Knocking noise on start up

[FZR400]

When i first bought my 3TJ the engine ran perfectly with no noises but stupidly enough i didn't check the oil before taking it on a 600 mile trip! Only checked it when i got back and it was off the bottom of the sight glass.
I gave it a service straight away but every time ive started it up since there is a knocking/tapping noise which initially lasted for 5-10 seconds but now lasts for a good few minutes. Its also started using a bit of oil.

Does anyone have any idea of what the problem could be? I know it could be a huge amount of different things but if anyone has any ideas of where to start it would be great :)

[KIYOSHI]

Sounds like you are into the realms of big end and main bearing damage or camshaft and journal damage or both due to oil starvation.

Cant really imagine cold engine tapping being anything else. The noise will reduce when warm as the worn components expand with heat. Check the starter clutch isnt sticking first just to be on the safe side though.

3tj engines have always suffered with oil starvation to the cranks and the crankcases themselves are often modified to help solve the issue.

Remove the generator cover, remove the spark plugs, turn engine to just before TDC on each cylinder, stick a screw driver down the plug hole and onto the piston crown. Then rock the engine back and forward slightly with a 14mm spanner on the end of the flywheel. Check each piston for any slop, this should give you an idea of any serious wear in the big end bearings.

The main bearing are a little more involving to check but more troublesome.

Removing the rocker cover and the camshaft bearing caps to inspect for damage would be the next step.

[KIYOSHI]

And if its started using oil its a sure sine of a poorly motor! Oil will either be escaping via worn bores and piston/rings or via the valve stem seals.

[Evilchicken0]

You need a compression test - it needs to be done "wet" and "dry" that should tell you something of what's happened.
I think your piston rings and barrels are .... fucked.

To repair that (if it is that) will be a top end strip, hone the barrels and rebuild ... but ... that's a lot. If you can't do that yourself then it's better (and probably cheaper) to find another engine.

[KIYOSHI]

Compression test wont give you a good idea of oil startvation damage to the crank and top end though. I think the most urgent thing would be to diagnose these first. Little point in stripping the top end, boring to +1mm and rebuilding only to find the crank is goosed.

As evil said, i think a new lump would be cheapest option. Where in the world are you?

[FZR400]

Thanks for the info, I'm based in London at the moment. If it is as serious as suggested then its going to have a wait a while as I'm a bit cash light for a while.
I was contemplating checking the valve clearances and cam chain/tensioner just to rule out those possibilities.

[Evilchicken0]

It could be the cam chain tinkling but if there's a knock then chances are it's the big ends. If it is and the engine is toast then it isn't going to harm it anymore by running it.

I'm in london too.