D
Deleted member 12676
Guest
The stock CDI is actually designed well for the purpose for which it was intended. Its use with these poorly
carbureted low compression engines is well matched. Why? It maintains a strong ignition advance all thru its RPM
range. That is good because these chinese carburetors have around 20% worse fuel atomization due to not having a
separate idle circuit. The larger than normal fuel droplets take longer to burn which requires more ignition
advance so that the peak cylinder pressure still happens around 15 degrees ATDC (for best engine power). Also the
low engine compression (around 95psi) requires more ignition advance. Only with more compression is less advance
needed because the fuel droplets will be closer together which allows a faster moving flame front for a more rapid
complete combustion, requiring less ignition advance for the peak cylinder pressure to happen around 15ATDC. Also
the weak spark allows for a slower burn, matching the needs of this poorly carbureted low compression engine.
But the engineers screwed up by not properly balancing the crank flywheels and the early ignition of these engines
contributes to engine vibration at high RPM. With a 10mm or 12mm carbide drill bit you can fix that though.
So if you are going to stick with the crummy stock carburetor and stock compression you should also stick with the stock CDI (although it is prone to failure). But then you are doomed to perpetual low performance.
If you want to get decent performance from these engines these are the steps I recommend:
1) increase engine compression by removing metal from the head mating surface
2) put on a good carburetor like one of these; Walbro, Mikuni, Dellorto PHBG
3) install the Jaguar CDI for less ignition advance which matches the previous improvements
If you stick with piston port intake (without reed valve) then I also recommend extending the intake manifold length. This allows a resonance effect to cram more mixture in at low RPMs for more power.
I am not a fan of expansion chambers because they have a very narrow powerband whereas these engines need things that allow a broad powerband since they have no gearbox. The only exception to this is my design of torque pipe which you are free to use to make one yourself.
There are other cylinder heads available but most of them raise the compression too much. I wouldnt buy one unless the seller can give a good educated guess as to what the final engine pressure will be with it. (Engines vary and so its impossible for them to tell what the final pressure will always be). Best target pressure is around 130psi (at sea level). Buy another upper connecting rod bearing also since the stock one is weak and wears out fast, especially with higher engine compression to push against.
Since the Jaguar CDI is good up to 8500 RPM I also like to see people use a rotary tool to raise their exhaust port and cut some of the piston skirt off (or lower the intake port) which allows the engine to rev higher than the stock 5500 RPM. You can change the ports 1mm at a time till you are happy with the top speed. Going too high (without a torque pipe) causes a regrettable loss of low RPM power.
carbureted low compression engines is well matched. Why? It maintains a strong ignition advance all thru its RPM
range. That is good because these chinese carburetors have around 20% worse fuel atomization due to not having a
separate idle circuit. The larger than normal fuel droplets take longer to burn which requires more ignition
advance so that the peak cylinder pressure still happens around 15 degrees ATDC (for best engine power). Also the
low engine compression (around 95psi) requires more ignition advance. Only with more compression is less advance
needed because the fuel droplets will be closer together which allows a faster moving flame front for a more rapid
complete combustion, requiring less ignition advance for the peak cylinder pressure to happen around 15ATDC. Also
the weak spark allows for a slower burn, matching the needs of this poorly carbureted low compression engine.
But the engineers screwed up by not properly balancing the crank flywheels and the early ignition of these engines
contributes to engine vibration at high RPM. With a 10mm or 12mm carbide drill bit you can fix that though.
So if you are going to stick with the crummy stock carburetor and stock compression you should also stick with the stock CDI (although it is prone to failure). But then you are doomed to perpetual low performance.
If you want to get decent performance from these engines these are the steps I recommend:
1) increase engine compression by removing metal from the head mating surface
2) put on a good carburetor like one of these; Walbro, Mikuni, Dellorto PHBG
3) install the Jaguar CDI for less ignition advance which matches the previous improvements
If you stick with piston port intake (without reed valve) then I also recommend extending the intake manifold length. This allows a resonance effect to cram more mixture in at low RPMs for more power.
I am not a fan of expansion chambers because they have a very narrow powerband whereas these engines need things that allow a broad powerband since they have no gearbox. The only exception to this is my design of torque pipe which you are free to use to make one yourself.
There are other cylinder heads available but most of them raise the compression too much. I wouldnt buy one unless the seller can give a good educated guess as to what the final engine pressure will be with it. (Engines vary and so its impossible for them to tell what the final pressure will always be). Best target pressure is around 130psi (at sea level). Buy another upper connecting rod bearing also since the stock one is weak and wears out fast, especially with higher engine compression to push against.
Since the Jaguar CDI is good up to 8500 RPM I also like to see people use a rotary tool to raise their exhaust port and cut some of the piston skirt off (or lower the intake port) which allows the engine to rev higher than the stock 5500 RPM. You can change the ports 1mm at a time till you are happy with the top speed. Going too high (without a torque pipe) causes a regrettable loss of low RPM power.