Breathing/oil burning issues.

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Guys.My 1970 750 Commando was finished few months ago.Engine was rebuilt by one of UKs most prominent builders.For first few hundred miles, bike ran perfectly.Started without fail, pulled like a train.
The engine has the CNW/Comstock breather mod fitted to lower case,returning back to top hat on oil tank.
There is a magnatek(?) oil filter fitted on return pipe.
An anti wet sump valve from Holland Norton works is fitted at oil feed.The breather from front of oil tank(circled in red) vents to a catch tank as shown.
Initially there was virtually no oil mist accumulated in the catch tank.
However it recently began to smoke from both barrels when hot.Not excessively but its there.Also, the catch tank fills with oil pretty rapidly.However, strangely oil doesnt vent from catch tank to atmosphere.
My engine guy and myself are pretty puzzled as to what maybe happening.
Hopefully you guys can offer some input.Thanks.
 

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However it recently began to smoke from both barrels when hot."

What comes to mind - If there is oil smoke in the exhaust it's either valve guides/seals or piston ring sealing...OR...the rocker arm spindles are installed backwards allowing excessive oil into the rocker boxes. If that is the case, the guides/seals will be submerged in oil and no matter how good the seals are, oil will enter the combustion chamber.
 
This just occurred to me - if the crankcase ventilation has "failed" - that is, not allowing crankcase pressure to vent - it could cause oil ingestion into the cylinders. Admittedly, I'm not sure how that would occur - the vent system would have had to clog almost completely...

But you could easily check - disconnect the existing vent system and just allow the crankcase to vent to the atmosphere and see what happens.
 
You can hear the reed breather ‘fart’ if you kick the engine over very slowly - that will confirm whether it’s working or not.

Not sure if that’s the best description, but hopefully you get what I mean.
 
You can hear the reed breather ‘fart’ if you kick the engine over very slowly - that will confirm whether it’s working or not.

Not sure if that’s the best description, but hopefully you get what I mean.

Well that’s EXACTLY how my kids described it when they first heard it on my Bonnie... “Daddies bike just farted !!!” followed by much giggling and general hilarity...
 
A head gasket failure will do this as the compression leaks into the pushrod tunnels.
Were the head bolts retorqued in time?
 
You can hear the reed breather ‘fart’ if you kick the engine over very slowly - that will confirm whether it’s working or not.

Not sure if that’s the best description, but hopefully you get what I mean.
I know exactly what you mean! If I remove exit pipe from breather,I would likely feel pressure exit as its kicked over too.Ill check this.thks.
 
Considering the engine was rebuilt, discount valve guides and if built by one of the UK's finest then discount rings also. That leaves rocker spindles - possible, many have rocker spindles move and rotate and only realise when they strip the head. That leaves the most probables, inlet valve seals popped off, or as above gasket. A lot can be checked and discounted without even removing the head, so the fix should be identified and fixed relatively easily.
 
The fact that your catch bottle did not catch oil before, but does so now, intrigues me.

Over the years, I experimented with several configurations of catch bottles, or rather, scrubber bottles to de-mist the vapour leaving my oil tank. I found no oil is deposited in a bottle unless the bottle is filled with an oil scrubber media, and the vapour flow is forced to travel thru the media before it can leave the bottle.

Transferring the above to your situation, no oil, or very little oil, in the catch bottle before your problem, is exactly what I would expect. If the bottle is now filling with oil, it seems to me the vapour entering the bottle is now very heavy with oil, and very likely laden with oil droplets. What can cause such a change in the composition of the oil tank effluent? Only thing I can think of is overfilled oil tank, but perhaps someone may think of something else.

I am not familiar with Commandos, but it seems to me your oil tank vent is at very low elevation. The vent on my Atlas comes off a tower ABOVE the oil tank roof.

HTH,

Slick
 
I think Jim’s head gasket leak has to be up there amongst the top contenders. I don’t know how experienced you are with Nortons or what your head tightening ‘program’ is, but if you’ve used a composite gasket it is AMAZING how much they crush and how much / how frequently you have to tighten them in the first few hundred miles. If this is all it is, then tightening it up now should sort it.

Alternatively, if it’s a new problem, what else has changed? Have you changed for a thinner oil, perhaps too thin?

Or could it be that actually the issue has always been there and it’s now becoming apparent because you are giving it more revs?

Incorrectly oriented rocker shafts would certainly fit with the latter option.

So, there are a few relatively simple things you can try before getting too ‘doom and gloom’...
 
@Madnorton depends who the UK’s finest is of course!

Some people hold your best mate Les Emery in high regard, and describe him as the UK’s most pre-eminent expert when it comes to the Commando!
 
Hi guys thanks for the input.I spoke to my mate today who was following me.He said that the oil coming from the exhaust wasnt under pressure.It was as he put it, just drifting out, as opposed to a "cloud" which youd expect if for instance engine was burning oil,(as in worn rings etc) Seems there is oil in pipes.Would also explain, why it happens when hot not cold.
 
I think Jim’s head gasket leak has to be up there amongst the top contenders. I don’t know how experienced you are with Nortons or what your head tightening ‘program’ is, but if you’ve used a composite gasket it is AMAZING how much they crush and how much / how frequently you have to tighten them in the first few hundred miles. If this is all it is, then tightening it up now should sort it.

Alternatively, if it’s a new problem, what else has changed? Have you changed for a thinner oil, perhaps too thin?

Or could it be that actually the issue has always been there and it’s now becoming apparent because you are giving it more revs?

Incorrectly oriented rocker shafts would certainly fit with the latter option.

So, there are a few relatively simple things you can try before getting too ‘doom and gloom’...
Nothing has changed.Bike was burning nothing at first, as I would have expected.Which is why its so perplexing.Im leaning towards head gasket not torqued!
 
Coomo, how many miles have you done since the rebuild and what head torquing program did you follow?
 
@Madnorton depends who the UK’s finest is of course!

Some people hold your best mate Les Emery in high regard, and describe him as the UK’s most pre-eminent expert when it comes to the Commando!

I some how doubt that, never met him and no mate - I work for his most hated opposition!!
 
@Madnorton I know who you are, who you work for and the various legal spats/piracy over the years.

I was trying to be humorous (obviously not successfully)
 
Coomo, how many miles have you done since the rebuild and what head torquing program did you follow?
about 300 miles.Re torquing.Due to some confusion,It may not have been done(complicated)Im hoping to check in week.
 
Check the easy stuff, retorque the head (make sure to loosen 1/8 turn first), then rocker spindle orientation (check with a piece of 12g wire with a tiny bend at the end)
 
A head bolt re-torque might fix it but if the engine has gone 300 miles without a re-torque the gasket is probably damaged.
 
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