Comet 250 idle problem

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

New hose to the PCV valve, which was also replaced. I made up new gaskets for the carb to adapter and adapter to manifold.

Click to expand...

Did you try shooting some carb spray around the carb mounting and all other vacuum hookups? Has it ever been real hot? Have you done a radiator test checking for combustion leaks? Sometimes you can see the bubbles in the coolant. Might be a crack in # six going into exhaust. You could try some K&N block sealer.

Well, I'm a rank amatuer but I'd be looking closer at that #6 cyl. Leakdown test, possible valve train issue. You've replaced everything I can think of that might cause it.

Howdy:

My first guess is that you have a major vacuum leak somewhere. The fact that it won't idle and screwing in the low speed air screw on the carb making no difference are clues. Is it an auto trans or manual? Is the crankcase closed and well sealed?

My next thought is that one or more valves are sticking open when running. They may not show up on a static leakdown test or compression pressure check. You might want to pull the valve cover to see what you can see. With those cylinder pressures you should have pretty good vacuum readings. You mentioned vacuum decreasing when you short out the #6 plug. Did you observe that with a vacuum guage?

Keep the info coming. Good luck.

Adios, David

karguyz":29a2dkq2 said:

vacuum decreases

Click to expand...

What is the Vacuum at idle, is it steady or jumping around?

How does the car run at road speeds?

A quick test run can run if your careful would be to get one of those ink jet cartridge refill hypodermics and fill it with gas. Then slowly and carefully shoot some done the carb with the engine running. If it gets better then your idle circuit needs attention. If it gets worse then you might be getting to much gas. No change , look else where.

I'd check and see if the fuel pump is doing it's job.
Maybe some trash in the tank?

When looking for vacuum leaks don't forget the distributor advance. It could leak too.

Plug the PCV valve and see if the idle improves. I've had these cause a vac leak condition right out of the box.

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

If you can rule out the carb, then there may be a valve train issue.
Maybe timing chain, or valves.

Checked the timing, no slack in the chain.

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 distributor 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.

Later in the day...................

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 weird 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

Well, it's back to running bad again. I drove it around on a hot day and was OK for about a half hour. Then is started acting bad and still misses something terrible at idle, even when cold.

Thanks for the ideas. I'll check the ground cable. It starts right up but maybe when it is running.....Had a battery that went bad on a Falcon six. When it got hot the plates would open up. Never noticed it until I made a short trip and car wouldn't hot start. Got a jump then when waiting for a train the idle dropped and the generator cutout relay dropped out and motor quit. It was running on the generator only when it got hot.

No slack in the timing chain.

Replaced the fuel pump yesterday, and rubber hoses. Now it runs bad from the get go, missed even before it is warmed up.

I put some Seafoam in the intake, crankcase and tank. Maybe that will loosen whatever is sticking and make it better or worse. I haven't run it for long after that. Will try to get it hot and circulate the stuff thru the system.

I'm about ready to change the head just for drill, or at least take it apart and change the lifters. It doesn't make any mechanical noise at all so I'm reluctant to tear it apart. Would eliminate any valve problems. Wonder if Pick-a-part has any cars this old.

Next step is to drain the tank and get some different gas to eliminate that.

This will be a good "What went wrong" article for Skinned knuckles when we fiigure it out.

John, still puzzled in Kalifornia

Are you sure it's not vapor locking? Does it always happen when its hot? The summer heat could be contributing, especially if you're in SoCal running around in the High Desert.

Darrell

You mentioned grounds, make sure they are all good, and don't forget the one at the back of the block.

karguyz":3gx7j16r said:

Next step is to drain the tank and get some different gas to eliminate that.

Click to expand...

