Dr Joe Ehrlich Motobecane - Refitting the engine.

Published on 23 September 2023 at 08:43
Motobecane d52 5 speed gearbox fitted

The gearbox in place in the frame, just held with temporary bolts until I could get to the engineering suppliers for the right ones.

motobecane d52 kick start mechanism

I forgot to order a new chain, the one that came with the bike is actually pretty good but it doesn't look very nice in rusty brown.

motobecane d52 engine test fitted

Once I had found the right cylinder head the engine fitted quite easily, I test fitted it so I could measure up for the spacers, which I knocked up on my trusty old lathe.

motobecane d52 moped fully restored

Just about done now, I am very happy with this.


With the engine now running sort of ok I finished fitting the gearbox and made a start on fitting the engine, it didn't take long for things to go wrong. I had tested the engine with the cylinder head that looked like it had probably had the most running, well it might have done but it turned out it can't have been the head that was on the bike when the record run was made. I discovered this when I tried to fit it to the frame and realised that the head cooling fins fowled the upper engine mount. When I compared it to the other heads I have only 2 of them have the fins reshaped so that they will fit on the bike. One of those looks unused so I concluded there was only one head it could be. So off came the one I had on there - it's only 4 nuts - and the correct head went on. It might be me seeing things that aren't there but I reckon this combination has slightly more compression. It certainly seems to run quite well, I think a bit better than yesterday's set up.


I went to my favourite engineering supply shop and picked up some bolts of the right size to properly mount the gearbox, fitted the chain and the footpeg bar. I filled the gearbox with oil while I was at it, just so I didn't forget later.

My next job was to make a load of spacers to secure the engine, I needed to make 4 plus a couple of bushes that go in to the cylinder head that convert the 10mm holes to 8mm ones for the upper bolt to fit snugly. Every part has been tested at this stage, the clutch is fitted but I don't have a cable for it yet, I have actuated it by hand and it all looks good. The gears all select ok, the brakes work, the kickstart works, the engine works. It's getting very close now, only a few bits are needed to turn it in to a working bike, I then have to sort the wiring.

Putting the engine top mount bolt in was an adventure as you have to hold the engine in one hand while threading a bolt through the two engine mounts on the frame, 2 spacers and the engine itself. I like to think the loud swearing helped because I eventually got the job done.

While rummaging through the box of parts I found a small cover plate that goes over the kickstart spring, I had forgotten about that so it had to be stripped, repainted and fitted, which required the kickstart lever to be removed. At least I thought it did, all it required was for the lever to be rotated, it does tidy things up somewhat. While I was at it I cleaned and repainted  the front number plate and refitted that, I also fitted the chrome finishing panel on the right hand side of the tank. I have a left hand one but it is in very bad shape so I either need a new one or to get this one re-plated - not a cheap option. My Brother lives in France so I will see if he can find one, he has a much better chance than I do.

the exhaust is a bit odd - it is a chopped up expansion chamber that has been grafted on to the original exhaust. I have a suspicion that this could be a major contributor to the extra power rather than just the boost ports alone. It is clear though that the boost ports flattened out the power band and made the engine much more usable. According to Ted Snook the engine went from 2.7 bhp or thereabouts up to 6.2, that's pretty good in a pre power valve world -  for comparison, a 1977 Yamaha fs1e of the same 50cc capacity produced about 4.8 bhp.

I have done a video of where it is at now, I hope to test ride it next week, still a few minor jobs to do ie throttle and clutch cables. Link to video here - https://www.youtube.com/watch?v=tuvac5_9-Ws&t=117s

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