Motobecane engine mount mystery solved

Legendre /
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Wouldn't you like your engine to sit correctly in the frame? Sure, we all would. I've been chasing a series of problems with the mounting of the engine in my 1977 50V Moby, for the last couple of months.

Everything about the way this motor was mounted was wrong - and the issues were bad. Variator and the variator cowling were both capable of grinding on the driven pulley, carb airbox was rubbing on the sidecover AND on the pedal chain, and the drive belt was clearly twisted and out of alignment. The motor was visibly sitting cockeyed in the frame. Worn out flexiblock mounts alone could not have accounted for the issues, and yesterday, I finally figured out the entire mess.

Issue #1 - The lower engine mount on the Moby is integral with the muffler mounting clamp. There is a spacer that spans the gap between the two halves of the lower mount, and it was missing. With the spacer missing, the previous owner tightened down the mount, which caused it to become badly distorted. Over time, the distortion worsened, to the point that one of the spotwelds on the mount broke free, and allowed the engine to further slip out of alignment.

Issue #2 - Somehow, some way, the upper flexibloc mounts came to be incorrectly installed. The factory special tool for installing / removing these mounts sets a precise installed depth for the parts - so either someone did the job without the correct tool, or somehow they managed to slip in their mounting bosses (doesn't seem likely). With the flexiblocs improperly installed, the upper mounting bolt was tightened, causing both mounts to flex, bend the mounting bosses, and left the motor improperly aligned on the top end.

I installed a spare spacer from a parts motor, and was able to bend the lower mount back into shape with a crescent wrench, a big screwdriver (used as a punch) and a hammer. On the top mount, I removed the factory spacers, and shimmed the motor into the (almost) correct position with some nuts and washers.

It's not perfect, but what an amazing improvement. The motor is still positioned a little too close to the offside, and the airbox lightly interferes with the pedal chain - but only over 25mph, when the variator is almost fully closed. I also can't install the right sidepanel on the bike - again, interference from the airbox. This arrangement will hold until I can get the special tool for the flexiblocs made, and get the flywheel & variator pulled so I can replace the lower mount with one that is undamaged. I'm not putting on many miles in the meantime.

The motobecane is tightly engineered, and doesn't respond at all well to mistreatment or carelessness. Coupled with the fact that they are clearly over-engineered, they are very easy to screw up - and in some very subtle ways. I have corrected dozens of problems on this bike over the last few months, and I think I can finally see the light at the end of the tunnel.

With some luck, it's not the lamp of a fast-approaching train.

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