[MABuckley]

Help!

I just replaced the short block on this motor and I cannot get it to run. I've adjuseted both intake and exhaust valves to 0.004 at 1/4 turn past TDC and no luck. It cranks over and kicks back on the compression stroke with smoke out the carb. So far it has not kicked over at all yet. What do I need to do next? If I could just get it to run I feel confident that I could tune it in but so far I cannot even get a rough run.

When I tried last night all I got was backfiring out the exhaust prior to adjusting the valves to .004. I can see the compression release lift on the intake valve but see no extra lift on the exhaust.

Please help.

[ballengerb1]

This may help you. However I think your valve adjustment is not correct. Read this site, especially the part about tappet adjustment. You will need your engine model number.

[ballengerb1]

I guess it would have helped if I gave you the site, dumb me. How are the valves properly serviced?

[MABuckley]

Thank you

[MABuckley]

I got as far as the flywheel and found I had sheered the key. This is good news, now I am confident I can get it worked out. Thank you again.

[ballengerb1]

Sheared the key on a bew short block? How would this have happened? Most of the time this happens when you hit a rock or stump under power.

[MOWERMAN2468]

I got as far as the flywheel and found I had sheered the key. This is good news, now I am confident I can get it worked out. Thank you again.

To set the valves it is 1/4 INCH past tdc, not one quarter turn past tdc, it is 1/4 inch past tdc.

[MOWERMAN2468]

Sheared the key on a bew short block?? How would this have happened? Most of the time this happens when you hit a rock or stump under power.

Ball, I will tell you how he sheared the flywheel key on a new short block. He set the valves at 1/4 turn past tdc, not 1/4" past tdc, and of course the 1/4" past tdc is correct. Therefore, it was timed at 90 degrees off. That is why the key sheared.

[DrRotor]

Hmmmm.

The flywheel is held in place primarily by the conical surface adhesion between the flywheel and the crankshaft stem. The amount of adhesion is a direct result/measurement of the torque applied by the flywheel nut; the key's primary purpose is to place the flywheel (and magnets) in the correct position so as to "time" the magnetic flux introduced (and interrupted) to the ignition coil(s). If the flywheel nut is left loose, the flywheel's rotational energy must then be transmitted from the flywheel to the crankshaft (and vice versa) through the only remaining path; the key. And the key will shear so as to protect both the flywheel AND the crankshaft. In other cases, sudden stoppage of the engine (i.e. hitting a stump or rock) will cause the flywheel to overcome concial adhesion and impose that energy on the key, subsequently shearing it. This allows the flywheel to continue rotating until it stops thereby preventing the crankshaft from having to bear excessive twisting moments (bending) between a very stationary rock and a rotating flywheel that very much "wants to continue rotating." BTW, keys are typically made of a softer material than either the crank or flywheel so that they CAN shear without damaging either the crank or the flywheel or pulley, or gear or whatever the application. A 20-cent key is quick, easy and cheap to replace versus the crank OR the flywheel.

Valve timing is only interrupted or changed if the timing gears are somehow compromised or crank/cam positions are relatively changed. (In other words, valve timing is incorrect only if the timing gear teeth are not matched correctly. Unless you broke open the crankcase and changed these gear positions, it is highly unlikely that valve timing is "off."

My guess would be the flywheel nut was not torqued correctly thus precipitating the key being sheared.

Incorrectly adjusted valve lash/clearance precipitates other problems more adequately addressed in the most excellent link these most excellent respondents have already provided. :)