CSC SG400 / Zongshen RE3 problems

ChopperCharles · 07-18-2023, 04:32 PM

So I've got a problem with my SG400. When the motor is hot, it will start to lose power randomly when at constant throttle input. Goosing the throttle is immediately responsive - at first. It's very mild, almost unnoticeable at first. It gets progressively worse the more I ride it, until I'm riding a bucking bronco essentially. When this happens, it feels like I'm dropping a cylinder.

There are no active or saved codes in the ECU. I've checked fuel pressure and it's a constant 35-37psi at all engine speeds, and is rock solid even when the problem is occurring.

My first thought was fuel pressure, but that's obviously not it. My second thought was a bad spark plug. Replacing the spark plugs with NGK Iridiums improved the situation for a while, but it came back just as bad in less than a week. So with that evidence, my third idea is a bad coil, especially since it seems to be heat-dependent and new plugs temporarily alleviated the problem.

Valves were adjusted at 5000 miles, which was 3000 miles ago. There was only the most minimal of movement, and half the valves didn't need to be touched. So I doubt any are tight now, and besides, in my experience tight valves don't cause these symptoms.

I've got an inductive timing light wired to the bike and strapped to the tank rack, and am just waiting for a night when I'm free so I can test this theory. I'll try the right coil first and if it's good I'll try the left. But if that's not the problem, what order should I investigate next?

Fuel injector potentially intermittently blocked by junk in the fuel line. Unlikely, but I've had it happen before.
Throttle Position Sensor
Plug wires/caps (no obvious arcing observed so far, but that doesn't mean it's not happening)
Pickup/sense coils inside the motor
Sidestand switch - not super likely because it feels like only one cylinder drops out, not both. But I could be wrong. Would this one should exhibit as both coils not firing during an event? Or would it be the fuel that is cut off, not the spark?
Bad fuel injector
Some other sensor or switch that I haven't thought of yet

Anything I'm missing?

I have a tablet running HUD ECU Hacker and the dongle and cables. I could keep the tablet in a backpack connected with a long USB cord, and pull over and start monitoring whenever it starts running intermittently. That might help diagnose the TPS or Sidestand, but not any of the other problems above.

Charles.

07-18-2023, 04:32 PM	#1
ChopperCharles Join Date: Feb 2018 Location: RDU, NC Posts: 677	CSC SG400 / Zongshen RE3 problems So I've got a problem with my SG400. When the motor is hot, it will start to lose power randomly when at constant throttle input. Goosing the throttle is immediately responsive - at first. It's very mild, almost unnoticeable at first. It gets progressively worse the more I ride it, until I'm riding a bucking bronco essentially. When this happens, it feels like I'm dropping a cylinder. There are no active or saved codes in the ECU. I've checked fuel pressure and it's a constant 35-37psi at all engine speeds, and is rock solid even when the problem is occurring. My first thought was fuel pressure, but that's obviously not it. My second thought was a bad spark plug. Replacing the spark plugs with NGK Iridiums improved the situation for a while, but it came back just as bad in less than a week. So with that evidence, my third idea is a bad coil, especially since it seems to be heat-dependent and new plugs temporarily alleviated the problem. Valves were adjusted at 5000 miles, which was 3000 miles ago. There was only the most minimal of movement, and half the valves didn't need to be touched. So I doubt any are tight now, and besides, in my experience tight valves don't cause these symptoms. I've got an inductive timing light wired to the bike and strapped to the tank rack, and am just waiting for a night when I'm free so I can test this theory. I'll try the right coil first and if it's good I'll try the left. But if that's not the problem, what order should I investigate next? Fuel injector potentially intermittently blocked by junk in the fuel line. Unlikely, but I've had it happen before. Throttle Position Sensor Plug wires/caps (no obvious arcing observed so far, but that doesn't mean it's not happening) Pickup/sense coils inside the motor Sidestand switch - not super likely because it feels like only one cylinder drops out, not both. But I could be wrong. Would this one should exhibit as both coils not firing during an event? Or would it be the fuel that is cut off, not the spark? Bad fuel injector Some other sensor or switch that I haven't thought of yet Anything I'm missing? I have a tablet running HUD ECU Hacker and the dongle and cables. I could keep the tablet in a backpack connected with a long USB cord, and pull over and start monitoring whenever it starts running intermittently. That might help diagnose the TPS or Sidestand, but not any of the other problems above. Charles.
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