Re: cub wont start

Posted by KEB1 on December 24, 2007 at 16:23:01 from (63.227.112.114):

In Reply to: cub wont start posted by javan on December 24, 2007 at 12:40:49:

Start with the basics. An engine needs three things to run, fuel, compression, and spark in the right sequence.

Compression: Was the engine torn down far enough that the camshaft may be out of time with the crank? Pull the valve cover, take the spark plugs out, crank the engine over by hand, should be able to watch exhaust to intake transition (exhaust closing, intake opening) coincide with piston at top dead center on the exhaust stroke. If valve timing appears to be correct, do you have compression? Should run (poorly, but run) even with really low compression. If you have low compression, check to make sure the valve clearances are set properly so that they're actually closing.

Spark: Now that you have compression and have confirmed the valve timing, check the ignition. Do you have a spark? If not, check that the point gap is set properly. If still no spark, check to make sure there is power to the hot side of the coil, and that the points are opening and closing. If you connect a test light between the coil terminal connected to the points and ground, it will light when the points are open and go off when the points close.

If you have a spark, check to make sure its timed right. Bring #1 cylinder to the top of the compression stroke (put a finger over the spark plug hole to feel compression). Make sure the distributor rotor is pointing to the #1 plug wire position. If so, set the distributor timing so that the points just start to open at top dead center. The timing won't be exact, but it'll be close enough it should run.

Fuel: Now assuming you have compression and spark in the right sequence, check for fuel. If you crank for a while with the choke all the way closed, the plugs should be damp with fuel. If they're dry, check to make sure you have gas at the carb and the float level is set properly. You can also check for a manifold leak by putting your hand over the carburetor intake and cranking. If you don't get good suction, look for a leaking intake manifold (probably a gasket) that would make the mixture too lean to fire.

As I stated at the beginning, if you have compression, spark and fuel in the right sequence it WILL run. Maybe poorly, but it will run.

Hope this helps,

Keith

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