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Old 04-16-2013, 03:26 PM   #1
Nath87 OP
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Joined: Feb 2012
Location: Bedford, NH
Oddometer: 47
Yamaha DT50 to 80 conversion

Hey guys, so I just got my girlfriends 88 dt50 going and she wants to use it to go trail riding with me. We live in NH and and its really the perfect bike for the riding we do up here(mostly class 6 roads) only problem is I would love if it had a little more power so I dont need to do so much waiting around. I'm thinking about boring it out to an 80 and putting a bigger jet in the carb anyone ever done this? is an 80 basically the same bike as a 50 with a bigger bore?
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Old 04-16-2013, 04:42 PM   #2
Roland44
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Joined: Mar 2013
Location: Alaska
Oddometer: 577
Quote:
Originally Posted by Nath87 View Post
Hey guys, so I just got my girlfriends 88 dt50 going and she wants to use it to go trail riding with me. We live in NH and and its really the perfect bike for the riding we do up here(mostly class 6 roads) only problem is I would love if it had a little more power so I dont need to do so much waiting around. I'm thinking about boring it out to an 80 and putting a bigger jet in the carb anyone ever done this? is an 80 basically the same bike as a 50 with a bigger bore?
It's probalbe possible but I will let the most expirience folks to answer this. I just want to see the bike, any pics?
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Old 04-16-2013, 04:48 PM   #3
JonW
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Joined: Apr 2009
Location: Sydney, Australia
Oddometer: 156
We used to do that sort of stuff all the time back in the day. Check out a few of the dutch moped sellers, they will have full kits for that including bigger exhausts, cylinders, pistons, gaskets, manifolds, airbox rubber upgrades etc etc

Autisa also did a big bore for the DT, not expensive if you can find one. Your issue with a rebore may be that you need to reline it anyway, may as well just get an already done cylinder.
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Old 04-16-2013, 05:10 PM   #4
cam14
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Joined: Jun 2010
Location: Texas
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Quote:
Originally Posted by Nath87 View Post
....is an 80 basically the same bike as a 50 with a bigger bore?
Did a little internet research for you. The answer is "no". The DT 50 has a bore and stroke of 1.59 x 1.54 inches making 2.99 CID.

The DT 80 has a bore a stroke of 1.93 x 1.63 inches making 4.83 CID.

Therefore, just added a larger bore will not get you what you really want. What would be interesting to find out is if you can swap the whole jug and piston from the 80 and bolt it to the 50 using the stock connecting rod. I did not do the math but should work out to close to 70 cc or so. I do not actually know if that would work and now that I've thought about it for a second, would require shaving the jug down to accomidate the shorter stroke. Also wonder if the 80 engine will fit in the 50 frame? All in all, sounds like more trouble than it is worth but that is up to you.
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Old 04-16-2013, 11:13 PM   #5
JonW
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Joined: Apr 2009
Location: Sydney, Australia
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The DT80 was just a DT50 with a bigger engine and i think 5 vs 6 gears, but that depended on the market. the DTMX versions were aircooled IIRC and were the same bike, but there could be different models and different motors over the years and markets. Either way a DT is a rare beast now... will you locate a Paris Dakar kit for it? very retro cool!
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Thanks to all who have supported the Omega PloProf book at DeskDivers.com - it's very much appreciated
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Old 04-17-2013, 07:18 AM   #6
Nath87 OP
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Joined: Feb 2012
Location: Bedford, NH
Oddometer: 47
Quote:
Originally Posted by cam14 View Post
Did a little internet research for you. The answer is "no". The DT 50 has a bore and stroke of 1.59 x 1.54 inches making 2.99 CID.

The DT 80 has a bore a stroke of 1.93 x 1.63 inches making 4.83 CID.

Therefore, just added a larger bore will not get you what you really want. What would be interesting to find out is if you can swap the whole jug and piston from the 80 and bolt it to the 50 using the stock connecting rod. I did not do the math but should work out to close to 70 cc or so. I do not actually know if that would work and now that I've thought about it for a second, would require shaving the jug down to accomidate the shorter stroke. Also wonder if the 80 engine will fit in the 50 frame? All in all, sounds like more trouble than it is worth but that is up to you.
Cam14, thanks for looking that up. I tried finding the stroke but couldn't. I can defiantly bore out the 50 or swap in a 80 top end and maybe even a 80 bottom end, if it will fit not sure how to find that out. my question now tho is, would that even be worth it? is the 50 carb going to be able to supply the extra air flow and if i bored the cylinder rather than swap it could the reeds/ exhaust port handle that extra displacement? Ive never made these big of changes to an engine so any advice is appreciated.
P.S. I love the Idea of putting a PD kit on this bike but I cant even find a good picture of one, never mind one to buy.
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Old 04-17-2013, 05:53 PM   #7
nedc
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Joined: Apr 2010
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Here's mine:


I have about $200 in it and it's a fun little bike. I looked into the big bore stuff when i first got it. Everything has to come from the UK and you would probably spend $400 or more to get it to 70cc. I decided it wasn't worth it. I can wheelie mine and i weigh 200 lbs. If i weighed 120 or so I could fly on this thing. Just keep the revs up and it is surprisingly strong for a 50cc.
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