Late 90's Wheelhorse 312-8 w/ Kohler M12 mods and repair questions

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  • / Late 90's Wheelhorse 312-8 w/ Kohler M12 mods and repair questions
Had this about 2 years now, and use the mower more as a mini tractor (made my own attachments for rear, like custom ripper, heavy duty box scraper, towing hundreds of pounds off dirt/rock at a time, etc., etc.,) but also still cut grass with it. Best $350 I've spent in a long time.

Anyhow, noticed the exhaust was getting loud like it had a leak (ear plugs masked it a bit) and discovered the exhaust manifold studs were backing out. Tightened one and it just snapped with less than 5 ft/lbs of effort. Problem is it snapped deep inside the block threads so getting it out might be a chore if remaining bit is seized.

1. Are the stud holes open-ended for the exhaust studs or are they sealed? If I have to resort to drilling it, I don't want it accidentally threading itself in the crankcase or something.

2. I didn't try removing the exhaust pipe flange from the block. But was wondering if the exhaust port is threaded for NPC pipethread exhausts? I know B&S does that on their small engines giving you both options to mount exhaust pipe on them.

3. Any ways to bump up power on these reliably for more hp/torque? Mill the head? How much should be removed and what compression ratio will it effectively have? Port passages with a dremel? Mod the carb? Free up exhaust with better flowing muffler? Does throttle blade have a stop that's adjustable? Would allowing an extra 100-200 rpm be too hard on the rotating assembly or be worth the extra bump in power? Also, already run 91 (or 93 if I can find it) octane in all lawn stuff so not worried about that.

Like I said, I use this more often than not as a mini tractor with heavy pulling, and while fixing the exhaust studs (and upgrading to a higher quality grade 8 or something) and changing fuel filter soon, if there's easy or free hp/tq to be had, why not?
 

mechanic mark

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  • / Late 90's Wheelhorse 312-8 w/ Kohler M12 mods and repair questions
I would first spray all studs with penetrating oil & let sit overnight. Purchase a couple left handed drill bits & go slow. Over 30 years ago a friend of mine removed a broken bolt or stud using a left handed drill bit in a Snap-On reversible air drill while working on a big block Cummins Diesel. Let us know how it goes, thanks, Mark
 

Richard320

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  • / Late 90's Wheelhorse 312-8 w/ Kohler M12 mods and repair questions
This^^^^

If you can find it, Kroil is an outstanding penetrating oil.

And I've often gotten lucky using left-hand drill bits by having them grab and unscrew the broken bolt.
 

bertsmobile1

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  • / Late 90's Wheelhorse 312-8 w/ Kohler M12 mods and repair questions
Max Hp means nothing when it comes to doing work
Torque is what you need and usually it is around 2,900rpm to 3,300 rpm.
making it go faster will just burn more fuel

Exhaust studs area PIA to do in situ
As that is a side valve the stud holes would usually be blind but there is no chance of drilling into the bore
lay the engine , stud hole uppermost heat the area around it with a big hot air gun or torch
When it is too hot to touch pour your penetrant down the hole then allow to cool down
Repeat as many times as you can
As the engine cools it will draw the penetrant

If you end up trying to drill it out, start with an end mill to square off the end of the bolt
I use a Dremel for this and then a ball ended bit to make the starting hole for the drilling
Drill a pilot hole about 1/2 the diameter of the stud then drill the biggest hole that will fit without damaging the threads.
If using a LH bit, crash the drill into the pilot hole the idea is to make it bite into the stud and jamb
Again drilling with the motor hot is likely to yield the best result.

