Just a quickie. Yesterday I finished building an MC21 300 motor for a friend, but found a rather naughty problem when I came to leakdown testing it.

Filled it up with 10psi and as quick as I could pump it in, it was squealing out. The good old soapy water revealed that it was leaking really badly between the carb insulator and the reed valves. The reed gasket was new, as were the valves, so I knew they were ok, but when I pulled off the insulator and stuck a straight edge across it, it was bent like a banana. Tossed that one in the bin, grabbed another which showed to be reasonable straight with the rule, but that too leaked. Then tried another and that leaked.

Two main problems. First is that over time the insulators have been fitted by someone who didn't use a torque wrench, and they've over tightened it, which bent not only the insulator but also the reed valves. Secondly. this insulator doesn't normally use a gasket as there's a seal built into it, but closer inspection showed that the seal was actually below the clamping face so was basically useless.

I eventually found a good insulator, which held the 10psi for a solid 10 minutes. Job done.

As for the torque setting on the insulator, normal torque would be 12N.m for an M6 bolt, but I used just 7N.m to seal it. This makes sure that the gasket does it's job without overstressing the part. On the leaky insulator I started at 6N.m and worked up to 12N.m and it made absolutely no difference. In fact, over 10N.m it got worse as it started bending even more.

Well worth a check, but don't get too paranoid about it. If in doubt a smear of gas resistant sealant will sort it out if it's not too bad. Messy and I can't stand using that goo where you shouldn't need it, but sometimes necessary to save your engine, your cash and your arse if it should sieze.