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        Project: 1976 Yamaha DT 400 Enduro/MX

In continuing with my theme of working with what were some of my favorite bikes from my youth, I have decided to build an old Yamaha. Or maybe 2....

I saw a 1976 DT400 on Ebay, and it was located close enough to make a pickup possible. I won the bid at $172. I knew from the beginning that the bottom end was a loss. It had suffered some sort of hydraulic seize, and the small end of the rod got twisted like a pretzel. It was fairly complete, that is to say it was a rolling chasis with a bad motor in it. There was also a "parts" motor from a 1974 DT360 that came with the deal, so i was hoping to use some of the parts from that.

Well I managed to split the cases on the 360 parts motor, but only after suffering a grievous injury to my thumb. You see, I was holding the case up with one hand and I had my thumb wrapped around the edge of the case. With the other hand, I was hitting the crank with a rubber mallet. Of course, one of my hammer blows struck my thumb knuckle, driving the crease of my thumb into the edge of the case. I got a nice looking cut, but my thumb was useless. It would not move at all. I figured I probably screwed up the joint somehow. I dont go to the doctors for broken toes or anything, but I was pretty sure this one needed some attention.

Turns out I severed the flexor tendon of my thumb completely. It took 3 and a half hours of surgery and about 12 weeks of recovery before I could really try to use it again. It works ok now, but doesn't bend as much as it used to. The injury cut my MX season short for 2004.

Ok, back to business....turns out I cant use the crank from the 360. It is a couple inches longer on the pinion side. DAMN.

About 2 weeks after figuring this out, I saw another DT400 on Ebay, this time it was a 1977, and again within driving distance. So I decided to bid on it to use the engine for parts. It was given a vague description such as "It stopped running and now it wont run and I don't know whats wrong with it." Ok. Well I won the bid at $105. It was very complete minus the road gear, but the motor was locked up. I was hoping for some luck in that the bottom end perhaps had survived, but it was not to be.

                                  This is my 1977 DT400 as it looked when I got it home. Ugly aint it?

The piston EXPLODED and of course the rod got tweaked because during it's last strokes, it had to share it's space with so many shards of broken wiseco. DAMN. Another bad bottom end! And guess what, the motor mounts on the '77 400 cases are different, so I could not have used the bottom end even if it had been good. The crank was different too! Turns out the '77 was not a good parts bike for the '76!


                                   Here are some autopsy photos of the 1977 400. Major carnage!

What was I to do? Ive got 2 400s, both with bad bottom ends. The 1977 had a "Mono Shock" on the rear, it is the tubular steel type that first showed up on the YZ in 1976 I think. The 1977 also had aluminum rims. Cool. But the 1977 was RED. RED for gods sake! Ive already got a RED Kawasaki. No problem, Ill use the 76 tank which is yellow and black, but no! The top frame on the 1977 was bloated to accomodate the monoshock, and the 1976 tank would not fit!


Ive got the frame painted on the 76. The stock shocks on the 76 were really very nice, but one spring was all pitted and the chrome was trashed. So I took the springs off and painted them YZ yellow. Looks better than rusted chrome! I put the old chrome rims with trials tires on the frame for now, to keep it off the ground while I decide whether to buy tires for the chromes, or use the aluminum ones off the '77.


I bought a 1975 MX400 bottom end from Ebay.. The guy who sold it said that the rod felt good, and that the item was in "good, useable condition". ASSHOLE! The rod was sloppy as hell. You could pull up and down on the rod and feel it clicking! Sloppy as hell vertically and side to side. To make it worse, the main bearings sounded a bit rumbly. It needs a complete overhaul to say the least. AHHHHHHHHHGRRRRRRRRRRR!!!!!


UPDATE: March 2005

I never learn. I bought another MX400 motor, this time, complete from Ebay. He said it was rebuilt at a local motorcycle shop, with zero time on it. Customer did not pay the bill, yada yada.... There was a new piston and bore in it alright, but it had a piston slap. Turns out the head gasket was too small. It was the stock gasket, and it had been bored 1.5mm over. No big deal right? Well, the guy who shipped this motor to me put it into a single play cardboard box with NO packing material whatsoever. The box was trashed, and the motor got banged up a little bit. Cracked a brand new clutch cable and smashed the throttle halves that came with it.

