Making a Lambretta exhaust quieter

I have a KBA exhaust, and it's pretty noisy. I have a theory that disrupting the air-flow near the exit point, and making it more turbulent, might reduce the 'pop' which is I think is the source of a lot of the noise. I also reckon this will have no impact on performance.

Now this is just an experiment at this stage, but I've dropped a discreet pop-rivet into the end of the exhaust to create the turbulence. I'll let you know a later date if it seems to have worked or not!



If it doesn't work then I'll have experiment with a baffle of some sort.

Gear oil change

While it was off the road I though it might be a good time to drain and change the gear oil. This hasn't done many miles but I was keen to see if I was getting any fuel contamination (indicating a leak via the drive-side bearing) or there was any alarming debris in there. I was relieved to see and smell everything was normal.



My oil-draining technique is a funnel and a plastic milk carton.

Lost compression

After about 50km of gentle commuting over a couple of weeks I noticed that the Lambretta had become difficult to start, and I'd had to adjust the idle setting and the mixture screw quite a few times in trying to get it all to work properly. So yesterday I decided to have poke around to see what was wrong.

My suspicion was aroused by what looked like oil on the fins where the head meets the cylinder. A check underneath revealed oil dripping from this area; this was pointing to a leak around the head gasket. So I did a compression test, and got a reading of 70psi. But what should it be?

Don't you remember Boyle's Law that tells us at a constant temperature 'PV = constant' where 'P is pressure and V is volume? So if you're compression ratio is supposed to be 7:1, then you're decreasing the volume of the gas in your cylinder (at atmospheric pressure, say 14.7psi) by a multiple of 7, which means the pressure reading at TDC should be a multiple of 7 higher than atmospheric, about 100psi.

Clearly I had a problem, and no wonder I was having to bump start the thing every day.



So I decided to remove the head and cyclinder with the engine almost in situ. By placing a trolley jack beneath it and removing the exhaust, inlet manifold and carb (just leave it dangling on the air hose), and then the engine bar you can lower the engine just enough to completely remove the top end. This is without having to undo any cables or wires, and I managed to leave my footboards in situ too (althoug I don't have a precious paint job!). Here's a pic of the oily fins after I removed the cowl. You can see all the others are clean but the top of the cylinder is clearly more oily:



Alarmingly, ny head nuts didn't offer much resistance when undoing, and although the head gasket wasn't damaged the cylinder base gasket had virtually disappeared:



I've no idea why my head nuts were loose. They either worked themselves loose (and I did notice that my wavy washers weren't very wavy at all), or perhaps I forgot to torque them up properly? I guess I'll never know, but when rebuilding it I cleaned all my gasket faces and this time used some sealant on the new base gasket but left the head gasket dry. I also made my wavy head nut washers more wavy in an effort to keep them tight.

After a long morning's work it was finished and back together, and to my amazement it started on the first kick! Time to take it out and do some plug chops.

With hindsight there was another symptom that should have alerted me to this problem earlier, but I'd simply made a mental note to 'keep an eye on it' - that was I seemed to get through a tank of fuel a lot quicker than I should have. Now I know why!

Note to self: Always have a spare set of gaskets for last-minute rebuilds.

Throttle return spring

The throttle handle on Lambrettas is returned to its stationary position by the slide return spring in the carb. In my opinion this is a big ask, and this became a problem when I fitted my new light switch. The cables for the switch were chunkier than the one I replaced, and I couldn't stop them interfering with the rotation of the throttle handle. It wasn't noticeable other than the throttle not returning when letting go, meaning a hairy moment when I forgot this on my first ride with the new switch.

To overcome this I fitted a spring from the end of the junction block to a nail inserted into the pin holding the nylon throttle wheel in place. The picture below shows the new switch wires and the spring in the foreground.



Not pretty to look at, but it works a treat, giving that extra bit of help to bring the throttle back to the closed position.

Adjusting the speedo cable

I never found the time to make the speedo work prior to my MOT, but this was ok as rather surprisingly you don't need one.

When I finally got the time to look at it it was apparent that the square-ended inner cable wasn't quite reaching home at the wheel-hub end. This was easily fixed by removing some of the plastic outer casing and winding the olive 5mm further along the cable.