Idle Adjustment Screw - Nothing happens (1 Viewer)

[scott74_40]

Details - '74 FJ40 with stck motor. Only thing not stock are the headers.

I adjusted my timinng to 7 Degrees and it idles well. She is holding right around 750 RPM at idle. Tried to lower that a little bit by adjusting it through the carb. Unfortuantely nothing happens when I try to adjust the idle using the Idle Adjustment Screw on the base of the carb. If I unscrew it nothing happens and if I screw it all the way in nothing happens. I had the carb rebuilt by Jm C. a year ago.

Any ideas?

Thaks,

Scott

 

[honk]

Which screw are you turning? There's one for idle speed, one for fast idle speed, and one for idle mixture. The two former are in the linkage and the latter screws into the carb at the base.

You don't adjust idle speed by adjusting the mixture screw, normally, but if that's the one you tried it should kill your engine at the spec. speed of 650 rpm when screwed all the way in and cause it to run rougher when screwed out.
If it IS the idle mixture screw that you're turning in hope of adjusting your idle down from 750 rpm...it won't. At speeds higher than normal idle the mixture screw will have little noticeable effect.

So.....Wrong screw?

 

[dgangle]

I second Honk. My first idea is that you are turning a mixture screw. If you are turning the correct screw make sure the linkage/bfly isn't binding or someother mechanical limitation. HTH

 

[Mr. Toad]

If turning your idle mixture screw all the way in doesn't kill the engine, then the engine is not really idling. Instead, it is maintaining an "idle" speed through the carb's throttle and non-idle jets. Sort of like giving the engine a little gas pedal to hold a certain RPM.

 

[REZARF]

You can also shoot some carb cleaner down the adjustment screw hole, just to make sure it hasnt had some fuel varnish up in there. Have you not had it up and running for a long time?

Jim C. carb was the best money spent on my cruiser!


Rezarf <><

 

[scott74_40]

Thanks for the help guys. I was adjusting the idle mixture (the one at the base of the carb). I did that to see if the idle would change at all. I remeber doing that in the past and I was able to get the idle to smooth out a little. I was just a little surprised when the engine did not stall out, as Toad Mentioned, when I screwed it all the way in?

I was able to make the idle faster using the screw on the backside of the carb. It is located up from the base but is connected to the linkage. (Hard to describe) As I screwed it in, the idle increased. I was able to unsrew it until it no longer hit the metal stop - but the truck idled at 750RPM.

I have not run the truck too much lately. I think the last time I drove it was 4 months ago. I do start it a move it in and out of the garage.

I will look at the linkage and see if it binding. In looking at the factory manual, it does not appear that taking the Idle Adjustment Screw out and cleaing things should be tricky? I guess I could try both.

Thanks again for the help and any other advice,

Scott

 

[65swb45]

If you are trying to get a lower idle speed and the idle speed screw is no longer affecting the idle, then something else is 'holding up the show.' The first thing I would suspect would be the choke linkage, which also has an adjustment screw. You might want to try backing that off as well.

If that doesn't work, try manually pulling on the horizontal linkage of the bellcrank towards the drivers side fender, which should also slow the idle. If this works after you take your hand off, then either something is binding to prevent this from happening naturally, or your return spring is weak.

Hth.

 

[scott74_40]

Well I tied a few things last night. Removed Idle Mixture Adjustment Screw and sprayed/cleaned opening just in case I had varnish build up. No change.

I actually removed the accelerator rod to see if that needed to be adjusted. No change in idle with that removed. The return spring seems to be OK. The entire carb was rebuilt by Jim C. about a year ago. Only 100 miles on the carb since the rebuild.

I checked for vacum leaks and checked to see that the base plate was tight. Everything looked good.

I am out of ideas.

Scott

 

[REZARF]

Scott-

Why are you trying to lower the idle speed from 750 RPM, that is a pretty good place to be? Just wondering...

If you are pretty smooth at 750, and you are not missing at all with your timing, are you just trying to figure this thing out or lower the rpms?

I would second the choke cable, you can disconnect it right at the carb with a little screw, it could be holding the idle at speed.

Just a thought, Keep us posted

Rezarf <><

 

[scott74_40]

REZARF,

The idle is not smooth and steady. It hangs around 750RPM then jumps between 650 and 800 RPM. I am just afraid that their is a problem in addtion or keeping it from getting to the idle circuit. If it did I would be able to stall it out using the idle mixture adjustment screw.

A picture would better expalin why I am so "retentive" about this but all of mine are to big to post.

I forgot to mention that I disconnected the choke last night. No help.

Vacum seems OK, but I guess I could check the it with a guage. That is about the only other thing I can think of.

I appreciate everyone's input.

Scott

 

[Pin_Head]

The reason that it is running at 750 RPM and the idle mix and idle speed screws have no effect is because the the throttle plate (either primary or secondary) is open too much, so that it is running off the transition slot fuel port and not the idle port.

It is either a linkage problem or the butterfly plate is physically stuck. If the choke high idle adjuster and the throttle positioner (if you have one) adjuster screws are not the problem, it is time to remove the carb and check the linkage and butterflies to make sure that they shut properly.

 

[honk]

Scott,
Not to single you out at all but I keep seeing from people that they can't post pictures because they're too big. Don't you have image manipulation software somewhere in your machine? There are several that are free to download that work pretty good and with them all (I think) you can resize images without loss of content or clarity that's noticeable online. One that I know works is this: http://www.irfanview.com/. Give it a try and reduce your stress level, eh?

 

[FJ40Jim]

Thanks for the help guys. I was adjusting the idle mixture (the one at the base of the carb). I did that to see if the idle would change at all. I remeber doing that in the past and I was able to get the idle to smooth out a little. I was just a little surprised when the engine did not stall out, as Toad Mentioned, when I screwed it all the way in?

Yes, that's odd.
But, let's focus on getting the idle speed down to some normal number (500RPM non-US or 650RPM Fed-spec). Then the idle mixture can be messed with if needed.

I was able to make the idle faster using the screw on the backside of the carb. It is located up from the base but is connected to the linkage. (Hard to describe) As I screwed it in, the idle increased. I was able to unsrew it until it no longer hit the metal stop - but the truck idled at 750RPM.

OK, you are describing the hi idle screw. Now you've gone and messed that up too.

The idle speed screw is hard to see. It is located right up against the backside of the carb, threaded into the cast-iron base way down low. Turn it out to lower idle speed.

On your '74 it has a mechanical throttle linkage. That means you can grab the gas pedal and pull up on it and force the carb back to idle if it is sticking somewhere.

 

[scott74_40]

Here are some pics that I was able to resize.

I was able to adjust the idle with the speed screw at the base of the carb on the back side. It is still rough idling. Did a vacuum check an it is pulling 15 in/Hg but floats between 16 and 13 in/Hg. Of course this correlates with the idle. When the idle drops vacuum drops and vice versa.

I still need to adjust the valves but outside of that, I hoping Jim C. can help.

Thanks everyone for your help!!!

Scott

 

[john0089]

That engine looks mighty clean!

 

[chitown40]

Seeing as you've already played with the idle mixture screw, where is it set at right now? How many turns out from stop? I think you should be starting around 2 turns out and then opening the screw until the engine speed no longer increases. Then you adjust down with the idle adjustment screw. More fuel will bring your vacuum (and your idle) up. Also might want to read the Vacuum 101 in the tech section as that might give you some ideas to check why your vac is fluctuating so much.