Setting Idle

[jeremywatco]

Trying to set the idle on my '73 302. Was idling around 1200RPMs. So I backed out the big idle screw at the throttle linkage and that did nothing to lower it. Noticed on the other side of the carb (the throttle arm goes through the carb) there was another small screw that also seemed to control the throttle stop. I adjusted this and the RPM"s dropped to where I wanted them (around 700). Any reason why there are two idle adjustments? I dont get it.

 

[B RON CO]

Hi, get s Chilton's manual for information on working on your Bronco.
You can set the idle speed on the left side of the carb after it is fully warmed up and the choke is held wide open.
The hard to reach adjustment on the right side of the carb is for high idle during warm up.
When the engine is warmed up you can adjust the idle speed and adjust the two idle mixture screws for the best idle.
The proper procedure is to set up the distributor, points and condenser, if you have them, and set base timing first.
Get a manual for reference and ask a lot of questions.
Good luck

 

[Broncomt]

The passenger side is used for fast idle when choke is not fully open. It sets against a stepped cam. Set that screw when choke is fully open to just clearing that cam. Then set normal idle on driver's side with the screw there. The passenger side screw should only touch the step cam when choke is not open.

 

[DirtDonk]

What they said Jeremy.
What happened in your case was either that the choke was not fully pulled off, the stepped ramp that the screw touches was either stuck, or improperly adjusted (they do get stuck fairly often when the carb is not cleaned and lubed regularly), or the screw you adjusted was already adjusted too high.
In that last case you fixed it.

If it was one of the others you may want to go back and un-stick it (a quick spray with carb cleaner first, then a thin lube like WD40 (which lasts awhile, but not forever), or fine out why the choke was not opening up all the way (was the engine warmed up when you did this?) or why the stepped lever was hung up.

And definitely check your timing and check where your vacuum advance is connected. This makes a difference.
Setting the final idle is a dance in circles. First you set timing, then you set the idle close, then you adjust the two idle/air mixture screws, then you set the idle again, then you adjust the two screws again. All to get just the perfect mix of speed and smoothness.

Good luck!

Paul

 

[edmedlin]

What Paul and others said. My 302 roller has a fairly aggressive cam, and I have AC, so my idle is touchy to say the least. I found that around 1150 cold and 800 hot is the sweet spot. When I first set it, I had issues with it surging up and down. I advanced the timing just a tad and cleared it right up. Mine is an auto with a 2200 stall converter.

 

[rydog1130]

I need to do this as well, but when I messed with the big spring loaded screw nothing happened? I have a unique carb, its got an automatic kick down with a manual choke.







I found this picture on here and I noticed a huge difference... what's the rod attached to the same spot as where I have my throttle springs? My old carb had a double spring set up that ran from front and back...thinking I may have something wrong here....?:??:??:??:?

 

[B RON CO]

Hi, rydog1130. The idle setting screw will not adjust the idle if the choke is closed and you are on fast idle, or maybe you have some vacuum leak.
Another important thing is you need a spring to pull the tranny kick down lever back up.
Good luck

 

[rydog1130]

Hi, rydog1130. The idle setting screw will not adjust the idle if the choke is closed and you are on fast idle, or maybe you have some vacuum leak.
Another important thing is you need a spring to pull the tranny kick down lever back up.
Good luck

You wouldn't happen to have a picture of that would you? I changed over to a C4 from my 3 on the tree and changed my carb to have the kick down feature....

 

[rydog1130]

I think I found what I need. I'm either going to need to track one down here in Michigan or order a new one from TBP.






or can I run my existing set up this way? I think I would just need to pop the spring running to the front and attach it to the kick down?

 

[DirtDonk]

The spring-loaded screw you mention is hidden from view in your picture, correct? It should be (like the one in the fourth pic) in a spot that looks to be underneath your kickdown lever.
If that's the case, when you turn it up/in like you're tightening it, you can get the idle to go higher, right? Then once you start turning it down/out the idle goes down, but only to a certain spot and no farther?

If that's the case, then you have at least one of the following:
1. Vacuum leak.
2. Choke linkage is still interfering even with the choke fully open (easy to see visually).
3. Throttle butterflies are not seated correctly in their bores. Happens a lot after full rebuilds where someone removed them.
4. Something else is limiting down travel, such as the trans kickdown linkage. Easy to test by temporarily disconnecting the kickdown lever to see if the throttle will then adjust farther down.
5. On some engines, excessive ignition timing will limit the low end of the idle, but that's pretty rare and is usually at least partially to do with some other issue with the carb.
6. The wrong PCV valve can act like a vacuum leak, but I have not found this to limit idle. More often it acts like a regular old vacuum leak causing the engine to run lean and bog down off-idle like the accelerator pump is not doing it's job.

There may be other things at work as well, and if so the others will fill in the blanks.
But most of them are easy enough to test while still on the vehicle at least.

Good luck.

Paul

 

[DirtDonk]

Sorry, did not see your pictures before I posted.
You caught the other "linkage binding" item that needs to be checked.
Are you saying your existing rod is not long enough and is holding your idle speed up? Did you experiment/test this by disconnecting the linkage and trying to set the idle?

Paul

 

[rydog1130]

Sorry, did not see your pictures before I posted.
You caught the other "linkage binding" item that needs to be checked.
Are you saying your existing rod is not long enough and is holding your idle speed up? Did you experiment/test this by disconnecting the linkage and trying to set the idle?

Paul

No I'm just sitting at work pondering whats wrong with it... last week I was home we tried adjusting that screw and there was no change. The truck will idle high when the choke is engaged and when its off the engine does calm down but I think its still a smidge high like 1000 rpm. most guys say 800 is a more acceptable range at idle. I started looking at throttle linkages and noticed most C4's have that direct rod linkage where mine is more of a mouse trap configuration of miniature linkages (green circle) that eventually connect to the carb. The vehicle runs and drives but I feel like the throttle is stiff. Im wondering if I need to ditch that larger spring that runs towards the front of the block. I added it because it was like that with the previous set up. I also noticed everyone has a small spring from the kick down towards the body of the carb which I'm missing....yellow line

 

[rydog1130]

I guess do I buy a linkage rod, and discard all the stuff circled in green, connect that big spring to the old linkage bracket (red arrow) and add a small spring from the kick down to the body of the carb (yellow line)?

 

[DirtDonk]

Yeah, '73 and later models (actually late '72 I think?) got rid of the whole extra bell-crank mechanism. Probably to clear the kickdown rod better.

And yes, you have a ton of spring there. That would definitely make the pedal push harder.
But stiff springs do not effect idle speed, other than to help keep it down when you let up on the pedal.

But are you only guessing at the speed? You can't do that unless you know ever subtle nuance of the vehicle. Every one sounds a bit different depending on lots of things like air cleaner, cooling fan style, exhaust type and stuff like that.
Yes, that stuff makes more of a difference at speed perhaps, than it does at idle, but still it's hard to tell idle speed accurately by ear.

And you do know your linkage is suspect anyway, because in general you should be able to adjust it so far down it will kill the engine. If you can turn the screw out all the way and it's still idling, then something is still holding the butterflies open. That, or you're getting fuel and air from somewhere else!

Paul

 

[rydog1130]

I’m going to replace that old throttle linkage with the updated accelerator bell housing and linkage rod.