Okay, well if anyone is interested, this is the story.
Guy came to me with a 4 year old Honda Izy & said it regularly tried to rip his arm off when starting it. (He'd had professional repairs done on it 2 years ago & this problem started shortly afterwards. He told the repair people there was an issue, but they said it was fine) the latest attempt at starting had snapped the pull cord so he wanted that replacing.
I put on a new cord & the mower started easily, first pull. Stopped the engine, re-started, no problem. Repeated the process half a dozen times without an issue, but the next attempt felt like the engine had seized!
Against my better judgement, I pulled anyway - it started but snatched the cord. Next attempt it went ok, continuing randomly with either starting easily or almost refusing to turn.
Slipped timing? That didn't make sense because the issue was inconsistent. Valve clearances, bent valves?
I took the valve cover off & found the strange plastic piece in a place it had no business being. Removed it, adjusted the valve clearances & put it back together. First few pulls it was perfect, then the issue returned, which is when I queried what this plastic bit was.
Having eventually found the answer myself, I messaged the guy to ask if he'd ever had issues with the governor. He said he had - that's what had been repaired initially! This lead me to believe the governor mech had been changed & a piece of the old mech left inside the engine. Either that or the part had come loose from the governor, but as the thing runs well once it starts that seemed unlikely.
I told the guy the only way to be certain all was well was to remove the engine, strip it right down to inspect it & then change any suspect parts. I suggested sourcing another engine, but he was reluctant & said he'd just use it like it was, he'd coped for 2 years.
I went back in for another look & this time spotted the problem. The plastic cam wheel has a wire lever, tensioned by a small spring. When static, the spring holds the lever in a 'closed' position & this swings a little node over, causing the valves to open a little more & relieve compression. Once the engine starts & the cam wheel spins, centrifugal force causes the lever to pull away from the spring & the node flips to expose a flat surface, restoring normal compression. The spring in this instance had been destroyed by the governor piece & so there was no compression release. If the engine had stopped on the inlet stroke, it would start easily, close to TDC was a different matter!
I replaced the cam wheel with one off a scrap machine & it now starts easily, every time. I can't be sure of the internals, but he doesn't want to go to the expense of a full strip down & it's run fine for 2 years so I can't blame him. I just hope he's changed the oil regularly though - those bits of spring won't have done any good!
