Atv burns oil only sometimes?

[Seriousbird]

Recently swapped a good top end with a bad drive output onto a engine with a bad top end but with a good drive output.

Engine 1 (bad drive output), top end was perfect and always ran good for me.

Engine 2 (good drive output), top end was toast but it has a output.

I swapped engine 1's top end onto engine 2. Didn't take the rings off all i did was everything from the piston to the valve cover. Sleeve, cylinder, valve system, cam, whole top end. Now that engine 1 is on engine 2 it runs good. Doesn't burn any oil, SOMETIMES.. i went on a fairly decent ride without burning a drop of oil then at one point i noticed i was burning it like a steam train. Drove off a bit more and it was like nothing happened?? It all happened on flat land. Im not overfull on oil. Any suggestions? The rings were untouched and are in their original cylinder. They did come out of the cylinder at one point.

[Mech]

I'd suspect the rings. There's a technique to prove it but it takes a bit of practice and experimenting with an engine to get it to show itself. The exact revs and throttle varies and has to be found by trying different throttle opening, opening speed, revs. and the speed you close the throttle. It doesn't need a lot of revs.

Warm it up and then start gently revving it up and letting it go back to idle. Only give it about 1/8 to 1/4 throttle and open it sort of gently, don't snap it open. When you let it off close it right off by hand, don't just drop the throttle but back it off like you were riding. Keep doing that, revving it and then letting it off. Don't hold it at revs and don't let it stay idling, just continually be gently opening and closing the throttle. After a few revs it might start smoking, if it does then try slightly more or less throttle and revs to try and get it to smoke worse..  If the rings are good it won't smoke at all, but if the rings are worn or soft it will start pouring smoke out the back. I can get a motor that uses a little oil between oil changes, or smokes a puff occasionally, to fill a yard with smoke by doing that.  It doesn't do any damage, just shows up bad rings.

 

Edited July 13, 2023 by Mech

[Gwbarm]

I suspect that when you removed the rings you removed the gunk seal between the piston and rings from oil sludge buildup that was masking the symptoms of worn rings, I have seen this happen before, Just cleaning up an engine with chemical cleaner. Did you check that the end gap was in proper spec when you took them out.

[Seriousbird]

9 hours ago, Gwbarm said:

I suspect that when you removed the rings you removed the gunk seal between the piston and rings from oil sludge buildup that was masking the symptoms of worn rings, I have seen this happen before, Just cleaning up an engine with chemical cleaner. Did you check that the end gap was in proper spec when you took them out.

I didn't check any gap, no. I don't have any gauges but its probably a good time to get some. Thats a great theory as i don't remember it burning anything before. Its from 2009 so it definitely has some wear here and there 

[Mech]

I agree with Gw.. that or you damaged a ring getting it back together.

It could be a valve guide seal too, but if it smokes badly when it does it, it's probably a ring..  the test will confirm it.

[Seriousbird]

8 hours ago, Mech said:

I agree with Gw.. that or you damaged a ring getting it back together.

It could be a valve guide seal too, but if it smokes badly when it does it, it's probably a ring..  the test will confirm it.

I did try that test after i couldn't replicate it in neutral. its really unpredictable, but seems more common upon engine braking or when i let off at higher speeds in general 

[Mech]

Given the circumstances, being it started after the swap, we'd have to suspect the rings, but, with those running circumstances you just described(high revs and throttling off) it could be the valve seals, and, sometimes, valve guides come loose in the head and let oil leak down around them. If it's smoking and not getting a fouled plug it will be an exhaust valve, but the usual thing is the inlet which actually sucks the oil down.

Either way it sounds like another strip down.. Check the engine breather, confirm the oil level and viscosity. Try going through a process we use to bed rings in, which is to accelerate from a speed/revs low enough to load the motor up, and do it giving it about half throttle, when the revs build so it's spinning freely but not revving any more than about where you might change up in a normal sort of ride, throttle off hard and let it run down to the initial revs again before repeating the process. Do  it in one of the higher gear so you get some load on the engine and keep doing it for about at least two minutes.. You'll need a good place to do this. That beds rings in and is what we do in the very first run after an engine overhaul. It's also important not to let new or unsealed rings sit idling..  It can let them start sliding on a layer of oil and never bed.

[Gwbarm]

Letting off at higher speeds does sound like valve guide seals, maybe a combination, but you can check it out when you have it down again.

[Seriousbird]

8 hours ago, Mech said:

Given the circumstances, being it started after the swap, we'd have to suspect the rings, but, with those running circumstances you just described(high revs and throttling off) it could be the valve seals, and, sometimes, valve guides come loose in the head and let oil leak down around them. If it's smoking and not getting a fouled plug it will be an exhaust valve, but the usual thing is the inlet which actually sucks the oil down.

Either way it sounds like another strip down.. Check the engine breather, confirm the oil level and viscosity. Try going through a process we use to bed rings in, which is to accelerate from a speed/revs low enough to load the motor up, and do it giving it about half throttle, when the revs build so it's spinning freely but not revving any more than about where you might change up in a normal sort of ride, throttle off hard and let it run down to the initial revs again before repeating the process. Do  it in one of the higher gear so you get some load on the engine and keep doing it for about at least two minutes.. You'll need a good place to do this. That beds rings in and is what we do in the very first run after an engine overhaul. It's also important not to let new or unsealed rings sit idling..  It can let them start sliding on a layer of oil and never bed.

Ill have to try that later today, its a cvt transmission so would holding the brakes do justice for the higher gear?

[Mech]

The brakes will put a load on.. if you can't find a hill.

That process working to settle the rings in though is far from guaranteed. I'd try it, and try just riding it with as much load as I could for a bit while I see if it gets better. Oil's cheap and it would be nice to not have to redo what had been an ok engine. Low to  medium revs with a load is what you want, and avoid idling which can just let oi get under the rings while they are still floating on a layer of oil. I've saved motors people have done up and then bought to me after a hundred miles to ask about because they were smoking.. They'd started them up after the overhaul and let them sit idling while they were checking things, or, they'd been riding them like a nana(sorry and nanas out there).

It's important when we overhaul motors that we start them and start driving withing minutes.. like two minutes. I make sure there's no water leaks and everything is right before I start them, then start them and have a quick check the oil pressure is there and no water or oil leaks are happening(as if), then get in and start driving as I suggest. I drive for at least five minutes then pull over and switch off and have another check on the side of the road, then get in and continue doing that settling in process. If we are paying attention, we can feel and hear overhauled engines start to run smoother, with less vibrations, and getting better compression over the3 course of the first five miles or ten-fifteen minutes..  It makes a huge difference to the condition and life of an engine doing it right, or wrong. The first five minutes of running are crucially important.