These engines have a restrictor plate between the carburettor & the inlet .
This plate chokes down the engine to reduce the output which is one of the ways they made several Hp engines using the same bore & stroke.
Some times they also changed the main jet and some times they didn't
When you pull the carb off you can experiment with opening it up.
If the hole is already full round then it has been maxed out
 
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  • / Late 90's Wheelhorse 312-8 w/ Kohler M12 mods and repair questions
Great info guys! Too cold to work on it atm but I'll look into a left handed bit and will use MAPP torch on it. Hopefully the balled dremel bit will be long enough to even reach the stud. Worse case suppose I could take a bolt and drill though it's center and use that for a bit guide.

4. The restrictor plate, is it basically a metal gasket they stuck a smaller bore on or is it an actual spacer between the carb and intake manifold? If it's a spacer, do I need to fab up a phenolic spacer or or grind out the restriction?

5. Any benefit from swapping in a carb from a M16-M20? Was looking at carbs online and seems they used a same 1.07" throttle bore between mine and the 16-20hp models, and only thing that appears different is linkage hook ups. Used ones seem cheap enough and I could rework the linkage or retrofit my stuff if possible. Have to assume a M20 would probably have a larger jet?

6. Any idea how restrictive the stock mufflers are? Was thinking of getting an automotive https://www.summitracing.com/parts/wlk-22229 and running a J off the muffler inlet then another 90 to the stock flange which I'd probably cut and weld to the 90. Mainly looking to cut the db down a bunch so I can run it with less noise but extra power would be great if frees some up. Just curious if anyone knows?

7. Do these respond to higher flowing or more surface area air filters? I know carb'd auto engines can be affected by the air filter. Just wondering if it's worth popping something else in there or opening it up, etc..

Planning on converting something like a Craftsmen tractor snow plow to have a front loader-style scoop, ideally running a winch for lift, as I have a ton of gravel and also dirt that'll need moved this summer and wanting to squeeze whatever I can out of it as I'll also be running tire chains more often than not. Not to mention the box scraper which weighs around 80 pounds + 140 from the cinder block ballast + a large mound of dirt/gravel being dragged. Surprised it was even doing it already with the stock tires and I'm light at 165.
 
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StarTech

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  • / Late 90's Wheelhorse 312-8 w/ Kohler M12 mods and repair questions
The ball dremel bit won't do any good; unless, it is carbide. Some exhaust screws and studs get heat harden to a point where only a carbide bit will drill them.
 
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  • / Late 90's Wheelhorse 312-8 w/ Kohler M12 mods and repair questions
The ball dremel bit won't do any good; unless, it is carbide. Some exhaust screws and studs get heat harden to a point where only a carbide bit will drill them.


I'll keep that in mind. Might get lucky and it'll move w/o much fuss.
 

bertsmobile1

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  • / Late 90's Wheelhorse 312-8 w/ Kohler M12 mods and repair questions
It is not economical to make a 3,75Hp , 4Hp , 4,25 Hp, 4.5Hp, 4.75 Hp etc etc engines by changine mechanical parts
Thus the use of a restrictor plate is a common method to get variable Hp in small increments to keep the mower customers happy
THus above, all of them would be made from a 5Hp engine choked down to achieve to lower Hp figures .
The the makers would go to a bigger bore to get engines from 5,? through to 10Hp
then stroke that engine to go 10 to 15
Then bore that engine to go 15 to 20 etc etc etc

Note number just indicative not meant to resemble any particular make.

Motorcycles and cars do the same thing it is all about volume production and making the biggest variety with the fewest different parts .
 
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  • / Late 90's Wheelhorse 312-8 w/ Kohler M12 mods and repair questions
Curious to see if the plate is still there.

Anyhow http://gardentractorpullingtips.com/a1engine.htm#headerpipes sells a ring flange that fresh exhaust pipe can get welded to and even offer welding a pipe to it if not wanting to cut the factory muffler pipe up if anyone else ever looks for this. Also apparently Kohler used the flange and threaded the exhaust port too, just not every engine.

At the bottom of that site's one page they offer 22hp M12 for $2k. Apparently they can get 53hp out of a M16 competition pulling engine @6500 rpm which is crazy.
 
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