Missing after warmed up

[menos]

I've got a 89 LX that I'm trying to get up and running. I've gotten the cooling system ironed out, which was the main reason it had been sitting.
Initial start is rather rough but I understand that can be normal for a carb'd vehicle. Once it gets warmed up it idles fine and the idle drops to just less than 1k after blipping the throttle.
It will drive around just fine for about 10-15 minutes then it starts running real rough. Idle wants to drop to less than 500 and I have to massage the throttle to keep it running. There is also a pronounced miss. When into the throttle it smooths out and will hold rpms however, there is also a rather frequent miss.

I know these carbs are a bear to work with but is there anything I should be looking at that only shows itself after the motor is heat soaked?

[2oodoor]

Sounds like the choke pulloff or choke heater is not taking choke fully off, or fast idle isn't working. Pull the breather off and check vacuum hoses for leaks and collapsed. Spray down the air horn and choke parts good with carb cleaner, make sure wire is attached to choke heater. Spray down the linkages and shafts good with wd40, inspect secondary diaphragm linkage sometimes they hang partialy open which dumps gas at idle.

[menos]

Took another look at it while it was acting up and noticed that the vacuum advance module was completely cracked through. I went to the yard and picked another one up. I installed it and set the timing while I was into the dist. With vacuum advance disconnected and plugged on a warm motor it looked like there was only about 5* of timing. I advanced it to something close to 15-20 but it was real hard to read it exactly. With every thing connected back it ran like a champ. Drove it for 30 minutes and it never stumbled once. My next step is going to be to look at the carb and make sure everything else is adjusted correctly. The guy I got it from said it had an idle problem so there's no telling what he 'adjusted' trying to fix it.

[djriprock]

Check the pre-heater hose. Especially since I'm assuming it's cold where you live like it is where I live. Carburetor icing is what I'm having a problem with. If that door isn't opening in the air intake to allow the warm air in, then it can ice the venturi's and cause you problems. I'm having issues with that myself.

[menos]

I've checked the choke and the fast idle settings. As best I can tell both are functioning correctly. I have noticed however that it seems to be running pig rich at idle when fully warmed up. What are the typical causes of a rich idle?

[Dr_Snooz]

Check your idle mixture screw. It controls mix at idle.

[menos]

What's a good procedure to set the idle mixture? The shop manual refers to a propane enrichment kit which obviously I don't have.

[Dr_Snooz]

Use your nose. Seriously. It's the most finely tuned instrument you have. I don't advocate breathing in large quantities of exhaust gas, of course, but you'll smell it when it's too rich.

[menos]

I think I've got the missing issue taken care of. Yet another advance module but I actually tested this one at the yard, new plugs and a new fuel filter. So far with 3 plus hours of driving around town and on the highway it hasn't done it again. Before I switched the fuel filter it was only showing about .7 psi on my gauge. Now it holds at 2psi.

My only real issue at this point is the cold startup. If I pump the pedal 2-3 times it will fire right away. However it will not hold idle. I have to stay in the throttle. If I don't it will keep slowing down until it sputters out. Eventually it will hold minimum idle on its own then it slowly increases as the motor warms up. It will eventually reach 2200rpm which is where my fast idle is set. A blip of the throttle will drop it back down to the base idle.

I've pretty much been through every test in the carb portion of the service manual. So far everything has tested OK or I've adjusted it to spec.

Any direction I should look in?

[menos]

OK... I think I may have it figured out. The vacuum driven choke opener wasn't opening the choke plate. I dissembled it and found a leak in the diaphragm. I used some brushable rubber sealant from permatex to seal it up temporarily. The opener now holds vacuum and is opening the choke plate. Car started on the first try and idled smooth even when it was dead cold. I've got another choke opener on the way so hopefully this will be the last thing and I can get this thing back on the road.

[Dr_Snooz]

I think you win the Carb-Master prize. Impressive work. Please stick around and help us with the other carb problems that come up.

[menos]

Well, looks like I was premature in considering my original problem of it missing at idle resolved.
Once the car gets completely warmed up it has a miss at idle. If the car is in park you can barely notice it but if its in gear its quite pronounced.
I've noticed that it seems to be worse the warmer it is outside. We hit 70+ here today and it is pretty bad.

It has new plugs, wires, cap, rotor and ground strap so I don't think it's electrical. The fuel pressure is rock solid at 2psi with new fuel filters.

Any other ideas?

[zenapup]

Well, looks like I was premature in considering my original problem of it missing at idle resolved.
Once the car gets completely warmed up it has a miss at idle. If the car is in park you can barely notice it but if its in gear its quite pronounced.
I've noticed that it seems to be worse the warmer it is outside. We hit 70+ here today and it is pretty bad.

It has new plugs, wires, cap, rotor and ground strap so I don't think it's electrical. The fuel pressure is rock solid at 2psi with new fuel filters.

Any other ideas?

Could be the igniter or the ignition coil, does it have a weak spark when the missing occurs?

[menos]

UPDATE:
I tested the coil which looked to be about as old as the car itself. I can't remember the resistance numbers I got but they were no where close to acceptable. I replaced the coil with a MSD Blaster 2 and put the correct heat range NGK v-power's in. Car has been running perfect for over a week and 500+ miles. Starts quick and easy, idles smooth and runs down the highway purring.

I'm crossing my fingers that nothing else goes south, because this is a fun little car to drive.