Rough running Cub

[Charlie M]

My 47 Cub lately has been starting up on only 3 cylinders but after it warms up for a few minutes it runs great. Any ideas on what it might be. I'm thinking maybe a valve needs adjusting? Found out today its #3 cylinder that is acting up. I found that by putting a timing light on each plug and there was no response on #3. If the cylinder is flooded would the plug not spark until it dried out? Motor was overhauled a few years ago with only a few hours on it. Ignition is a magneto. Would a bad spark plug act that way?

 

[grandpa Love]

Valves are really easy to check and adjust. Unhook the throttle rod on carb , unbolt the valve cover, slide it out and adjust. On a magneto I always unhook the coil wire before adjusting the valves, don't want it starting while turning the hand crank.

 

[Fritz Maurer]

Crank it with the no. 3 plug out and see if it blows anything out of the cylinder, after sitting however long it normally sits to exhibit this problem.

 

[Janicholson]

Different answer! If the timing light is not blinking when on #3, the distributor cap, might be bad causing a failed spark, or the points are misadjusted too close, or the rotor shaft bushing is worn causing the #3 breaker cam bump to not open the points. Jim

 

[rustred]

if your saying u have no blinking on #3 plug thats not a common problem. and the first thing i would check is the plug wire , as in swap one around and recheck. plus i would be very leary of that plug even though u say u have no spark happening at it. why not remove the pugs and look at them then tell us what u found. have you checked the wire at the mag to make sure it ok and not all green and corroded? and yes if the pug is wet it will not fire more so with condensation on the electrode and tip connecting them.