Topic: Fluctuating oil level in the bike, bizarre mystery!

[markreimer]

I've recently rebuilt my 74 cb750 engine, as well as the oil pump with Elan's kit. The bike runs very nicely, probably better than ever since I bought it 6 years back.

I'm having a weird thing with the oil level in the tank though.

After I rebuilt the engine just before winter, I primed the pump, added the usual 3.5 litres of my favourite oil, verified I had oil flow and pressure, and started the bike up. I was able to run it maybe 100km before the snow fell, so I put it away.

This spring I was doing some other spring maintenance,  fired the bike up and rode around the block a bit, then came back in for an oil change. I checked the dip stick and found it was almost dry.  But my pressure gauge indicated I had proper pressure the whole time, so I didn't panic. It was only running a few minute anyway, and there WAS oil in there...

When I drained the oil, it seemed to have a bit less than 3.5L to my eye, but I didn't actually measure it. I put fresh oil/filter in, measured it accurately, went for a ride, shut the bike off and put it on the centre stand, then checked the oil - Bang on, we're all good.

Fast forward to today - I check the oil after a 10km ride and it's massively OVER filled. About an inch from the cap, and I've got oil spitting out the overflow tube that runs down by the swing arm. The whole ride I had good pressure on my gauge (right side of engine), so I know it's circulating as it should, but what's up with the fluctuating oil level in my tank?


Once again - oil pump was fully rebuilt with new seals/shaft seal/o-rings etc. Pressure is around 65psi at start-up, and slowly falls as it heats up, always maintains at least 10psi at idle. No leaks anywhere.

[markreimer]

The incriminating evidence after idling over this puddle


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[markreimer]

Had a thought. If indeed the oil level was correct at my last change (which I'm positive it was), and after letting it sit for a couple weeks and coming out to ride it again, finding the level higher, perhaps some of my floats are not sealing properly, and fuel is leaking through the carbs, past the rings and into the oil pan, resulting in a higher 'oil' level after riding.


Last week when I rode, the bike back fired and sputtered at start up, with a poof of black smoke. I figured it had somehow flooded, though the petcock was left off and I confirmed it does not leak. But I suspect the fuel in the fuel lines could still sneak past the floats and into a cylinder. If it did this a couple times, that could result in a higher oil level if I wasn't being careful.

Today when i rode, I started the bike up after having run it totally dry, to see if it started smoothly with no backfires. It did, started smooth and without argument, and off I rode. That makes it sound like I've got a leaky float issue to me, which also means my oil will be contaminated. Time to head off to the oil store and do a change tonight. I'll keep running it dry the next few rides to see if it affects the oil level. And also clean up my float seats while I'm at it...

[flybox1]

Nope...your carb bowls would drain out the overflows first....unless you've purposefully blocked them off.

[PlungerWaffle]

If its really bad you can sometimes smell the gasoline in the oil. You could give it a sniff nod see if you notice anything. If might just confirm your suspicions.

[markreimer]

Doh! Of course they would... I forgot about those. And no, I have not blocked them off. I suppose I could verify that they are actually working and not blocked from age/grit, but it's unlikely all 4 would be like that.


I guess I just screwed up the oil change last time or something. The oil doesn't smell gassy at all, and only has maybe 60 miles on it, looks brand new. Maybe I'll start by sucking some oil out the tank till it's at the proper level, then checking it regularly over a few rides and report back.

[markreimer]

Yeah just went and did that. Must have looked funny seeing someone stick their nose in their oil tank. Smells like oil, no hint of gas to me.

[754]

If you had fuel in your oil, you could maybe smell it. I think you can feel it with your fingers.
If it is in there, dont run it.
Yes, it does happen that it can get in there..

[PlungerWaffle]

Does the fuel level only change when the bike is running?

[markreimer]

I don't smell anything, but how do you feel it with your fingers? What does it feel like?


Regarding the fuel level, it stays the same. My petcock is not leaky at all, and I always shut it to the 'off' position when parked. I tested the petcock again recently, and it does not leak a drop. So at most, the fuel in the lines between the tank and the carbs, would be all that could leak down into the bowl, and overflow.

How would the fuel make it into the cylinders by the way, considering there are overflow tubes?

[markreimer]

I guess if I had a float that was stuck open, it could drain the fuel left in the fuel lines faster than the overflow could handle it. I've never found fuel stains under the bike before, or evidence of fuel in the overflow tubes (clear lines), and the bike runs 100%. If it was stuck open, I'd expect issues at idle.

