The intake is quite different.
First you need to ensure the port is capable of handling the volume.
Compare the dimension of the port window cross section area (width x height) to the dimension of the carburator bore cross section area (PI x radius squaired). To save on the maths here is the rounded numbers.
12mm carb = (3.14 x 6 x 6) 113mm
13mm carb = (3.14 x 6.5 x 6.5) 133mm
14mm carb = (3.14 x 7 x 7) 156mm
15mm carb = (3.14 x 7.5 x 7.5) 177mm
16mm carb = (3.14 x 8 x
The stock intake on the engine was 21.2 wide and 8.5 high (180mm) which is good for up to 15mm carb.
To introduce a 16mm carb you would first need to add volume to the intake cross section area.
As there is a good difference between the stock and target timing it is safe to raise the ceiling of the intake to 9mm and widen the intake to 22.5mm which will give you 203mm.
When measuring the intake you need to measure the port width and height inside the port.
One method to accurately get the measurement inside the tract is to press a ball of plasticine against the port from inside the cylinder and measure the plug of plasticine which went into the port.
When cutting this material from the intake you will first need to match port the intake to the start of the cylinders intake tract.
Make sure that the cross section area of the match port will be equal to the cross section area of the carb.
From that point of the intake tract you will need to blend the shape to meet the port window.
After this preparation work you would then proceed to modifying the intake port timing event.
You would use the degree wheel to find 65 degrees before top dead center and mark a line on the piston around the entire intake port.
You then need to remove the piston from the engine.
Extend the vertical lines (sides of the intake port) down to the edge of the piston skirt.
Measure the width of the port marked on the piston and locate the center of the port floor.
Mark an arc between the points where the vertical lines meet the edge of the skirt, using the port floor mark as the center of the arc.
The material within the arc is the material which requires removing.
Using a file or the rotary cutter you then need to cut the skirt of the piston.
Finish off the cut by rounding off the end of the arc where it meets the skirt.
The rounded corner will help to prevent the corner from damaging the cylinder wall.
Once the arc has been cut you need to slightly bevel the bottom of the piston and the edge where you have removed material.
The bevel edge is crutial to prevent the piston to act like a scraper and remove oil from the cylinder wall.