My craftsman 21 hp b and s engine.

JohnDeereFan8899 · Oct 4, 2009

I have a craftsman lawn tractor, 21 hp briggs engine, and the starter doesnt want to turn the engine through its compression stage. I tried getting a new battery, didnt work, so now Im looking for answers to what my problem could be.

jinman · Oct 7, 2009

I would disconnect the spark plug wires and see if the starter will turn the engine over normally. If it does, you have to suspect spark timing. If it does not spin the engine, then you have to suspect that it's a defective starter: dragging or bad brushes. Since you tried a new battery and didn't mention doing anything to change spark timing, my guess is you may be in the market for a new starter.

I have an old '74 single vertical cylinder 16HP B&S engine that is a bear to start. I have to have a fresh, well-charged battery to get the engine to turn over. The starter really struggles to get the engine through the compression stroke. I suspect a new/rebuilt starter would also serve me well.

atschirner · Oct 8, 2009

You may also want to check the valve lash. On a number of B/S twins an out of spec exhaust valve will keep the compression release from working correctly. Your engine model and type numbers would help you find the specs and procedure to measure and adjust the lash.

Scooby074 · Oct 8, 2009

Pull the plugs and see what happens. How does it wind over?

What are the condition of your wires to the starter and solenoid?

THere is a compression release as mentioned, that can get messed up if the valve lash is too much.

JohnDeereFan8899 · Oct 9, 2009

The starter only turns the engine to its compression stroke. Then it just wont turn it at all. So i have to turn the air cooling fans at the top all the way back so the starter gets enough power to go through the compression stage.

Scooby074 · Oct 9, 2009

Pull the plugs and see if it will wind over at approiate speed.

It could be a electrical problem. Ie: the starter's not getting enough current.

Check all the wires and especially the ends where they attach to the solenoid and battery, ground, starter for corrosion. Clean contact area with emery cloth,.

carpenter383 · Oct 9, 2009

atschirner said:
You may also want to check the valve lash. On a number of B/S twins an out of spec exhaust valve will keep the compression release from working correctly. Your engine model and type numbers would help you find the specs and procedure to measure and adjust the lash.

I second this, I had the same problem with a briggs with overhead valves on an mtd.

alchemysa · Oct 11, 2009

carpenter383 said:
I second this

I third it. I had the same problem. Drove me nuts. I enquired about buying a new starter before I found out about the valves on this site. If you don't adjust them it eventually gets worse. You have to get the valve clearance just right or the engine won't get past the compressions stroke. I've played with engines most of my life but I never thought such a tiny valve gap adjustment could make such a difference.

If its like mine, the valve adjuster is a small allen screw inside the rocker nut.

(Actually I never got too serious about buying a new starter. Have you priced those things!!)

heehaw · Oct 13, 2009

i would check the battery connections an cables: you might not be getting enough amperage thru the wires: put a voltmeter on the starter connection, an if it drops dramaticly when you try to start the motor, you aren't gettin enough power where you need it.
heehaw