Stuck FIP idle screw?

[enriquegarcia]

I've searched the forums but can't find my specific problem:

On my '68 280SL, the knurled knob on the back of the FIP (for adjusting idle mixture) seems stuck. I've adjusted it several times in the past with no problem, but now it will neither push in (to engage the internal slot) nor pull out. I can easily turn it (both CW and CCW) for about 1 1/2 turns end-to-end before it tightens in either direction, but with no detectable "clicks". Also, with the engine running, the knob is turning. So it seems to me that something inside the FIP has slipped and the knob is now always engaged. Before I start messing with the FIP, can anyone offer suggestions? I also recently detected a coolant leak that seems to come from somewhere near the rear of the engine. Could this be a related problem?

Thanks! 

[Tyler S]

It seems somehow the adjuster screw on the inside of the FIP has backed itself out and is now jammed against the adjustment knob. This can happen if the knob is turned too far counter clockwise and the screw and detent spring no longer make contact with one another, or the knob was engaged while the engine was runnning. Either way the back of the FIP will have to come off to re-thread the screw in its home. Assuming it wasn't damaged.

[Benz Dr.]

I agree but you will only need to remove the large nut that holds the screw and not the complete cover. This is really more of a collar that screws on. You will need a large socket to remove it. Some oil will drain out of the threaded hole so you should place a pan under your car the catch it. You should be able to turn the adjustment screw once the cover/nut is off.
There's a small O-ring that prevents oil from leaking out from around the shaft of the adjusting knob and prolonged spinning from being engaged with the adjusting screw will destroy this seal so you could end up with a leak. I don't know how you would fix it but I imagine it comes apart somehow.

[enriquegarcia]

Thanks for advise. After clearing away as much as possible of the hardware that impedes access, I'm stuck until I can locate a 42mm socket to remove that big hex collar around the knurled knob. If that fails, it seems that the only way to remove the back cover of the FIP is to get the entire FIP out of the car. Hope I don't have to go there! Thanks again.

[enriquegarcia]

Problem fixed after I got a hold of a 42mm box wrench to remove the hex collar. Tyler's diagnosis was spot on: I had probably turned the knurled knob too far CCW so the internal screw came out beyond the spring detents and jammed into the internal end of the knurled knob. Those spring detents are what give the characteristic "clicks" as the hex head of the internal screw turns against them.

Thanks very much for your usual excellent advise!

NOTES:
- A 42mm socket would have been better as there is barely enough space for a box wrench to get a good grip on the hex collar
- A small mirror is essential to see what is going on inside the open hole
- With collar removed, about a cup of oil came out the hole, just as Dr. Benz said; the small O-ring was OK, probably because the engine had not run long in this condition