G'day folks,
So even though I've got compression, it won't run? Why's that? And is my only option here to hone it and install an oversized piston?
If so, my question then is... if I have good compression now, what's the difference between having it now and having it with a honed bore and new piston?
Compression in naturally-aspirated piston-port two-stroke engines like the Victas [there are other types of two-stroke valving, which I'll disregard here] has two forms. The engine has to be good on
both of these to run properly.
That is, the familiar combustion chamber compression, which is easy to measure with a gauge; and
crankcase compression.
If you turn an engine in good condition over by hand, you will be able to feel resistance as the piston approaches Bottom Dead Centre [BDC] - this is due to the crankcase compression.
It isn't usually possible to measure this directly, but leak-down rate testing can be done, with a pressure pump, gauge, and suitable port adaptors. The piston has to be positioned so that the inlet port is open, and the transfer and exhaust ports closed, for this test.
Combustion chamber compression is most influenced by piston ring condition, and their fit to the bore.
Crankcase compression depends most on good shaft seals, and good fit of the piston body and skirt to the bore, assuming the crankcase joints aren't leaking.
Straight-fuelling of one of these engines very quickly results in excessive piston skirt wear, thus the engine loses most of its crankcase compression. The amount of scoring visible on the skirt can be a useful indication of this wear.
This leakage has two effects:
1. Loss of crankcase
vacuum on the up stroke of the piston, so the engine can't take in a full air/fuel charge.
2. Loss of the crankcase
compression on the down stroke that is required to transfer the charge to the combustion chamber, via the transfer ports. And incidentally, to purge the dregs of exhaust gas from the previous charge.
If such an engine has lost crankcase compression, the only effective repair is to restore the piston to bore fit to original specification.
This is best done by reboring/honing, and replacement of the piston and rings with oversize parts to suit.
