86 LX runs fine, but idling on 3 cylndrs

[registered]

Hope someone has a clue, I've been working this problem since it started 3 days ago.

1986 Accord LX, car ran fine, but carb started freezing. Replaced bad air control diaphragm on air cleaner, and everything good as new again, for about two days.

Suddenly warm engine idle dropped to about 500 and rough. Pulled plug wires and found that pulling #3 wire had no effect. Pulling any other wire killed the engine. Spark was obvious at the distributor cap, but #3 not firing.

Checked compression #1, #3, #4 195 psi, #2 180 psi. Switched #3 and #2 plugs. Still not firing on #3. Changed plug wire, #3 still not firing.

This is only obvious below 2000 rpm or so. Above that engine accelerates strong and smoothly.

What could cause 1 cylinder to not fire at low rpm?

Any ideas appreciated.

[AccordCodger]

It may well be that your are ALWAYS firing on 3. As the RPMs go up, it runs more smoothly and makes you think that it's running normally.

[andrewken]

change your spark plugs and wires

[PlumcrazEbody]

did you say youreplaced the distributer cap? Try that... there isnt much more thn that to those carbureted motors. Is your car burning oil at all? When you pulled out the plugs what color was the end of that cylinder's plug? was it covered in oil? If the cylinderisnt dead, it really only seems like it could be the cap, wire or plug... but you tried most everything ican think of but let us know if you figure it out....

[registered]

The mystery is solved. A google search turned up several references to 1 cylinder missing at idle, most suggesting vacuum leak.

I took the air cleaner off, and there was one more vacuum connection coming of the rail than there were hoses to attach. I faintly remember a cap on one of the connections, and my service manual showed what looked like a capped line.

Capped that line and reassembled and all is well. The capped line (#6) is right next to the #8 vacuum line going to the air control diaphragm which I disconnected , but I swear I didn't touch the cap. And it ran fine for a couple of days, so I don't know how or why the cap would have fallen off.

I wouldn't have expected a vacuum leak to cause a single cylinder to not fire unless it was near that cylinders intake.

Thanks for the suggestions.