Driving the DT50

Trying to remember where I left off, I think I had torn down the DT50 and found the leaking shaft seal behind the clutch. Most of the engine was in super good condition, now that i've worked on a couple of these dirtbikes I'm even more impressed, as they get rode hard and put up wet, as they say.

First thing I did when I tore it down was tackle the top end, the paint was peeling and there was some minor corrosion. Someone was trying to fix it and had pulled the head, wrecking the gaskets, so it had no compression. I figured I'd do a proper job of it, so I pulled it all apart and took a box of grimey, crusty junk to the engine rebuilders next door for hot tanking.

Not the greatest 'before' picture but generally showing the condition of the parts.

 Back from blasting and masked for paint. The total cost for the hot tank was $40 for 7 pieces, kinda high but way worth it to get everything back totally ready for paint.

And after 3-4 light coats of VHT caliper paint. Same stuff i use on heads, amazingly tough with a nice satin finish.  Unfortunately my paint curing oven wasn't set up so i couldn't bake the paint. A tiny tiny bit of chipping occured at the head gasket sealing surface and now there is a little rust there... damn. That stuff works much better when you can bake it.

Since all the bearings, crank, etc. looked great, and there was no center gasket included with the rebuild kit, and it is a vertical split case.. and I didn't want to open a whole can of suck... I didn't bother to split the main case. Plus sometimes these vertical split moto engines can require special tools to reassemble correctly and i'm not feeling that.

I did have the clutch cover off, clutch out, replaced all the clutch-side-case seals including water pump, crankshaft, kick starter, shifter, and oil pump. Re-assembled all the clutch side parts with liberal application of red loctite, and replaced all the stripped out phillips-head screws with stainless allen head cap screws.

Cheap insurance, and since Yamaha is a company that is still in business (what a rare treat!) I was able to get parts from PartZilla who took forever to ship them (I'm pretty sure they special order it in from Yamaha so it takes 2 weeks) but everything came in OEM yammie bags and was correct and complete. Very nice. Plus they have all those wierd rubber frame bits that you can't ever seem to get. And their prices were very good. All told the bill was just over $50 for all new seals and some tank dampers, and a couple oddball screw thinges.

The gasket kit came from Treatland.tv  and it is an Athena brand gasket set. It was super complete-o with some bonus gaskets for different model year DT's and everything I needed, except for the oil pump gasket (which was pretty easy to fab) and the center case gasket, which, if you really need that badly you can just order from Yamaha/partzilla.  The head gasket is my only gripe, the OG gasket is MLS (multiple layer steel) which lots of Jap OEM's use, especially for aluminum-on-steel applications (I.E. Mazda DOHC 4 cylinder, aluminum head on a cast iron block). The Athena gasket is a cheaper quality carbon/graphite composite thing with a steel fire ring.

I dunno a whole lot about this kinda stuff, but on the EJ25 Subaru motors, the OEM graphite composite head gaskets are trash and prone to failure, the fix is to use an MLS gasket... making me think that there is significant reason to buy the OEM head gasket, especially if you plan on doing performancey type stuff.

Not too shabby for a days' work. This is 95% complete. Main thing missing when this photo was taken is the airbox.. that was a tough one.  Chad in Canada send me one along with the sidcovers. I was able to fill her up with 10-w40 moto oil and coolant and she fired right up, LITERALLY on the first kick. So cool.

Since it was already broken in, and running lean with no air box, I just let her warm up and make sure there were no leaks. Well, there were a couple leaks, the coolant temp sensor was dribbling, and 2t oil was puking all over the place because i was missing the M5 screws to hold the oil pump on.

But other than that success! Everything works as it should, adjusted brakes, and re-torqued the muffler bolts, the head bolts and the temp sensor, did a quick check to make sure everything was free and happy, and put her away.


The next couple weeks we were moving.  Then my truck broke down so I got a fire under my ass to get this thing working. The truck still is running on 5/6 cylinders, but that's a different story.

I put on a new chain (420 Chain x 110 links- good to know because some places list 428x120 chain as the replacement ) Patched the front tire, put on the airbox and installed a homemade air filter cut out of window air conditioner foam sheet.

Oh what else? I dunno.  Replaced quite a few little random missing screws, etc. Got the oil injector hooked up and working, but i'm still running a little oil in the gas because i don't quite trust it and there are a couple air bubbles in the line.

So yeah, its pretty fun. Still a few things to do, replace the rear brake pads, true up the rear wheel a bit better.  The rear tire has a huge gash and everyone keeps saying 'hey do you know there is a huge gash in your rear tire' ... ha.. I have a gazelle to put on there but it would just be sooooo ugly.. maybe. who cares

The forks might need some oil. The clutch cable is wackadoo way too long and all mushy.

It probably needs to go up one click on the main jet and might not be the worst idea to have a front tube without 74 patches on it.


But man, does this thing GET. It will pull a pretty stupid wheelie in 1st gear (up to about 7 mph)  and hop the front wheel in 2nd. It gets up to 45 real quick and eventually climbs a good bit past 50.  My commute is 16 miles each way of 60 mph 4 lane slab, and it handles that surprisingly well. The little engine loves 10, 11k and just sings along quite happy. Very well balanced and zippy.

If I have more time perhaps I'll do a better write up on all the research i've had to do to find parts sources and stuff for this. Its not as well supported as the AM6 motor, but there is pretty good aftermarket for it in europe and south america.

Its kinda funny, I'm not in love with the thing and i'm not sure why. It does everything pretty well. Its basically exactly what we all want our mopeds to be, good brakes, good suspension, stable, lightweight and plenty of power clean up to 10k rpm. Yet somehow its just not as much fun as a moped. You've always got to shift, you're usually either frustrated you cant go faster, or aggravated that you are being slowed down by traffic. If you don't wind the piss out of it, it dogs shifts, and if you do wind it out you, it blasts through gears so fast you are constantly shifting.

Its tough to say really what would make it better, if anything. It would be a real blast to have a 150 lb bike with about 2x the horsepower this has, or even just a little more torque. It would be a monster with a 200cc thumper like an xr200, but as it is, its just not that great to drive.

If i end up keeping it, I'll probably paint the horrendous rattle-canned plastics in a kenny roberts yellow paint scheme, to go with the black. Maybe even throw the cheap 70cc kit on it or put a pipe on it. Hard to say.  Its the kind of bike that is so close to being awesome, yet mysteriously just not that good and I can't quite put my finger on it.  At this point i've got a grand total of maybe 400 bucks into it, including fluids, so its not too hard to justify keeping it, but the 1500 I could probably sell it for could buy me a couple bikes that i'm more into, so its a tough call.