Ask Me Help Desk - My Briggs and Stratton 12.5Hp motor won't start

Hi, I have a 12.5Hp B & S vertical shaft engine in my mower and last weekend the plastic starter cog (the one rivetted to the underside of the flywheel) lost a few teeth and it would not start. So I replaced the cog with a metal one and reassembled it.

When I returned the flywheel I made sure the it was aligned properly on the main shaft and the key on the shaft and flywheel was inserted correctly. But when I try to start it, it won't fire. I have tested the spark and it appears to be OK. It looks a little weak but that could be normal.

I am pretty sure its not a fuel problem as I have also sprayed ether into the air filter and all it did was backfire once.

Apart from removing the flywheel and changing the big starter cog I have not altered anything else so I am at a loss as to why a motor that used to run fine now won't start.

If anyone can help with suggestions I would be very grateful.

Thanks

Did you remove the magneto? (the box the spark plug connects to)

First make sure the aluminum key in the crankshaft is completely intact and not sheared. Second, you got the key seated in the grooves of both the flywheel and crankshaft as this is what keeps the engine in time. Third if you removed the magneto make sure when you reinstall it you allow proper clearance between the magneto and flywheel. This could be the result of your spark being weak. I cannot recall the clearance setting off the top of my head, however a standard business card will work.

To set the magneto loosen the bolts on the magneto with the magnet on the flywheel away from the magneto, insert the business card between the flywheel and magneto. Then turn the flywheel until the magneto is centered over the magnet, this will pull the magneto down against the card. Next tighten the bolts on the magneto and carefully spin the flywheel to squeeze the card out from between the flywheel and magneto and you will have set you clearance.

Quote:

Originally Posted by hawkeye088

did you remove the magneto? (the box the spark plug connects to)

First make sure the aluminum key in the crankshaft is completely intact and not sheared. Second, you got the key seated in the grooves of both the flywheel and crankshaft as this is what keeps the engine in time. Third if you removed the magneto make sure when you reinstall it you allow proper clearance between the magneto and flywheel. This could be the result of your spark being weak. I cannot recall the clearance setting off the top of my head, however a standard business card will work.

To set the magneto loosen the bolts on the magneto with the magnet on the flywheel away from the magneto, insert the business card between the flywheel and magneto. Then turn the flywheel untill the magneto is centered over the magnet, this will pull the magneto down against the card. Next tighten the bolts on the magneto and carefully spin the flywheel to squeeze the card out from between the flywheel and magneto and you will have set you clearance.

Hi Hawkeye,

Yes I did remove the magneto and I put it back with the sort of clearance you suggested. The strange thing is that when I took the magneto off, one of the 2 bolts holding it in place was missing but the engine was running fine then with just the one bolt (just wouldn't start because of the teeth missing on the cog). So I found another bolt and put the magneto on with 2 bolts. It should have been an improvement but now it doesn't run.

With regard to the key on the shaft/flywheel I am sure I inserted that correctly.

One thing that I am wondering is this. To get the flywheel main nut off (the 24mm one) I had to use an impact wrench as it was oxidised in place. Could I have caused any other problems by using the impact wrench on the main shaft? Just a thought.

Removing the nut with the impact wrench will not cause any significant harm, however, I am wondering how you got the flywheel off. Typically the flywheel has to be removed with a puller.

I am also assuming this is on a riding mower. Correct?

Quote:

Originally Posted by hawkeye088

Removing the nut with the impact wrench will not cause any significant harm, however, I am wondering how you got the flywheel off. typically the flywheel has to be removed with a puller.

I had to make up my own puller as the standard 3 legged ones I had would not fit. There was not enough clearance to get under the flywheel with the ones in my garage. And beside that the 3 arm ones would have damaged the starter cog even more and I wasn't sure I wanted to do that. So I welded the puller cross piece and arms (with hooks) together and attached a large nut and bolt through the centre of the cross piece. Screwing the bolt down pushed against the main shaft. The flywheel came away just fine with a mild tap on the flywheel to give it a nudge.

And yes it is the motor for a ride-on mower.