Hi, not a major problem as my truck (69 F100 with a 390) runs beautifully once warmed up, but I'm having problems getting the fast idle to engage. The carb is a rebuilt Autolite/Motorcraft 2100 with a Motorcraft electic thermostat.

I've been working on the carb today, but to no avail. Here's what I did...

First I checked the choke pull-off clearance by turning the choke housing all the way over to fully close the choke flap and then depressing the pull-off diaphragm rod. This opened the choke flap about 1/8", which I believe is about right.

Then, with the choke rotated around again into the fully on position but with the pull-off engaged (so about 1/8" open), I adjusted the screw that goes into the top of the plastic thing so that the fast idle speed screw hits the V mark on the fast idle cam.

Then, I rotated the choke so that flap just fully closed and tightened the 3 screws to set it in that position. I then tested the operation by manually opening the choke flap, applied some throttle and the choke flap snapped back into the closed position. So, as far as I can work out, these are the choke adjustments completed.

But upon depressing the throttle once and starting the engine, it kept cutting back out because the RPM was too low. The choke flap closes but the fast idle cam doesn't engage. The cam only seems to engage if I rotate the choke much further anti-clockwise, beyond all the index marks. But the choke flap is being held closed quite tightly by the spring in this position and only opened about half way after 5 mins plus of running when I tried it set like this.

So, why won't the fast idle engage with the choke set so that it just fully closes with a cold engine? Any suggestions much appreciated. Thanks.

 

The fast idle speed screw not only has to touch the cam, but also push on it. In order to open the throttle a tad to give you a faster idle speed. I would turn that screw in about 2 revolutions more to start with. Then adjust it to get the idle speed you want. On the choke cap set it to the middle hash mark. Then come winter you can see if the choke unloads to fast or slow. It'll prolly end up 1 mark rich.

 

Hi, thanks for the reply. But, the problem is that the cam doesn't engage at all so it only runs on curb idle even when choke is fully closed......it only seems to move around to the engage position if the choke is adjusted on much more tightly, (like I had it when I set the fast idle index), which I don't think can be correct.

Should I maybe try setting the fast idle index with the choke set so that the flap just closes (i.e. the normal operating position) and not with it rotated 90 degrees anti-clockwise, which pulls the choke on really tightly, but is what I understood to be the correct procedure?

I was following the instructions in this thread, but maybe misunderstood that part...

https://www.ford-trucks.com/forums/7...ml#post6422219

 

9F577 is the plastic fast idle cam. Has it broken?

Originally it was white, it turns yellow over time.

 

Appears intact, it's what I'm screwing the fast idle index adjuster into. I'll take a close look tomorrow to make sure it's completely intact, but I think it is.

 

The 9B538 is the fast idle speed screw. It screws in to the 9538.
Here is a pic of them on a later model 2V I have. But samo,samo.




And here is how the choke linkage should look from above.

 

Thanks....mine looks pretty much like that. I'm now wondering whether I simply don't have the fast idle speed screw turned in enough, as you suggested in your first reply. My initial thought was that the cam is in the wrong position because if I turn the choke more I do get fast idle, but I guess that could be happening as the screw may be just far enough in to contact the highest step (but not where the V mark is). I'll look more closely tomorrow.

 

The screw in the 9F577 clocks the cam. Whether or when the 9B538 screw hits the next step higher on the cam.

 

All sorted...very simple in the end...9B538 screw wasn't wound in enough...I've got it running pretty nicely now, though still maybe some more tweaking to do. It starts up on fast idle but drops too quickly to the lower step so the rpm is too low for a few mins. Will have to rotate the choke a little further. I'm at 1 rich now, so will probably try 2 rich.