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MG Midget and Sprite Technical - Crankcase breathing problems

My newly built 67 midget has a crankcase breathing problem, I think! Oil is throwing out of the rear end of the crankshaft and out of the dipstick tube.

I’ve attached a photo that I hope helps to explain the set up. I have connected a Catch Tank to the oil separator. The Catch Tank breathes into the rocker cover and we’ve attached a small vacuum hose between the top of the Catch Tank and the front carb to create some negative pressure.

The rocker cover is an after market chrome fitting with a breathing pressure cap (just visible in the photo). We’ve removed the cap while running and it makes no difference. The carbs are the early type designed to go with an emission control valve so fitting the later “Y” piece breathers is not an option. We’ve blanked off the inlet manifold where the emission control valve would otherwise fit.

Present thinking is to simply beef up the hoses from and to the Catch Tank (there is a step down between the oil separator and the Catch Tank hose) but do we also need to beef up the vacuum (negative pressure) and if so how? The hose between the front carb and the Catch Tank is the maximum internal diameter that will fit on the front carb vacuum take off.

Is this the correct approach or should I refit an emission control valve connected to the Catch Tank, or, something entirely different and if so what?

All thoughts, comments and suggestions will be gratefully received.


BillMex

No1: make sure it's not overfilled with oil.
No2: A new engine will have increased crankcase pressure until the rings bed in. Do a series of full throttle runs up a steep hill in 3rd gear, from 1500rpm up to 3000rpm. If it's a new cam don't let it rev higher or lower until after about 30 mins of running.
Mike Allen

Shouldn't the catch tank be higher up? I've mine by the windscreen washer bottle.
Brad (Sprite IV 1380)

That vacuum port on the carb is for ign advance, and is very small. In effect, you have sealed the system and have no breathing at all. You should have a vented cap on the rocker box, and the hose from the catch tank feeding into the air filter box to approximate the early type breather system. Or the catch tank hose going down under the car for the really early road draft type. Or put the PCV valve back.

FRM
FR Millmore

I can't see any vent on the catch tank.
John Collinson

Frm is correct...DUH! Has he ever been wrong (LOL)

where you are pointing to is for the dissy adv. port...

Some Options ...

you will need to ither get newer carbs with the brass ports and the "Y" peice or make shift add them in

you might try porting to the intake manifold instead of the front carb

another idea would be to tap both of the black spacer blocks that fit between the carbs and the intake manifold ... but do so at a 45 degree angle away from the flow, and on the bottom side of the blocks as thats where most of the vacume flow will be....Im not 100% that will work...But its an idea

one thing to remember...this engine will probable continue to leak at the rear seal but most likely not during idle or low revs...but mostly when driving down the freeway at high revs....why? Because carbs butterflys will be wide open meaning alot less vacume is generated. Look into vizards bible, he has a nice diagram for using the exhauste pipe to create vacume at high end revs....but you have to have a fairly non-restrictive muffler/system...such as CHROME SIDE PIPES (LOL), or an original classic cherry bomb muffler. (see there website by the same name) other wise the back pressure will close the gulp valve shutting down the system

If the engine does leak, its just one of those things that has to be expected and lived with...hey at least you will always know theres clean oil in the sump

One last note...you might try putting in an in-line PCV valve into the rocker cover that way the vacume flow only goes one way

Prop
Prop

I suggest removing the plug from the intake manifold and installing a PCV valve, as this is how the car was probably configured originally. By "emission control", I assume that you are making reference to the PCV set up. "Emission Control" over here includes an air pump, fuel vapor canister and, on 1500 models, a CAT.

The A series will always leak at the rear scroll. Even if you adapt a lip seal to the rear, it will leak soon enough as well.
Glenn Mallory

I'll agree with JC - no vent on the catch tank so how does the air get out?
Daniel Thirteen-Twelve

Many thanks for all these helpful suggestions. I think we're going to be busy for a while.

