FRENETIC IDLE WHEN WARM

Some years ago i bought a 1983 Range Rover. The engine ran fine, but it struggled a little at tickover. The first thing to upgrade on these V8 engines is the ignition system. Points and capacitors are good from a roadside-repair-point-of-view. Otherwise they've got no business being in an eight cylinder car. Only the slightest bit of wear in the distributor shaft will result in continously varying dwell angles and ignition timing, independently for each cylinder! I fitted a LUMENITION optical electronic system (which wasn't very expensive), and things improved instantly.

But then another fault surfaced. When the engine had warmed up, the idle suddenly rised to 3000 RPM! And stayed there. Not funny at all, you cant engage D or R at that idle, it's not healthy for the gearbox.
The trick was to switch off the ignition, wait for it to nearly die, switch on the ignition again and quickly put it in D. But this method could potentially damage the exhaust system. (If you wonder how, there's a small test you may perform on your own. Drive down a long hill. Engage top gear and swtich off the ignition. At the end of the descent, switch on the ignition... Feel free to email me and tell how loud the bang was :-)
Another trick was to pull the choke lever and wait for it to nearly die, push it back in again and quickly put it in D.

This engine was equipped with a pair of Zenith Stromberg CD 175 carburettors. The same type Saab uses on the single carb 2 litre. I've had half a dozen cars with this type of carburettors, both in single and twin carb form. It's an ingeniously simple design which works well if serviced on a regular basis. They're nice to work on and hardly ever goes terribly/suddenly wrong. I went through the carbs in my head, over and over, and could not imagine what caused this strange fault. It really shouldn't be doing this! I suspected a vacum leak somewhere, but however hard I looked I could not find it.

At this point I had not yet accuired a workshop manual, so I asked a friend (who also owned a Range Rover) for help. THAT'S WHEN I LEARNED ABOUT THE POPPET VALVE. It's a spring loaded deceleration valve on the throttle valve which opens when manifold vacum rises. The purpose is to bleed air into the mix during engine-braking, when the mix otherwise goes too rich. In other words, an emission control add-on.

What happens is that the spring in the poppet valve sometimes (Hah! Often, if you ask me) looses its tension. Or even breakes. So when my engine got its ignition system sorted with brighter sparks, the idle rose to a normal level. Which was enough to trigger the sloppy poppet valve springs. Which in turn opened, and gave the same effect as pressing the accellerator a little.

THE CURE
The workshop manual suggests that worn springs should be replaced by new ones. I dont quite think so!
At least not on MY car. My friend and I unbolted the carbs, let them have some air, and soldered the popped valves solid! They never gave me any trouble again...

SUM UP
You take a perfectly functional device (like these carbs), start hanging on it all sorts of add-ons and afterthoughts to compensate for various silly demands, and you end up with a complicated heap of parts.
A poppet valve sorts under the category "extra-unneccessary-possible-source-of-faults gadget".