Cub Backfire Issue

[Smarg]

Greetings! I have a '48 Cub...all of a sudden it will not start, and backfires badly when trying.

It was recommended from a local antique tractor club member that I use Marvel Mystery Oil to free up the valves which could be sticking.

How would I do that? There is a plate on the side of the engine which I believe exposes the valves. Would I open this up and pour the oil inside and then turn the engine over? What is a recommendation to free up sticky valves, and how would I permanently fix the issue?

Thanks

[Eugene]

You put some of the Marvel Mystery Oil in the fuel tank and some in the crankcase, the engine oil. No need to remove the valve chamber cover.

Compression test results?

When was the last complete engine tune up?

Backfiring is usually a symptom of an ignition related problem.

[staninlowerAL]

In order to get a visual assessment of the valves you will have to remove the head and/or valve cover. You can determine if the valves are moving as they should by removing the valve cover and observing the action as the engine is turned by hand crank or starter. Also check and adjust valve tappet clearance as needed. The next step is to remove the head and check for burned/pitted valves and seats or carbon or other foreign material between valves and seats. If this needs repair, a machine shop services might be required. Good Luck, Stan

[Mike in Louisiana]

[quote="Smarg"]Greetings! I have a '48 Cub...all of a sudden it will not start, and backfires badly when trying.

Since it was running, Check and make sure that the distribuator is not loose. If it has a mag check the gears , are they striped.

[John *.?-!.* cub owner]

Before you get too carried away, with the switch off turn the engine over a few times with a hand crank. You should feel compression each 1/2 round. If not continue with the suggestions, as you probably do have a valve sticking. If you feel good compression 4 times per 2 rounds, look elsewhere. A 48 originally had a magneto, and if yours still has one, even if an external coil has been added, some of the teeth on the rotor or pinion gear could have broken or stripped causing it to be out of time causing the spark to go to the wrong plug and wrong time.

[ScottyD'sdad]

Sometimes the spring on the points, breaks. (I've had it happen to me, and have seen it happen to others.)
Ed

[Smarg]

Thanks for the feedback, guys. I'm copy/pasting and then printing these responses before I go back to the shop. Wish me luck!

[Mike in Louisiana]

May the luck of the Irish be with you.

[smallfarm]

Any kids around to switch the plug wires??