this is a 9rta 76 engine. i am having problem with one of the showing low compression pressure. the peak pressure is normal.fuel valve and exhaust valve operation is ok.what can be the resons for low compression pressure.

There are only so many places that a cylinder can leak.  However, just because you "renewed" some items, it doesn't mean that they might not be the culprit.  Additionally, there's still the spark plugvalve and any valves, if it has any.

It seems hardly possible that, if the low compression pressure is related to a leakage, you'd get normal pressure from combustion. compression 13-14 bar? combustion 150-160?
Any leakage would certainly interfere with resulting combustion pressure. As mentioned in previous post, I'd check or swap pressure gauges.

The rta engine has been in production for a large number of years nad as such has been subject to continuous development. Without more information it is difficult to nail down but I would add the following to those made earlier by Waross

The engine has both VIT and VET both of which if not operating correctly will affect peak pressure. I would check for slippage of the fuel cam and lift of the fule pump. I would check the condition of the cam followers ( I have seen  a number of failures), the condition of the pump espcially the anti dribble device. I would check that no bespoke fule limiters of pump lift devices have been inadvertently activated.

Apologies if this has been mentioned but some korean built rta's suffered from excessive bore cooling and porr scavenge demisters resulting in huge wear rates in the cylinder ( I know of a batch of engines running liners out in 5000 hours with piston ring packs lasting as little as 250 hours. Yes I do mean two hundred and fifty.

Operating the enginer with one unit significantly underperforming leads to extremely serious torsional vinbration and critical speed issues and you should consult the manufacture for advice on operating the engine in a degraded condition.

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