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Engine breathers

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Just out of curiosity ,does anyone know why the breathers on the 750/850 engine's are on the exhaust camshaft instead of the inlet, as on the smaller twin engine's. Other than that they are quite similar. 

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hello, now you can get crankcase reed valve breathers that fit to the bottom rear  of the crankcase,    yours  anna j

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Hi Dennis,

 

I only have the one camshaft in my bike. not sure if you would call it exhaust or inlet!

The lightweights did (do) have two.

Tony

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I suffered a bit of brain fade when I wrote the first post. I am in the middle of rebuilding a Triumph engine and got my camshafts confused. I meant to ask why the breather on the 750 was moved to the front of the engine as it's tidier looking at the back .

 

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I've always presumed that increasing bore when they made 750 out of the 650, the breather drilling in the crankcase came in conflict with the bigger diameter cylinder.

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From what I remember, under sustained high speed, the crankshaft would keep the oil up towards the back of the engine and thus get blown out of the breather with obvious consequences.

So it was move to a timed breather on the end of the camshaft.

Tony

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And then in 72 they moved it to the back at the lower part of the cases and at high revs more oil went back to the tank via the breather than the oil pump. That lasted 1 year and then came out of the old magneto location in the timing chamber.

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The '73 on has three large holes connecting the crankcase with the timing chest.

 

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Hello  Now has For breathers   Norton fitted a inlet Rocker cover breather  to the Norton Manxman 650   via  a Banjo fitting  the part numbers are available,    yours   anna j

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I bought a new stainless banjo breather outlet connection, but it doesn't want to go back to the tank, it just points downwards at the chain.

Was it just chucked on the road in 1961?

 

Steve

 

In reply to by steve_marshall1

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Hi Steve.

My late 1960’s 99 has an upward facing spigot/pipe on top of the chain guard and the breather pipe fits into that spigot. It looks as if it’s designed to lubricate the drive chain but I don’t think it’s a great idea as any oil will contain nasty stuff that wouldn’t do the chain much good.

Hope that helps

Tony

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Acidic gases and condensate dripping on the chain, maybe why they did not last long in those days.  

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1) The 750 engines lost the breather to the rear of the cases due to the changes that were made to accommodate the larger bore and greater crankshaft throw. The attached photos help with the explanation. In the P11 engine pic you can see that the old breather tunnel in the drive-side case has been machined down to allow the  750 barrel spigots to sit deeper inside the cases. The Mercury drive-side case pic shows the step in the back to allow for the greater crank stroke. Possibly there would have been a conflict of integrity if a large breather hole had arrived in this area to cope with 745cc of waste gases.

2) On many of the pre-1960 Dommie engines the breather was just a length of Copper pipe exiting the back of the case that pointed at the ground. Later models had a longer pipe that dripped onto the gearbox sprocket. Later still the breather pipe was extended to run to the top inside of the oil tank. This might have seemed a good solution at the time but in reality the waste gases pressurised the oil tank which then leaked out of its cap.

3) Anna is quite right about the first Manxman bikes having a breather from the Inlet Rocker Box. This actually works quite well and helps prevent gasket and rocker spindle cover leaks. It doesn't work if the engine has a pressurised rocker feed.

 

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