[PlungerWaffle]

If you leave the fuel on after when your done riding does it make a difference in oil/fuel level?

[PeWe]

Important to check oil level when engine has been driven fo at least 10 minutes and up to temperature. The oil level can sink when cooling off. I think some oil pumps leak some oil into engine. My first oil pump did that over night since I bought the bike 2 years old and 17.000km on the meter, I doubt the seals were bad, probably pump cast.

It sank over night. No problem at all. I changed pump  about 85.000km later just for sure last spring. A pump with less than half milage. I did the Elan kit on it. This pump keep the oil in tank. I see no advantage with that.
It was easier to change oil before since the oil could flow out thru oil pan oil bolt and no need of starting engine to let the pump flow out the oil from tank.

3.5L oil is with bone dry engine like after a total restore. (Remember the 0.5 liter already in,  trans, over chains, crank etc during assembly.)
When changing oil, all oil will not flow out, less than 3L. I did a change last weekend and did my best to take out all oil. Filled with less than 3L but still overfilled, sucked out 3 dl with a large syringe with hose lenght reaching top level exact when fully pressed to oil tank.
Correct refill level at oil change should have been 2.5L to start with, start engine and run it warm, adjust with a deciliter or max 2 if needed.

I think oil might leak a little when fully warm when it is thinner. I think the oil I have in now do that, 10W-40.

I always switch off fuel when parked, maybe not for a short stop. 1-2 hours, off since I do not trust that carbs will not overflow.

[markreimer]

Thanks PeWe. I remember filling the oil tank with 3.5 litres according to the little scale on the side of the oil jug. This was after rebuilding the oil pump, so it should have been pretty close to as dry as possible I think. What is strange is that after I did that oil change, I ran the bike for a few minutes, then checked the level. It was  exactly at the full mark. And then the next time I ran it, it's an inch OVER the full mark. I understand when people check it while it's running, or it's sat for a long time, and it seems too low because it's seeped down into the case. But for me, it's the opposite. I haven't checked the oil since the last change, and now that I do, it's too much. So either I somehow got a screwy result when I changed the oil, or there is something else causing the oil tank to fill up too much.

I wondered if maybe the pump was pushing oil into the tank, but not into the engine fast enough, causing the oil to 'build up' in the tank, but my pressure gauge is down stream of the pump, and reads normal.

Tonight I'll drain the oil and measure how much comes out. That should help narrow it down!

[flybox1]

After your next few rides, disconnect the fuel tank from the carbs (and run a overflow line from the pet to a mason jar or empty fuel can)....and note your oil tank level.
A sure fire way to catch the culprit.

[markreimer]

I'm not sure I'm following. You're suggesting I remove the fuel tank and petcock from the equation by disconnecting it at the petcock, right? And you're suggesting it because it's possible the fuel tank is leaking past the carbs and into the crankcase while parked? If so, I'm positive the petcock isn't leaking. I had the tank full of fuel, off the bike, and sitting over a piece of cardboard with a bowl under - nothing in the bowl, no stains on the cardboard, after a week of sitting. It's a OEM new petcock I installed a couple years back.

If I understand it correctly, the oil tank level cannot rise while the bike is parked, since it relies on the pump to supply it. If anything, it could only change levels by dropping if oil drained past the stopper in the pump.

Thankfully I've never had that problem, and just replaced the stopper when rebuilding the pump due to an unrelated oil pressure issue (low pressure at startup on very cold, sub freezing startups. Problem disappeared after pump rebuild).

So I would presume that after I ride, disconnect the fuel tank from the carbs, and check the oil tank level, it would be impossible for the oil tank level to change? Unless at the most, fuel in the bowls somehow drained out past the float pins, into the cylinders, then into the block, but I don't see how that would be possible without the tank connected and having faulty floats and plugged drains.


So far as I can figure, having more oil than when I initially checked it could only be due to operator error  during the fill up, or maaaaaaybe I forgot to shut off my petcock once and don't remember, causing fuel to somehow drain into my crankcase without running out the overflow tubes.

