magg wrote: ↑

Sun May 22, 2022 1:30 pm

Plugs blackened but not wet and engine not running suggests no fuel. Have you tried spraying fuel into the carby throats while cranking the engine, or placing the palm of your hand over the throat of each carby in turn while cranking the engine (make sure the float bowls are full). Either action should provide enough fuel to get a response.

I tried this earlier today. There was no difference in response at all.

Upon opening up the engine we inspected the valves.
All eight intake valves were worn and razor sharp. Probably the original ones that went in 30 years ago.
there was also some - very little - sideways play in two of the pistons. Mechanic thinks it's a compression issue.
Could these be the causes of my hard starting issue?

Low compression can be caused by poorly sealing valves and by poorly sealing piston rings.

Maybe you have found the issue - you can lap the valves perhaps unless they are really knackered? I did mine on the NC30 years ago and it was very therapeutic. Easy to do, take your time and maybe that will do it.

You can them perhaps compression test it - follow the standard methods given in Haynes manuals.

It's been a while guys, the parts I needed took too long to arrive.

The good news:
replacing all valves and all piston rings fixed the hard starting issue. Bike started and ran eventually. Rode it around 10kms and parked it. Next morning, cold start took some figuring out because she would fire up and revs would fall right back down to zero, but adjusting the idle screw finally got it to idle on high revs until the engine was warm and then I turned it back down. Later I read that the choke was designed for that.

The bad news:
riding around earlier this evening, in second gear, a sudden grinding sound began in the engine and the bike jerked to a stop.
Tried to fire her up again but the grinding sound persisted so I shut her off and pushed the hefty beast home.
Anyone know what I could be looking at in terms of damages?

Have you kept an eye on the oil level? It's not unusual for oil to burn off while bedding in new rings.

MadMotoUK wrote: ↑

Tue Aug 02, 2022 8:37 am

Have you kept an eye on the oil level? It's not unusual for oil to burn off while bedding in new rings.

We didn't check the oil. Opening the cylinder head, we found two broken valves on cylinder 2, which damaged the valve seat. Disheartening stuff. A very costly repair is ongoing.

I couldn't locate OEM inlet and outlet valves for my NC31 so I went for a Chinese replacement kit.

They fit well enough and seemed to fix the hard start, but two valves on Cylinder two broke mid-ride one evening.

the piston smashed the broken valve bits into the valve seat, damaging it.

We noticed that the semicircle clips that lock the valve in place were deformed. Could the valves have slipped out of position after failure of the locking clips?

What could cause such a failure? Is it down to the cheap quality of the replacement valves?

You can tell the valves broke before they dropped out? Perhaps the retainers failed or were not seated correctly.