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#7667
dave bushell
Participant

LOAD EXHAUST NOTE AND HUNTING TICK-OVER UPDATE
I followed Roger’s advice and fitted a restrictor to give a 1″ dia. outlet. This eliminated most of the tendency to hunt at tick-over but did not reduce the sharpness of the exhaust noise by very much. Retarding the ignition did little except reduce the responsiveness of the engine.
Being not entirely happy with the outcome, I decided to remove the barrels and pistons and check for any “tuning” that had been carried out. (I had bought the rebored barrels and new pistons from a club member earlier this year). The porting on the barrels compared to my other barrels did not appear to have been interfered with but on measuring the pistons I found that, although the gudgeon pin and piston body height were normal, the deflector height was +0.040″ higher than a standard piston. This obviously has raised the compression ratio, and although the pistons didn’t hit the head, they did close down the spark plug gap! I don’t know who manufactured the pistons, but they are very similar to pistons that I had purchased from Tim Sharp for my Brum Scott, i.e. no oil grooves around the body of the pistons. On my Brum I had to fit two gaskets to prevent the pistons hitting the cylinder head! I wasn’t sure at the time whether the pistons were taller or whether the cylinder head that I had fitted ( a Scott rally purchase) had been machined down).
I can either machine 0.040″ off the top of the pistons – it would be a straight cut – or fit a compression plate. I have two plates available, one 0.022″ thick made of copper, and one 0.040″ thick made of aluminium. If I fit the latter, do I need some form of gasket, other than the head gasket, on the other side of the plate, or should I fit two head gaskets which combined , would obviously be greater than the extra 0.040″ in piston height. I don’t like the sharp, and to my ears, louder exhaust note for a road going machine if I stick with these higher compression pistons.
In the mean time, I have fitted a well worn pair of pistons and barrel and the exhaust note has returned to it’s original level and the tick-over no longer hunts.
Your opinions will be gratefully received.
Dave