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MG Midget and Sprite Technical - Oil blowing out of filler cap

I have a 1972 midget with a 1301 (+30 overbore?) fast road engine/ The engine is fitted with roller rockers and an alloy rocker cover.

A couple of times now, after a 'spirited' drive I've found oil sprayed onto the inside of the bonnet from the vent hole in the oil filler cap.

I've checked the oil level and it is within the acceptable level.

Has anyone had this problem occur? Or any suggestions as to why it might be happening? To be coming out of the top of the rocker cover it would almost appear that the cover is full of oil. Even if it isn't full and it is just oil from the inside of the cover being forced out of the hole it seems to me that the pressure needed to cause this is likely to be above that which I'd want in the engine.

Thanks,
Dave
Dave Thrussell

Dave,

What breather system is fitted? The engine should have a 'closed circuit' system that has a hose connected from the timing cover to the carburettors, and the 'vented' oil filler cap.

The principle is that the vent hose to the carburettors, in conjunction with the vented filler cap, creates a partial vacuum in the crankcase and keeps the oil/fumes inside the engine. Varying degrees of success are achieved!

Richard
Richard Wale

Hi Richard,

That's what I've got set up.

Forgot to mention it's a single HIF44 carb fitted on a Titan manifold - not sure if that will make a difference or not.

Cheers,
Dave
Dave Thrussell

Is the alloy rocker cover fitted with the correct original black plastic filler cap/breather unit?

Replacement covers often have pretty chrome ones that do not work correctly with the breather system.
richard boobier

Dave,
does the hose from the timing cover go direct to the HIF44, or is there anything else (eg a PCV valve) also fitted? It shouldn't have one if connected to the carb; only when connected direct to the manifold. It sounds as if you have an obstruction to the "suction" side of the closed system
GuyW

Dave,
the other thought that occurs is that at high revs the amount of blowby may well exceed the suction rate provided by the carb through the extraction system, allowing a positive pressure to build up. If this is combined with perhaps an excessive oil flow from the rocker shaft this might cause your symptoms.

I assume that your roller rockers are fairly new and that their bushes are therefore in good condition. But what about the rocker shaft? Was that replaced as well? A worn shaft would allow more oil to escape into the rocker cover and although it won't actually fill up, there would naturally be a fair bit splashing around in there at speed!
GuyW

This is the effect of blowby on my engine when the extraction system was temporarily blocked off as an experiment. And that is at barely more than tickover speed. At high revs I have no doubt that the rubber glove would be the size of a hot air balloon and maybe have provided me with a new form of transport!

(The gauge in the background was measuring the suction provided by the HIF carb)


GuyW

Thanks for the comments, answers to the questions below:-

Yes Richard, the rocker cover has a chrome cap with a breather hole in the middle. Not sure if an original cap would fit - the cover is the same as the one on Guy's engine in the photo posted. If the original caps have better ventilation I assume the pressure wouldn't build up as much so the oil wouldn't be shot out the top, but would it not just cause an oily film to be pushed out instead?

Hi Guy, Yes the hose goes direct to the carb. The rockers and shaft were replaced together and, although second hand, both appeared in good nick. So did your rubber glove test prove anything in relation to pressure build up versus suction from the carb? Any suggestions as to how I could increase the suction?


I could take this as a sign to keep the revs down - but where's the fun in that? :o)

Cheers,
Dave



Dave Thrussell

Dave, regarding the cap ventilation, keep in mind that the cap is for inbound air, not to vent crankcase pressure out.

Just thinking out loud here, did that single HIF come from a car equipped with only one carb, or is it half of a set of two? I am thinking the HIFs have some sort of restrictor to limit the air taken in to vent the crankcase, and if the carb is half of a set of two, it may not draw enough air to vent the crankcase. In a single HIF application would the restriction be larger?

Charley
C R Huff

Dave,
My experiment with the inflating glove was to establish that there was a positive pressure build up, even at fairly low revs, even when the system was open to atmosphere at the timing cover connection, but not connected to the additional suction provided by the carb.

At the same time I measured the suction at the carb, which turned out to be fairly modest; 1-2 bar as far as I recall. So not very much suction is actually needed to keep the system operating correctly.

You are right about my vented chrome cap. But I don't think that cap has a large enough hole and I have since reverted to using a (less attractive) vented plastic cap. I think that for the extraction system to work properly the size of the venting is critical. If too large, the system draws air in too fast (i.e. too easily) and doesn't create the required negative pressure in the crankcase. If the hole is too small then the system is strangled and doesn't pull enough volume of air through to purge fumes from the crankcase. But I don't think any of this helps answer why yours is blowing back through the filler cap.

Do you suppose that either the gauze filter in the timing chest canister, or the rubber hose itself, is blocked?
GuyW

Charley may be right about the single / twin version of the HIF carb.

