I have recently rebuilt from top to bottom a Yam XS500C, 1977 vintage.

The engine was totally stripped down and rebuilt step by step - I'd better point out here that I'm not a mechanic, I just take my time, follow the books and use a bit of 'common'

The bike started on second push of the starter button - it's not running right , but I can work on the carbs and timing later.

The major problem is that the oil pressure light stays on when running - Not good!!

The 'book' says turn off the engine immediately, trace and rectify the fault - that's it

I've tested the pressure switch - remove, compressed air blown in, multimeter to check circuit breaks when under pressure - all OK

The correct amount of oil had been added (about 4ltrs with a dry engine)

So what do I do next??

I am presuming the oil pump is working as the tacho drive works - they are both driven by the same shaft - and checking the pump is a last resort as the engine has to be totally stripped down to remove it - and I did check that all was working when I assembled the engine.

So, where do I start as a 'first resort' ????? Can anyone help and point me in the right direction, things to look for or check??

Top and bottom of it is - the last thing I want to do is remove the engine - so what can I do before I have to do the last thing I want to do - are you following this

Hope someone can help me please

Could be many things, but first thing I would look at are all the gaskets that have oil passages in them to make sure one did not get flipped, blocking an oil passage.

Thanks Houghmade - will do the checks on gaskets as I gradually strip the engine down to find the fault

Bit of an update though - thought I'd check the compression again to check the 'poor running' problem. The results were intersting:

Right cylinder 140psi just as it should be according to the manual.

Left cylinder 105psi way off what it should be!! I rechecked both cylinders, twice, with the same result.

There's loads of questions I could ask but I'll stick to topic, so .....

Could the low compression on the left cylinder contribute to, or even cause, the oil warning light to stay on?

Just trying to narrow down the problem, anyone any thoughts?