B&S lawn mower 12H800 hard knocking but runs

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So, I have this Lawn Machine with a B&S 12H800 motor. Probably 20 years old. Not neglected but maybe not best maintained. The other day, while doing leaves, it developed a noticeable knocking sound and clanks when trying to start. It will run. As winter is here for the next four months and the outdoors activities will decrease, I was thinking of taking it apart to repair. I've done a head job on my Neon and other things, I should be able to do this, right?
I did the carb last year. Rest of mower, such as deck is fine, although wheels getting looser.
1. Is there something like a engine re-build kit for this or do I just purchase the needed gaskets? Head/oil pump.
2. Are there things you want to replace while your in there?
3. Guesses as to what parts making the noise?
4. How do I find the torque spec's for various parts? Is there a decent manual or website for this? I;m not afraid of doing my homework.
Thanks for looking.
 
looks like typical standard B&S flathead with walbro (clone of course) side-draft carb.

Things you need to check/verify:

(1) check oil level, also smell the oil, if it reeks of fuel, you have fuel dilution problem (oil too thin, causing rapping noise).

(2) check flywheel timing key. If the key has shifted (even a bit), get a new one and replace it. Nothing more pronounced when it comes to all sorts of issues with B&S flathead timing.

(3)ask yourself if the engine has ran low on oil before. excessive big-end side crankshaft to conrod clearance will lead to rapping (or so-called knocking sound)

(4) fresh fuel?
 
Oh and BTW: pretty much most of the B&S single cyl consumer grade engines (incl your 12H800 series) are what they referred to as "cool bore" engines (aluminium all the way, no iron liner); and crank to con-rod has no bearings to begin with.

proposed service life of these engines are targeted to 1khrs max., with most of them worn out around 4~800hrs (based on the ones on our local dispose depot, and oil burning/staining situation from muffler outlet).

They are meant to be disposed of when worn, no use rebuilding them (for you cannot hone/bore an alu bore head w/o causing more issues).

Q.
 
So many freebie mowers on craigslist that have good engines but other things wrong with them (or just wont start so they throw them away. It might be easier to find one of those and replace your motor.
 
Originally Posted By: cat843
Someone ran it without oil.


Most likely, which means at the least it needs a crankshaft rod and set of rings...

While I haven't repl rings in probably 40 years, unless scored badly the older aluminum bores were generally fine with a re-ring..
 
Briggs flatheads are free and plentiful. Decent engines, but not worth any money beyond a few pieces. Part it out for the $50 worth of sellable parts and find that same engine on hundreds of freebees on Craigslist. Too many around to waste time working on one of the most common engines in the world. Take the carb, primer assembly, flywheel coverings and ignition and sell them. Take the rest of the $1.00 worth of scrap and buy a coffee with it.
 
Pull the head.Last year I had a screw come off the throttle butterfly and get sucked into the engine. It was fine then suddenly started knocking before I shut it down. Check the flywheel key also.
 
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Thanks for all the ideas. The takeaway is if it cant be fixed, enjoy taking it apart and learning. I'll take it apart and see if I see something. Maybe it's just the blade and key part. Onto to researching the key thingy.
 
Has it ran more than a few SECONDS with the knock in it? If so, It is probably NOT the rod, as usually once you can hear a knock from the rod, they break within seconds. I would bet on a loose blade bolt. Put a LONG wrench on it and pull hard. I have had many people say (when asked ) they checked the blade bolt, and when I put a wrench on it I turned it another 1/2 turn, and the noise went away.
 
Originally Posted By: asand1
I have heard of conrod caps cracking, and the piston slapping the head for a bit, before going nuclear.


What is a conrod cap?
 
Originally Posted By: Lubener
Originally Posted By: asand1
I have heard of conrod caps cracking, and the piston slapping the head for a bit, before going nuclear.


What is a conrod cap?


Connecting rod cap... usually the big-end is split in two and secured with two bolts.

TM-5-4320-302-14_523_1.jpg
 
Originally Posted By: asand1
I have heard of conrod caps cracking, and the piston slapping the head for a bit, before going nuclear.

Yeah a few years back one of my old Briggs pulled that stunt... Running fine & BLAM! Locked up solid, wasn't any in between... At least if it banged the head(no evidence), it came apart in the next 1/2 revolution...
 
After researching and replacing the ignition key, sound was still there. Did some small mowing, then decide to ...

CHECK THE BLADE. It was lose and I could not get it to tightening any more, so I removed it with an impact gun and installed a newly sharpened blade. All better.

Thanks to everyone who took the time to answer. It is appreciated. And I learned how to replace the ignition key!
 
I wouldn't put any money in it. Wheels are worn too.Change the oil and run it till something happens. Why part it out as suggested, it runs. I had an engine which knocked slightly. I made the mistake of changing the head gasket, the added compression made matters worse and the rod threw.
 
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