This is Jimmy glenn's word on porting. this is in the context of a stocker Briggs racing engine, but you can get some ideas. Also following is a picture of an eyebrow shave on a big valve briggs.
Porting of any motor takes careful thought and generally not a heavy hand. This is very true with the Briggs motor in the stock classes. BIGGER is not better, flat out. What you are always looking for with porting is improved AIR FLOW. Big ports do not mean better air flow.
General rule for porting the Briggs motor is to leave the size of the intake port alone and work on the upper left hand corner. I have developed a easy method for getting started on any intake port. Take an old carburetor that you don't care about and bolt it up to the port with the back plug knocked out. If you have done carb work out of my previous articles you will have in your possession a .690 reamer. Take the reamer and insert it through the carb bore until you hit the left hand side wall of the intake port. Now rotate the reamer using a wrench or vice grip until you have cut a good bit of metal out on this side. What you are doing is making a straight shot through the bore of the carb into the port.
Remove the reamer and carburetor and rough finish this side with a 5/8" deburring bit. What you want to end up with, is the left side opened to the existing side wall. You'll want to move the cut slightly upward toward the valve seat. BE CAREFUL not to hit the seat as the tech inspector will throw you and your motor out if you do.
After roughing out the basic cut, we want to move to a sanding disc. You can use the small 1/2" disc that come for the Dremel tool or I really prefer the larger sanding arbors available from MSC. These are great for smoothing out the cut and slightly radiusing the upper portion of the intake port. You just want to radius this edge for a slight improvement. Be careful not to damage the leading edge of the port.
Don't mess with the hump on the right hand side of the intake. This is a support area for the carb bolt. If you remove this, you WILL kill air flow. You will also do damage if you hog out the lower right hand side of the intake. It's ok to lightly remove casting flaws, but nothing heavy here. Again don't enlargen the overall size of the intake runner. As it comes stock, it is less than the .880 maximum size allowed by WKA but still much large than the bore of a blueprinted carb (.695). Keep in mind that as air and fuel move through the carb bore and hit the wider opening at the front of the carburetor and enter the even larger port, air speed will DECREASE! If you don't believe this look at a stream sometime where a small area of flow runs into a larger still area. You will see how the flow of water slows when it hits the open area. Air flow is the same.
To finish up the port, you can make you a simple tool for using scotch bright to polish the inside of the port. I very much like the finish scotch bright puts on the port. It's not super smooth and this is exactly what you want. A smooth surface will not help your air/fuel mixture. It will not help air flow as there is little surface tension anyway. What you want is a slightly rough surface to keep the air/and fuel mixed. Many builder for larger engines will actually put bumps in the intake runner to help keep the fuel and air stay mixed in low pressure areas. This is not legal in WKA stock classes. Fuel has a tendency to collect in these lower pressure area as large droplets. Large drops of fuel do not burn as well as a small atomized mixture. For fun take a spray bottle of water and spray a slick glass surface. You will see the water collect into larger droplets.
The tool for using a small square piece of scotch bright is easily made from a 4" 3/8 bolt. Cut off the bolt head and then cut a 1" slot in one end big enough to put the scotch bright into. Put this into a drill and have at it. You will end up with a dull looking port but who cares if it flows well! Scotch bright is easy to find in any grocery today.
On the exhaust side, we will not do much. The port is already too big. All I do today is take out the bump in the upper right hand corner. That's it.
A good header will mean more to you in improving exhaust flow than any porting you can do. Some general thoughts on headers:
small diameter headers will help torque. (.930 <)
large diameter headers should be used only in limited classes.(.990 >)
multi-stage headers do help over a standard single size of pipe.
Use a .910 or smaller for the purple plate class, with .930 working well in the red and gold plate restricted classes.
Don't be afraid to try several different headers with a single carb/cam setup.
Changing cams will most often cause you to re look at the header you are using.
Look to Robinson's Torque tubes and Dover Power for some very good headers in all classes. I have used these for years with excellent results. They are high quality pipes. They also have pipes made specifically for muffled classes.
ut:. Where can I get a new one?