3HP BRIGGS MODS/TWEAKS?

TheSignGuy

Member
Nov 24, 2014
452
7
18
seattle, wa
Looking to get more torque out my Briggs without risk of blow up or snapping a shaft...

3HP

3600 rpm

4.4 torque rating.

3/4 shaft

Trans/pulley system pre setup:

Clutch Small pulley: 3.5 inches

Large drive pulley: 15inches (whizzer sheave.)

Belt estimated size by calculator:

Pull centers: 20 inches

SP X LP x PC = 70.1 BELT SIZE.

ESTIMATED STOCK MPH PRODUCTION ON 32.3 CALCULATED


"Low end torque needed for hills?"
 

Tony01

Well-Known Member
Nov 28, 2012
1,388
824
113
sf bay area
Follow these steps to get the most out of it:

Stage 1: Get a $6 tachometer off ebay. DONT! get the similar looking one off amazon. get the china one from ebay.
Remove governor and plug all holes with short bolts and flat washers and red loctite, or thread the holes and put a screw in.
File your timing key .036" about halfway deep to advance timing 6*.
rod blows at 6200rpm, flywheel blows later, so just keep the rod safe
Install a 1-3/4" pulley, mount the 15" sheave on a 24" wheel (no jackshaft? do this like i say!) and run a belt tensioner setup for your clutch.
buy the tecumseh h35 carb with adjustable jet, and h35 intake manifold, and air filter assembly. figure out the carb linkage, it aint hard. put a cable stop bracket on one of the headbolts and run a spring between there and the arm on the carb, if not using stock carburetor.
make sure your breather is working properly and the gasket is installed correctly allowing oil to drain back into the block
make or buy a custom exhaust header that bolts to the motor, not screws into exhaust port, and make the pipe at least 14" long same diameter.

Stage 2: Do the above, then: Buy a new head gasket:
lap valves in, set lash to .005/.007
Shave head .025" to .030" but no more, BUT if your new head gasket is copper or aluminum thin .010" don't do the shave. Stock gasket .047 shave head .030".
use grinding bits and burrs to smooth out exhaust port. Round the inner corners on both ports and smooth all big bumps and burrs. Smooth out those exhaust threads.
Use plug indexing washers to point the spark plug open end at the exhaust port.

These things should take you to 4-4.5hp, [email protected] which is about the highest speed you'll reach with 4hp.

If you insist on using the clutch, the pulley on that clutch is actually 3.2" not 3.5, and you'll need to run a 16" rear wheel to even get close to a usable ratio, in fact it will still be too tall to climb hills, so the auto clutch isn't going to work for you in this case.
 
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TheSignGuy

Member
Nov 24, 2014
452
7
18
seattle, wa
Follow these steps to get the most out of it:

Stage 1: Get a $6 tachometer off ebay. DONT! get the similar looking one off amazon. get the china one from ebay.
Remove governor and plug all holes with short bolts and flat washers and red loctite, or thread the holes and put a screw in.
File your timing key .036" about halfway deep to advance timing 6*.
rod blows at 6200rpm, flywheel blows later, so just keep the rod safe
Install a 1-3/4" pulley, mount the 15" sheave on a 24" wheel (no jackshaft? do this like i say!) and run a belt tensioner setup for your clutch.
buy the tecumseh h35 carb with adjustable jet, and h35 intake manifold, and air filter assembly. figure out the carb linkage, it aint hard. put a cable stop bracket on one of the headbolts and run a spring between there and the arm on the carb, if not using stock carburetor.
make sure your breather is working properly and the gasket is installed correctly allowing oil to drain back into the block
make or buy a custom exhaust header that bolts to the motor, not screws into exhaust port, and make the pipe at least 14" long same diameter.

Stage 2: Do the above, then: Buy a new head gasket:
lap valves in, set lash to .005/.007
Shave head .025" to .030" but no more, BUT if your new head gasket is copper or aluminum thin .010" don't do the shave. Stock gasket .047 shave head .030".
use grinding bits and burrs to smooth out exhaust port. Round the inner corners on both ports and smooth all big bumps and burrs. Smooth out those exhaust threads.
Use plug indexing washers to point the spark plug open end at the exhaust port.

These things should take you to 4-4.5hp, [email protected] which is about the highest speed you'll reach with 4hp.

