Strange breathing problem!

[Welung666]

On my way back from the national I had the smell of burning oil just as I came off the motorway so pulled into McD's car park and had a look! It appears that whilst coming back at around 70/75 the crank case pressure got up a bit (a lot actually) and forced oil up the rocker breather pipe drenching my nice new air filter and slowly dribbling down the air filter inlet pipe and on to the hot exhaust! It now does it cold when I 1st set off in the mornings to work!

Any suggestion/ideas as to why this has suddenly started happening?

[Pikey]

You might find that the good run that youve just done has probably cleaned the engine of a build up of internal moisture (lots of short runs?) and now blocked the breather pipe (with an emulisified oil) which exits the front tappet chest cover, on the back of the engine. This might explain the build up of engine pressure and the only exit for the air which unfortunately seems to have carried oil as well is out the rocker cover breather and into your filter. Take the tappet chest cover off and give it a flush through.

[Welung666]

Mine is a 63 so no tappet chest breather. IIRC it comes out of the tming chain cover but I'll check it later at work

[Packedup]

Timing chain cover breather is a 1275 thing isn't it? Not to say someone hasn't fitted a 1275 cover to a 1098, but I'm sure it's not a standard fit.

You might find your engine is just breathing very heavily at constant high revs, the fact the rocker breather is obviously still breathing suggests that to me, bearing in mind many engines only have the one breather (Triumph OHVs for example only have the rocker one) and don't tend to spew oil out for fun...

[Willie]

If you have t he breather pipe from the rocker cover to the air FILTER then you MUST also have the upturned swans neck pipe on one of your
tappet covers( that is the INLET ). You should also have the SOLID oil filler cap i.e. no wire filter in it. A further point is that you should check
the actual size of the hole in the pipe on the air filter inlet, It should be
about 1/8" only!! There are some identical looking filters which have a much larger hole and this allows too much oil to enter and ruin the filter element.

[bmcecosse]

Unless it's a 1275 it should (MUST!) have a tappet chest breather. It' s far better to route the pipe so it either connects to the carb (later models) or comes up INSIDE the air filter casing. Even so - this all sounds quite extreme and could very well point to piston ring problems.

[Welung666]

I had a look at work and it's the swan neck off the tappet chest cover with a solid cap but the rocker cover pipe to the air filter looks about 11/16" could I not use a rocker cover without the outlet and a mesh cap instead or add a filter to the pipe between the 2?

[Stig]

My recon. engine came with the swan-neck tappet chest cover and a pipe out of the rocker cover. What I did was to block off the rocker cover pipe with a short piece of hose and a suitable bolt in the end and connect my existing (carb) breather hose to the tappet chest cover. There's a mesh oil filler cap of course. It seems to work as it uses very little oil now.

[bmcecosse]

It should have two breathers - you should unblock that hose asap!!

[Stig]

Eh, why? Surely it's getting air in via the filler cap and that's being sucked out via the tappet chest cover. If I unblock the pipe on the rocker cover then air will just go into and straight out of the rocker cover and not directly vent the crankcase at all. All I've done is to try to replicate what I had on the original (1969) engine.

[Packedup]

I often wonder why I bother on these topics, but anyway...

You want to suck the cakky oil vapour/ combustion blow by gasses out of the crank case. Or at least have them vented to air.

Contrary to certain opinions, the closed breather system was introduced as an emmissions limiting device, prior to that the tappet breather would have a tube hanging in the air flow under the engine bay and the fumes would release into the world. Closing that system so the fumes went through the carb was done to burn the hydrocarbons and be kinder to the fresh air all around us. There's also the benefit of that bit less crank case pressure this way, reducing the amount of oil that sneaks past that oh so effective scroll on the rear of teh crank.

The downside is the engine is pulling in old fumes and low octane oil vapours rather than fresh air and fresh fuel. Of course, if the engine is still healthy enough the vented cap will be allowing an amount of fresh air in, but the engine is still sucking on it's own vapours o an extent!

Now, the more breathing an engine has the better. Far more knowledgeable and experienced people than I have made their reputations putting the pros of venting to air (or through the exhaust with a fancy valve) into print along with other extremely well researched information about increasing power, economy or both. So steering clear of that area of contention the point is whether you're sticking the breather into the carb or not, there's no point blocking off a breather. But you also seem to be confused as to what they do - If you have more than one they're still "outlets", not inlets! So if you're running the closed breathing system, you have *all* the breathers T pieced together and fed into the carb/ air filter. not one fed in and another open to the air!

In other words, if you've got a rocker breather and a tappet one, get a T piece and some hose and link them. Route the other part of the T with more hose to the air filter/ carb. Or invest in a valve, some steel tubing and a bit of rubber, and have some real crankcase ventilation out the exhaust! ;)

[Stig]

My point was that the air would find the path of least resistance, i.e. from the oil filler cap through to the rocker cover pipe. I agree that if there's positive crankcase pressure it'll come out anywhere it can - including the oil filler cap, after all there are no non-return valves anywhere. If the carb is sucking nicely then you'll hopefully get slightly reduced crankcase pressure. Depends how much blow-by there is (it's a fairly new engine) and how much carb suck there is. I frankly doubt that connecting the rocker cover pipe as well as the tappet chest pipe would improve things in my case.

[bmcecosse]

No - the tappet cover breather should be left open to allow a measure of air into the engine (to help remove water vapours) - and the rocker cover breather should go to the carb. Anything sneaking in through the oil filler cap is a bonus. Most caps are 'solid' these days. But getting back to the original poster - the oil fume there sounds far too much to be normal - probably rings/pistons/bores trouble.

[Stig]

Well, I seem to have started a bit of an argument here! I don't like the idea of an open pipe allowing unfiltered dust to be sucked in thanks, I'll stick to what I've got as it's the most leak-free A-series I've every owned.
Agreed that Welung's engine sounds a bit worn but as long as he works out some way of getting those fumes sucked into the carb after the air filter then the nasty smell should stop at least.
Welung, did you replace the air filter and clean out the oil (from the filter housing and the breather pipe) and still get the problem?

[Willie]

Packed Up, I believe you are mistaken if you think that the tappet cover pipe is an outlet. With the correct rocker cover pipe breathing into the air filter and the accompanying SOLID oil filler cap the tappet cover pipe is the only means of allowing air IN to the engine. BMC I think you are mistaken too if you think that most oil filler caps are solid nowadays. I find that I cannot buy a new solid cap only the filtered type.

[aupickup]

if its the non vented cap you want willie esm sell them

[Willie]

That's useful thanks

[bmcecosse]

Hmm - must admit -I have never bought a cap - huge stock of rocker covers all with perfectly good solid caps! They never wear out.

[Roni]

I saw in an old publication, a manual, a description of the pipe on the side of the engine. It was called the " road draft tube ". In the description of how it worked it said that the foward motion of the vehicle would draw air out of the tube, to be replaced by the air drawn in from the air filter and through the motor. It was a puzzle to me when I first read that as to how that could work as I knew by experience (mayonaise,blocked oily air filter, etc) it didn't. Getting oil sucked/blown into the filter area would not have been so bad in the days of oil bath air cleaners.
I'm not going to say at the moment what system I have at the moment except to say that it works. I have a clean, dry engine.
It seems there are many answers to the question on how the engine breather system should work.

[bigginger]

But surely only one (depending on the system) if you're keeping to factory spec?