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Engine Losing Revs

I have a Barrus Shire 50HP engine with 200Hrs clocked. The strangest phenomenon started to occur quite recently. Whilst cruising the tickover dropped from 900rpm to 500rpm and obviously the engine vibrated viciously and cut out. This happened a number of times, so I decided to increase the tickover to 1250rpm. Now when I start the cold engine it revs to 1250 until it heats up then drops back to 850rpm. This is a 'sticking plaster'cure but I have lost 400rpm?

Asked by: Mike Holyoake  | 4.25pm, Thursday 9 October

WW says:

An interesting question; it sounds unlikely since you have managed to adjust the idle speed, but is the throttle control fully against the idle adjusting screw when the engine is cold. Also check that the throttle cable moves freely and is unaffected by the engine heat; sometimes the control cables are routed close to exhaust pipes.
Check that the load on the engine is not changing as it heats up; before starting the engine check that the propshaft rotates freely, when the rev drop manifests itself; stop the engine and check that the propshaft still rotates freely. If there is a change in the rotation resistance, look at the alignment of the engine, the mounts and the sterngland.
Although the installation is quite new, the problem could be a restriction in the fuel flow to the engine. Check the fuel filters are completely free of debris and that the fuel pipes are clear and there are no fuel leaks. If possible check the operation of the fuel pump; disconnect the output and operate manually or spin the engine with the starter.
If the problem persists after these checks; then contact the engine supplier or Barrus as it could be a problem with the engine governor that controls the injector pump.

Rupert Smedley  | 6.27PM, Monday 13 October

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