If you insist on using the clutch, the pulley on that clutch is actually 3.2" not 3.5, and you'll need to run a 16" rear wheel to even get close to a usable ratio, in fact it will still be too tall to climb hills, so the auto clutch isn't going to work for you in this case.
Wow thanks I kind of realized because I'm a 700c I however will be the assisted rule,

Instead of machining buy a bigger motor, exhaust wise I can free up some extra boost I prefer not to unrestricted it to the point where I can't explain to law enforcement.
 

Tony01

Well-Known Member
Nov 28, 2012
1,388
824
113
sf bay area
I did my 3.5hp head .030" and my first 212cc head .070" with sandpaper, took me 1.5 and 3 hours respectively, lots of sweat and Gatorade. I also hand filed my own high lift cam for the 3.5hp only took me 6 hours. Buy 60gr sandpaper and 3m spray glue and stick the paper to something flat I used granite
 

curtisfox

Well-Known Member
Dec 29, 2008
5,155
1,578
113
minesota
Tony is right! one thing would also help is longer intake manifold, will give more low to mid range. Also your gear ratio is to high, if you want to run the clutch you need a jack shaft 3.2 to about 6" then 2" to the 15". Had a Monark twin when i was a kid and had a setup like this, also Whizzer is setup like this also. Both had the jack shafts up under the seat, sorta out of the way and fit regular bike frames............Curt
 

TheSignGuy

Member
Nov 24, 2014
452
7
18
seattle, wa
I did my 3.5hp head .030" and my first 212cc head .070" with sandpaper, took me 1.5 and 3 hours respectively, lots of sweat and Gatorade. I also hand filed my own high lift cam for the 3.5hp only took me 6 hours. Buy 60gr sandpaper and 3m spray glue and stick the paper to something flat I used granite

That's basic modding but worth it looking to squeezing on some newer parts though would defiantly bring the old dog a new heart.

Now for the head shave what did you use to measure the clearance?


Mine is a 80302 model.



As well for the carb, I know when you put the carb farther way, from the exhaust makes better fuel efficiency by keeping the fuel from boiling. That would be good.
 
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curtisfox

Well-Known Member
Dec 29, 2008
5,155
1,578
113
minesota
The Tecumseh h35 would be the best upgrade, fully adjustable or a Mikuni. Advance the timing a little would help also.........Curt
 

Tony01

Well-Known Member
Nov 28, 2012
1,388
824
113
sf bay area
The exhaust doesn't heat the carb, the motor block heats the carb. It is fighting the cooling caused by the suction. My intake on the 212 gets ice cold when running even in hot weather, with condensation on it.

You don't need to worry about clearances with only a .030 shave. I used a 6" dial caliper for initial measurements and then a dial indicator with a custom mount on each valve to measure lift at durations
 
Feb 2, 2017
155
6
18
California
Don't they have aftermarket valve springs available? That would help with floating valves and increase RPM thus increasing speed. Look for aftermarket rods too, these 2 combo with definitely help you on your way to achieving light speed.
 

TheSignGuy

Member
Nov 24, 2014
452
7
18
seattle, wa
Don't they have aftermarket valve springs available? That would help with floating valves and increase RPM thus increasing speed. Look for aftermarket rods too, these 2 combo with definitely help you on your way to achieving light speed.
Now "cross generation" swaps sounds like problems waiting my motors about 20years old.
In retrospect, it's 10 in age....


But it's overall health is tip top shape for a stocker.
 

Tony01

Well-Known Member
Nov 28, 2012
1,388
824
113
sf bay area
Damn do you guys really want me yi repeat myself? Fack just read up and do the mods then keep posting. Not the other way round I spent way too much time on that post for this shht

You want the power, put the work in
 

TheSignGuy

Member
Nov 24, 2014
452
7
18
seattle, wa
Damn do you guys really want me yi repeat myself? Fack just read up and do the mods then keep posting. Not the other way round I spent way too much time on that post for this shht

You want the power, put the work in
LOL TRUE!

I JUST GOT BRIGGS CURSED OUT LOL.
 

Tony01

Well-Known Member
Nov 28, 2012
1,388
824
113
sf bay area
Believe me dude better to learn how to make the most outta the little 3hp before you go to a 212. You can go to a bigger motor but the torque will destroy everything on your bike that you setup for the 3hp.

Yes I was somewhat drunk when I posted that but it was cause simply upgrading to a larger motor is a nightmare in which you have to upgrade everything else too. It took me really too much time to figure that out, but I can tell you right now my upgrade $200 front brake setup, 4 piston hydraulic with metallic heat sink pads and 203mm rotor, and a custom fox shock, is not enough for my modded 212
 
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