This is something I have only done to a few engines, but with positive results. Most people aren't willing to do it, or have never thought of it. I'm going to give a quick explanation for anyone who would like to try it out. Of course it is a procedure that should be done by someone with at least a basic knowledge of the engine, but you don't have to be a guru!

First off have the engine on a clean bench, the oil should be drained from engine to prevent a mess as well. You will need a good set of sockets/wrenches of course, some pliers / cutters/ a die grinder or some small round files ( powersaw files) some different grades of sandpaper, 220, 400, and 600 would be a good selection. A valve spring compressor is also handy. Compressed air is also a help but not necessary. Wear safety glasses if you are using compressed air and while handling springs. Some small stiff wire brushes and some varsol / paint thinner will also be needed.

First of all, remove all external pars from engine, shroud, coil, carb, throttle cable, etc. Then you will have to remove the head, and valve cover / breather, being careful to not damage the gaskets as they may be re-usable, although I recommend getting new ones for how cheap they are. Then you will have to remove the valves, (remember safety glasses!!) a spring compressor will be the best bet, but a good flat screwdriver and a steady hand can carefully compress the springs enough to remove the keepers. Be very careful with the springs, as they can fly out if you slip. Be careful also not to lose the keepers and not to mix up the intake / exhaust parts. Now slide the valves out and set them aside, preferably in separate plastic bags along with their respective keepers / springs to keep them clean and organized.

Now that the engine is bare, make sure the piston is at TDC to keep debris out of the cylinder. If you are experienced with engines you may even want to dismantle the base and remove internals to be safer. Now you can take a wire brush and remove the loose carbon that may be in and around the ports, on the piston and combustion chamber (cylinder head) and valve faces if necessary, being careful not to gouge the metal as it is only aluminum, and especially careful not to damage the valve seats!! Once it is clean, you will notice how rough and angular the ports are, which is very restrictive on fuel / air flow and exhaust flow. Whether you have a die grinder or files, start by lightly grinding away and the roughest edges, Try to round them off, slowly, take your time, and do not go too deep. I recommend only taking off 1/16" to 3/16" (one sixteenth to three sixteenths of an inch) as you don't want to damage the casting. After rough grinding it till you are satisfied, you should clean the filings out. Then you can take some 220 grit sandpaper and start smoothing it out more , and working your way down to 600 grit to give it a fairly smooth finish, remembering NOT to touch the valve seats!!

If you have a newer Weber carb with the separate intake pipe, you can also perform this procedure on that pipe, as it is also very rough inside. You can even polish the combustion chamber on the head, as it is usually rough. This will not only help flow better, it will also prevent a lot of carbon deposits. Motors I have performed this to stay nearly carbon-free even after hard use and oil burning!

Now you should thoroughly clean out all debris and residue with compressed air (wear safety glasses!!!) or paint thinner, take a light colored rag afterward and rub the areas down till they stop dirtying the rag. Make sure to get inside the valve guides as any leftover debris will quickly destroy the valves!! Use a small dowel with a rag wrapped around it and push it through both guides many times till they are spotless. You can even use a good vacuum around the piston and ports to suck up anything that was missed. If any debris does make it's way around the piston, carefully rotate the engine till the piston goes down a bit and carefully remove the debris, do not let the piston back up till all debris is removed as it may damage cylinder wall / rings / piston.

Now comes a tricky part, (wear safety glasses!!!) hopefully you have a valve spring compressor ( I don't lol ) Place one valve at a time, coating them lightly with engine oil, remembering which valve goes where and use the proper keepers for each valve. This may take a few tries if you are not used to it, having a buddy help is very useful here. If you DON'T have a spring compressor, here is what I do, I don't recommend this method, but it will work if you have no other choice. Place each spring in a vice carefully compressing it, and use some small solid copper wire (telephone cable size works) or small zip-ties, tying the spring in 2 places on each side in the compressed position. Then you can place it over the valve, insert the keepers and while holding pressure against the keepers, snip the wires / ties with a pair of side-cutters / snips being careful not to let the springs jump out. Once again, if at all possible use a spring compressor! And always be careful with springs!

Now that the worst is over, you can re-install your head and external engine parts. Torque head bolts to specs if you have them, if not just tighten them as evenly as possible in a criss-cross pattern ( they are not very fussy) Make sure everything is assembled properly, install motor on machine, and fire it up!!

You may not notice much difference, but it will effect performance, it may idle a little roughly at very low idle, but it will have better acceleration and smoother running in the long run. If you have to re-adjust your carb for proper mixture, do so.

