Hi All, Here's a problem that we can't figure out. Car is the 72 Comet, 250 (4.1) six. It runs OK at speed and does not miss once it is moving. Lately the idle quality has gone WAY down. We just can't keep it running. Very intermittent. The carb idle adjustment seems to make no difference, like there is a massive vacuum leak. It just acts like a couple of cylinders are cutting out intermittently. Here is what we have found and done. 1. Rebuilt the carb and even replaced it with another one, fresh rebuild off Ebay. The replacement carb has more of a flat spot on acceleration but idle quality is the same. We put in a new carb base gasket. New fuel filter. Gas is good, not stale. 2. We had a Duraspark II distributor on it, with the blue grommet. The idle timing was moving about a bit so I took it off. The spark plugs all looked about the same- no evidence of fouling, gapped at .040-.045, maybe a bit lean. 3. We put the old points distributor on it, set the dwell and timing. New wires, new plugs. Timing at idle (high idle- maybe 1500) is 10-15 degrees. It speeds up when you move the timing but still has that miss. The vacuum and centrifugal advance both work well. No leaks in the diaphragm of the advance unit. 4. There is a log type fitting under the carb, connected to the intake manifold. The hose going to the vacuum controls for the heater/AC doors was cracked so I just plugged the fitting. Line to the vacuum modulator for the C4 trans was good, no leaks. The shifting is good also, so that isn’t the problem. 5. The PCV valve was replaced, no change. If you take the valve out of the valve cover the poppet is bouncing back and forth like it is supposed to so the PCV system is not at fault. If you put your finger over the PCV valve the idle doesn't change. 6. Tried to find a vacuum leak with both WD-40 and propane gas methods. It didn’t make any difference. 7. We are using the original ignition coil. It is the only thing we haven’t changed. I would think if that were the problem it wouldn’t run well at high speeds. I may jumper from the battery to the hot side of the coil to bypass the ballast resistor and ensure full voltage to the coil. Please help, this is Mikey’s first car and he really likes having something different to drive, especially one he works on for himself. This car used to run good, but something has changed. I’m losing confidence on this vehicle. I’ve been working on Fords for over 40 years and this one has me stumped. karguyz
Pull the plugs and see how they look...Look for oil fouling.Sounds like you may have a bent pushrod or burnt valves...Do a compression test as well while the plugs are out.Also...You stated the timing bounces around...Could be a bad chain or the gear on the dist is worn/spinning on the shaft.Also a used out distributor can cause the timing fluctuation.Pull the dist cap and wiggle the rotor/dist shaft side to side.If there is any appreciable movement,the dist is worn out.Also check along the intake log for any open vacuum nipples or rotted freeze plugs on the ends of the log. Check to be certain the EGR valve is working properly...If the pintle on it is clogged with carbon(not uncommon) it could be partially opened causing the poor idle quality as well.Good luck!!!
That's an idea, rotted freeze plugs. I'll clean the engine so I can see more of what is going on. No EGR valve, this has no smog stuff, except the PCV valve and the charcoal cannister for gas vapors, which is not part of the engine systems. Thanks, karguyz
It's fixed!. When I took off the adapter between the carb and manifold there was the answer! There was a piece of gasket missing which let in lots of air and destroyed the vacuum. When we first got the car back on the road last year it ran better. Over time the oil and dirt around the engine had combined to form sort of a sludge which filled up the space where the gasket was missing. When I cleaned up the engine compartment and washed away the dirt the vacuum leak got worse. I scraped off the old gasket, wire brushed the manifold and adapter then made a new gasket. Put everything back together and also replaced the cracked pcv hose. It started right up. We had to spend a while resetting the idle lower and adjusting the mixture. Now it's running like it should. Steady idle and no flat spots on acceleration. Thanks for the suggestions. karguyz
