‘73 FJ40 Slow Idle speed problems

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S

Scotty2

Joined
Dec 24, 2019
Threads
3
Messages
80
Location
Florida
I’ve got a F155 engine w a 2bbl Aisan carb that was jetted for 8000’ and now I live at sea level. I rejetted it and rebuilt the carburetor using a Haynes manual because the carb kit had no instructions. Now I can’t perform the “lean drop” method of idle adjust. My slow idle adjust screw is completely “in” with no more adjustment. I read that a grossly wrong Fast Idle adjustment could cause this so I moved it around to try to create some movement room for the slow idle adjust. No change.

I could really use some ideas! Anybody seen this situation before?
 
Have you done carb work before? No, well its easy to miss things putting it back together, especially with not so wonderful of guidance provided by Haynes.

If it runs I would start with checking for vac leaks by spraying the outside of the carb junctions with like WD-40 and see if the RPM's jump.

Somewhere around here there are links to buy or perhaps download toyota manuals - well worth the effort and expense IMHO. It will have correct starting positions for you to tune from.

I would pay attention to the needle and seats - they can be easily damaged by gorilla's tightening them down - nothing worse than to try and operate a ox acetylene torch with "ruined" controls.

I have a 2bbl Rochester that came on the rig when I got my 72 in 82. I had to work on the accelerator pump leather once a long time ago. I bought a gallon of carb cleaner and an ultrasonic cleaner to do a job on it soon - It still works but I'm sure its do for some maintenance by now.

Good luck.
 
If the idle speed screw is non-responsive, chances are your cable is too tight.
 
Pighead said:
What year carb? and pics help...
Click to expand...
Sorry I forgot that. It’s a 1973 FJ40 and based on the numbers on the carb it’s the original carburetor. It’s a CA unit with all the emission control equipment intact (I think). I’ve replaced all the vacuum hoses because they were pretty stiff and brittle. I cleaned the carb thoroughly before installing the kit. I’ve rebuilt Rochester Quadrajets before, but never an Aisan. I’ll attach some pics of the linkage side of the carb (firewall side). I took the carb off again to see if I put something on “backwards” so the pics are of the carb on my workbench. If you’re wondering what that white residue is on the linkage it’s a WD40 white grease I sprayed on the linkage to make sure nothing was binding. I’ve wiped it off now. What a mess!

IMG_0634.webp


IMG_0633.webp


IMG_0635.webp
 
65swb45 said:
If the idle speed screw is non-responsive, chances are your cable is too tight.
Click to expand...
That actually makes sense to me. When I say”unresponsive” I mean that the screw is threaded in so far that the spring around it is completely compressed and yet the end of the screw isn’t making contact with the metal stop it’s supposed to touch. I was actually thinking about bending that stop to force it to touch, but adjusting the cable length would achieve the same thing. I’ll try it!
 
Pighead said:
What year carb? and pics help...
Click to expand...
Pighead
I’ve had some major success, but I still have a couple of questions.
First the success. I had a vacuum leak at the carb base. Nothing fancy to fix it. Just tighten it up. Had to buy some crowfoot sockets to get to the valve cover side of the carb due to the anti-pollution stuff in the way.

I just adjusted timing for max vacuum and now Im running 18-19 InHg of vacuum. Woohoo. No idea how much in degrees I advanced timing, but there seemed to be a sweet spot so I clamped the dizzy down there.

Idle was slow. My slow idle speed adjust screw was fully “in” with the spring completely compressed. I resorted to adjusting the throttle cable jackscrew to get idle to 650-700.

Took it for test drive and it surges in 2nd gear. But when I stop it idles! And when I turn it off it doesn’t diesel. Woohoo.

Now the 2 questions:
1. Is it normal to have to use the jackscrew to adjust slow idle? Or is the jackscrew a tool to allow the slow idle speed to be adjusted into a usable position, like midrange so idle can be tweaked both up or down?

2. I regularly find that after I rev the engine, the slow idle speed screw doesn’t seat itself on the tang it contacts. Is there a helper spring somewhere that is weak, or a lubrication issue? Or something else?

I really appreciate your help. Thanks!!
 
Pighead said:
1) I have no experience with jackscrews.

2) you should have a return spring somewhere on the carb. Mine is a different, a 10/73 carb on a 12/74 55

View attachment 3593752

View attachment 3593753
Click to expand...
Interesting. I will attach a pic of the jackscrew that fits between the carb linkage and the end of the throttle cable, for your viewing pleasure lol. I will visit the local Ace Hdwe to survey their spring inventory! Many thanks!
 
I feel like I need to close the loop with you because you helped me during my carburetor issues. It turns out that a “simple rebuild” of my 2bbl Aisan so that I could re-jet it for sea level instead of Colorado’s 8000 feet, wasn’t so simple after all.

The main thing I learned was that the idle circuit is so dependent on strong engine vacuum!! My problems began because after my carb rebuild I just didn’t tighten the base plate nuts enough. Because of the smog control tubing, I couldn’t tighten them easily until I bought a set of crowfoot sockets. They are great for this application!!

Once vacuum was restored to normal, I had to adjust the throttle linkage jackscrew to its maximum, and replace the idle speed adjustment screw with one about 1/2” longer. THAT finally got the machine to a point where I could (at last) perform the Drop Idle Adjustment to the carburetor. Before, the idle mixture screw had no effect on rpm because of the vacuum leak. I’ve finally got it idling fairly well at 700 rpm.

The idle sounds a bit fast to my ear at 700, so I might tweak it down.

It runs well enough that I took my wife out for a drive and an ice cream at the local parlor to celebrate!!

A side benefit of getting the vacuum restored so that the true idle circuits operate, is that when I shut it off, it dies immediately instead of that annoying dieseling it was doing before. The shutoff solenoid works!!

Long story. But thank you all for being generous with your knowledge and experience!!
 
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