Below is a timing diagram I created to show you what I think needs to be done on top of what Wayne said yesterday (close the mixture valve a bit earlier).
- In this diagram the engine rotates anti-clockwise.
- Ignition (yellow) is 30 deg BTDC (Before Top Dead Centre) the gray line.
- Then the power cycle starts (left half of diagram) and here, still a bit early, the exhaust valve (blue) starts opening.
- During the exhaust cycle (right half) exhaust is pushed out, and when the piston is almost at the TDC (Top dead Centre), the mixture valve (red) starts opening.
- Then in the intake cycle (left half), the exhaust valve (blue) closes, and the whole cylinder is loaded with new mixture.
- In the compression cycle (right half), the mixture valve (red) closes to late, resulting in the cylinder spitting out mixture back into the carburetor.
I really have the feeling that the overlap at the TDC is sort of right:
- IVO: inlet valve opens 10 – 20 deg BTDC
- EVC: exhaust valve closes 10 – 15 deg ATDC
- IVC: inlet valve should close 30 – 40 deg ABDC
- EVO: exhaust valve should open 30 – 50 deg BBDC
Wayne said, to “close the mixture valve a bit earlier” and I agree with that. But that means that the exhaust valve is going to open to early in the power cycle, back to were I started .
- The LSA (Lobe Separation Angle) on this engine is to big, can be improved by moving the CAM rollers closer together. This will delay the opening of the exhaust valve, and close the mixture valve earlier.
- The valve open time is to long, can be reduced by making the CAM lobe narrower.
A lot of work, but that is going to give me improvements, definitely.