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MG Midget and Sprite Technical - Oil Catch System 948

Second thread today (sorry!)

modified 948 engine (+0.060, flat top pistons, 12g295 head, standard cam (likely to upgrade to fast road) - preivously a 649, twin HS2s -> heading to H4s)

currently an open system with
original rocker top, unvented cap, breather pipe
tappet chest breather with rubber pipe down to near gearbox : open
Mechanical fuel pump: blanked off

Id like to close this system to allow me to use it on track days etc. I currently don't suffer from crank case oil leaks etc.

Would just plumbing the rocker breather, tappet chest breather +/- a third from the fuel pump take off to a vented oil catch tank on the passenger side wing (UK spec) be an acceptable plan? any perceived problems?

DJ Paul

Your proposed breather system sounds fine.
May I ask why you think you may need H4 carbs on a 1 litre engine? The HS2s will run to over 100bhp on a 1293cc one.
David Smith

from 'Tuning S.U. Carburetters' booklet (available on web as pdf download) -

'An alteration in size of carburetter should only be necessary if the breathing capacity of the engine has been substantially increased.'

'If larger instruments are to be fitted then the next size should be used; it is very rare that an engine's breathing capacity is increased to the stage where it requires an increase of two sizes. Remember if a larger instrument is fitted to a power unit which does not really merit this increase in carburetter size, the piston will not reach its full travel - the only benefit achieved will be a more impressive underbonnet appearance!'
Nigel Atkins

Translating what nigel said for us avg folk. Which he is dead on

An engine can only process a certain amount of air/fuel and will not accept any more once that limit has been reached

A carb will only provide enough fuel/air that the engine ask for and not a drop more

If the engine only wants 450cfm ... thats all the carb will deliver even tho the carb is capable of 850cfm

The problem with a larger carb then is needed is the opposite of what you may think... larger is better, when in fact the reality is smaller the better

With a large carb think of it as a 4 inch pipe as the carb going to your engine the hole is so large it won't suck fuel into the engine and it wont atomized the the fuel very well there is to much room and that room can't be sucked out very efficiently

If we go the other way and hook up a pipe as a carb and make the diameter 1 inch the engine will try to suck a bowling ball thur the 1inch pipe with a lot of force but if the carb is to small it wont suck enough fuelair into the engine

So it's a balancing act you want the carb just big enough so it wont STARVE for food but you want it small enough to act like a vacuum cleaner and suck that fuel/ air down into the cly with as much force as possible

If you go to duel HS4 carbs what will most likely happen is you will use a massive amount of fuel and it will leak every where cuz it can't mid with the air and top speed will be around 60 mph cuz it can't atomized the fuel very well and there will be very little suction from the carbs into the engine

It's a crazy day that you need to go up 2 carb size from orginal factory spec

So your HS 2 carbs are fine.. add some trumpets onto the carbs and you will get better more curvature air flow into the carbs instead of a 90degree turn and into the carbs for extra performance

God, did I really spell negils name correctly at the top opening

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On your oil catch tank system... I would not vent the catch tank... that will create a vacuum leak same as leaving the oil dip stick open ... giving you lots of oil leaks at the rear oil seal

Make sure you have a PROPERLY vented oil cap then I think your system will be fine

Did you see guys coffee can catch tank... that is about as perfect a catch tank as you can get using DIY elbow grease

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Just for clarity...

I'm assuming your going from the catch tank to the a 2 way splitter then to both carbs to create the suction out of the crank case?

Btw... you may need to check all the gaskets are sealing tight to avoid any air from being sucked into the crank case such as sump, valve cover front engine plate dip stick and dip stick holder ECT ect.

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its a 948 so the rocker cap is currently unvented

do I need to plumb the catch tank back to the carbs? can it not just breath to atmosphere via a filter?

The current plumbing is:
unvented rocker cap
open vent on the rocker top to air
open vent on the tappet chest to air via rubber hose down side of block
mechanical fuel pump blanked off as replaced with electric.

DJ Paul

Thank you for the carb advice. I understood it was borderline for a 948 but thought with the upgrades it may be appropriate - the simular spec sebrings in period ran H4s as offered by DHMC.

Going off topic but in terms of extra breathing before I stripped it apart again it was running
948 + 0.060 flat tops giving a 9.3:1 compression ratio I think off top of my head and 999cc of displacement.
12g295 head
1.3 roller rockers
HS2 twins on K&N cone filters
3 to 1 manifold with short straight side exhaust with small silencer
Fully balanced bottom end
standard 948 cam (previously had something ridiculous in it - 300/300 spider drive cam that was unmarked - cam on cam at 3000 and reved easily to 6500 before lost my bottle)

achieved 52bhp at wheels in that trim at 4800 rpm - still climbing, we didn't take it higher at the time as was still bedding the engine rebuild in but feasibly another 1200rpm in it which would peak out on the current curve at about 60bhp. so 30-40% ish increase on power from standard

next upgrade steps would probably be something like an SW5 cam and H4s but open to opinions - not something going to be doing soon as plenty of other work to do first.
DJ Paul

I know next to nothing about engines and less than nothing about Sebrings so please take my remarks now as just general and not aimed at yours or anyone else's engine. Similar or same spec engines will have different requirements and outputs becauae of the engine builder, assembly, parts specification and finish. Racers particularly have different approaches of how to achive the best result even within (or not) regulation restrictions.
Nigel Atkins

If your current system is a simple breath to atmosphere system and is working OK, then just install your catch tank and allow that to breath to air in the same way. No further modification is needed, unless you are wanting to change the breather system entirely, or it is leaking excessive oil from the rear scroll. But there is nothing wrong with the early passive breathing systems.

