Hi All, Here’s a problem that you may be able to help with. Car is Mikey’s 72 Comet 250 six. Over the past couple of weeks the drivability has gone downhill, to the point where it just is not a viable vehicle. It misses something terrible at idle, very erratic and may or may not stall. It acts somewhat like a massive intermittent vacuum leak. The carb idle mixture adjustment can be screwed in all the way and it doesn’t seem to make any difference. First I replaced the carb with another recently rebuilt Carter RBS. Same thing. I checked all the hoses and made sure there were no leaky vacuum diaphragms, transmission modulator, heater control valve, distributor, etc. New hose to the PCV valve, which was also replaced. I made up new gaskets for the carb to adapter and adapter to manifold. I checked the compression, all 140-150 except number six which was 110. It read the same last year when it was running better. Replaced the spark plugs. The old ones looked good on the insulator, nice light tan, but black around the outside. Number six looked just like the others. The timing was checked, it hadn’t moved, about 15 degrees BTDC. The car had a Duraspark II conversion with an Accel coil. I replaced the whole ignition system with the stock points distributor and black Ford coil. Jumpered directly from the battery to the coil to eliminate wiring problems. No difference. So it isn’t ignition related. When I short out each spark plug individually the engine speed goes down or rather the vacuum decreases. It’s hard to tell because it idling so poorly. Number six is firing just like the others, speed drops when it is shorted out. The engine has NO internal noises; lifters are quiet also. The oil has been changed recently. It doesn’t use any water so don’t think the head gasket is bad. Only 67 thousand miles, far from worn out. Anybody have any ideas? I’m open to suggestions, except to replace the motor with a V8. John and Mikey, at a loss in Kalifornia
Like I said, the timing is about 15 degrees, at whatever RPM I can get it running at. The engine is intermittent, not just slow idle. Something acts like it is changing. If it was ignition it woulldn't act this way. I'll move the distributer back and forth to see if it helps any. The fact is that something has gone bad, the timing wouldn't just change all of a sudden, especially with the Duraspark. To answer the cap and rotor question. I replaced the complete ignition system, plug wires, and whole different distributer of an earlier (points) design. Thanks for the suggestions, John and Mikey
when you said that you can screw the idle screws all the way in and it still runs tells me that its geting fuel from where it shouldnt be at idle. take the air cleaner off and look down the carb as its idleing. you shouldnt be able to see any fuel coming out the boosters. the float levels may be to high. what idle rpm is the motor at?
I used to have the same problem with my 250. With a long going over of every hose i finaly kinda fixed it. I ended up also turning the idle down a good deal. But the problem i had was a bad ground and a leaky hose. With the cars this age you have to really be thuro. I cant think of anything else besides a gorund
Still sounds like a vacuum leak to me. I would check the carb gaskets and intake gasket while running by carefully spraying some carb cleaner on them. The carb cleaner will fill the gasket surface momentarily, if there is a gap, and the engine will smooth out and then return to poor idle.
Thanks for all the ideas. Pcv valve plugged makes no difference. I think I'm running at too high an idle for the idle mixture control to be effective. I made up a block off plate to go in place of the carb. Had an adapter to the air compressor screwed into it. Then I pressurized the maniflod with the compresssor.I had the external vacuum lines ( carb modulator, heater control valve, etc) blocked off. There were no leaks heard of found with soapy water. I thought that the carb adapter to manifold seal might be faulty. Again, nothing definitive. The carb seems to work Ok. I pulled the float bowl off carefully to check the fuel level. The gas looked clean and the level was OK> Changed the fuel filter just for grins. This is too simple an engine for there to be anything major wrong without it being obvious. Any more ideas? TIA, John and mikey
Weak coil...It heats up and then acts up... Cracked throttle plate (carb base) may not show till engine is hot... Cracked adapter plate at base of carb, once again may not show till engine is hot. Does the choke work or is it sticking closed. Turning mixture screws in and car still runs says vacuum leak... Crud in mixture orifices. Points not adjusted well or condensor failing once its hot... Unless you are running a coil meant to take a full 12 volts...(you hot wired it right) you will kill the coil or are allready doing so. Points ignition systems run the coil at 6 to 8 volts (hence the resistor wire in the ignition circuit) running em at 12 volts isnt good for the condensor or coil... Good luck, hope this helps you a bit.
That sounds like my old 250. It never did idle. You had to keep it running by depressing the accelerator pedal slightly. My eventually solution was to ditch the 250 in favor of a 302. Sorry, not much help here as I never could solve the problem.
Well, I did the smart thing and had someone else take a fresh look at the problem. For some reason my brother was able to get it fixed. A combination of things probably. 1. The distributer was set up with a quick advance curve. We set the idle as low as it could go then just before it died checked the advance. Eventually came up with an idle reading of about 12 degrees at the lowest setting. 2. This must be a smog carb set to run very lean at idle. The idle screw had to be out over 5 turns and was ready to fall out. We stretched the spring and put in a couple of washers so it would hold a setting. So far it is working fine but it is too hot to do more testing. Will see how it works when the motor is cold and warms up. Thanks for the answers. John and Mikey
Well, it's back to acting up again. I drove around in the hot afternoon with the air conditioner on. It idled a bit rough but did not stall even though it does not have an idle up solenoid connected to the compressor line. After a couple of short stops in various stores it started acting wierd again, rough idle and stalling. There are NO mechanical noises like lifters, etc. Something is getting out of whack and once it starts running bad it doesn't stop acting up. I'm going to try the Seafoam treatment and see if that does anything. Can't hurt, except for all the smoke.... John and Mikey
