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.