Oil pressure troubleshooting

I have an 81 CM400 that I have had apart quite a few times. The last few times I have had it apart and back together again, when I start it up, the oil pressure light comes on. I have replaced the oil pressure switch, and I have had the oil pump apart. The oil pump looks mint. I blasted the engine with air and looked in every little place to make sure nothing is blocking up an oil port. The engine is clean inside.

So I was wondering if anyone has any ideas as to what could be setting the light to go on. I fresh out of ideas, and I don't want to run the bike too long, in case it really is a problem with low oil pressure. Don't wanna cook the engine. Should I just suck it up and get a new oil pump?

Thanks!

Have you taken the valve cover off to see if oil is actually getting up to the head? Also, you mentioned you had the engine apart, if so I believe with these engines the base gasket can be put on incorrectly. If put on 'backwards' then the gasket actually covers the oil orifice. Also, worth checking is where the oil pressure switch is located. Unscrew it and make sure the passage is not blocked. On my old 400 this passage was blocked by a bit of silicone that the previous owner used to seal the cases.

At the risk of stating the obvious, there are two possibilities - low oil pressure OR a dud switch. Try a different pressure switch first and if that also turns teh light on, I'd be thinking top end oiling issues.

I have not taken off the oil cover, but I will check that out. The gasket is good, not backwards. I will also check in the port that the OP switch screws in to. Did not check that yet.
I have already gotten a second OP switch, but it was used, so I should prob just buy a new one once I runs those other checks.
Thanks b'yes.

I took the valve cover off and there is some oil up there. So I ran the engine and a little bit of oil was slung out, or splashed, but i dont know how much oil should be up there. Any ideas?

If I remember right, the oil pressure switch is on top of crankcase on the main oil gallery?
It takes a little while for the gallery to fill up and then develop pressure.
You haven't left out an 'O' ring allowing oil to drain back?
Oil level is correct (your not screwing dipstick in when checking?)

Also maybe a bit obvious, but does the light go out after the bike runs for a few seconds or is the problem that it remains on?

you need to actually see what the pressure is with a pressure gauge.
does the light stay on all the time? running and not running? if so the wire is shorted to ground somewhere. you can static ck the switch(resistance) and then use regulated air(do not use high pressure air!) to see if it trips(ck both sw to get an idea of low pressure actuation point) and at what pressure. if the switch is in the gallary on the case and top end gasket was blocking pressure(downstream) switch would see pressure and light would stay off......now if you left out orings or parts that is a whole other issue but by cking actual pressure you can verify..

good luck joe @ vcycle

Just dug the manual out.
Honda doesn't give pressure or method to check pressure.
It does have oil switch behind 'bump' on top of clutch cover connected to mail oil gallery AFTER the oil filter, etc so it will take a second or 5 to get a reading.
Switch is probably low pressure (<10psi)
Check the wiring isn't damaged at screw in connector.
It will probably be worth removing clutch cover and checking oil pump drive chain
Re-set balancer shaft adjuster quadrant while your in there
78 doesn't have plug to adjust without removing cover, we used to re-set quadrant on all of them at first service (500miles)

Check and clean the oil pump pick up screen, they get blocked from poor maintainance (and silicon sealer )

you can plug in at the pressure switch in front of the engine. pressures should be aprox 50psi cold and 10 hot at idle and 50+ at cruise speed ...prv is set at 55-75 psi which can be another source of low oil pressure. sounds like he already checked pump so i would think chain is ok ... is flange gasket ok ?
joe @ vcycle