hgas Posted April 6, 2019 Report Share Posted April 6, 2019 Just picked up a 2016 300 evo I’ve heard stories of the main bearings going , any idea how much use or life they’ve got before they give up , there’s definitely a bit of a ‘whirring ‘ sound coming from the engine which I’m told is a sign of them on their way out but the bike I’ve bought is in very condition and not had hard use can anyone shed some light on it ? Quote Link to comment Share on other sites More sharing options...
lineaway Posted April 6, 2019 Report Share Posted April 6, 2019 I have a `16 that certainly has some time on it. I had not even thought of mains yet, but it could happen soon. I had a 2010 that I had to put mains in just before I bought the `16. Somewhere along the way the newer bikes come with the bearing shields left in the bearing. So the original factory grease is supposed to lube the bearing for life. Ha, that is not going to happen. That is why you run two stroke oil. There are plenty of 40 year old two strokes running around on the original mains. Dumb idea thinking they could make bearings better by not letting two stroke mix lube them. 1 Quote Link to comment Share on other sites More sharing options...
feetupfun Posted April 6, 2019 Report Share Posted April 6, 2019 There is a reason why they leave the shields in. In the 1970s the Japanese two stroke trials engines had a ventilation system for the main bearings. There was a little port leading from the mouth of the crankcase to the space between the bearing and the crank seal (on both sides usually). Suzuki is an exception with their fancy CCI system on the drive side bearing. The Spanish trials bikes of the 1970s don't have that ventilation system. Guess which bikes the mains lasted for 40 years and which ones didn't last that long at all. Anyway, the bearing ventilation idea was ditched when brands like Sherco decided that a super-slim engine was worth sacrificing main bearing life for (the ventilation ports took up valuable space between the bearings and the seals). The first Shercos had a reputation for the main bearings lasting only a short time (one season) and this didn't go down well with owners so they tested the idea of leaving the bearing shields in place and found it gave a longer life to the mains. I'm fairly familiar with 1970s Bultaco motors (which have no main bearing ventilation) and from what I've seen when pulling them apart to replace the main bearings (which is every Bultaco I've ever owned), the bearings appear to have failed due to the balls and races having to continually squish little bits of carbon. 1 Quote Link to comment Share on other sites More sharing options...
feetupfun Posted April 6, 2019 Report Share Posted April 6, 2019 1 hour ago, hgas said: Just picked up a 2016 300 evo I’ve heard stories of the main bearings going , any idea how much use or life they’ve got before they give up , there’s definitely a bit of a ‘whirring ‘ sound coming from the engine which I’m told is a sign of them on their way out but the bike I’ve bought is in very condition and not had hard use can anyone shed some light on it ? Betas make a whirring noise even when everything is working perfectly because they have straight-cut primary drive gears. My 1976 KT250 also has straight cut primary gears and also makes that great sound. When your mains fail if there is any noise it is a rumble. Beta mains usually last for many years of normal trials use. Quote Link to comment Share on other sites More sharing options...
lineaway Posted April 6, 2019 Report Share Posted April 6, 2019 And all this goes back to what fuel and oil you burn and how clean your intake stays. I have seen hundreds of cranks through the years and not all oils leave the lower end nice and neat. Quote Link to comment Share on other sites More sharing options...
hgas Posted April 6, 2019 Author Report Share Posted April 6, 2019 Ok so it sounds like it should be ok , the bike isn’t running as well as it should do I’m going to give it a bit of a strip down make sure the carb is all clean and then drain the oil and put some decent oil in , I’ve asked on another thread but does anyone know optimum jetting sizes for the 300 ? Quote Link to comment Share on other sites More sharing options...
dan williams Posted April 6, 2019 Report Share Posted April 6, 2019 I’ve only had to have main bearings done once on a bike that sat in storage for the better part of a year. Just sitting did more damage than regular riding. When the Beta mains go there’ll be no question as it’ll sound like a rock crusher. The whirring noise is on every Beta I’ve ever owned and after a while you just tune it out. Quote Link to comment Share on other sites More sharing options...
markparrish Posted April 7, 2019 Report Share Posted April 7, 2019 Yes - whirring Beta good, grinding and rumbling Beta bad! I replaced my mains because they were badly worn (and also causing it to be difficult to start) and just yesterday I also replaced cylinder and piston - which was cause of piston slap. Now whirring nicely and running very smoothly! 2 Quote Link to comment Share on other sites More sharing options...
hgas Posted April 8, 2019 Author Report Share Posted April 8, 2019 Thanks for your help everyone you’ve definitely put my mind at ease I can just get on it and enjoy it now , one thing I will say is I stripped the bike down last weekend and was very impressed at how the beta is put together , very solid and no flimsy bits 1 Quote Link to comment Share on other sites More sharing options...
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