John Deere D120 21hp Briggs OHV very hard cranking, breaking metal pinion starter gears.

[cyberslick18]

Hey everyone, like many of you I figured I could restore a clapped out John Deere I got for a song and a dance and it keeps finding new and exciting ways to not run when I need it to.

So, I bought it from the original owner with 400 hours on it, says the starter was stuck up and he was done with it. Didn't bother to tell me he had obliterated the mowing deck somehow, but thats another story, and my fault for not inspecting.

I replace the starter, and with a jump pack I was able to start and use the mower fine for a week or two.

Last week, it broke a pinion gear when trying to start. Just exploded into 2 or 3 pieces. It was a metal pinion gear with a plastic inner piece to ride on the plastic threaded spindle from the starter motor. I replaced it with the old original pinion and it started hard, but would still start up and run fine.

Then it ran for maybe 10 minutes, and stalled out. It would start back up but then die out. I checked all of the fuel supply and ultimately decided the fuel pump was weak, maybe perforated. I replaced that and it ran like a champ for one day. The next day I start it, and the pinion gear exploded again. At $15 a pop for cheap ones and more for OEM, this is getting old.

The engine is extremely hard to turn over by hand on the compression stroke, and I can hear it burping out of the air intake box (with the air filter off) when I get over the stroke. I adjusted the valves two times: One time as the B&S manual says, by finding TDC and then continuing 1/4 down (I believe to better match the ignition to the camshaft decompression function, which I know very little about), and also by simply alternating the valves closing and opening and setting the gap there as well, .004 for each as is spec for my model. Neither have helped the engine turn over via the starter. With the spark plug out it turns by hand considerably easier.

What are some other things I can check? I'm going to double check with a different fresh battery.

 

[StarTech]

First have verified the ACR is actually working? Looks to be a 330000 series Briggs which have the 793880 camshaft the ACR is bad about breaking due to poor design. Also note it sometime damages the governor too.

 

[cyberslick18]

First have verified the ACR is actually working? Looks to be a 330000 series Briggs which have the 793880 camshaft the ACR is bad about breaking due to poor design. Also note it sometime damages the governor too.

What is the ACR? The camshaft decompress feature?

How do I test it?

 

[ILENGINE]

Remove the valve cover adjust the valves to proper clearance-.004-.006 and then roll the engine over a few times by hand both to double check the clearance and also to make sure the intake valve which in this case is the bottom valve in the head bumps off seat slightly before TDC compression stroke. No bump then compression release is broke on the camshaft and pieces are laying in the oil sump. Also be aware that the OEM camshaft has been on nationwide backorder for several months with no ETA, and the aftermarket camshafts can be hit and miss for quality.

 

[StarTech]

What is the ACR? The camshaft decompress feature?

How do I test it?

Yes it is an Automatic Compression Release feature of the camshaft.

To check if it working. You rotate the crankshaft two complete turns. As piston approaches TDC compression stroke the intake (lower valve) rocker will bump open if the ACR is working. If not present then the ACR has broken off the camshaft. Note: Normally the valve clearances has to be set correctly but you have already done this.

There current after market version of the 793880 camshaft but are not made very well. Several I tried would even fit in the camshaft journals as just were .001" oversized nor would the oil pump shafts as they also were .001" oversized.

Now the OEM (Briggs) has superseded the camshaft with PN 84005207.

BUT as IL noted they are on back order with no ETA. Just another Briggs screw-up.

 

[cyberslick18]

I will check this when I get home. Just from memory though and based on how it's behaving I sounds like the ACR is not functioning.

Is there a work around to modifying the existing camshaft itself or some other modification to the engine to achieve a similar function?

When I can get the thing to actually turn over and fire up it runs well enough for my purposes.

edit: Thanks for the quick responses really appreciate it

 

[cyberslick18]

Quick thought, probably dumb:

The nut on the top of the flywheel assembly is a 15/16.

Any reason I can't just put an impact driver on it and spin it to fire up? Is it a reverse thread that'll spin off if I try to send it clockwise or anything? I assume the magneto is what provides and times the spark, so if the key is on and I can overcome the compression stroke fast enough it should fire?

 

[Scrubcadet10]

Depends... it's possible to cause some damage to the crankshaft and threads if you use an IMPACT big enough to spin it over fast enough... but i think if the starter is having trouble spinning, a corded or cordless drill will have just as much trouble.

 

[StarTech]

Not to mention over tightening the flywheel causing damage to it.

Besides an impact is just like a hammer drill hammering things. A good 1/2 hp drill probably will work but it cost is near what the camshaft is.

You could the mower graveyards for a blown engine that may have a good camshaft still.

 

[rdedrick]

Using a drill you may get you hurt. When the engine fires up it may be spinning possibly at a higher RPM's than the drill. You may not be able to release the socket which has a 1/2 drill attached. So there's that.