Non-Return Valve Spring

A weak spring is good, it only has to hold back an 18" column of oil but open when the oil pump pulls on the oil. There are horror stories about these valves especially the manually operated ones. I have used an automatic inline valve adapted from an airline for 20 odd years with no issues and chenged the spring to a weak one. But you need to follow a strict regime.

1. When you disturb the oil line for any reason then when you reassemble it it must first be primed with oil.

2. At the first start up you need to check the oil returns to the tank and do this for long enough to be certain the oil coming back is fresh oil not the dregs from the sump. I start the bike and then sort out helmet and gloves before checking the oil and watch the oil return for 2/3 mins.

When these valves do fail they seem to fail be sticking shut after a layup hence these 2 recommendations cover the high risk problems.

The 72 Combat I am working on is currently having an inline valve added as I have a milling machine but after the pump not before, this is a safer option as a gear pump pushes better than it sucks.

In saying the gear pump pushes better than it sucks the only experience I have of an inline non return valve sticking shut was on a C15, the BSA factory valve after the pump stuck shut, luckily I was running clear plastic oil line and saw the oil flow stop and then reverse. So no valve is foolproof and so I will be fitting a 5psi limit pressure switch to turn a warning light on.

Like John says the "suck" (even if it can technically be called suck)on the suction side of a gear pump is very poor. A 12" head of oil from the tank produces about 0.4psi of pressure, obviously the valve needs more pressure than that to open or the oil would just flow past. If the bike has been sitting for a while, the valve effectively oil tight and the gear pump empty having drained it's contents into the sump through leaks, it's unprimed and at it's worst point to "suck" open a stuck valve.

Under perfect conditions a pump can only "suck" at 14.2psi or more correctly the atmospheric pressure it takes to push the liquid into the space vacated by the liquid being pumped out. A worn, empty gearpump is not perfect conditions.

My most ridden norton wet sumps badly, even after a week half the tank will be in the sump. I just kick it and ride it. I can't imagine in it's 50+ years that someonebothered to drainthe sump before every ride after a short break, or on any of the thousandother norton heavy twinsnot used for a daily commute. Any problems are usually caused by 'special' plumbing features or deviations from the original setup, which whilst not perfect, invariably works.

I run the oil tank at just above the minimum level, less oil to heat up, if the bike has been stood i look in the filler, if it's below the gauze, tip a bit in so the engine doesn't start dry. Go for a ride. when you next check the oil, if it's too high drain some out of the tank drain, if not, all good. Saves grovelling on the floor and scuffing up yer winkle pickers before heading out to beat up some mods.

Executive summary.

I said leave it in but follow 2 steps to ensure risk of failure is minimal.

Martyn said leave it out and let the engine wet sump and the oil pump will clear the sump on it own (as the return side has double the capacity of the feedside)

My suggestion still contains a risk albeit IMHO minimal, Martyns has the risk of the primary crank main seal blowing out, also if all the oil is in the sump the feed side has to wait for the return from the sump to get the oil level up high enough. But a simple check of the oil level in the tank before first start of the day is enough to cover that one as Martyn suggested.

So you have a 50/50 choice, both will work but both carry risks.

just a thought a "small" restrictor in the breather exit from tank would give the pump a positive feed ???

No that will not work, also as a 72 had a one year only crankcase configuration which is a bit of a disaster you should put a reed valve on your breather outlet to only allow air out and not back in plus change the pickup point for the oil return from the front to the rear of the case. A 72 at high revs the oil all ends up at the back of the crankcase and the pump return stops returning and the breather pipe become the oil return. They also dropped the sump filter but its not possible to correct that error except by drilling a series of small holes in the back of the case instead of one large one over the relocated oil pickup to act as a crude filter.

I would agree about the crankcase breather, any sort of extra pressure in the cases can produce oil leaks in unexpected places. Like the third fin down on the cylinder head

That type of non return valve does not cope at high revs when you most need it to work, you need one of these

http://www.ebay.co.uk/itm/YAMAHA-YZF-R-125-R125-2008-2014-EGR-EXHAUST-AIR-VALVE-/172536241252?fits=UKM_Make%3AYamaha&hash=item282bf61c64:g:AmgAAOSwax5Ypw61

It has a reed valve so will easily cope with the high revs and fits the 72 breather hose.

Near to the breather outlet, the breather has a long rubber hose connected to it, put the breather in that hose near to the breather outlet, you may need to shorten the hose to get the run correct.

John's first two points in his initial reply are spot on. With an unprimed pump, not drawing oil you will get approximately 11 miles before a nasty tapping noise turns into a £500 bill. (I just happened to note the distance to my break down 2 years ago en-route to the Wistanstow classic bike show on Sunday just gone) The oil line might stay primed for a couple of weeks but not a couple of months, in which case you might as well just drain the crankcases back into the tank. It's easier than re-priming the oil line.

Near to breather outlet 'on engine', the oil tank has the breather inlet. The closer it is to the crankcase the stronger the signal is from air movement, as air compresses, and the more effective the valve is.