Dirt Rider . Net Text Version Home
Pages: 1
MX400 airbox
(Click here to view the original thread with full colors/images)
Posted by: Colorado
Hey all!
Do I have to drop the swing-arm off the monoshock to remove the double sided airbox of a '75 Yamaha 400MX? It sure looks like the only way to get the box through the frame, but strikes me as a strange design quirk. Or am I missing something?
Posted by: Colorado
I'm not getting any responses, so have been looking elsewhere. Now I'm even more confused. Pictures of a '75 MX400 usually show a dual shock design, and a 'curvey, rounded' tank --- silver paint.
My bike is supposedly a '75 MX with a '75 DT400 motor (newly rebuilt). But it IS a monoshock! It's a more primative monoshock than my '77 and '78 IT400's, which have the shock inside of the frame. The MX/DT cross has a more traditional top frame tube, which the monoshock mounts beneath. The swingarm is built from heavy, solid metal flat stock, instead of tubing. The forks are old school, with the axle under the tubes instead of on the front of them. The gas tank is aluminum, with a more 'coffin' shape. The rear brake backer plate has a strut benetath the swingarm instead of the receiver slot for a lug inside the sawingarm that the '77/'78 have.
The airbox, which is my current dilemna, mounts to a carb which comes straight off the back of the cylinder and intake (my newer 400's have the carb angle off to the left) through a short (2 1/2") rubber coupler. It (the airbox) is a T design, with two boxes and filters, one to each side of the bike, both feeding back into the 'cross' part of the T.
Do we have any Yamaha nuts who recognize any of this, and can help me out? I know the MX400 was very popular in it's last year, before the YZ completely displaced it. Was there more than one version in '75? Do I actually have some other mid seventis Yamaha with a DT400 motor stuffed in?
Any help would ... well ... help!
Posted by: zoommx
Sounds like you have a 75. I recently got one from a friend. Needs some work though. Plus I haven't done any work on it yet, so I wouldn't know about the air box removal. It does have 2 air filters though.
You can go to yamaha-motor.com to compare what your parts look like compared to other bikes/years. The mx400 has 510 as the first 3 numbers of serial number.
Good luck, Roger....movmx.com #491
Posted by: Colorado
Thanks Roger.
The serial number begins 510, so maybe the monoshock came out later in the '75 model year. I decided not to pull the airbox. I was ony going to in order to clean and paint (quick paint, with a spray can) the frame. For some reason the last owner put the rebuilt DT400 motor, all painted and pretty, into a ratty old MX400 frame that must have been sitting behind someone's garage for years (except that the seat is perfect, which doesn't match with the stored outside look of everything else) without even washing the bike first. He sprayed camo paint on the tank and fenders over red paint someone before him did with a brush. It really looked like hell, but I bought it for what he had in the rebuild, and got a facotry DT400 service manual, parts catalog and two good tires to replace the dry rotted ones with it.
I unbolted the box and moved it around in the frame enough to clean off the grease and carbon from a bad silencer connection, then sprayed some Rustoleum on the frame to make it look a bit better. Stripped the tank to bare alluminum, and rear fender to white plastic. I think I'll slap some tuning forks decals on the sides of the tank and call the alluminum look good. I've got a white Preston Petty mudder in fair shape that I'm putting on the front. It's shaping up into a decent looking old bike --- kind of cool, like the first Elsinors.
The monoshock has no sign of leakage and seems to dampen well, so I'm leaving it alone for now. The forks are giving me kittens though. I've never had so much trouble getting old seals out or new ones in. I'll have to buy a little bit bigger socket to drive them in with or I'll ruin them. The weird, watery swamp mud I found in one fork was unlike anything I've seen in a motorcycle fork before. All in all, I hope to have it ready to ride by next weekend --- though I've got a brutal work shedule this week.
Text Version Home
vBulletin Copyright ©2000 - 2009,
Jelsoft Enterprises Limited.
vB Easy Archive Final ©2000 - 2009
- Created by Stefan "Xenon" Kaeser
