Vintage Dirt Bike Q & A

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PostPosted: Fri Dec 07, 2012 12:37 am 
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Joined: Thu Dec 06, 2012 11:17 am
Posts: 4
This is my first post here and I just found the site today, so haven't had much time to look around or figure out the search function. I've got 3 hobbies, or more like obsessions. First is old muscle cars from the 60's. Second is vintage racing outboard motors, the big ones. Third is vintage bikes and dirtbikes. Unfortunately I have a lot more depth and quantity in the cars and outboards than in the dirtbikes.

My first bike was a Yamaha DT1E which I bought new in 1971. Later I bought a MX cyl, pipe, 21" wheel and other stuff, basically converting it to an MX except for the close ratio transmission. A year or so later I cut the frame on it, adding the 2 1/2 or 3" extensions, dropping the engine cradle which lowered the center of gravity and also gave the swingarm better geometry. I also messed with the porting some and ran various heads and a bigger carb. '73 rolled around and I bought a new 250MX. In 74 I had a chance to buy the ONE 250YZ that the dealer was able to get and not long after was able to buy a second one that a mechanic for another dealer had bought for his own use and couldn't make the first payment on when it came due. In '75 I got the only YZ250 monoshock that the local dealer was able to buy and buy now I was doing all the port work for the dealer's customers who were racing the bikes. I was a pretty good wrench and my stuff was fast, but I never had the stamina to be a decent motorcross racer. My stuff did really well in the dirt drags they held at the track on motocross days and I ended up sticking with that.

Those bikes are all long gone, BUT...................... several years ago I ran across another '75 YZ 250 mono, kinda in pieces, but thinking it was my old one I bought it. It wasn't, but I started a resto anyway and it rekindled my interest. Since then I've accumulated a 73 250 enduro, a pair of SC500's a '78 RM400 Suzuki and a 90's 250R. Years of searching for my '74 & '75 YZ's have been fruitless. I've always wanted a true '70 or '71 360MX and look around on E Bay fairlly frequently.

Last week I saw on E Bay what appeared to be a 70 or 71 360MX engine, although it was not advertised that way. It definately had the MX cyl, with the fins removed on the right side for clearance on the expansion chamber. It is factory, and not hacked off with tools as I've seen of others. The seller also had the frame with number matchin the engine, and a few other MX parts, but some old enduro stuff mixed in too that obviously came from another bike.

So after all that, here's the question: Is there a way to tell for sure if this was actually a factory RT1MX by the serial number, or merely an enduro that somebody stuck an MX jug on years ago. Another thing that seems odd........... even though the engine and frame numbers match, there is a switchkey bracket on the left of the frame that is below the left front side of the tank. My '71 DT1E had the switch up between the tach and speedo, but this one had a round swingarm, so I do not believe it was pre '71 unless somebody swapped it out. It also had a 21 inch front wheel with it, although that too could have been from another bike. It does have a 71 and older style front brake hub, unlike my newer stuff that's conical.

Sorry for the long ramble, but trying to determine if this is worth a full blown parts search and resto effort to complete as a factory MX.


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PostPosted: Thu Dec 13, 2012 1:25 pm 
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Joined: Tue Oct 16, 2012 1:35 am
Posts: 1
First post for me I have this bike and the engine and frame numbers should start with 365 and be matching (if that bothers you?) MX came out for 2 years 73 and 74

Anyway here you go

http://dirtbike.off-road.com/dirtbike/r ... 21792.html


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PostPosted: Fri Dec 14, 2012 8:09 am 
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Joined: Thu Dec 06, 2012 11:17 am
Posts: 4
chamber66 wrote:
First post for me I have this bike and the engine and frame numbers should start with 365 and be matching (if that bothers you?) MX came out for 2 years 73 and 74

Anyway here you go

http://dirtbike.off-road.com/dirtbike/r ... 21792.html


Actually the 125, 250 and 360 versions of MX bikes from the factory came in 1970 and were designated 125M, 250M and 360M and picked up the X in 1971. I'm not familiar with the early 125 MX prior to the '74 125YZ but the 70 and 71 MX's were basically factory gyt kit'd enduros, sharing the same frame, tanks, and other major components. They came from the factory with no lights, full knobbies, 21" front wheel, number plates, and other subtile differences from the Enduro. '72 was the first year that the MX was totally different from the Enduro.

This is the most comprehensive list of Yamaha Enduro and MX serial numbers I've ever seen: http://www.yamahaenduro.com/showthread.php?t=7844

Here is a factory 1971 360MX. The 70 model would look almost identical with the exception of yellow stripes instead of red and steel lower fork tubes instead of aluminum. The front fork brace and Webco cylinder head are aftermarket. This is identical to the bike I am restoring with the numbers matching, although I am missing some components including the pipe, seat and both tanks.

Image

This is a factory '71 250MX. It is cosmetically identical to the '70 model with the exception of the front fork lowers.

Image


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PostPosted: Fri Dec 14, 2012 8:30 pm 
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Joined: Mon Dec 10, 2012 7:27 pm
Posts: 30
I remember when those bikes came out. Those two examples are much cleaner than the factory supplied. Cool bikes.


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