But thinking further about the hole size in the vent cap. The system is fairly finely balanced and needs to cope with quite big variations in blow-by pressure as the revs change, whist also purging the crankcase and maintaining a negative pressure to help minimise oil leakage at the rear crank scroll seal.

Suppose your vented cap isn't allowing enough volume air in. Then the oil fumes are not sufficiently purged and an oil "cloud" builds up inside the rocker cover. Then when the revs peak the blowby overwhelms the crankcase pressure, blowing some of the oil cloud out of the vent.

By comparison, just suppose that with a larger vent, the system properly purges oil fumes and doesn't create such a cloud of oil in the rocker cover. As a result, at peak revs it may still reverse the venting out of the oil cap, but what is blown out is less oil-laden. And with the correct cap there is an in-built gauze filter to trap oil droplets. It would be easy with your car to test this with one of the correct plastic vented caps. It might be a simple solution - if you are lucky!
GuyW

Guy,

Would you care to correct the 1-2 bar reading as that is impossible unless you were running the engine in a hyperbaric chamber. To achieve even 1 bar vacuum at sea level would require a very good pump and a car engine isn't, maybe 0.9 bar.
David Billington

Going from memory Dave - which isn't very reliable these days! I see your point so I will see if I have a proper record somewhere and report back. Probably just got my units wrong!

I think the main point is that the suction provided at the carb is actually very low.
GuyW

Got it David! You were right. Not 1-2 Bar, but 1" to 2" Hg of vacuum. So very little suction is necessary,

See photo taken at the time.

The other mistaken memory was that the glove inflated at low revs with the breather pipe open to air. That was not the case; it only went "positive" at moderately high revs, but this might accord with Dave's oil blowing out of the filler cap.

<<Raising the revs, but still with no breather pipe vacuum applied, on my engine blow-by begins to overcome the rate of flow through the breather pipe system (slightly longer and more complex than standard)such that the glove can just about maintain a semi at 3500rpm. A less worn engine would presumably run to higher revs before this happened.>>

Its from this mega-long thread from 2013.
http://tinyurl.com/h6eb3vb

Not recommended, unless you have several days to spare!


GuyW

Guy,

Thanks for the correction, that makes much more sense. Please don't take my comment as a criticism, I just felt the units had to be a mistake.

Regarding the oil spray out the cap, in my experience that would indicate too much oil past the rocker bushes and excess blow by. I've run my engine with rebuilt rockers, new bushes and new hardened shaft, and there seemed to be little in the way of oil in the rocker cover area but all was lubricated well as it covered about 65k miles before being taken off the road for repair to the body and no significant wear in the rocker assembly. My neighbour had problems of oil being blown out from the rocker cover vent on his Morgan (Ford X flow) and that was definitely excess blow by, corrected by an engine rebuild in the end as on inspection all 8 compression rings were broken, likely due to a previous owner that did a Morgan road going race series.
David Billington

Looks like I'll be looking through my old parts to see if I've still got the original filler cap then.

I don't think the gauze is likely to be blocked but will try removing the breather pipe and see if I can tell.

The carb was originally off an MG Metro so was always a single set up.

I'll have a play with the cap and venting and see if it makes a difference.

Looking on the bright side, the bonnet won't rust from the inside as long as I keep my right foot down occasionally!

Thanks again for the comments and suggestions - happy to hear any more ideas if anyone has any. I'll report back if the cap makes a difference.

Cheers,
Dave
Dave Thrussell

I think the chrome vent hole is a red herring... i have the chrome cap with no issues plus these engines dont seal all that well they suck air in from the rear birds eye seal the oil dip stick hole and the crack case oil pan gap area so unless you know those areas are sealed i dont think the chrome cap is an issue

as you say... its a positive crank case issue

i do know for a fact that a broken piston landing will create a gyser effect out of the chrome oil cap

my guess is you got some engine gaps sucking in air and its not allowing the evac system aka carb to do its job and suck out the engine thur the timing cover causing a positive crwnk case that not allowing the oil to drainoff the top of the cly head fast enough

or the carb is not hooked up to create enough suction

or... i made this mistake... you have port vac which measures ariund 9 bars and the timing cover vent hose has been replaced with something soft and the 9 bars are sucking so hard its collapsing the hose ...i replaced my timing cover hose with transmission hose becuse it wont colapse under suction


prop
Prop and the Blackhole Midget

Lots of ideas! It will be interesting to see what the final cure turns out to be.
GuyW

No problem David B. You were quite right to pull me up on that wrong units claim! And combination if blow by and worn rocker gear was my first thought too.
GuyW

Should I even bring up the subject with Prop sucking 9 bars?
David Billington

This thread was discussed on 04/04/2016

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