1983 KTM 504 MXC
The Austrian Mystery Bike

Cifu Motorsports Home / Motorcycles / 1983 KTM 504 MXC / Identifying the KTM

After deciding to buy the mighty 504, I had to figure out exactly what it was. This sounds easy but it's not. The vehicle ID number clearly identifies it as a 1983 KTM. The engine is definitely the original as are the other parts of the bike. The problem stems from photos in 5 different magazines which show different features of the bike.

To the best of my sleuthing, I believe there were 2 different versions of the 504. The first was the KTM 504 MXC. It had the 43 mm Marzocchi forks and 300 mm of travel at both ends. It was designed as a motocross or desert racing bike. The second version was the KTM 504 GS. The GS model was an enduro version that had slightly shorter travel 40 mm forks and matching suspension in back. It had the same engine as the MXC but it was targeted at the woods rider. It also came with a full compliment of enduro parts like a centerstand, lights and available speedometer. In some countries, it was sold as a street legal bike. In fact, when I finally did get to look at a parts manual, it even showed available passenger pegs and brackets!

The different bikes shared similar frames and the MXC had conspicuously empty brackets where the GS model's enduro gear would attach. These included regulator / rectifier brackets, brake switch brackets and turn signal holes in the rear sub-frame.

The specs would suggest that my bike began life as the MXC version (with the larger forks). When the time came to convert it to street use, some of the GS parts were likely fitted like the (now) unused speedometer drive unit. The problem is that my rear sub-frame is very different from ALL of the other 504s I've seen in magazines, though it is clearly a factory produced item. Many  details like the stampings at the top of the seat stays, match the 504s in the magazines. The strange part is the aluminum standoff brackets installed at the bottom of the seat stays. I don't have a clue why they are on there but the stays are clearly not thick enough to just bolt into the tabs alone. I suspected that it could have been a poorly substituted sub frame from another KTM model but it DOES have a regulator bracket. Despite having this bracket, it lacks the other airbox mounting tabs that are clearly seen on the magazine test bikes. Mine is also missing the rear brake light switch mounting bracket, even though it's clearly visible on ALL of the magazine test bikes and preview photos.

There is also a question of plastic side covers. The magazine test bikes used a triangular side cover and the rear fender served as a number plate. Even on the 1984 600 cc model, it used these sidecovers. Despite this, mine clearly has the FIM style number plates used on the 1984 KTM 495 two stroke motocrosser. I would have just written this off to owner's whims, but, there are very nicely finished mounts, welded to the frame to support them. Again, the proper mounts for the normal triangular side panels are conspicuously absent.

The carburetor is another source of mystery. All of the 504 tests showed a 36 mm Bing carburetor, mounted to a rubber manifold. My bike has a  larger Dellorto carb (I haven't removed it yet to measure the bore) with a clamp-type mount on an aluminum manifold. Yes, this too could be written off as aftermarket changes but there are factory looking punched clearance holes in the frame which line up perfectly with protrusions on the Dellorto. A stock looking rubber boot connects to a stock looking K&N air filter but the airbox shown in all the tests is missing, as are the mounting tabs needed to attach one to the frame.

Of course, there is the issue of color. My frame is white, even though ALL the magazine tests of every KTM from that era showed an orange frame. There are deep scratches all over my frame and forks, yet not a single trace of orange paint is to be found anywhere. I seriously doubt that anybody who would do such awful work on the exhaust and electrical system, would bother to strip EVERY bit of orange paint from the frame AND forks, just to make it white. My seat is also blue with KTM painted on the flank even though the tests showed a black seat with no KTM markings.

Five different exhaust systems were shown on each of the bikes in five months. I still can't determine which one was the final production part. You would think that the earlier ones were pre-production but my system looks like one of the early clamped systems (no heat shield on the header), with a badly retrofitted, late model 4" SuperTrapp. My silencer appears to have very factory-looking mounts with uncharacteristicly welded tubing leading from the header. I suspect somebody must have tried to mate an early header with a later silencer. Still, I don't know why a production bike with a 1983 serial number would have received an early production system in the first place.

Exhaust Systems: an overview

Open Megaphone

3 Inch SuperTrapp Silencer Without Heat Shield on the Header

4 Inch Upswept SuperTrapp Silencer Without Heat Shield on the Header

4 Inch Horizontal SuperTrapp Silencer With Stamped Heat Shield on the Header

4 Inch Horizontal SuperTrapp with Formed Megaphone

Basically, I am very confused by this bike. It clearly has numerous differences, not shown on ANY of the other 504 test bikes, yet the dealers can't seem to help identify WHY it's different. The only clue I have is that somebody at KTM mentioned something about a Colorado 500 model while we were trying to track down a replacement exhaust system. Someday, I will discover that it was an early prototype model and worth thousands of dollars. If you know that to be true, please don't tell me.

Cifu Motorsports Home / Motorcycles  / 1983 KTM 504 MXC / Identifying the KTM