Well I hope this is helpful for anyone who wants to try, it is very time consuming but nonetheless very worth it. If I have missed anything, or if anyone has suggestions please feel free to say so! Also for anyone goes through with this, let me know the results! And also remember, you are doing this at your own risk, I cannot be held responsible for any engine damage that may occur during this procedure. Good luck!

Sounds awesome. Nice tutorial. The valve springs put me off doing this a little though

I read at a lawntractor racing site that when polishing the intake, it should be left a little rough- least on briggs motors- can polish the carb all y want till its smooth inside. Something about briggs liking a little turbulence flowing in.

I also herd a 10HP head will fit a couple other motors- to increase compression. Yeah with an aluminum motor, you can wreck it in a hurry if youre not careful. I considered minor polishing the intake on my 18.5 opposed twin- to make it flow better- but with the cast intake manifold i cant see wasting my time. Id like to make a custom intake to run my scooter carb- bet that would flow awsome.

Good info, saved it to the harddrive for future use. It made me curious though as to what all could be done to massage an opposed twin, flathead/L-head.

I've never had the chance to do an opposed twin (I call them 'boxers' like a Porsche lol) but I'm sure it would work just as well, just double the work!

It might be easier though. The valves are in the block right? So you shouldn't have to remove them, just pull the heads off and take it away from the rest of the motor. Less cleanup that way.

DOH!!! Well that probably turns it from easier to harder real quick doesn't it, haha.

if i done this i would only do it to a I/c Engine for as Briggs aluminum bores may last a good while, but i don't like that type of cylinder i like a cast iron bore, last much longer and is very shinny when well used and you can re hone it too unlike aluminum you would have to have it re hardened for as honing would wipe the hardened surface right off.
Also to increase compression MIll the head, thinner head gasket and some high grade bolt to have good torque. I snaped my briggs 3.5hp head bolt with a 3/8 drive ratchet, though i really didn't try to and the bolts were 19 years old.
Also I have a set of DUAL valve springs but idk what i want to use it on. I can't do the twin cause its a set for a single cylinder but i could get another set to do the twin. but the dual valve springs just help with keeping pressure in the combustion chamber, faster snap back for higher rpms, which would then result in thrown rod unless i put in some racing rods. but thats alot of money
also i would like to see roller lifters like in my car except my car is hydraulic but have solid roller lifters,
but idk

It all comes down to 2 things : money and power- you can stuff the motor full of racing parts and dump a ton of cash into it- a tech i used to work with had a wheelhorse pulling tractor with a 8HP kohler - he said he dumped at least $8000 into the motor alone. OHV motors would be easier to mod then flatheads , with the valvetrain outside the motor.

Alot of tech tips can be had from lawntractor racing sites - i went to a lawntractor race a few years ago, the super mods were running 22HP v twin vanguards bored out to well over 70HP - things were way to fast for that small track.

But also with speed/power comes expensive breakage- alot of highly modded motors tear them selves apart after a short time.

I suppose its one reason that id like to see how fast i can make a tractor go with a stock twin cylender running the governer- its possible to get in the 50mph range with the right gearing.

And usually a better way to get more power- get a bigger motor- i went from a single 14.5HP OHV briggs to a 18.5HP opposed twin on the mod murray and that thing flew- too fast for a nearly stock tractor.

That reminds me of another question I have had for a while now. There are many seemingly identical motors in lawn tractors with dfferent power ratings. My opposed twin fr example only makes 14hp, while I've seen the same engine as high as 19hp, maybe higher. Are they larger displacements internally? I come from a car/truck background and can understand the same block having different displacements and therefore different power outputs, compare a Chevy 305 with a 400 small block.

Same bore, longer stroke, good to know thanks.

Probably why there was a lawsuit again tecumseh- people were suing over the 'actual' horsepower ratings were under what the motors stated - alot.

Yeah all the small briggs motors- 10 to 13HP all seem to be nearly identical - why alot of parts swap between them.

Ive got a 13HP on my 99 murray ( not original) and a 16.5HP on my 2003 murray ( original)- oddly the 13HP seems stronger then the newer motor- i dont really like the plastic shroud on the 03, if the flywheel ever comes apart- that plastic wont stop it, least with metal it has some sheilding.

I have a few twin opposed motors- makes me wonder why theres a 17HP, 18HP and an 18.5HP - probably isnt much difference in actual horsepower between them- id really like to see what power my 20HP onan will do- IF i can get it running ( cranks- no spark, just have to tinker with it)- itll be the biggest motor in the fleet.

Seems like all the twins have about 20 horsepower, even the 13 horse twin. Just different stickers.