New to flat heads looking to make low mid range beast

Ap1985

New member
Hey guys I have a 5HP Briggs from the 70s. Im looking to build a low and mid range grunt motor. I read that porting the eyebrows is really for high rpm, but what can I do for low and mid range. I was thinking just clean the ports up, port match the carb, and then drill and tap for a regular muffler instead of the threaded pipe that's in there now, maybe a 4hp head, a timing key way, and then a good cam. Is there a such thing as roller rockers for flat heads? I've read that it's not a good idea to grind the threads out of the exhaust port completely but I have to believe those ridges hurt flow, what about epoxying them and then grinding them smooth? Im not sure what else to try?
 
No roller rockers .
They used to make an insert for the exhaust to reduce the port and fill the threads.
4hp head is said to reduce power .
Stock the cam in them pulls down pretty good maybe a 89-7 would be better .
A bored or flowed carb should be on the list .
 
Last edited:
There are no rockers in these engines (unless you convert it to overhead valve cylinder head.)
As far as the port size, both ports are too large already. Smoothing the exhaust threads will only make the EX port larger -- epoxy will work if it's the type that can handle constant 1000*+. We use an aluminum thread insert that threads into the port and reduces the size/volume, and can then be ported to make whatever size you want. On the intake side, there will be no "port matching" to the carb, as the carb bore is considerably smaller than the port entry already. Now, if you go with a bigger carb you could certainly do that. You will definitely want to bore the stock carb if you are not replacing it. These engines are asthmatic to begin with -- let it breathe and it'll make considerably more power. Then work on the inside radius of the port just under the seat. The transition area between the IN valve and the cylinder can be smoothed and/or dropped (eyebrow) to help with flow considerably. Don't worry about the loss of compression (CC's) as these are very low compression engines anyhow. Forget the 4HP head. If you can find a 5 HP with a shallow center, that'll work better, then just valve relieve (pocket) the head for the lift of cam that you are installing.
Now, on to the cam. If you are putting any of the .233" lift "slapper" cams that are designed to loft the valve, you had better plan on welding the lifter bores, possiblly installing lifter guides, chamfering the bores, and using heavier springs (stock exhaust style) with spring pockets cut. Better yet would be to use the automotive style retainers and locks so that you do not float the valves at all. If you want a bigger lift cam, then select that cam grind without lofting the valves as the stock classes need to. The 112-alt is an excellent cam for short tracks and bottom end. If you want a less invasive cam install, consider something in the .315" lift range where you won't have to do as much clearance work or pocket the head.

We still work on these engines all the time in our shop.
We have everything that you would need in stock and offer quality machine work.
If there's anything we can help you with, please feel free to give us a call during business hours.


-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
32 years of service to the karting industry ~ 1Cor 9:24
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
 
Raptors eat predators not to mention they are not from Ccchhhiiinnna!!! Or how ever Trump says it. lol
Lol it was Chinese rods imported by Briggs that was responsible for the billet rod being allowed 🤣
Briggs has a plant in China, and plenty of the parts come from there.
 
There are no rockers in these engines (unless you convert it to overhead valve cylinder head.)
As far as the port size, both ports are too large already. Smoothing the exhaust threads will only make the EX port larger -- epoxy will work if it's the type that can handle constant 1000*+. We use an aluminum thread insert that threads into the port and reduces the size/volume, and can then be ported to make whatever size you want. On the intake side, there will be no "port matching" to the carb, as the carb bore is considerably smaller than the port entry already. Now, if you go with a bigger carb you could certainly do that. You will definitely want to bore the stock carb if you are not replacing it. These engines are asthmatic to begin with -- let it breathe and it'll make considerably more power. Then work on the inside radius of the port just under the seat. The transition area between the IN valve and the cylinder can be smoothed and/or dropped (eyebrow) to help with flow considerably. Don't worry about the loss of compression (CC's) as these are very low compression engines anyhow. Forget the 4HP head. If you can find a 5 HP with a shallow center, that'll work better, then just valve relieve (pocket) the head for the lift of cam that you are installing.
Now, on to the cam. If you are putting any of the .233" lift "slapper" cams that are designed to loft the valve, you had better plan on welding the lifter bores, possiblly installing lifter guides, chamfering the bores, and using heavier springs (stock exhaust style) with spring pockets cut. Better yet would be to use the automotive style retainers and locks so that you do not float the valves at all. If you want a bigger lift cam, then select that cam grind without lofting the valves as the stock classes need to. The 112-alt is an excellent cam for short tracks and bottom end. If you want a less invasive cam install, consider something in the .315" lift range where you won't have to do as much clearance work or pocket the head.

We still work on these engines all the time in our shop.
We have everything that you would need in stock and offer quality machine work.
If there's anything we can help you with, please feel free to give us a call during business hours.


-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
32 years of service to the karting industry ~ 1Cor 9:24
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
Dude............................You impress me.
 
There are no rockers in these engines (unless you convert it to overhead valve cylinder head.)
As far as the port size, both ports are too large already. Smoothing the exhaust threads will only make the EX port larger -- epoxy will work if it's the type that can handle constant 1000*+. We use an aluminum thread insert that threads into the port and reduces the size/volume, and can then be ported to make whatever size you want. On the intake side, there will be no "port matching" to the carb, as the carb bore is considerably smaller than the port entry already. Now, if you go with a bigger carb you could certainly do that. You will definitely want to bore the stock carb if you are not replacing it. These engines are asthmatic to begin with -- let it breathe and it'll make considerably more power. Then work on the inside radius of the port just under the seat. The transition area between the IN valve and the cylinder can be smoothed and/or dropped (eyebrow) to help with flow considerably. Don't worry about the loss of compression (CC's) as these are very low compression engines anyhow. Forget the 4HP head. If you can find a 5 HP with a shallow center, that'll work better, then just valve relieve (pocket) the head for the lift of cam that you are installing.
Now, on to the cam. If you are putting any of the .233" lift "slapper" cams that are designed to loft the valve, you had better plan on welding the lifter bores, possiblly installing lifter guides, chamfering the bores, and using heavier springs (stock exhaust style) with spring pockets cut. Better yet would be to use the automotive style retainers and locks so that you do not float the valves at all. If you want a bigger lift cam, then select that cam grind without lofting the valves as the stock classes need to. The 112-alt is an excellent cam for short tracks and bottom end. If you want a less invasive cam install, consider something in the .315" lift range where you won't have to do as much clearance work or pocket the head.

We still work on these engines all the time in our shop.
We have everything that you would need in stock and offer quality machine work.
If there's anything we can help you with, please feel free to give us a call during business hours.


-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
32 years of service to the karting industry ~ 1Cor 9:24
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
Who makes the auto retainers and locks?
 
Back
Top