NE0 Posted February 12, 2011 Posted February 12, 2011 Hi guys, I’m up at the bar looking into the bottom of my empty glass! Let me buy you a drink, and tell you the problem with my DT175MX I’ve only recently purchased it to go with my CB400/4. see both bikes here It’s a 1978 registered early 79. Its got 19,300 on the clock, seems genuine as it came with MOTs dating back past 20 odd years. It had a rebore at 14,300 including 2 main bearings (not the big end). In the past week or so I’ve noticed it has a rumble under load, it’s a vibration which travels through the foot pegs, take the revs off and the rumble disappears, go back up through the gears and it becomes more noticeable. I’ve only had it a few weeks so in all the excitement I may have not noticed the rumble, but as I’ve ridden it more its certainly there! My thoughts are is it’s the big end….... So what will you have?
Moderator Cynic Posted February 12, 2011 Moderator Posted February 12, 2011 Hi guys, I’m up at the bar looking into the bottom of my empty glass! Let me buy you a drink, and tell you the problem with my DT175MX I’ve only recently purchased it to go with my CB400/4. see both bikes here It’s a 1978 registered early 79. Its got 19,300 on the clock, seems genuine as it came with MOTs dating back past 20 odd years. It had a rebore at 14,300 including 2 main bearings (not the big end). In the past week or so I’ve noticed it has a rumble under load, it’s a vibration which travels through the foot pegs, take the revs off and the rumble disappears, go back up through the gears and it becomes more noticeable. I’ve only had it a few weeks so in all the excitement I may have not noticed the rumble, but as I’ve ridden it more its certainly there! My thoughts are is it’s the big end….... So what will you have? Check the chain and sprockets, especially the spring loaded tensioner as they sieze and the chain goes tight when you sit on the bike. Secondly check the engine mounting bolts are tight. Last time you checked the plug, etc Finally, is it when the revs come off or just the amount of throttle.
Moderator Airhead Posted February 12, 2011 Moderator Posted February 12, 2011 I would check the ignition timing properly and by the prescribed method if possible, before pulling the engine to bits. Anyhow my bike has 38000 on it and I dont have big end problems yet (touch wood)
NE0 Posted February 12, 2011 Author Posted February 12, 2011 Thanks for the quick replies guys, Neither of you wanted a pint then? I'm not over keen on taking it off the road just yet to rebuild the engine having only just got the bike. So I won't be stripping it down in a hurry and even more so when I don't need to.. I will check those items tomorrow and get back to you all, (providing I can get the time!) some more details for you... Cynic, it is only noticeable on throttling or cruising, soon as you throttle off and remove the load on the engine the vibration stops. On tick over it sounds just like any other stroker. I did wonder if it was speed related so i took it out this afternoon, and no its not. You can ride at 20 or 40 an its only noticeable when the engine is throttled up!! (Gee isn't that what happened to the shuttle Challenger? you are clear for throttle up....Kaboooom) cut the revs and it goes. Its quite a harsh vibration which you feel through the pegs first and then it feels like it makes the frame vibrate. Don't get me wrong its not shaking, its a rumble vibration. I did take out the plug a few days a go just to check , it was not oiled up but a nice brown. Its not difficult to start, one or two kicks, but certainly harder if the engines hot. If any of this helps? Cheers...don't forget your pints guys....
Moderator Airhead Posted February 12, 2011 Moderator Posted February 12, 2011 Another thing worth considering is the piston. i have taken measurements of the weight of cheap pattern pistons against genuine ones and found them to be heavier, this could lead to increasing vibration with increasing engine speed. Consequently i would only fit quality (mitaka) pistons or the genuine Yamaha ones. Mitaka can be bought from PJME.
YPVS TONE Posted February 13, 2011 Posted February 13, 2011 It's also worth checking the engine mounting bolts are tight and as cynic says make sure the chain and sprockets are in good condition. On the DT125MX I had I found the best way to get the right chain tension was to sit on the bike,hold down the tensioner and make sure you have some freeplay in the chain. You say that the mains have been done but you can check the LH side one at least by taking off the LH engine cover and put both hands around the flywheel and make sure you have no up and down movement.If this is OK you may have to take the head and barrel of and check the big end. Tony.
NE0 Posted February 14, 2011 Author Posted February 14, 2011 Well Guys you certainly know your stuff. My bikes cured!!! All three engine mounts were loose, the back top one was only finger tight! the other two were not as bad, all three have now been torqued up. Plus the swinging arm nut was not as tight as it could be, nearly a full turn before the torque wrenched 'clicked' I guess the clues were there in my intro , the frame had recently been stove enamelled, like past 6 months and clearly whoever did the work didn't tighten it all back up again. this prompted me to check all the other bolts , one bottom fork nut was also loose! All in all I'd say the vibration was 95% gone. It's not as smooth as my 400/4 so my expectations are it bit misguided! I expect you get a slight vibration from a stroker (trouble is its been 34 years since I was 16 and riding them!) But I'm a happy man again! so the drinks are on me , what will you be having guys?
Moderator Cynic Posted February 14, 2011 Moderator Posted February 14, 2011 Well Guys you certainly know your stuff. My bikes cured!!! All three engine mounts were loose, the back top one was only finger tight! the other two were not as bad, all three have now been torqued up. Plus the swinging arm nut was not as tight as it could be, nearly a full turn before the torque wrenched 'clicked' I guess the clues were there in my intro , the frame had recently been stove enamelled, like past 6 months and clearly whoever did the work didn't tighten it all back up again. this prompted me to check all the other bolts , one bottom fork nut was also loose! All in all I'd say the vibration was 95% gone. It's not as smooth as my 400/4 so my expectations are it bit misguided! I expect you get a slight vibration from a stroker (trouble is its been 34 years since I was 16 and riding them!) But I'm a happy man again! so the drinks are on me , what will you be having guys? If the DT and your 400/4 are ever and i mean ever comparable on vibes the engine in the 400 is gonna blow. Singles always vib a bit, esp little strokers as there is no balancer . A buzz through the pegs and the bars is to be expected but if it sends your toes numb you need to investigate. Unfortunately you live in the real world , if you fancy the squires weekend we could all have a drink.
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