High idle no matter what (1 Viewer)

[georgiacruiser87]

Crunk it up this morning. Still idling super high at 2000 and no changes while adjusting the idle speed screw.

I just ran all new tubing for the Desmog so I don’t think that’s the source of a vacuum leak. I did have to take the manifold off during the Desmog. I feel like a leak in the intake manifold would be noticeable.

I’m stuck with the ideas of the throttle blades not closing all the way or a timing issue.

If I learned anything from the previous owner… it’s that he touched everything and nothing was done right haha

 

[WartHog73]

Since you desmogged, recommend checking all your lines and double checking they are routed correctly, things are capped correctly, and as others have said no vacuum leaks. Also, did you rebuild the carb (and assume OE/Aisin)?

My linkages got switched (to this day I don’t know how (I think when it got painted but my engine bay didn’t get touched and no one should have messed with it). Took me two months to figure it out. However, it still ran like a top - I just couldn’t go past 35 on any hills as secondary would never open. The idle however was fine.

I hope you are able to figure out the culprit soon!

 

[georgiacruiser87]

Yeah that’s my next step is chasing vacuum lines.

It’s an Aisin carb. I did rebuild it and obviously took it apart. I’m always skeptical of the little linkages on the choke and throttle that are attached to the carb but the primary is closed at rest and when I manually open it and let it go it comes to rest back closed. (If that makes sense)

 

[Skreddy]

I have another question. Is this what my carb is supposed to look like at rest? I feel like the blade being altogether vertical is a problem? I can close it but I have to turn the choke mechanism counter clockwise 90 degrees. It’s spring loaded so it goes back to the picture.

View attachment 3861151

The blade that’s vertical is the choke. If you pull your choke knob out, it should close that blade.

 

[Skreddy]

For the high idle, you’re sure you are adjusting the correct screw for idle speed?

 

[CruiserTrash]

Yeah that’s my next step is chasing vacuum lines.

It’s an Aisin carb. I did rebuild it and obviously took it apart. I’m always skeptical of the little linkages on the choke and throttle that are attached to the carb but the primary is closed at rest and when I manually open it and let it go it comes to rest back closed. (If that makes sense)

With the air cleaner off like it is, look down the throat of the carb. The primary is opening and closing properly … what about the secondary? You’ll need a flashlight.

What about the AC idle up screw? It’s on the side of the carb with the float bowl, facing the front of the truck. There are two vacuum pots on that side, each with a linkage rod that connects to the butterfly valve shafts. You’re looking for the shaft towards the valve cover - you’ll see a screw there. So: down low on the base of the carb, the side with the sight glass, towards the valve cover.

 

[CruiserTrash]

And with the air cleaner off … may as well start over and do a lean drop tune on the carb.

Is your timing set to stock 7* BTDC? Don’t touch any screws until you do that.

 

[georgiacruiser87]

And with the air cleaner off … may as well start over and do a lean drop tune on the carb.

Is your timing set to stock 7* BTDC? Don’t touch any screws until you do that.

The cap is still on the idle mixture screw. I may have found something though? With the air cleaner off and still getting high idle I started checking lines. The pcv that goes into the hic was loose. Popped that on and put a tiny hose clamp on it and boom lowered the idle.

I now can adjust the idle but it’s backwards or I’m backwards? When I turn the screw on the rear of the carb closest to the valve cover it lowers the idle. So now I have idle at around 650 but when I pull the choke it only goes to maybe 1000

 

[georgiacruiser87]

For the high idle, you’re sure you are adjusting the correct screw for idle speed?

Yes

For the high idle, you’re sure you are adjusting the correct screw for idle speed?

i believe so. I’m turning the one at the rear of the carb sort of in the middle. Hard to get to unless you’re climbing half way in the engine bay. Funny enough after finding a leak at the HIC to Pcv line, it idled down and I was able to adjust it. But (and it’s a big one) what I believe is the high idle speed screw is now determining my idle speed with no choke.

 

[georgiacruiser87]

And with the air cleaner off … may as well start over and do a lean drop tune on the carb.

Is your timing set to stock 7* BTDC? Don’t touch any screws until you do that.

Checking the timing tomorrow. I have a really bad flat spot when revving the engine around 1500. From what I’ve read there may be a pin or bushing that’s crumbled inside the distributor. I plan to take it off and look through it or send it off for someone to rebuild at some point.

The caps never been off the idle screw so I’m wary.

