Sudden high idle

[Papay]

Need some ideas here.  My 5k has a fairly new Aisan type carb, which I run without a choke.  Up until yesterday it was running smoothly.  While driving, the idle speed suddenly shot up to at least double.  After getting her home, I suspected the throttle cable may be hung, but it wasn't.  The linkage is working normally, bolts and hoses are tight.  I've backed out the idle speed screw, which only lowered it few hundred RPM.  

This car is equipped with some kind of a control solenoid, which the original Aisan did not have.  Could this just be buggered?  I'm thinking of just removing it to see if it makes a difference.  What else could cause the sudden change in idle speed?

 

[davew7]

The control solenoid, which the original Aisan did not have is a anti-diesel solenoid, that closes the throttle when you turn off the ignition. You should be able to adjust it fore and aft at the throttle, which would effect idle. But first check the choke. Make sure that the choke  is fully opens after the engine is up to temp. Then verify that the linkage to the fast idle cam is correctly adjusted.   I would also check for a vacuum leak. Davew7

Edited December 7, 2019 by davew7
added vacuum leak

[Papay]

On my carb that solenoid is in a threaded bore.  About a week before I posted this, I had found the solenoid wasn't tight.  So while I had the carb off for cleaning, I went ahead and tightened it.  I just figured it being loose couldn't be good, but at the time there was no idle speed issue.  Since posting this in early December, I have replaced the cylinder head, along with intake and exhaust manifolds.  I checked over the carb while I had it off, and removed a couple of linkage pieces that were still in place from the choke mechanism.  Just in case the little tooth cam was interfering with the throttle linkage.  To be clear, I don't have a choke plate at all, the entire choke system is removed owing to the warm climate here.    

After getting her all back together with the new head, idle speed was normal.  I figured it must have just been an air leak somewhere, and now everything is new and tight and fresh.  Then today it happened again, exactly like before.  Driving along, come to a stop and now the idle speed won't return to normal.  I poked around to check for loose hoses and the like, but its all as it should be.  Tomorrow I'm going to experiment with unplugging the idle solenoid, and bypassing the little bi-metal valve there on the side (the rectangular thing).  

Edited December 26, 2019 by Papay
pic

[davew7]

A wild a$$ guess is that you are bleeding air into the carb. Try pulling the solenoid and cleaning the port area and plunger, allowing the solenoid to close off the air bleed port. Could also possible be a function the bi-metallic strip out of adjustment not allowing an electrical contact switch to function. Davew7  

[altezzaclub]

Is this photo before the head job or after?  I hope its before because that bolt is not good!

Anyway, it sounds like a leaking gasket or hose or carb part.

I thought the solenoid was either open or closed so it shouldn't affect idle speed, just whether it idles at all or not. I've never had to work on one.

Certainly because it doesn't happen all the time it suggests something involved with the warmup procedure or some hole that expands as it warms up.  How and when does it go back to normal idle?

 

[Papay]

41 minutes ago, altezzaclub said:

Is this photo before the head job or after?  I hope its before because that bolt is not good!

 

Anyway, it sounds like a leaking gasket or hose or carb part.

I thought the solenoid was either open or closed so it shouldn't affect idle speed, just whether it idles at all or not. I've never had to work on one.

Certainly because it doesn't happen all the time it suggests something involved with the warmup procedure or some hole that expands as it warms up.  How and when does it go back to normal idle?

 

That's the 'before' photo haha, no I got all new bolts remember.  There is no leak anywhere in that intake system, and since the mystery behavior also happened before my cylinder head swap, its gotta be in the carb somewhere.  I had another thought while falling asleep last night:  something sticking in the secondary.  Both times this has occurred have been after heavier acceleration.  Anyway this morning I'll poke a bit before heading out, and snap a recent pic! 

[altezzaclub]

" That's the 'before' photo haha, no I got all new bolts remember. "     That's good, I thought it would be..

" something sticking in the secondary " Good idea!

[Papay]

Took a video, can't figure out how to make it a gif.

Anyway here you can see all the new stuff.  Ordered actual Toyota part numbers for all that stuff, but I feel it was worth it.

[Papay]

May have solved this.  I had the carb off today and found a little grain of something that was holding open the secondary just a crack.  It had kind of glued itself to the wall of the throat right where the butterfly closes.  After assembling and reinstalling, it was back to its normal low-low idle behavior.  So I cranked in the idle stop and added a half-turn on the idle mixture screw (no choke remember).  

[altezzaclub]

That's all it takes, some tiny thing to cause an air leak.  It must have slipped in through the filter somehow, easy to happen when your doing other things to an engine.

[Papay]

Ran fine this morning on the way to work, just have to keep it alive with the gas pedal for the first minute or so.  Interestingly, the cold behavior was better with the leaky secondary.  I'm wondering if there could be a happy medium to be found, unfortunately the secondary is not really adjustable.  Looking at it, the only thing controlling its position is a metal-to-metal cam profile in the linkage.  Kind of cheesy.  Maybe with some careful bending I might get it to open just a hair instead of fully closing.  Another option would be drilling a small hole in the butterfly, but that's kinda permanent if I get it wrong.