Hey all, Im new to the forum so I'm not to sure how this works. I have a question on my 02 Big Bear 400. When I first bought the bike it would burn some oil. I took it apart, did a top end rebuild on it with no problems. A few months later it started burning oil, but only on startup. So I took the top end apart again and brought the head and jug back to the machine shop, got it bored along with new piston and rings. Installed everything back and after 9 miles it started to smoke on startup after it sat overnight. I ordered a new cylinder jug along with piston and rings again. Installed everything and the same thing happened again. I went to my local Yamaha dealer and talked to the mechanic who informed me that my cylinder head had to have a crack in it. I ordered a used cylinder head that had been gone through. I just installed it 3 days ago and now with 7 miles on it, smokes on startup after sitting overnight. I have done many rebuilds before with this one being the only one I have ever took apart more than once. Every time I tore it apart there would be a puddle of oil on top of the piston and Idk where it is coming from. Everytime I tore it back apart rings were all separated according to Yamaha spec. Please any info would be greatly appreciated. Thanks

 

I'd pull the cylinder off and check first for as broken oil ring. Next I'd check the clearance between the piston skirt and the base of the cylinder against the specs in your manual. If this is the second head(with new valve guide seals?) I'd bet too much clearance was allowed on the bore job. Don't have the specs, but if like Polaris they should be fairly tight on bore jobs.Polaris shows .0006 -.00181 (.015 - .045 mm) normal piston to cylinder clearance with .0024 as the service limit of the piston on the 400 and 500 engines.

 

Thanks for the reply. I actually had the machine shop do this the last time I had it apart and he informed me everything looked fine. He was stumped at this saying he couldn't find anything wrong. Obviously something isn't right somewhere. I think that I've had this machine apart 5 times and still having the same problem. Its been to 2 different machine shops that say nothing is wrong. I don't understand pooling of oil on top of piston. Yes new valve seals

 

The oil has to be running downwards into the cylinder, when going, it burns away so all oil is coming from above the cylinder. With two different heads it is unlikely to be from the head (but possible, particularly if the oilway back down is blocked).

Yamahas have a piece of rubber tube in a stud hole, fitted rather like a ring dowel between the head and block. Presumably this is for the oil. Have you lost it?

 

Merryman, here is what I know about the oil and feel free to correct me if I'm wrong cause I could be. I was informed that the right rear head bolt is where the oil enters up in the head. I was told that the oil drains back from the holes on the side of the head where the cam sprocket is. Does this sound correct? That right rear head bolt has the ring/dowel there where I can see a hole where the oil comes up. Now that ring has an oring around it which is there and has been replaced every time. I know the head is getting oil cause there is a screw on the head to bleed/check oil consumption in the head. Two days ago I drove the atv for 15 minutes or so and the next day I went to fire it up and there was a cloud of blue smoke and then cleared up after a minutes or so. Today I went to fire it up and there was no blue smoke at all and I let it idle for a few minutes. It seems that I get a cloud of blue smoke the next day on startup after I ride it the day before. If I just start it without riding seems like it doesn't do it? Make any sense to anyone? Any suggestions would be appreciated.

 

My apologies, it is the 350 that has a tube, the O-ring does the same job on the 400. The symptoms seem like valve stem seals but, with two different heads, and I am sure you must have fitted new seals anyway, I can’t see it being them. Could a breather be routed wrongly, feeding condensed oil vapour directly into the carb? Though, with a new piston and rings, there should not be much vapour. Sorry I can’t be of more help, you may have to live with the problem.

 

when you replaced the head, did you replace the valve seals? as the previous post said, it sounds like valve seals leaking down into the combustion chamber. If you let your machine sit for a while, meaning several months, the seals can dry out and leak small amounts of oil . It doesn't take much oil to produce a fair cloud of smoke. If you replaced the head with a used unit, it could also have sat for a while.

 

So today after not starting the atv for over a week, I started it with a big cloud of smoke that filled a 28x28 garage. It cleared up after a minute or so. Is everyone thinking valve seals? I definitely know that the oil has got to be coming from the head. It only does it after sitting for a day or longer. I'm just curious if that is too much smoke for it to be valve seals? Any suggestions woul be great. Is there any way that I can tell its the valve seals without taking the head off the bike?
Thanks all!

 

I replaced valve seals and had the guides checked on the atv. Put everything back together and still getting smoke on startup for a minute or so. Today I tore apart the whole top end again and found all the rings to be within spec. I did notice that on the sides of the ring that some of it was worn down to all silver while some was only worn halfway silver and halfway black coating. Is this a cause for concern? Maybe the break in wasn't done properly? Any help would be appreciated!
Thanks

 

I'd go back as I mentioned on checking the piston to cylinder clearance. If the rings are worn irregular around the piston usually that means too much clearance was allowed. This engine if like the Polaris 330 or even the Sportsman 500 has a piston to cylinder clearance of .0015-.003 max on the 330, .0006-.0018 max on the Sportsman 500. .0024 is considered worn out on the 500! Out of round cylinder allowance is .002 on both models.This may be the reason oil is accumulating on top of the piston. Excessive oil blow by on the piston and rings. Service manual should give you the specs on clearances so you can check them against your piston/cylinder readings.