 

[georgiacruiser87]

Like I replied to a few earlier. She’s running at a lower idle after I found a leak at the Pcv to hic hose. Dumb luck. But now the high speed idle screw is altering my idle speed with no choke.

 

[CruiserTrash]

Checking the timing tomorrow. I have a really bad flat spot when revving the engine around 1500. From what I’ve read there may be a pin or bushing that’s crumbled inside the distributor. I plan to take it off and look through it or send it off for someone to rebuild at some point.

The caps never been off the idle screw so I’m wary.

The distributor bushings are certainly crumbled by now, but that's a problem for future you - it's not the cause of this current issue. IT will just give you too much advance at high rpm, maybe some light pinging on the highway. But forget that for now.

If the cap has never been off the mixture screw, that carb last got tuned at the factory. It's time, especially after you rebuilt it. Gently drill, try to grab it with pliers. Get it out of there without damaging the mixture screw itself. When adjusting the mixture screw with the lean drop method, you'll screw it all the way in then back off X number of turns. When you screw it all the way in stop when you feel the lightest pressure. Screwing it in tight will deform the end of it and then you'll have other issues.

I'm assuming you have the 2F Engine factory service manual? The lean drop procedures are in there, along with the setup procedures you need to do after rebuilding a carb (before the lean drop). If not, look at the top of this web page for the "Resources" tab. You'll find the manual there.

 

[Skreddy]

The cap is still on the idle mixture screw. I may have found something though? With the air cleaner off and still getting high idle I started checking lines. The pcv that goes into the hic was loose. Popped that on and put a tiny hose clamp on it and boom lowered the idle.

I now can adjust the idle but it’s backwards or I’m backwards? When I turn the screw on the rear of the carb closest to the valve cover it lowers the idle. So now I have idle at around 650 but when I pull the choke it only goes to maybe 1000

The screw on the firewall side of carb, closest to the valve cover, is the high idle screw.
Here’s mine below, carb is slightly different variant so won’t look exactly the same but screws should be. Blue is high idle, red is curb idle or standard idle speed.

 

[georgiacruiser87]

The screw on the firewall side of carb, closest to the valve cover, is the high idle screw.
Here’s mine below, carb is slightly different variant so won’t look exactly the same but screws should be. Blue is high idle, red is curb idle or standard idle speed. View attachment 3861353

Correct. So the blue is somehow adjusting my curb idle speed and the red for all I can tell is doing nothing. Maybe I have a linkage screwed up?

 

[georgiacruiser87]

The distributor bushings are certainly crumbled by now, but that's a problem for future you - it's not the cause of this current issue. IT will just give you too much advance at high rpm, maybe some light pinging on the highway. But forget that for now.

If the cap has never been off the mixture screw, that carb last got tuned at the factory. It's time, especially after you rebuilt it. Gently drill, try to grab it with pliers. Get it out of there without damaging the mixture screw itself. When adjusting the mixture screw with the lean drop method, you'll screw it all the way in then back off X number of turns. When you screw it all the way in stop when you feel the lightest pressure. Screwing it in tight will deform the end of it and then you'll have other issues.

I'm assuming you have the 2F Engine factory service manual? The lean drop procedures are in there, along with the setup procedures you need to do after rebuilding a carb (before the lean drop). If not, look at the top of this web page for the "Resources" tab. You'll find the manual there.

Will work on this and report back

 

[Skreddy]

Correct. So the blue is somehow adjusting my curb idle speed and the red for all I can tell is doing nothing. Maybe I have a linkage screwed up?

Here’s pics of the stock carb from an 87, still smogged.

 

[Skreddy]

Here’s the AC idle up screw @CruiserTrash referenced.

From what I can see between your pics and my carb, linkages look correct.

 

[CruiserTrash]

Here’s the AC idle up screw @CruiserTrash referenced.

From what I can see between your pics and my carb, linkages look correct.

View attachment 3861371

Thanks for the assist. Couldn’t grab a photo of it until later.

 

[CruiserTrash]

Correct. So the blue is somehow adjusting my curb idle speed and the red for all I can tell is doing nothing. Maybe I have a linkage screwed up?

Red is the warm idle screw - the main idle screw you want to work with once everything else is good. Blue is the high idle limit screw (sets the rpm with choke engage essentially) - perhaps that’s doing something to elevate the idle when it shouldn’t be?

 

[CruiserTrash]

It’s all in the manual too