Motobecane 40T 50cc first start

There’s nothing like the sound and smell of a brand new engine.

The bike started right away which is always a relief and a good sign. Today was a very hot day with triple digit temperatures. I’ll wait for cooler temperatures to tune and test ride the bike. I kept the idle high because there’s no clutch. With a big main jet for safety and break in, the bike is a little rich at low rpm. The engine feels good though with lots of power and should make that light bike for a motobecane an other 50cc ripper. We’ll tell people it’s a 70cc, shhh… 😉 The chain makes a weird sound because of those new after market front sprockets with teeth that don’t always line up perfectly. I need to work on that or just let it wear itself down.

I still need to hook up the lights, tune the carburetor and variator + launch lever. Then we’ll send the bike to the body shop for a new paint and seat upholstery. This is probably the last time you see the original orange paint. Once the bike is completely done, I’ll probably post some photos and videos of test runs with the lucky owner.

Engine Set Up:

Stock cases, doppler crankshaft, 50cc doppler kit, doppler head, doppler stock 2 petal reed block, doppler intake manifold, 19mm dell’orto carb, stock cdi, doppler exhaust, doppler er3 variator, stock engine spring, doppler belt, doppler pulley, transmission 11×56 for now.

Configuration Moteur:

Carters origine, vilebrequin doppler, kit 50cc doppler, culasse doppler, clapets 2 lamelles origine doppler, pipe admission doppler, carbu 19mm dell’orto, allumage origine, pot doppler, variateur doppler er3, ressort moteur origine, courroie doppler, poulie doppler, transmission 11×56 pour l’instant.

The electrical wiring is done with headlight, tail light, brake light, regulator and kill switch. I’ll use some heat shrink tubing to hide and protect the wires. The bike is now ready for a strip-down and paint job.

Exclusive photos of the freshly painted frame at Melvin’s body shop, 2 more coats to go.

Videos of 1st start and test runs:

Morning break in session 1/2 throttle up and down the hill with cool down period between each round. The bike will get a final tuning after the paint job.

Test run:

After 2 years of honing my tuning skills I’m finally having fun blasting with those 50cc variated french mopeds. I think that once you’ve learned the fundamentals of 2 stroke tuning and that you have a good formula to build fast engines using cheap stock 50cc moped parts as much as possible, it’s time to stick with the basics and stop searching spending time experimenting with crazy 70cc or even scooter parts. Why try to reinvent the wheel, riding is more fun. “Gonflage des cyclomoteurs” is a good vintage book out there that brings you back to the basics of 50cc moped tuning wrote by the 80’s master tuner Didier Thomas who now works for Doppler but unfortunately it’s in French.

I modified the Er3 variator and replaced the doppler belt 13.2 mm with the wider malossi belt 14.3 mm. The bike should have better low end + better top speed using the variator’s full 100 mm range. By pressing on the launch lever we’ll be able to push start the bike and compensate for a more slippery belt sitting all the way down flush against the variator shaft. Once we’re able to get a real heavy duty clutch pulley on the market, nothing’s gonna touch those fast motobecane 50cc stock cases or be more fun and fuel efficient, especially if we start using 50cc h20 kits.

Quick tip:

I recommend to always disassemble a brand new Doppler Er3 variator before installation to check and lubricate the weights arm pins, grease the arms head and damper yellow plates, tighten the 2 guide pins with a drop of blue Loctite (or they’ll fly off at high rpms) and grease them.

50cc engine set-up tested on the video above available at







I’m doing the monthly ride tonight with the motobecane av10 50cc Bidalot to test the malossi belt with the modified Dopller Er3 variator and 11 x 54 transmission.

Quick tip:

When you don’t ride your fast moped very often, it’s important before each ride to always check your brakes, tire pressure, chain tension and maybe grease the pulley’s bearings and the 2 variator guide pins + check for loose bolts due to vibrations on those french variated bikes. As a general rule, use regular thick grease for slow moving parts like variator guide pins and liquid grease for fast moving parts like pulley bearings. Also, always carry a tool kit, extra spark plug, belt, exhaust springs or emergency wire.

Back from a night ride with the Latebirds moped gang and awesome views of LA city lights from Mulholland Drive.

Before the ride I tested the malossi belt. Compared to the doppler belt, the variator felt heavier and harder to control with the launch lever. It feels like I’ve lost some snap and 10% of the acceleration probably because of the different cross-section shape of the belt, I may test that belt again with smaller weights. I couldn’t push the bike to full speed because of the traffic but it feels like the malossi belt pulls a little more than the doppler belt and would increase the top speed. At that point with 65+ mph top speed, the bike is fast enough to ride in the streets of LA and I would probably choose the best belt for quick acceleration over 70+ mph top speed.

The good thing about those 50cc Gr1 stock cases engine 19 mm carb @ 12,000 rpm max is that you could almost race an entire season without having to disassemble the engine for a check-up. That’s not the case with a faster 50cc Gr2 or Gr3 engine bigger cases + bigger carb @ 15,000 rpm like my yellow peugeot 103 RCX malossi cases 21 mm carb 14,000 rpm. After 3 or 4 race weekends that bike is way overdue for an engine disassembly and full check-up, looking for signs of detonation in the combustion chamber, wear of top cylinder nikasil, piston + ring + wrist pin bearing, rod and crankshaft bearings, seals, variator guide pins + weight arm dampers, etc… to prevent an engine failure. I really need to do it before any future race or monthly test run but I’ve been lazy and not looking forward to it since everything fits so tight on that bike and makes the engine dismount a little more difficult.

It’s always funny to see people’s reaction in their cars when I tell them it’s a 50cc moped. They should make a gta moped game version of this. That’d be so rad.



  1. tomahawktuning

    The tires are 2.50 with good clearance.
    The 2.75 worked but almost too close to the frame even after doing some grinding.

  2. Tracy

    Awesome! Reminds me of my bike!
    Gr1 ish it totally lasted thousands of miles. It was a half time daily rider (split with slimer) and lasted for 8 months or so. I just noticed that there was some play in the main engine bearings and am going to rebuild again and change out the reeds. Other than that my engine is still in good condition as a 60 mph 50cc! Viva la 50!
    It looks like you’ve been busy! Lots of bikes this year so far!

  3. tomahawktuning

    Thx Tracy. Yay, I’ve been super busy.
    Building that bike reminded me a lot of your moby.
    Same Doppler 50cc set up as your bike but with stock cases instead of your polini cases.
    The stock small volume cases actually feel better with a 19 carb.
    Knowing how much you ride every day, I’m glad to hear that your bike still hits 60 mph and that the doppler kit lasted that long even though we built it for street / race track.
    Probably less problems than with previous 70cc kits.

    That’s great news!

  4. eric

    do you think points would work as well as the cdi setup? also, does the treats cdi have a limiter? thinking of getting cdi but I’ll hold off if I can get similar results. thanx Christophe!!

  5. tomahawktuning

    Try with your points for now but it won’t be as good as the stock cdi (no limiter as far as I know) or any cdi.

  6. Eric

    the malossi belt is a little too wide. so wide you don’t need to use cardboard on the er3. too much tension on the variator so it won’t idle. think i’m gonna go back to the doppler.

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