IIWIYS that would be the next thing I eliminated since you've done the fuel pump and the carb. Crud in the tank as Jackfish mentioned is a typical problem. It usually will affect performance sporadically. Drive fine after sitting quite a while then eventually crud plugs up the sock on the pick-up tube after extended driving and fuel delivery becomes a problem...(seafoam in the gas tank might excaserbate issues). You can 'thoroughly' drain, clean and flush the tank and pick-up assembly or anunconventional work around is get your hands on a temporary fuel tank like a boat fuel tank and temporarily plumb it in place in the trunk.

Noticed in your first post you had duraspark but went to points...'direct wired to battery'...don't points need to be operated with a resistor and not see full 12v...but since you said it made no difference I'm guessing the problem was there before when you had duraspark and did not go away with points set up. Are you using the same 'coil'? A worn coil could show itself in high heat circumstances.

Is the car running hot? Any backfiring through the carb or out the back? That 20% diff compression reading on #6 is concerning but don't know if I'd pull the head just yet. As David mentioned you can check for stuck valves by pulling the valve cover and inspecting their range of movement while running. Seafoam in the crankcase might resolve it, if in fact that turns out to be the issue.
Good luck!

We replaced the tank with a NEW one a couple of months' ago. Don't think that any coating in the tank that would dissolve would still be in there now.

When I changed the ignition I also went back to the original coil. Yes, the points require a ballast resistor so they won't burn out but a full 12 volts to the coil for a minute or so won't hurt. What I was doing was eliminating the car's wiring as a source of the problem.

It doesn't run hot, new radiator last month. Even with the air on and stop and go traffic never went over 205 degrees. Thew fact that it runs bad now when it isn't even warmed up leads me away from a heat related problem.

The line to the fuel pump is a rubber hose. I might just feed the gas into the pump from a separate can directlt to eliminate the tank and plumbing.

The number 6 cylinder has always been low. If it was bad it wouldn't come and go.

Back to checking the grounds.

John and Mikey

karguyz":18tlnd9l said:

It doesn't run hot, new radiator last month. Even with the air on and stop and go traffic never went over 205 degrees. Thew fact that it runs bad now when it isn't even warmed up leads me away from a heat related problem.

Click to expand...

John & Mikey...was curious about operating temps as a symptom...not a cause. Along the lines of what was suggested above crack on exhaust port/super heating or burnt valve. Asking about backfire issues through carb or tailpipe along the same diagnostic lines. Sometimes a backfire in either direction can indicate an ignition issue and feels different (to me at least) than a vacuum leak or missing cylinder.

Gas can is a good idea, or you can also check after the pump for rythmic delivery of about 2-4oz every couple secs into a can or jar.
Good luck on checking your grounds and finding the issue

Frankenstang":nycvbqk8 said:

A worn coil could show itself in high heat circumstances.

Click to expand...

karguyz":nycvbqk8 said:

I drove it around on a hot day and was OK for about a half hour

Click to expand...

This seems to me to be a coil problem. They go bad. At first they have to heat up so they only show up on hot days. After a while they get worse and thier self heating is enough to cause problems. At last they get bad enough they run bad all the time.

Coils are cheap, beg borrow or steal one and try it before you start ripping your engine apart.

X2. He said that he put a black Ford coil on it, but he did not say it was new.

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.

Click to expand...

Well, we spent most of the day troubleshooting and redoing everything we did before..

Pulled the plugs they were black.

Replaced the spark plugs, again, and wire to coil. Jumpered directly from battery negative to engine ground while running, no help. Timing hasn't changed. Pulled one plug wire at a time to see if it was related to only one cylinder. couldn't tell, it was missing so bad.

Ran it out of gas and pumped new gas from a can through fuel pump directly to carb, thus bypassing the gas tank and lines from it. No help.

It acts like there is a bad ignition problem but the timing light is steady.

Compression test shows all ok except six is low, rings bad. The leakdown test results were good for all except number six which was blowing air bigtime past the rings and into the oil pan. But the spark plug from number six didn't look any different from any of the other cylinders.

Maybe another new coil will help, done everything else.

Ideas?

John and Mikey

m