Fumbling around with my DKW Hummel

Several years ago I purchased a 1957 DKW Hummel moped. Being too slow to ride safely in modern traffic, I really only intended it to be a display item, but I did want to actually run. When I bought it, it did not run as the fuel tap was clogged. I only got around to replacing the fuel tap corks in February 2022 and instantly the moped fired straight up and I took it for a spin around the neighborhood. I reported on this here: https://dkwautounionproject.blogspot.com/2022/02/my-1956-dkw-hummel.html 

But the fix did not last. After another short ride the engine just would not fire up. After much frustrated fiddling, including replacing the fuel tap corks, I put the moped aside. Why invest so much time in such an impractical vehicle? I have many other more important projects to work on.

The fundamental problem is that access to the fuel outlet and tap is extremely limited. It can only be done by feel alone.

However, the Hummel problem kept niggling away at me. I needed to find out why it stopped working, so six months ago I drained the fuel tank and replaced the original fuel tap with a new reproduction fuel tap. I had avoided buying a reproduction fuel tap for a long time as I have found the quality of modern fuel taps to be generally poor, in addition to being extremely expensive at over 100 Euro (with postage to Australia around 85 Euro). That's a ridiculous expense for a novelty vehicle, but I felt I needed to give it shot and so bit the bullet. When the tap arrived, it proved less than satisfactory. The sieve that sits atop the tap and inserts into the fuel tank was too wide to fit through the M-12 sized outlet. I tried to crush it a little to make it fit, but this did not work at all and the sieve wire ripped up its seam. The lip of the sieve also prevented the M-12 bolt fitting to the petrol tank outlet. I was able to fit the tap without the sieve, but squeezing the on/off tap through the frame was very difficult. After fitting, I took the moped for another test run and after a few coughs and splutters, the engine steadfastly refused to run again. Back to square one.

This is the modern reproduction fuel tap - four parts broke after fitting - the sieve, the bronze tube, the plastic mechanism linking to the on/off switch and the fuel nozzle that connects to the plastic fuel line. Quality workmanship at exorbitant prices!

After more frustrating fiddling, I decided to remove the new tap and refit the original tap. This was an enormous problem as the tap was extremely hard to reach and the on/off tap that protrudes from the frame could not be removed easily as it is held in place by a split pin that is almost impossible to access. The removal had to be done in phases as the whole mechanism would not come out as one piece. To my great frustration the tap broke in several places - the on/off switch snapped off and the bronze inlet tube that inserts into the fuel tank and theoretically sits above the layer of residue broke. Fortunately I was able to flush the broken tube out of the tank with a flush of acetone. This troubling exercise however revealed the ultimate cause of the problem. The interior of the fuel was badly rusted and encrusted with decades of accretion. I had expected this but was hoping against hope it was not so as the Hummel 115 doesn't have a separate fuel tank; it is an integral part of the frame and cannot be removed.

So I needed the clean the tank 'in-situ'. I experimented with several different solvents and acids. Several doses of cleaning vinegar helped wash through a lot of the grit, but did not really treat the tank interior. Several different solutions of different acids were tried, but the last soaking of a citric acid solution gave the best result. After a week of soaking, I drained the tank and let it dry for a week before washing the tank through with acetate. This dislodged a lot of accretion. I cycled through the wash several times until the rust colour finally disappeared.

So now I had a 'cleaned' tank and two broken fuel taps. I tried to reassemble the original tap using some spare parts from the reproduction tap. Of course, the reproduction tap is completely different sized to the original so that proved fruitless, but it gave me an idea - why not dispense with the original tap entirely and run a fuel line directly from the fuel tank through an inline filter with an inline tap? Great idea! The new tap had a fuel nozzle that could fit directly onto the fuel tank outlet, so I mounted that in place and attached a fuel line to the nozzle and.... the nozzle broke in half. What the hell? Another case of a brand new fitting being of such poor quality that it simply snapped in two when a rubber fuel line was pressed onto it.

I did not want to buy another fuel tap so I dug through my boxes to spare parts and managed to find some parts of an MZ fuel tap for my IWL Troll scooter. The nozzle for that tap roughly fitted into the fuel outlet so I was able to jury rig a new fuel line and plug it into the carburetor. It was by no means pretty, but as soon as I turned it on and started pedaling, the moped fired right up again and I was away. I'm quite amazed at how well the little engine runs despite being unused for several years. I managed to take on a test ride around the neighborhood twice....

Yes, you have to unlock the side panel to switch on / off the tap, but at least it works.

That all said, the set up is very much temporary. The MZ tap fittings don't fit correctly so petrol leaks all over the place, but the fix generally works. I'll try and fix the leak and make the set up more robust. I know, it's ridiculous, but.... It is satisfying to hear it run again.