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Hi,

So, I recently completely rebuilt a Briggs & Stratton 21hp platinum engine(model #: 331877-1234-B1) It had thrown a connecting rod and punched a hole through the crankcase. I ordered the parts I needed which included and connecting rod, crankcase/engine block, crankshaft, and an oil slinger/governor. I endlessly watched YouTube videos to make sure I put it back together correctly. The rebuild went very smoothly. I got the engine back one the mower, added the correct amount of oil and filled up the gas tank. Turned the key and she started right up and ran very well after I adjusted the carb. I let it idle for 15 - 20 minutes to get all the new parts broken in. Fast forward about a week, I had used it to mow the lawn a couple times and hauled some tree branches in the trailer I have. I was riding along in my front yard at about half throttle(not mowing) when suddenly the whole engine started shaking and clicking/knocking so I quickly shut it down to evaluate the problem. After I found nothing out of the ordinary, I tried starting it again. The engine cranked for about one revolution but then stopped(seemed like something was wrong with the compression release) I tried again and it slowly started turning and after about 5 seconds it began turning faster and surprisingly the engine started. Only problem was that the engine was still shaking and knocking.

One other thing I tried was removing the spark plug. When I did that it cranked over fine but I could still hear a knocking sound.

I have no idea what could be wrong and I wanted to ask for your opinion before I tear the whole engine apart again.

Thank you in advance!
Jeremy

Jeremy, check your valve lash. Be sure your valves are set correctly and that you didn't bend a push rod. I've even seen push rods fall off because valve lash wasn't properly set. It's minor repair if you bent one and they are very easily to adjust. Good luck bud!

Tinker Ty said:

Jeremy, check your valve lash. Be sure your valves are set correctly and that you didn't bend a push rod. I've even seen push rods fall off because valve lash wasn't properly set. It's minor repair if you bent one and they are very easily to adjust. Good luck bud!

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Ok, I will do that and let you know what I find.

My only question is, would a bent push rod or poorly adjusted valve clearance make the whole shake and knock?

Jeremy

Is this a v-twin engine or single cylinder? Shake is relative to the scenario, so yes and no. The knock, yes. It's hard to tell you exactly because everybody hears knocks and sees shakes differently. ?

A 33 Series engine is of course a single cylinder engine. Sure does not sound like a valve issue to me. You bought a new crankshaft? So you had to remove the old counter weight ass'y and reinstall it on new crankshaft? Sounds to me like your problem is there.
Walt Conner

Tinkerer200 said:

A 33 Series engine is of course a single cylinder engine. Sure does not sound like a valve issue to me. You bought a new crankshaft? So you had to remove the old counter weight ass'y and reinstall it on new crankshaft? Sounds to me like your problem is there.
Walt Conner

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Yes, I got a used crankshaft from eBay that came from a good running engine. It also came with the counterweight already installed. Everything spun very freely when I was assembling the engine. There was also very little play between the connecting rod and the crankshaft journal.

Also, just in case you are thinking that I got the wrong parts, let me assure you that I made sure to match all the part number up with the ones I got from the Briggs and Stratton website.

Thanks,
Jeremy

Tinker Ty said:

Is this a v-twin engine or single cylinder? Shake is relative to the scenario, so yes and no. The knock, yes. It's hard to tell you exactly because everybody hears knocks and sees shakes differently. ?

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It's a single cylinder.

I'm not getting a whole lot of input. Maybe I have to figure this out by myself.

I checked the valve clearance and pushrods, everything was fine. The flywheel key isn't sheared so the the spark timing is good. I guess the only thing to do now is take the oil pan/sump off and see what's going on with the internals. Good idea?

Jeremy

sounds like you might forgot to install the oil pump drive link and now aluminum transfer again on the journals. Just a thought.

StarTech said:

sounds like you might forgot to install the oil pump drive link and now aluminum transfer again on the journals. Just a thought.

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I'm 99% sure I reinstalled it, but that is a possibility. Thanks for your input!

EngineRepair15 said:

I'm 99% sure I reinstalled it, but that is a possibility. Thanks for your input!

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but did you confirm smooth operation of the oil pump itself?

its not uncommon after an engine failure for aluminum transfer to the pump gears and they dont turn to pump and just spin as a complete lump not pumping any oil

i see no way to avoid going back in at this point because you need to stop guessing and find out for sure whats knocking

next time buy a used mower with a bad deck and good running motor of the correct or larger hp size for $100-$150 and be done with it

repairing an engine that had a major failure is never a good idea because very often things like this happen after you have put good money into them

keakar said:

but did you confirm smooth operation of the oil pump itself?

its not uncommon after an engine failure for aluminum transfer to the pump gears and they dont turn to pump and just spin as a complete lump not pumping any oil

i see no way to avoid going back in at this point because you need to stop guessing and find out for sure whats knocking

next time buy a used mower with a bad deck and good running motor of the correct or larger hp size for $100-$150 and be done with it

repairing an engine that had a major failure is never a good idea because very often things like this happen after you have put good money into them

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Thank you for you input! Everything was turning smoothly when I was assembling the engine.

I found the problem. The compression release mechanism failed(which is why it was hard to crank) and some of the metal from the compression release chipped a tooth off of the cam gear(I think that's what was causing the clicking/knocking sound. Also, since the gear tooth broke off, it slipped a little which through the timing off a little and I think that's why the engine was shaking, because it was firing at slightly the wrong time. I hope I'm making sense.

I inspected the whole engine and nothing else is damaged.

Thank you for all your help!

Jeremy

And ACR failure are common on the 793880 camshaft as it can happen at anytime.