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Alan L Alan L is offline
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Join Date: Apr 2006
Location: New Zealand
Posts: 3,261
Pat, One of those is the original (you can just make out some scores) and the other I made.
The metering is defined by the lower edge on the top skirt - this is the defining line over the slits - ask me what happens when the skirt lip is fractionally out of round.
The circumference above the lip on the head of the piston is the 'seal' if you want to call it that. ie the only thing that seperates your WUR pressure and system pressure. The tolerance is about 1 thou in the bore. Even at 1 thou you could imagine fuel leaking thru a gap that big with 50 psi behind it. If you have wear, or a score, the flood gates open. So, your cleaned up piston may have moved freely - as it needs too, but what leakage did it have? I suspect this is a major part of trying to tune ageing CIS systems - the AFR range simply becomes too wide to optimise with the limited options available. You could postulate the leakage is constant , therefore tunable. But as you fling the piston up the bore under WOT I suspect the leakage allows the piston to overfuel - ie degrades the WUR/system pressure relationship - which was stable at idle, or low opening.
Old 06-15-2008, 08:13 PM
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