My oil pump to carb line won’t stay filled lately as it had in the past (see photo), is that OK if I don’t suspect any holes and the pump and bike overall are running fine?1976 DT175. The pump works fine and spec flow rate was confirmed a month back. I can prime all the air bubbles out by spinning the hand gear, but once it’s running they come back during idle when it’s not pumping as hard by design. Thanks. 

imgur.com/a/t483aFx

Just my thoughts but line looks way too large in diameter but might just be thicker skinned than original. Yamaha use a black line for a reason to stop you worrying. You'll be good. If the ball & spring at the base of that was not seating good the line in theory would fill up from gravity of the oil tank being higher than the feed into the cylinder depending if the oil tank level is plentiful. Feed out from the tank base & feed into the cylinder are possibly about equal without looking. Sorry, long story going nowhere. 

Thanks. I saw a related post about this also so I tend to think it’s not a big deal as long as the line isn’t like completely dry when I’m running the pump at full stroke. I tend to prefer the clear line also just to keep an eye on it, but you’re right it’s easy to look at it too much as a result, haha.

I would be curious if anyone else sees and has opinions.  

How long does the line take to 'fill up' when running? If you have to ride a few miles waiting for that line to show the oil right up to the carb that might be a concern? If you start it up and it "fills back up" within a minute I would not be so concerned.

I think there are some one-way valves that Yamaha and maybe other makers made just for these type of lines to prevent the oil in the line flowing back to the pump (I'm thinking SeaDoos). Not saying you need one of these at all. As pointed out, most of these lines are black so no way of telling what's up within.

It fills up initially or doesn’t take miles of riding, but seems like once it reaches the carb there’s backpressure that makes it jump back down in spurts keeping air in the line (mostly at idle). It does prime up again though and at throttle (higher flow rates) it does keep up better I think. Just didn’t know if backpressure from the carb was normal or to be expected and I didn’t notice it weeks back when I was testing it out. 

The one way valve is a good idea, do you happen to know of a product for that? Thanks for the input. 

If the check valve at the pump isn't completely shutting off , you get those symptoms .
 

That’s worth looking into, but I also did hand prime it back up and the fluid level wasn’t dropping with the bike off. Only when running and air flowing through the carb at low throttle this seems to be an issue. 

If you pulled the line at the cylinder & held it vertical it would fill to the top then you'd see the pulses if you held it vertical. If you pulled on the oil pump cable you'd see big pulses all with the motor idling. When the line is bent over & plugged onto the open fitting at the cylinder with sucking & blowing going on past it I doubt it would ever look totaly full when running.
Pretty normal stuff. For them that are worried I wonder what the outcome would be if the check valve they have at the pump outlet was somehow placed right at the cylinder. Oil tank being higher than the fitting at the cylinder it should stay full. Food for thought, source for discussion. Maybe not.

I agree with RT325.  You're fine.  The flow is miniscule at idle and your larger than stock clear hose is looking exactly as expected at idle..  

There are reasons they use the tiny black hose.  

Try this...  put a fairly large diameter clear hose on the end of a faucet and route it uphill a little and then horizontal to mimic your oil line routing.  Turn the faucet on really low.. just a drip.  Hose will fill up and water will drip out the hose but there will be a visible air bubble near the exit end.  Turn up the flow and the air bubble goes away.

Now put a teeny tiny clear hose on the faucet and turn it on again really low so it's just barely dripping again.  No air bubble.

Another similar place this happens is with clear fuel line and/or filters.  People will swear no fuel is running because the clear line looks empty.  But it runs fine.  Fix again is to get rid of the clear line.  
 

If it were me i would install the correct small black hose and be done with it. No see, No worry.