Bump clearance

[Tony Dunkley]

Just to conclude this thread . Tony was 'on the money' with the diagnosis of crank case pressure causing this noise. Just by cranking the engine over with the oil filler cap removed the noise is gone. when I installed the engine I ran a 6ft small diameter breather pipe to the stern of the boat which is causing back pressure in the crankcase. If I reduce the length of this pipe to just outside the engine bay and increase the diameter I think all will be ok. The engine is obviously creating more crankcase pressure since being stripped down for some reason. Not blow by because the effect is the same when decompressed. It seems to be running well with the repitched prop. Might be a bit too fast at tickover for some sections of canal now though. Any way thanks for help on this.

 

For want of a better way of describing it, the noise is pulses, or peaks, of momentary crankcase pressure due to the configuration of the cylinders.

In the horizontally opposed flat twin layout, both pistons move inwards to BDC at the same time, which significantly reduces the volume of the crankcase. The crankcase volume increases again as both pistons move outwards towards TDC creating a 'panting' effect/sound at the crankcase breather.

You don't get this effect on a conventional 'in-line' twin, because the crankcase volume remains constant with one piston travelling upwards in it's bore whilst the other one is moving downwards.

The flat opposed twin configuration is also the reason for the exhaust note sounding more like a 3-cylinder and very different from a conventional twin. The flat twin fires on every single revolution of the crankshaft, whereas the firing sequence of a conventional twin is ~ fire ~ then one and a half turns of the crankshaft ~ fire ~ half a turn of the crankshaft ~ fire, . . . the second cylinder always fires half a turn after the first one, instead of the one full turn separation with the flat twin.

[Sea Dog]

For want of a better way of describing it,...

 

What a cracking follow up to an already very informative previous post! It's not gonna help me much with my Beta 43, but I just thought it worth mentioning that it got wider recognition than just from the receiver of the advice who is doubtless grateful. Nice one Tony! (+1 greenie)

[Tony Dunkley]

What a cracking follow up to an already very informative previous post! It's not gonna help me much with my Beta 43, but I just thought it worth mentioning that it got wider recognition than just from the receiver of the advice who is doubtless grateful. Nice one Tony! (+1 greenie)

 

Thank you for those kind words.

 

I'm not familiar with the Beta 43, but if you can tell me what the base engine is, and how it's misbehaving, then I may be able to make some useful suggestions.

[Sea Dog]

You're welcome. The Beta 43 is based on a Kubota, but I'm pleased to say mine is, as always, behaving impeccably, so no help required from me... at the moment!