 I pulled the clutch cover to inspect the clutch side, and I found some broken pieces of metal in there! It was a tooth from a gear, and part of the shifter linkage! Go beat up pretty badly in shipping, I guess the kick lever and the gear shaft took some major hits and broke the crap off inside. To top that all off, again, the main beaings in THIS motor were shot. Whoever put a new top end on this thing without attending to the mains bearings should have his ass kicked. The guy I bought this one from said he would give me a partial refund, but weeks went by, and no refund. I filed a complaint, but no word yet on the results. At least the crank looked alright. So now, Ive got yet another motor thats useless. I am waiting on getting seals and mains for this MX motor, and then I will put it together. 

Ive also decided to scrap the 1977 400 all together. I used the aluminum rims on the 76 DT/MX and I used the forks and front wheel from the 1976 on the bighorn! Heh heh.....

SO now, Im gonna have what will be a 76 DT400 chasis with the full MX400 motor/carb/trans. Ought to be a fun wheely bike!

I dont think it will be ready for th start of the 2005 season which is now a couple weeks away!. 

UPDATE February 2006.....

Finally, the pieces are coming together. Got the new parts in for the MX engine. I had three of almost everything, and the motor was completely in pieces. Somehow, I managed to assemble the thing even though it has been almost two years since I took it all apart. Other than the new parts, I used the best of each piece that I had. I chose the best set of cases, the best transmission, the best crank, etc. I just hope I didn't forget anything!

I've got the motor in the frame, and I hooked the electrics up. I got spark right off the bat which was a great relief, because the old Yamaha CDI systems are fickle. I also sold two complete stator assemblies over the last year to get some cash. Luckily, the one I kept works! (Ive got extra coils and ignitors and CDI boxes still) I also had sold all the stock DT carbs I had, which is another scary thought. I have an MX carb which is HUGE! I could pass my hand thru it for god's sake. Don't you know the rubber intake boot is a little crappy. They all get that way and even though I've got three spares, they are all alot smaller, being from the DT models.

The shame of it all is, I really dont have the cash for new fenders, new side covers, new cables and all the other finishing touches that the good people of the world use for a vintage ride. I've had to scrap together cables, fenders and such. I'm even using an old chain! Its not going to be all that pretty, but at least the important stuff is primo. New Bearings, New seals, New bore and piston.

Im not too far away from being able to start it up.


Well, here it is. Done. Well, that is to say now the real work begins. There are a few bug to work out. There was a rough spot in the acceleration. I changed the needle position which helped a bit. I think I may try a new pilot jet. The carb had been flooding which didnt help. Then somewhere thru the day it developed a I had some kind of vacuum leak or something. The handling is all wrong too. I dumped the thing on  an off camber section because it is just not as sure footed as my Bighorn. Yet, anyway.

UPDATE: 7-14-06

I've missed most of this season already working on my house, and trying to sell it, but I managed to take the DT/MX out to a pratice track today. I fouled a plug right away, but after replacing it, it ran fine. Its different than the Bighorn for sure. The Yamaha does not have the off-the-line grunt that the Bighorn has. However, if you leave the power on long enough, it gets up and screams. I could not find the end of third gear on this track where I was riding, but the RPMS were way up there. I think Id of ended up in a third gear wheelie if there had been a long enough straightaway.(The power that the '75 MX400 engine is making is more like a modern bike than my Bighorn.)

The handling is MUCH better, now that I lowered the rear end to where its supposed to be. (I had fabbed up some riser brackets for the rear shocks to cant them forward and raise the seat height, but it handled like crap with those in place!)

The forks are a little beat. The first 2 inches is useless, it sags that much. Wimpy enduro springs in there no doubt. I will fix that soon. All in all, I think its ready for the next race.It seems to be just a bit less tiring than the bighorn to ride, although you have to work a little more to make sure the power is there when you need it.

2009 UPDATE: Well wheres the time gone? I FINALLY got my older Brother to come to a race a couple of years ago, and he was hooked. We came to an agreement that he would "borrow" my Yamaha 400 and at the next race, he got the bug so bad, he bought the Yamaha from me! I didnt mind selling it to him because I was still racing the bighorn, and building the KX400. My Brother's made a much better bike out of it, and having him racing again is such a thrill for me. More on that and some more pics on an upcoming page.



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