Idle hunting, 1974 280, Solex 4a1

[juha_teuvonnen]

The car also has ignition issues (missing at idle) which I plan on fixing before getting serious with the carb.

When I bought the car it would not idle at all. Found unplugged air pump (it seized, so I left it unplugged). I found and plugged an air leak, it was a loose vacuum connection to the defunct air pump. I put a can of carb cleaner into the tank and cleaned the carb using carb cleaner spray. The car started to idle ok, just a little high. It would die in gear while on the brakes, for example at a stop light. I looked at the vacuum-operated idle mechanism which according to the book is responsible for adjusting the idle while in gear and it was badly out of adjustment (would not engage the throttle at all). Drove it to work and back yesterday, it did not die at stop signs, but idled a little high. I replaced the float chamber ventilation hose, the one that connects float chamber to charcoal canister this morning. Drove it to work, and the car started hunting for idle.

I am planning to disconnect all vacuum gadgetry completely, plug the vacuum take-off and see if it idles. If it does I will start connecting the vacuum valves one trying to identify which subsystem causes the hunting. It looks like there is some kind of feedback loop in the system, just not sure what causes it.

Has anybody successfully cured this behavior (idle hunting) before?

[Maki]

Paivaa, Juha.

As you mentioned, it pays to get the ignition straight on these cars before messing with the carb. The early 70s were the dark ages for emissions control, and the worst of the technology was in the ignition. A lot of the vacuum stuff is meant to adjust spark timing, BTW -- at least it is on my W114.

Pertronix is a nice electronic ignition insert for this car. It fits right in the distributor, with no addtional external boxes or wires. Here's a link to the catalog: http://www.pertronix.com/catalogs/pdf/ptx/2010/ptx2010_ignition_conv.pdf

I have a 71 250 (a little different, I know) that used to surge slightly at neutral throttle. It turned out to have a sticky fuel shutoff solenoid, an electrical device on the carb that was meant to prevent dieseling. It was cutting the fuel flow off to the rear carburetor (twin Zeniths on my car).

[kraakevik]

The dwell and timing must be close to perfect before attempting to adjust the carburetor. You're on the right track in isolating all the vacuum inputs/sources to the carb.

Try blocking off all vent and vacuum hoses except the connections in front, which should be used to control the idle diaphragm--the control arm should be against the screw throttle stop at idle--with the car in drive the diaphragm shaft should keep the control arm off the throttle stop and maintain revs at 900 rpm or so using vacuum to modulate movement.

You can adjust the shaft out and in using the threads and locking nut and then regulate the diaphragm idle by turning the big nut/spring mechanism in or out. If someone has messed with the mixture screws in front it may be necessary to adjust them--but only after the timing and the basic carb functions have been set.

Good luck




Tim Kraakevik
kraakevik@voyager.net
Four M110's

[juha_teuvonnen]

I have put in some wrench time this week-end, and here's what I found:

1. Idle shutoff valve connections were bad. Ignition on, pull the wires a little and the valves would click. Fixed that.
2. Air injection system completely rotted out. Check valve has fallen completely apart. Blocked off air injection for now (exhaust manifold without air injection is on the way).
3. The car idles better, but still dies when I get into gear. Adjusting the vacuum-operated idle control helped somewhat, but not good enough.
4. Blocked off the vacuum line that goes to the EGR. My gut feeling tells me that the EGR is probably leaking, It's probably in sad shape, just like air injection.

At this point the game plan is:
1. New (well, technically used) distributor with crane setup, new rotor, cap and wires. Get the timing adjusted.
2. Remove exhaust manifold, plugged cat and EGR. Intall manifold with no EGR or cat.

Any pointer as to where to buy the downpipes would be appreciated. I have one pipe tha goes from the cat to the rest of the system. Looks like I will need two downpipes from the manifolds, these are two piece (pre-smog and Euro style). Worst case, I can always use my MIG welder to fabricate them, but if they are reasonably priced, I'd rather save time and buy them. I am only interested in new manifolds.

Also, how should I adjust the ignition on the crane setup, should I use the settings for pre-smog engines or Euro engines? According to the shop manual my W114.060 supposedly has high compression pistons, per Chilton it's low compression, but I trust factory manual more than I do Chilton. I am removing all vacuum operated gadgetry, which makes me think that Euro spec should be ok.

I am also contemplating removal of the vacuum pump. Should I connect the brake booster directly to the manifold?

One more question: Does anybody know how to retrofit W114 with electric power locks instead of vacuum-operated ones?

[bwostosh]

Leave the vacuum pump on with a new diaphragm. Strip all the smog stuff. Crane is good I use them. Set timing advanced until pinging upon slow accel. then retard until correct.

Try to get exhaust manifolds from a fuel injected non smog motor. just like a header system.