One or two additional points that may influence the thinking here; the car has been completely rebuilt and has only done around 60 road miles since passing the MOT. I'm using Valvoline racing oil. We've had the 1275 engine bored to +40, fitted a 735 cam, added a stage II head and an LCB exhaust.

Does any of this make any difference to the suggestions?

Clearly the catch tank set-up is wrong and we'll need to take some action here and for my peace of mind I think we'll do a compression test.

Thanks everyone.
BillMex

Bill,

My experience is that the blow by with new rings is very great in the first 50miles or so but reduces dramatically after and around 100 miles is what the engine will do for much of its life after, once the rings have bedded in.
David Billington

>I'm using Valvoline racing oil.

Be SURE to add ZDDP. The cam and lifters failed last year after using this oil for almost 40 years. Other engine parts were in exceptional shape. It appears that ZDDP content was greatly reduced be Valvoline.
Glenn Mallory

I got to go with glenn and use the ZDD additive... make sure the oil is 20/50W viscosity, these engines really perfer this viscosity...only during the dead of winter would I go to 10/30W
Prop

This is all very helpful. I've never heard of the ZDD additive but will find some - pronto! I'm encouraged by David's reassurance that the new rings can take a while to bed in and will persevere while working through the many great suggestions and ideas provided.

My biggest problem now is working out a logical order for the many checks, tests and changes that need to be tackled.

This rebuild has taken over 17 years and I have to say that the car is great to drive. It makes by tired 72 RWA feel very second hand. However, my two mentors, Dave and Mike, and I only spend around four hours a week on the car, two on Monday evenings and two on Thursday evenings so it may take a while to go through all the recommended checks and changes. I will post progress reports in the hope that this experience will be useful to others.
BillMex

ZDDP is a vital anti wear additive for classic engines and is not present in modern oils intended for use in cars with cats.

Here's a quick link to a page giving more details
http://www.volvoclub.org.uk/castrol_with_zddp.shtml

There are also special oils for use whilst running in a new engine. They help to bed the rings in quickly

Oils are cheap compared to an engine rebuild so its always worth getting the right one!

Hope this helps

David
http://vintageman.zenfolio.com


David Southcott

Thanks for the link David. While I'd not heard of ZDDP it seems that the Valvoline Racing oil I'm using (20W-50 mineral) contains ZDDP according to their information sheet - www.valvolineeurope.com/uploadedFiles/1725.pdf.

It seems that this should be OK?



BillMex

Bill, have you had the VR1 in it from start up? You should really use running in oil for the first few hundred miles, I never knew of this until I read about it on here, it will help bed the rings in and stop breathing and oil burning problems.
John Payne

Hi John, yes the Valvoline has been in from the start. To be fair someone did say (after we'd finished the build and filled the engine) that I shouldn't use it until the engine has run in but it didn't seem right to deliberately let the engine effectively "wear" more than need be before using a better quality oil.

Perhaps I was wrong?
BillMex

Modern engine designs operate, in the main, with higher piston ring radial loadings and much tighter tolerances,thus the oil specifications include additives that cater for these loadings plus emission criteria.
Older engines need to be 'run in' using oils that do not inhibit the piston ring to bore 'bedding in' process.
Using a very high specification 'modern'oil from the start will probably lead to unnaceptably high oil usage and ultimately bore glazing unless you are very lucky. Reverting to a higher spec oil after getting the rings 'bedded in' is usually OK practice.
PJ HOBSON

one thing to consider also is you dont want to use Hi lube oils (for lack of a better word) for the 1st 500 miles as they will glaze clyinder walls and you will get ba blow by and will have to tear down the engine and re-hone the cly. to break the glaze so the rings can bed in, Its widly debated to not treat the engine vary gingerly during this period as abuse tends to help the break in period....I think a K-mart brand of 20/50sae will work perfectly for your needs, then use the high end oil

If you drain the oil call around to various shops to see if they have an oil recycler/cleaner machine...not sure how good that works but worth a look into. as Im sure that racing oil wasnt cheap

Prop
Prop

This thread was discussed between 22/01/2010 and 23/01/2010

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