Either way, I need to change the oil tonight. I'll start there, check my overflow tubes, go for a ride, and report back

[markreimer]

Oh and let me take a moment to pause and say, she might be a bit smelly and leaky, but sure is pretty




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[flybox1]

So far as I can figure, having more oil than when I initially checked it could only be due to operator error  during the fill up, or maaaaaaybe I forgot to shut off my petcock once and don't remember, causing fuel to somehow drain into my crankcase without running out the overflow tubes.

Fuel flows from the fuel inlet at the carbs, and right into the bowls. 
From the bowls past the jets into the carb throats....but only when a vacuum is present. 
If the fuel rises without vacuum, the overflows are there to drain away excess.  Just like the overflow drain on a tub or sink.

[markreimer]

And it wasn't till I took this photo that I realized the right side of the rear tire is totally covered in oil. Jeez!!! Good thing I love a couple miles away down a straight road.


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[markreimer]

Just returned from draining a chunk of oil out of the tank and had an interesting experience.

I removed about 1L of oil from the tank. I checked the stick and it was about 1/8" below the 'full' mark. Awesome, lets fire it up.

I fired it up and the level dropped substantially, to the point that it was maybe only 1/3 up from the 'low' mark. Wow, how odd...

I let the bike continue to idle, and checked the oil level every 30 seconds or so. It slowly came back up, until it settled about 1/4" above the full mark.

I'm thinking is that when the bike starts up, the oil pump sucks oil out of the tank quickly, especially with all the new seals etc, causing a momentary drop in the tank level. There won't be much sitting in the oil pan to pump back to the tank until the oil has fully circulated, so once that happens, it begins filling the tank up again.

If that stands to reason, then it makes sense that during my last oil change I would have filled the tank with oil, ran it a minute, checked the level and found it to be low, then drastically overfilled it, and didn't bother to check it again afterward. The proper thing to do would be add 2.5-3L of oil, bring it up to temperature, and then fill up to the 'full' mark.

It looks like I overfilled the tank by over a litre of oil. So weird, I've done many oil changes on this bike and never had an issue till this last one. Very curious. The bottom end on my motor was swapped out in fall though, and thats when I started with the strange oil pressure and oil level issues, which is why I rebuilt the pump.

[Nic]

With the filter drained and changed my K2 takes just under 3 litres. I always check  oil level when I get home from a good long ride.

[markreimer]

Good to know, thanks!

I just rode home after washing the oily tire down.

I left with the oil around 1/4" above full.

Arrived at home, opened the cap and the tank was very full, probably 1" from the top. ?

Fired it up to see if the oil was circulating nicely. It slowly went back down to 1/4" from the top. I'm wondering if it's possible that the pump is just pumping too much back into the tank or something? I mean it should be limited by how much is available in the pan to pump back, right? Maybe I'm barking up the wrong tree. It's just so odd how much it's changing on me.

[ekpent]

 It did not overflow again though correct ?   I just posted a thread about my oil tank overflowing on my recently resurrected basketcase Blackie. The breather hose was completely blocked coming off the valve cover causing the oil tank level to rise and pressurize and spew oil out the tank breather,separator hose like your did. Did not do it at idle just under a good hard load. Other things that can cause that to happen are bad or broken piston rings and maybe the vent at the lower crankcase back. Not saying any of it is related to your problem but the next time your gas tank is off may want to run some good compressed air down that breather hose from the valve cover to see if its partially blocked.

[PlungerWaffle]

I know you dont want this one but could something be wrong with the pump? If it has been rebuilt recently then perhaps something got put together or adjusted wrong.

[markreimer]

No it did not overflow or spit any new oil on the back tire. Other than the level changing, everything seemed to be working fine.

So the breather hose on the valve cover, which vents to the atmosphere, was blocked, pressurizing the crankcase, and that pressure pushed up the small hose leaving the back of the case into the tank? That's interesting, I'll pull those hoses tonight and check them.

Shouldn't be bad rings, I just replaced them with 2nd oversize rings, new valve guides/seats cut etc. The bike hauls, exhaust is clean looking, just the oil tank level. I'll look into that hose.


PlungerWaffle: who knows, certainly might be. The pump is really dead simple so I'm not sure what could be wrong that would cause this. It only goes together one way (ie couldn't end up swapping the two rotors accidentally or something) and the pressure gauge indicates that it's working properly. I do have a second pump handy, so if I can't figure it out after changing the oil, checking the vent tubes and confirming it's not fuel/carb related, I guess I'll swap the pump. Not really a huge hassle anyway.