2005 Bruin 350cc Oil pumping into airbox

[BruceG]

Hi guys. I have a problem where the quad will start and run well for about 5 minutes and then it dies. On inspection the airbox has oil in it (quite a lot). The oil is pumping through the cylinder head breather hose. This problem first happened suddenly. I have since done the top end including, piston, gaskets, rings and valves. I have also done the carb (not that I thought that this was the problem). The compression seems normal. The only thing that I can think of, is that I did not grind the valves well enough. Anyone experienced this? If so, how did you fix it? Any other thoughts?

[Mech]

The most common reason for that oil in the airbox, and especially if it starts doing it suddenly, is that the sump is full of fuel. A sump full of fuel would also cause the engine dieng once it starts to get hot. I'd suggest checking the oil level as the first thing.

A lot of bikes also have restrictor valves or orifices in their breather hoses, and in some models if that restrictor blocks up it can cause problems like the oil in the airbox.

[BruceG]

Thank you for your insight. I will check on the oil in the sump when I get a chance. 

[Kcommins]

Check the oil level first as the other post said if over filled or if fuel leaks into the bottom end this can happen. 
from my own experience with my kfx700 after I did a big bore kit hi performance cams and a rev box this problem all of a sudden showed up. This was due to excessive air movement inside the cylinder bottom end and there was not a breather box to catch any excess oil that will blow out with the moving air. 
to fix this problem I got a breather box from a Yamaha raptor and installed this in line with the breather hose then it also has a drain and I had to use a extra unused port on the other cylinder for the oil to drain back to the lower end. 

[Gwbarm]

That sounds like piston ring blowby, how long since you did your top end job. If you are using synthetic oil it takes longer for the new rings to seat.

[BruceG]

Thank you for this info. The Quad is at my holiday home, so cannot check oil levels etc at the moment. It was not overfilled as the bike was not using oil and therefore had not been topped up in a while. However, it is possible that fuel got into the oil and I will check this. I have just done the top end so it is possible that the rings are not yet seated properly. If this is the case, I am not sure how to fix this as the bike will only run for a minute or two before dying. I will check the breather pipes for blockages etc. 

[Mech]

It had the problem before the overhaul, so it's probably not blow by or bad rings. You'd have to fit the wrong size rings, or break a top ring, to get enough blow-by to cause the filter housing to get oil in it.

And, if it starts easily, it's got enough compression. So.. blow-by, by itself, shouldn't be a problem once/if you get the engine running. Neither blow-by, nor oil in the filter housing should cause the engine to stop after only a few seconds. If the air-filter was so full of oil that it choked the engine, then it probably wouldn't start easy, and it wouldn't start easily after the oily filter had flooded it of fouled the plug. If the oil in the filter housing, and the cutting out, are related, it will be fuel in the oil.

There is a chance though that the two are not related. If you have a timing light you could connect that up and watch it to see if you are loosing spark. If you don't have a timing light you can fit a plug to the plug lead, and then rest that against the fitted spark plug so that the spark has to jump the first gap to fire the second plug. If the spark is good it will do that jump and the bike will run, and if the spark dies you can see it on the first/external plug.

 

[Gwbarm]

Agreed! None of these problems should cause it to only run for a few seconds and quit.

[BruceG]

I changed the oil and the problem went away. How does fuel get into the oil?

[Gwbarm]

That should not have fixed anything but diluted oil, it should still run with gas in the oil just doesnt lubricate well and toast your engine.  Generally gas gets into the crankcase when the needle and seat is bad and lets gas seep by and into the crankcase.

[Mech]

The fuel starts evaporating out of the oil once it gets just slightly heated and the fuel filled fumes drowns the engine... just like flooding.  Seen it plenty of times. At the same time the thin oil, carried with this excessive amount of fuming, gets past the usual oil/air separation systems and fills the air-box with oily fumes which separate out in the box.

The float needle allowed the sump fill, but may not be the cause.

Float needles can seal plenty well enough for maintaining the fuel level during a days use, but let enough leak through to flood an engine over night if the fuel tap doesn't get turned off. If the needle valve worked perfectly it wouldn't happen, but a lot of needle valves that seem to work just fine during the day can fill the sump if the tap leaks.

I'd check the fuel tap because that test is so simple.. Turn it off, pull the hose and watch for a drip. I'd open and close it a few times checking it closed every time. If the tap is working ok then I'd check how the bike runs and consider cleaning the carb, but I'd check whether the float level seemed to be ok and was ok during a days use.. It is possible that someone forgot to turn the tap off one day, or, the tank breather didn't work ad pressurised the tank and caused the problem.

 

 

Edited May 3, 2024 by Mech

[Gwbarm]

Good call Mech! I have never seen this happen but generally the first thing i do is check the oil and if its over full or smells gassy or feels just not oily, change it immediately. Glad you got that sorted out.

[BruceG]

Really great advise and info. It all makes absolute sense. I did have a problem with the float needle in the beginning. I replaced it and reconditioned the carb. This is also when I did the top end. Thanks guys

[spock58]

It doesn't hurt to ensure that the fuel tank is venting properly, if it doesn't then the pressure can cause carb flooding. Some Yamaha's have gas caps that use ball bearings in them to prevent fuel loss if the machine tips over. These can get stuck and create problems. I'm not sure if these type of caps are used on their quads but I've found them on the off road bikes. The caps can be taken apart and the steel balls removed.