1973 Calif. car, L-48, auto. A/C numbers matching, engine has never been apart, not even the intake has been off, well over 100K miles. 90% of the smog crap has been removed, either by me or P O. P O replaced the carburetor, with this one; 17054919
1350
I rebuilt the carburetor about 2 years ago.
oil pressure @ idle..........35/40 lbs. off-idle, 55/60lbs.
water temp, after 30 minutes driving......195
vacuum @ idle..........21"
All sparkplugs are firing
Choke is wide open
Fuel tank looks very clean inside.

New; fuel pump, fuel filter, air filter, valve stem seals, oil pump, sparkplugs, wires, cap and rotor. timming chain & gears,
egr valve.
Pertronix Ignitor II with flame-thrower coil, full voltage to the coil. EGR valve is functioning properly.Probably left something out, but here's the problem........

When driving under any, or all the above conditions, all of a sudden the engine will start to idle very rough when stopped at a light, almost to the point where it will stall.. No warning, I just take my foot off the accelator, when coming up to a light and it starts this rough idle. No black, white, gray smoke out the exhaust.
The only thing I can pin down is it occures right after I take my foot off the accelator and the intake vacuum is high. Usually when coasting up to a red light. I keep tapping the gas (to keep it running) untill I can get into a parking lot, where, after a few minutes, it starts to run fine again.
Generally speaking, the car runs great, fires right up, idles fine when cold, gets great gas milage, but every once in a while it just gets this hair across it's azz.

Any suggestions?

 

Partially clogged fuel filter , gummed up carb especially if the car isnt used often , float level in carb not set right , sticking floats in carb , malfunctioning fuel pump occasionally ???? Id run a can of SeaFoam cleaner slowly down the carb venturis at idle as well as in each fuel bowl vent and the tiny air vents for both the primary and secondary. Then take it for a spirited test drive and see how it performs. Let us know what the problem specifically was.

 

Vapor lock? Notice you're in CA...we have some of the most diabolical gas in the world. Mine did that once...just started idling all poopy and then just died. I let it sit for a few minutes and restarted it by holding the pedal to the floor...it went away. Might want to check to make sure the "sock" on your fuel pickup isn't blocked/clogged

 

Well I wasn't going to say anything, but two times it happened right after filling up at a mobil station.

 

After you do what "TheycallmeDave" suggests in post #2 I'd start looking electrical. Remember the old saying...., 90% of fuel problems are actually electrical. I'd get rid of the petronix (temporarily) and go back to the original points set-up and go from there. Coils can act funny sometime as well after they get hot so if you have your old coil I'd swap it out with what's in there now. Just my 2cents.......

 

Remember your engine compartment gets hotter before it gets cooler after you turn off the engine (no air flow and no water flow and no fan). Because it only takes a few minutes to add fuel to your tank the coil may be hotter when you start your engine again.... which might be enough for it to start acting up. More of my 2cents..

 

Well, as a fellow owner of a carbureted C-3, stay away from Mobil, Arco and 76. I've had the same problem in years past. Try Chevron or Shell. Computer cars can self adjust to the crappy burning characteristics of many fuels but not our old dinosaurs. Also if you used a cheapo rebuild kit for the carb rebuild, the float could be sinking causing an over rich condition.

 

it certainly sounds like its trying to vapor lock ..... what kind of fuel pump did you put in ??? and check the routing of your fuel lines to make certain it's away from the exhaust .... ????

 

Sounds like vapor lock to me too. Like Jims66 said the engine gets hotter while it's off for those couple minutes.

 

Thanks guys, all good advice, I'll start looking into some of the items. Hard to say if I'll see any change soon, as this problem dosen't occur very often. This car is my DD, except in bad weather and is only street driven.

 

Be sure and let us know so it will help others down the road.

 

I have exactly the same problem in my '75. A week ago I noticed a rough idle when slowing down to stop signs, felt like it was going to stall but didn't. It ran fine for a few days then suddenly stalled out at a stop sign and wouldn't start back up. It eventually started a minute later when I was flooring the gas, and revved right up, but would just die again at idle. I had it towed to a nearby mechanic, who said it had a stuck secondary float which he tapped on, and it worked fine again right after that. I picked it up, drove a half hour home with it running fine. I took it out the next day (today) and it feels stally at stop signs again. Now that I have the Haynes manual I'm poking around at the carb but I have no idea what I'm doing. I'm going to try carb cleaner, and see if I can perform any of the checks you guys mentioned in this thread. Failing that, I already have a date at a mechanic for tires so I can see if they'll check/rebuild carb if needed. No clue how much time and cost that will be.

 

Bad gas. I'd bet they have water in their tanks. Happened to me with Chevron a few years back.

 

Okay I just tried pouring some SeaFoam Motor Treatment into the carb, which was messier than I thought. I took the air cleaner off, started the car, put it in neutral, wedged something on the accelerator to keep it at 2000rpm, and slowly poured half the bottle of seafoam in. Because of the radiator fan blowing like an airplane engine it sprayed seafoam all over. I got most of it in, though, and cleaned up the rest. Shut the engine off for a few min, I'm going to go run it hard shortly. Hope things improve.

 

After treatment it definitely sounds healthier, and hasn't stalled out so far. I'll report back in a week or two.

 

Sounds like a vaccum leak. That will cause rough idle and stalling. Check the brake booster lines, headlight and heater control vac lines and vac advance lines. plug them off one at a time at the carb base or manifold and see if that smooths out the idle.

 

I said I'd check back in...

My problems haven't come back since putting Seafoam in the carb and gas tank. My mechanic suggested a new carb, though.

 

Next time put the Seafoam in a squirt bottle and a 3/8" hose connected to the pcv connection at the front of the qjet, it will suck it in.

 

Yeah I knew I must have been doing something wrong. I'm not even sure what the PCV connection is to be honest, but next time I go this route (if I have to) I'll look it up in my Haynes manual. Thanks for the tip.