Getting a 1962 DKW Hummel Running Again
The DKW Hummel is a two-stroke microcar from Auto Union, the company that eventually became Audi. You will not find many of these on the road. Finding parts is the first problem. Understanding how the two-stroke twins are supposed to behave is the second. I have spent more weekends than I care to count with one of these under a blanket, so I am going to lay out what actually works and what does not. Start with the carburetion. The Hummeli uses twin Bing carburetors mounted on a manifold that is prone to cracking if you have ever torqued those nuts down without a feeler gauge. My suggestion is to remove them, send them to someone who specializes in two-stroke carbs, and have them rebuild the jets and reseat the needles. If you try to adjust these while they are on the car, you will spend three hours turning screws and end up with a rich condition that smells like unburnt oil. That is normal for a two-stroke, but it makes diagnosing other problems impossible. The ignition system is a contact breaker setup with a centrifugal advance. The points wear quickly, and the condenser fails more often than people expect. Replace both every time you open the distributor. Use a fresh set of points, but do not reuse the condenser even if the old one tests within spec. I learned this the hard way when a car I was working on would run perfectly at idle and then misfire harder the faster you opened the throttle. Turns out the original condenser had an intermittent breakdown under voltage stress that a bench test did not catch. A $12 part saved me four days of guessing.
The two-stroke engine requires pre-mixed fuel or an oil injection system that is actually functioning. The Hummeli uses a separate oil reservoir with a mechanical pump that meters oil into the intake. This pump is located inside the crankcase and the drive mechanism is a simple cam lobe. If the oil line is blocked or the pump diaphragm is cracked, you will destroy the engine within twenty miles. Check the oil line from the tank to the injection point before you even attempt to start it. A flow test takes five minutes and prevents a $3,000 engine rebuild. The transmission is a four-speed manual with a cone clutch. The cone clutch is the weak point. When it glazes over, you lose engagement and the car will rev without moving. Relining a cone clutch is doable but tedious. The cone itself can develop taper wear that makes a new lining impossible to seat properly. If the cone is worn, you need to either refinish it on a lathe with a specialized tooling setup or replace the entire clutch drum assembly. The latter is nearly impossible to find new and used ones are usually worn too. Refinishing the cone is the practical route if you can find someone with the right tools. Bodywork on these cars is steel, but it is very thin gauge. Rust does not just eat through the floor pans, it eats through everything below the beltline. The wheel arches, the sills, and the lower door panels are all common failure points. I once replaced a entire lower door section on a Hummeli that had been patched twice before with body filler. The filler had trapped moisture against the metal and the rust had spread behind the patch in a way that was not visible from either side. You have to lift the carpet, remove the weather stripping, and inspect the metal from behind whenever you see surface rust. What looks like a small repair is often a much larger one once you open it up.
The electrical system runs at 6 volts. This is not a suggestion, it is a requirement. If you convert to 12 volts without upgrading the wiring, the increased current demand will melt the original harness over time. The original wire is far thinner than modern equivalents and the insulation becomes brittle with age. A full harness replacement is recommended even if the existing one appears to work. A short circuit in a 6-volt system is less likely to cause a fire than in a 12-volt system, but the damage to the wiring and connected components is just as bad. I replaced a complete harness on a project car because the headlight switch had failed and drawn enough current to degrade half the circuits in the dash panel. The switch cost eighteen dollars. The new harness cost about two hundred and took a full day to install. Brakes are hydraulic with drum brakes on all four wheels. The master cylinder is shared between the front and rear circuits with a warning light that activates when pressure drops. These master cylinders are prone to internal leakage that does not always produce visible external seepage. If the brake pedal feels soft after sitting for a few days, rebuild or replace the master cylinder before you bleed the system again. Bleeding a second time will not fix a leaking master cylinder and you will waste several quarts of brake fluid. Parts availability is the overarching issue with any DKW Hummel restoration. Some items can be sourced from Audi specialist suppliers since the platform shares DNA with early Audi models. Other components are fabricated from scratch or cannibalized from donor cars. There are now a handful of specialty fabricators in Europe who make reproduction parts, but lead times can run six to twelve months and pricing is not cheap. Joining a DKW owner club and monitoring their parts exchange boards will save you considerable money and time compared to searching general classic car marketplaces.
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The car is drivable once sorted, but it has limits. Top speed is around eighty kilometers per hour and acceleration is leisurely at best. The two-stroke engine produces a distinctive exhaust note that some people find charming and others find annoying within the first five minutes. Cooling is air-cooled, which sounds fine until you are stuck in traffic on a hot day and the engine temperature rises past the acceptable range. Keep an eye on the temperature gauge and pull over if it climbs into the red. Two-stroke engines are tolerant of heat but not infinitely so. If you are considering this restoration, be honest about the time and money involved. A complete, concours-quality restore can exceed the value of the car by a significant margin. A driver-quality restoration with a running engine, functional brakes, and a decent paint job is a more realistic goal and still requires roughly eight hundred to twelve hundred hours of labor depending on the starting condition of the project. The reward is driving something that almost no one has seen, but the path to get there is long and filled with obstacles that are specific to a car produced in very limited numbers during the early 1960s.
