2005 Yamaha Bruin Burning Oil and Bogging.

[Evil MD]

Hello out there,

I recently picked-up a 2005 Yamaha Bruin 250 for the wife. When I went to check it out, it started on first pull, idled nicely, drove it around, essentially no problem. The previous owner had recently put in a new Chinese made carb, and all seemed well. I got the Bruin home, unloaded it, and fired it up. Started to drive it around and noted that it would idle fine, but would bog and whitish smoke would come out of the exhaust when throttle was applied. Did some research online and there was talk of possibly a stuck or bad ring, bad oil, or leaking head gasket. Recommendations were to first change out the oil before proceeding further. I changed out the oil and put in 10W-40 synthetic with 20% Marvel Mystery oil ( was recommended on a couple of sites, but not sure if this was the best idea). The old oil in the machine was thick and black, obviously not having been changed in a long time. I also put is a new plug. After starting up the machine, it idles fine with no smoke. I can even drive it around in first gear at low RPMs with no smoke or bogging. If I increase to about 1/2 throttle or shift into second gear, it starts to smoke (whitish), bogs and dies. I've check the plug, and there appears to be black oil residue on it. I am not sure where to start, as I am not sure if it is one issue or a combination. I've seen four strokes drive around burning oil and not bogging. There appears to be some correlation between the bogging and the smoking. I am not sure where to go with this next. Do I swap out the oil with the Marvel Mystery oil in it for pure synthetic? Do I try putting in 20W-50 to see if this resolves the issue? I half question if the reason that the ATV drove fine when I tested it was that the old oil was so thick and dirty that it could not leak past the rings. Or am I looking at performing a top end rebuild with new gaskets and rings? Total frustration here with a new machine to me. Any thoughts would be appreciated. Thanks.

[Ajmboy]

Is the air filter clean? The carb may need to be adjusted but worst case, it’s consuming oil. Any difference if you adjust the choke?

[Evil MD]

I looked at the air filter and thought it looked fairly clean, but to help clarify if the air filter was a problem, I took the ATV out for a spin without the air filter. Obviously this was part of my issue, as now at least the ATV does not die with acceleration and I was able to upshift through all the gears without dying. Overall, about 70% improvement. Thus said, it starts to bog at higher RPMs, and again, this was related to a white smoke coming out of the exhaust. I think my next step is to change the oil again and put some good stuff in without the Marvel Mystery oil and then try tuning the carb. If once I get the new oil in and have the carb tuned, and there continues to be white smoke, I think the next step will have to be a top end job.