phase of your cold engine it will not reach that rpm and this is fine.


There are also a lot of people that will do an empirical method and just start their car cold and choke in regular working order, and set the fast-idle at whatever speed the engine feels happy by ear.