It was only as power increased on later models that they began to experiment and then refine a positive crankcase ventilation system. i.e. one that positively sucks air in at the vented filler cap, through the engine internal voids and then out, using one or other point on the induction vacuum.

However, if you are competing there may be regulations requiring even a breathe to atmosphere system to have all those breathers plumbed in to a closed system, including the catch tank vent. You would need to check that point.

The problem with your intention of plumbing the present 2 orifices into one catch tank, is that this may begin to restrict the free air movement (2 into 1 system) and this could then cause a build up of pressure and make the rear scroll seal start leaking. A bit of experimentation is needed. Add a catch tank, open vented initially and see how that works out. You could always then connect it through to an induction tract vacuum source later on.

Sorry DJ,

I'm a bit confused as to what your wanting to accomplish on the breathing upgrades

I'd get a copy of the race regulations that focus on the oil system and see what they want if it's a sealed or a open vent system

I was looking at this as a modificatikn tona closed system from an open ... But yes I think your good as is as you describe.... the trick is will the race officials ?

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If your truly married to a HS4 Carb.... perhaps just a single carb instead of dual carb.... or a single Webber

Look for a good couple of books Daniel Stapleton has a good midget book out and David his are

I can't remember the names of books for both men , but Amazon will display them with no problem... lots of insight from both books


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A vented to atmosphere catch tank should be ok for UK track days so long as it's of an acceptable size for the organiser.

If you have a catch tank that recirculates the air then it will be defined as closed breathing system so the catch tank size will be irrelevant

The catch tank does have to vent to something - either atmosphere or back into the induction system. The vast majority are simply vented to atmosphere. Guy is correct that the venting mustn't be restrictive and that usually means a large enough opening on the tank itself. It is also possible to put non return flapper valves on all the breather pipes but even then they do need to pass enough blow-by to avoid creating pressure in the crankcase.

Hey Danial,

Do you by chance happen to know if there are any good books on the topic


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I ran a 998 with a 649 and twin HS4's back in the early days and it leaked like a sieve, but my CR was over 12/1. I think if the engine is dry it's clearly been built properly working well,and you should plan stay with open breathers. This is more or less what I've ended up with now - a non vented cap, and the timing cover and rocker box plumbed into a catch tank on the inner wing. On this 1330 with a 286 and rear seal. I've found it doesn't collect anything, and it doesn't lose oil either.

f pollock

DJ (Dan) - I hope it gives you a little warm inner glow that no one seems to appreciate the difference between H4 and HS4. (Like Little Willie and his H20/H2SO4 differentiational problem?)

In fact all SUs, as well as coping with being undercarbed can work remarkably well when used in a situation where - theoretically - the engine is overcarbed. Unlike a fixed-venturi carb (Weber or Dell’Orto) they effectively compensate – the piston doesn’t rise so much.

(SUs also do not waste engine vacuum - that most precious of all commodities - unlike the idiotic PCV system which has no place on any car where performance is at all important.)

It was probably Eddie Maher of Morris Engines Branch, rather than the Healeys, who identified that twin H4s hit a particular sweet spot on the A-series engine. The 1959 works Sprites at Sebring and the first two disc-braked road test Sprites used the newly-introduced HS2s, but H4s were spec'd for the BMC Formula Junior engines. In fact racers in other formulas stayed with H4s for many years despite their being essentially a pre-war design.

The H4 was reckoned to give better low-down torque, thought to be due to the slightly longer inlet tract within the carb body. But the H4 may also produce chunkier atomisation – as David Vizard has observed, the A-series does not generate its best power if the droplets are too fine.

For security purposes, once you have read it, please eat this message. The secrets of the Fellowship of the Sebring Sprite must remain safe ...
Tom Coulthard

I don't know tom,

Your talking about finding part that are pushing 70 years old... this engine is a bored out .60 over 948, so there is going to need a massive amount of modification unless you can find the old works parts that were sd to the public in the 60s

Yes you can as far as I know put H4 carbs directly onto a 1275 intake manifold and the attaching bolts all line up where as the HS4 CARBS the attaching bolts are offset as I recall so you would need to cut up 2 manifolds and tig weld them into a frankinstine creation

You would have to modify the block and A 1275 head to play with each other or

Come up with some kind of spacer attachmettattachment
To marry the 2 parts togather

And I doulbt doubt this would be a very steerable engine.. as it would need to be radically modified to take advantage of the extra fuel charge

Also it would need to be ran WOT to see the main advantages... the carbs will only deliver what the engine needs and not an ounce more

For the kind of money he is looking at he could do a mild 1275 and get the same results or drop in a 1.5 k series and have a real demon on the wheels

There is going to be a butt load of rolling road time to set the carbs up, not to mention an on sight dissy guy to continually work over the curve to pair up the carbs... the expense would be insane

Agian... I'm the wrong person to ask any real detail on this as everything I know about H4 carbs and there history I just posted

If anything get one of those old super chargers that came from the works team
.. they come up on eBay all the time, they went the best like an eaton, but I'd think the performance would easy match the performance of 2 h4 carbs and the engine needs o be as close to stock as possible for those old blowers to work at top speed and cooler to lock at under the bonnet lid


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we had a pair of H4s for sale at Race Retro the other weekend, made serious money...
David Smith

A washer bottle from a 1500 Midget works well as a catch tank and fits neatly under the nearside wing, its intended location.

Dave O'Neill 2


It looks great... if you did it agian, would still vent the valve cover

Does the 4 inch ally tube do a good job cooling the carb

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This thread was discussed between 26/02/2018 and 03/03/2018

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