Diablo Shop 503

[Go Go Diablo]

The Diablo 503 motor is fairly unique being a Hirth 190R. The one thing that makes it unusual is the very small tapered output shaft measuring 24mm at the large end of the taper. The Hirth 54R motors in the Diablo 500 & 502 also have this small tapered output shaft. Common motors of the day had standard 25mm and 30mm tapers. I can only guess that Bolens requested this special taper on both the 54R and 190R so that they could supply a common clutch which is also special which I'll explain later.

Another motor that works in a Diablo without modification is the Hirth 191R. This was a far more common motor used in other sleds. It is basically the same motor except it has a larger standard output shaft. If you use this motor, you need to find a clutch that will fit the larger taper.

For the sake of keeping this topic a reasonable size, I will wait till another day to explain the process of building a motor.
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[Go Go Diablo]

The original starter used on a Diablo is a Bosch 001 158 004. The "158" series starters are fairly common and the last three digits designate the nose cone which turns the starter in the desired position as needed. The 004 starter on the Diablo is meant to put the starter under the motor and keep the onboard solenoid close to the motor.

The issue these starters often have is they get moisture in them. As the flywheel turns and pulls air past the motor, if you are driving in powder, this snow can get into the housing of the warm motor and melt with the water droplets going down and into the starter.

So check your starter out closely. If needed, pull it apart and clean it up reseating the brushes. If the starter is beyond repair, you can look for a 001 158 002 or a 003 starter and take everything off of it's nose cone and put it on your 004 nose cone for a replacement starter. These starters turn in the same rotation as the 004 starter. If you use a different starter, check the rotation of it before you install it.
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To help with the moisture issue, drill a hole in the motor housing to give the water someplace to go other than into the starter. See hole below.
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The motor goes in from the top and between the handlebars. I leave the top two rubber motor mount out to give me some room to move the motor around. Once the lower motor mount is in place over the three rubber motor mounts, then add in the top two motor mounts and bolt everything down.

This is a good time to hook up your exhaust pipe. Tighten the manifold to cylinder head bolts evenly and tight. Then tighten up your flex clamps.
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The headlights used on a 503 are a bucket style WIPAC light as opposed to the flush mount lights used on the 500 & 502. It also uses a trailer style lamp rather than a festoon style lamp like the 500 & 502.

I like to sandblast and paint the housings keeping the socket and the ground tab free of paint. Then solder the ground wire on to the tab rather than just having the tab wrapped around the ground wire prong.
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Assemble the lights using dielectric lubricant on the inside of the socket and lamp. The 503 uses a thick rubber gasket between the lens and the bucket. If you don't have them, you can easily cut them out of a piece of rubber. I drill out the little chrome tabs to 3/16" so I can use a long machine screw to mount the headlight instead of that small screw set up that is original.
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Cutting the holes in the nose cone is always a challenge. Put the nose cone in place.
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I use a Dremel with a drywall style bit and follow the outline of the rectangle opening. Then drill through the nose cone with the 3/16" bit to add the two mounting holes.
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Then install your first headlight.
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Do the same for the second headlight. After they are installed, put a dab of loctite on the nuts inside the hood.
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[Go Go Diablo]

Install your brake system.

If the splines on the brake rotor are worn but still intact, you can use the rotor to failure. The brake rotor is softer than the shaft and is meant to be the "sacrificial lamb" of the two. Put the aluminum spacer on, the rotor and then the E clip.

Clean the steel bushings that go into the aluminum housing of the gearbox making sure they slide freely. Lubricate with grease. Then assemble the brake making sure the brake push pins are free and lubricated.
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When all together and tight, the brake rotor assembly should be loose in the aluminum gearbox housing so the caliper can track with the location of the rotor.

Hook up the brake cable such that the lever is just left of high noon. This sets the lever up against the push pins. Then from here on out, adjust the castle nut as needed to set your brakes. Make sure they are not set too tight or the brakes will heat up quickly. There should be clearance between the brake pads and rotor and yet set that you have plenty of brake when pulling the brake half moon handle.
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The Voltage Rectifier is a simple AC to DC converter with a fuse inside. They are originally designed to be mounted with the terminals down but on a Diablo, they are mounted terminals up which creates an issue. When the snow get on them, it melts and leaks down into the body by way of the vent holes by the terminals.

Clean up the assembly repairing terminals if you need to. Check the fuse to make sure it is good and has a good connection to the terminals.
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Test the diodes with the diode setting on your meter. When you test a diode, test with the positive and negative lead of the meter one way then the other. A good diode will have no resistance in one direction and some resistance in the other. Check both diodes.
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Drill holes in your cover so if water gets in, it can get out. They act as the new vents as well.
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Wrap some black electricians tape around the diode and nuts so if the cover gets bent over, these do not short out.
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Install the cover and wrap more tape around the seam. Then fill the vents with RTV to eliminate the path for water to get into the housing.
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With the hood, handlebars, motor, dash and headlights installed, now you can tackle the wiring. If you need a wiring diagram, let me know and I can email you one. For restorations, I use all new color coded wires.

- Brown is ground
- Black is ignition for killing the motor
- Yellow is lighting and wires from the lighting coil to the voltage rectifier
- Red is the hot wire
- Green or blue is the starter solenoid wire

If you have an old original key switch, toss that in the garbage. The old original key switches were known to fail and often burned the wiring back into the motor. When you put the key switch on, use a stack of washers behind it so you can really tighten up the face nut. Otherwise, the switch will certainly come loose on you when you are out on the trails.

I use a switch harness as well for two reasons. One being it protects the hot terminals of the key switch when you are swinging a plug wrench around that area. Second being it makes for a good place to make your wiring connections to the rest of the sled and hide that all when done.
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When you connect your new wires to the terminal block on the motor, put solder on them otherwise they may break with the vibration.
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With the harness for example, I run the black wire from the motor terminal block into the harness and use a butt connector within the harness to hide the connection. You can do the same for most of the other wires as well. When done with the harness connections, you can wrap black electricians tape around the harness to close it up and keep it tight.
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You will want to add a fuse between the battery and the hot wire. I use a 15 or 20 amp fuse. I also use these nice red terminal protectors for over the battery's positive post and the post on the starter.
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Run a lighting wire and ground wire to the ski seater using a 2-prong plug.
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When done, you have nice clean wiring.
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[Go Go Diablo]

Install the hood scoop (Nacelle) as shown.
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Carbs on the Diablo are either Tillotson HR3A or HR14A. Other Tillotson carbs work as well as long as the linkages are set up properly.
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I not going to go through the process to rebuild the carb on this post. I'll save that for another day and another post. But needless to say, the carb should be rebuilt. Then, install the heat shield and carb. You should use a special 13mm wrench cut and welded such that it has a very steep angle and allows you to get the carb base nuts very tight.
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I like using this low temp Tygon fuel line with an inline filter. Also, use a short piece of this same fuel line for the pulse line between the carb and crankcase nipple. If need be, the nipple on the carb can be turned downward to keep this pulse line as short as possible. I use hog style clamps on all the connections.
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Routing a choke cable through the original path of going through the upper corner of the top motor mount is difficult. For this Diablo, I ran the choke cable around the side of the motor and came at the choke from the opposite direction. This requires the choke lever to be reset. This set up is working very well.
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