I was wondering if anyone could help me out here, I have an 18 hp flathead twin cylinder briggs engine on my small airboat that i built. I've been thinking about what to do to get more power. I got a spare set of heads and want to mill them down to raise the compression. I would also like to make a thinner headgasket out of sheet stainless. My other ideas are to move the spark plug to the center of the combustion chamber, use an offset flywheel key to advance the timing, and experiement with cams to get more valve lift. If anybody has any experience with building flathead briggs engines, please post what you know. Thanks

 

This is just a suggestion. . .

Do a search on the net for the 'kids dragster' information. They are using small mini-bike type engines in their 'mini-dragsters' and there is a LOT of aftermarket stuff available.
I think Jegs has some stuff, too.

I once re-built a 5 hp mini-bike engine and grabbed an A.I.R. pump from my brother-n-law's SS 396 Nova, that he took off. (Track only use.)
I was gonna use it to make a type of 'blower' for my little engine, but, I lost interest before I got it done. (I discovered girls!!!)

Never did determine which pulley combination would yield the most boost, etc.

 

You used to be able to get flywheel keys that were "sheared" in degrees... I used a 2 degree in a 5hp briggs that I shaved the head on. That thing ran great. The best thing is run pipe the same size as the ports for exaust.

 

the jr dragters use a 5 hp 1 cyl block. don't know about the twins other then flat heads you have to be carefull as to how much you mill off or the valves hit the head. use model clay in the chamber then turn the motor over to see how much clearance you have valve to head. surprised you did not use a old 4 cyl. plane motor

 

I built a couple of really strong engines in my day. The last one I did was when I was in high school. I shaved the head, ported, polished intake and exhaust, polished the head, gasket matched the manifold to block, 3 angle valve job, welded additional lift and duration on the cam. Had to run a 14hp carb because I couldn't get the stock one to flow enough. That think was so strong I shattered 3 transmissions at idle. Twisted the input shafts off of two of them and shattered gears in the third one. The third one would yank the whells off the ground. in fact it threw me off of it once. I never had the chance to dyno it. Oh yeah, these engines like to run on old mothballs too. They won't idle though.

 

Back when I was into all that, I was working on a turbo for a 3 hp briggs and stratton. trhen I too discovered girls aren't into guys that like to soup up old lawnmowers.

 

Here are a couple of things you can do.
Clean up the intake ports, smooth out any parting lines,and rework the ledge under the valve seat, there is a lot of restriction there.Do the same to the exhaust port. "Size" them both to the intake and exhaust gasket.
You don't need to use the key in the flywheel either. Take it off and clean the maiting surfaces well. Put a dab of valve lapping coumpound on the crank and lap the flywheel to the crank. Clean it all up real well and put it back a "little" advanced. You may have to change it a few times till you get it where it runs best, remember, A LITTLE AT A TIME.
You can also "lap" the head and deck with some coumpound and run with out the head gasket, this takes a lot of work, so you may want to go to an industrial hardware store and get some thin copper sheets and make your own head gasket. Some craft stores carry the copper sheets too.
Don't try to run it on gas. Use Methanol. When you raise the compression (thin head gasket)and advance the Timing, gas just won't get it done. You will have to open the jets in the carb up to at least double the original size as well.
These little engines can be awsome if you take your time and work with the timing.
Get a degree wheel and learn how to degree the cam. There is a lot of horse power there too. File a little off the back side of the lobes and reset the valve clearence. this will give you more lift. After you fool with them for a while you will figure out how to increase the duration as well. ( you will probably have to use new valves to get the clearence back, and you can close it up a little).

 

fun things to play with. I built a wheely-Waggon out of a lawn tractor once, just as an excuse to play with a B-n-S motor.

Try this site for preformance parts.

http://www.briggsracing.com/home.html

 

Amen to that. I used to be proud of my souped up mower
Then i learned that girls like an 11th grader to act like an adult. Have to keep my mower projects quiet around them girls

Is it an opposed twin? You can use a torx driver on the govoner to tighten up the govoner spring for more RPMs

good luck

 

Deck the block, Lower the valve seats, Machine the ports, Bore out the carburetor, Get shorter rods, bore and hone the cylinder, high performance coil, hotter spark plug (depending on your environment), racing oil, and many many many many more.