Badgers Posted June 7, 2012 Posted June 7, 2012 Hi, This is my first post having just joined. Thanks for the warm welcome DirtyDT. I am in the process of buying a DT175MX 1981. It is claimed to be a stock bike with a 15T front and 49T rear sprocket. However with this setup it only does 50MPH at 7000 RPM (Claimed by dealer). By my reckoning this means it will only do about 57MPH on the redline. The dealer is in the process of fitting a 16T front sprocket as I intend to use it mainly on the road. However surely the top speed stated by yamaha of about 65MPH should be possible with the stock gearing of 15T and 49T with out redlining it? Even if the 16T sprocket cures the issue surely it is just masking a problem elsewhere? It has been advertised as fully restored mint condition and the only other things I can think of that would alter the gearing or present this problem would be. 1) incorrect internal gearbox ratios (not sure if this is even possible). 2) Smaller diameter rear tyre. (Looks right for the bike) 3) Speedo reading incorrect. It will probably read fast as most do but not slow surely! 4) I didn't have the nerve to ask the dealer if he had used sixth gear. I have ridden the bike but there was no opportunity to give it a handful so am being advised by the dealer. Any advice or comments would be much appreciated. Simon
Moderator Airhead Posted June 7, 2012 Moderator Posted June 7, 2012 they arent fast by any means, i have seen about 65 on mine but i'm far too sympathetic to keep doing that! cant remember what i have for a front sprocket though it's been on there for so long now!!
blackhat250 Posted June 7, 2012 Posted June 7, 2012 Simon" dealers back then stated top speeds that were optimistic , There was no radar speed traps to accuratly measure speed, The average saloon car barely did 80mph, , So 65mph from 17bhp. wasn"t bad ,,, Remember Adverts for the Z650 stating how fast it was, Well it was all media hype,
busyeddie Posted June 8, 2012 Posted June 8, 2012 I think I had a 16 on mine last time it was on the road and got an indicated 70mph running a quite tall trials tyre regularly. I am in the process of rebuilding it atm, and have noticed my new sprockets are 15 - 49. I will see what its like. 15 49 should see more than 55 mph, but as airhead said there not quick, nor would you sustain that speed if you cared about your bike.
Moderator DirtyDT Posted June 8, 2012 Moderator Posted June 8, 2012 Hi Simon, 1) incorrect internal gearbox ratios (not sure if this is even possible). Could this be some gears from an older model? a 125 or possibly a TY - Not sure if any of those would fit but there should be pleanty of gears around if you decided to strip the engine. 2) Smaller diameter rear tyre. (Looks right for the bike) The only real way of curing this is getting the hub re-spoked and rimmed. Getting a good second hand wheel is almost impossible. 3) Speedo reading incorrect. It will probably read fast as most do but not slow surely! Did the dealer mean that the mileage is incorrect? Different speedos or a little bit of clocking would do this and most dealers would state that unless they new the history of the bike. I wouldn't worry about it. NOS speedo come up on ebay. If it is the actual top speed, it is to be expected. They never were racing machines. 4) I didn't have the nerve to ask the dealer if he had used sixth gear. I am not saying a lot of dealers lie but...........................................................................what answer would you have liked to have heard?
Badgers Posted June 8, 2012 Author Posted June 8, 2012 Thanks for the replies. I appreciate the bike is never gonna be a rocket ship and to be honest when I test rode the bike it appeared to be fine. However the dealer rode it first as a demonstration before I did and he said it appeared to be too lower geared for road use. In the area we were in it was not possible to get it above 35mph. This was with a 14T front 49 rear. The dealers comment concerned me as I wanted a completely stock bike. He assured me that a 15T would sort it and I was happy that it was being returmed to stock gearing with the impression that this would realise a top speed of 65 (for short durations). After the change he reported a marginal improvement of 50 mph at 7000RPM. This equates to 7.14 MPH per 1000 RPM in top. This is 57 mph at the redline of 8000 RPM Which still seems a bit low to me. Simon
Moderator DirtyDT Posted June 8, 2012 Moderator Posted June 8, 2012 49/15 is right for the E,F and MX but it only produces 15BHP @ 7000 RPM. I posted a gearing calculator on another thread. Can you use that to see what should be happening? What is non stock with the rear wheel? My TS only has 23BHP
busyeddie Posted June 8, 2012 Posted June 8, 2012 I guess a Ty gear cluster would fit, as the engines are very similar. Can you check the engine and frame numbers match? And if so what prefix? Maybe the dealer was taking it easy and actually the tachometer over reads?
Badgers Posted June 8, 2012 Author Posted June 8, 2012 I haven't bought the bike yet so I dont know a lot of the info required for the gearing calculator and online info is sparse and often conflicting (I've seen specs claiming the bike has 15, 16 and 17BHP). The gearing issue has developed after I paid the deposit. I now don't know whether to back out or proceed! I bought through autotrader but it was also advertised on Ebay. http://www.ebay.co.uk/itm/Yamaha-DT-175MX-SOLD-SOLD-Fully-Restored-Bike-Mint-Condition-Classic-Investment-/140640919499?pt=UK_Motorcycles&hash=item20beda1fcb
Moderator DirtyDT Posted June 8, 2012 Moderator Posted June 8, 2012 It looks nice but for that kind of cash I would want it to be perfect. That is assuming that the dealer has got things right about the gearing.
Moderator Airhead Posted June 8, 2012 Moderator Posted June 8, 2012 yes thats an awful lot of money isnt it,
busyeddie Posted June 9, 2012 Posted June 9, 2012 It's an outrageous ammount of money. I know im tight, but id want a nice looking RD for that kind of money. Gadgets you can have my DT for £1500 and I promise you it will be more fun. Because you wont mind scratching it :-p
Moderator Airhead Posted June 10, 2012 Moderator Posted June 10, 2012 Badgers I did 70mph at the 8000rpm redline today 16/49 sprockets
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