SQ4 - maintenance

 After a run to Norwich today, 55 miles at 27.4 mph average speed (according to my new Beeline sat-nav) the Square started making an odd noise on the way home, a bit of a squeak from the engine sounding perhaps like a leaky head gasket. I stopped for coffee at the lovely Itteringham community shop:

After going on to reserve on the way home, I thought I would have a look at the plugs and do a compression test and the picture shows the result. All four plugs are the same colour, dark chocolate, all dry, numbers two, three and four all gapped to 18 thou but number one was gapped to more like 25-28 thou, which I corrected.  

Compression figures are good and consistent at 130-140 psi.

Restarting the engine after I had put the plugs back in revealed that the squeak had gone. Now I am worrying about the oil pressure on the last part of the trip home...

FH - looking for problems

 I snatched a few minutes in the workshop today to look at the cylinder head leak and the oily exhaust on the Huntmaster. It passes a little bit of smoke from both cylinders but nothing persistent and only when pulling away. To start the investigation and to try to eliminate the valves and valve guides and further home on the problem I thought I would do a compression test. 

I started with the drive side, which is the side that is showing oil in the exhaust pipe. This revealed a figure of something like 65 psi with the cylinder dry. Perfect compression at 7.25 to 1 should be giving us something a little over 90 psi.

This didn't improve after several kicks.

The timing side shows us something like 95 psi. The exhaust on this side is dry.

I ran a bit of oil around the top of the drive side piston in an attempt to improve the sealing and to reveal either a head gasket problem or perhaps a valve problem.

This wet test brought the compression up to 75 psi - still a bit down on where we should be. The rings have only done around 145 miles but given that the timing side is working well there is obviously a problem here, and combined with the oil leak around the top of the cylinder we are looking at a head gasket leak.

Looking at the plugs I wasn't surprised to find that they were both dark and wet. The last journey the bike had made finished with a few minutes riding at very slow speed through town - on the pilot jet. We will lean this out next time.

Riding back from the railway yesterday along the main road revealed that the bike will maintain 45 miles an hour at just under a quarter throttle - so until we are properly run-in we are not going to be able to do anything other than look at the carburettor slide cut away at the very most. We can adjust the pilot air again but we won't be able to examine the mixture from the main jet.

Compression Ratio

I know the head has been skimmed, so I measured the combustion chamber volume with a graduated syringe. It measures 38 cc. Adding 6.63 cc for a head gasket nominally 2 mm thick and 2.7 cc for the dish in the piston gives me a combustion chamber volume of 47.33 cc, and a compression ratio of 6.26:1, quite an increase on the 6:1 stock figure.