Weak Firing Cylinder #5


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By alanfreeman - 13 Years Ago
Now that I resolved my sticking valves problem, this 1961 292 is still running a bit rough. I have determined that #5 cylinder is firing, but very weakly. When I touch the exhaust manifold next to each cylinder's exhaust port with the engine running, each is too hot to touch except for #5 (front left bank) which only gets warm to the touch. Also, when I pull the spark plug wire from each plug while the engine is running, the engine lulls back slightly except for #5 where the engine does not react to pulling the wire. I checked everything I can think of. Compression in #5 cylinder is 125 pounds. I tried a new spark plug. Although the wires are brand new, I even tried a replacement of the #5 wire with no difference. The cylinder is getting a strong spark. Anyone have some ideas that I may not have thought of? Thanks, Alan S. Freeman
By Hoosier Hurricane - 13 Years Ago
Alan:

Since you have checked the obvious, one possibility is that since you have had sticky valve issues, is that the intake pushrod for #5 has bent and jumped off.  You would still probably read compression, but no air/fuel getting into the cylinder.  Another possibility is a vacuum leak.

By alanfreeman - 13 Years Ago
Thanks John. I pulled the valve cover and the #5 valves are opening and closing properly. The sticking valves were the intake and exhaust on cylinder #7. Why would an vacuum leak effect only cylinder #5 and no others? Thanks, Alan
By GREENBIRD56 - 13 Years Ago
See anything amiss inside the distributor cap? Something swapping wires might not catch ......... cracked plug? Seems electrical to me if it passes the usual mechanical inspections.
By Hoosier Hurricane - 13 Years Ago
Alan:

If the leak was in the port to #5 it could lean that cylinder enough that it wouldn't fire.  Only place for that to happen is at the manifold/head juncture.

By bird55 - 13 Years Ago
Along with what Hoosier suggested, with the engine idling, try squirting a little mist of something like wd-40 around the intake to head juncture at #5 if the engine changes speed or stumbles it's vacuum leaking there. don't go crazy with it, fan might blow it around too much. let us know what you find out.