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Technical | How To / 500ccm hard to start due to high compression
« on: June 23, 2024, 08:15:44 PM »
Hi, I hope someone can guide me in the right direction regarding my engine problem. There are several posts in this forum mentioning 500 engines being hard to crank, but I have not read any solutions for this problem. Assuming Kymco ATV, scooters etc. using the same engine or at least block, top etc.
I bought a 500 MXU (2009) with unknown engine problems a year ago. I split the engine and found a broken bearing for the balancing shaft. After replacing both bearings, I reassembled the engine while measuring every component inside the engine according to the workshop manual. Everything was according to spec. After some cranking, the engine started and was running smoothly.
The problem is the high compression in the engine, making it almost impossible to crank over. It sounds like the battery is flat and will only crank twice to the compression stroke. The recoil starter is impossible to use when the engine is cold, but once it’s warmed up I can start it with a small pull.
My understanding is that these engines only have a kickback decompression mechanism on the exhaust cam, so it doesn’t really do anything during the compression stroke.
So far, I have checked/replaced these things trying to get rid of the problem, but so far no luck:
- New battery
- Changed exhaust cam (The old was according to spec)
- New brushes in the starter motor
- Checked timing
- Connected 12V from a tractor directly to the starter motor
Any advice or suggestions would be greatly appreciated.
I bought a 500 MXU (2009) with unknown engine problems a year ago. I split the engine and found a broken bearing for the balancing shaft. After replacing both bearings, I reassembled the engine while measuring every component inside the engine according to the workshop manual. Everything was according to spec. After some cranking, the engine started and was running smoothly.
The problem is the high compression in the engine, making it almost impossible to crank over. It sounds like the battery is flat and will only crank twice to the compression stroke. The recoil starter is impossible to use when the engine is cold, but once it’s warmed up I can start it with a small pull.
My understanding is that these engines only have a kickback decompression mechanism on the exhaust cam, so it doesn’t really do anything during the compression stroke.
So far, I have checked/replaced these things trying to get rid of the problem, but so far no luck:
- New battery
- Changed exhaust cam (The old was according to spec)
- New brushes in the starter motor
- Checked timing
- Connected 12V from a tractor directly to the starter motor
Any advice or suggestions would be greatly appreciated.