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MG MGA - Oil out of rocker cover breather

Help appreciated re following .
I recently drove my 1600 150km on a very hilly road and fairly warm day . I have twin radiator fans but had forgot to switch on .
Going up a long climb I noticed the water temp going up very high ( plus smell ) and pulled over . The tmp was over 240 and after a while I released the cap and coolant overflowed. I waited and then refilled and restarted and all seemed fine .
Two days later ( next drive) on return drive ( with fans on!) all seemed fine but after 45 mins at a stop i noticed fumes coming from under bonnet and oil was coming out of the breather/filler cap and onto exhaust .Oil level still fine .
I drove the car home without real noticeable oil loss ( but more oil over rocker cover from breather) and have just started an inspection.
1st basic just done was a cylinder pressure test results..
Cylinder 1 110 psi
Cylinder 2 93
Cylinder 3 120
Cylinder 4 43
I speculate either a piston ring leak or a valve stem leak in Cylinder 4 .
The mga is fitted with a Judson supercharger and next step seems inevitably to remove head peripherals and get head off.
Before doing so I would really appreciate any advice re likely cause and also any further checks I can run before removing the head
Neil Ferguson

just puck a few squirts of light oil into Cyl 4 and compression was 120psi for a short while .
Neil Ferguson

If you are running a Judson with the standard Holley 1904 or 1908 carb, you will eventually have engine problems because you will be running too rich at low RPM and too lean at high RPM. The carb only has a fixed jet and an accelerator pump that doesn't do well with high revving use (It was intended for low RPM engines in US pickups....)

I would suggest you have damaged a piston because you are running lean. This is common with running lean on a supercharged engine - which is why the Judson MGA has a reputation as an engine killer - it is!

There's no way around this with the Holley carb, so I designed a new manifold to fit a HIF44 car from SU, which is the same carb used on the original Moss supercharger kit. This works like. dream and allows tuning to be spot on at all RPM. The details and manifold drawing drawing can be found here:

clancy.ch/Judson_MGA_files/Judson_MGA_HIF.pdf
Dominic Clancy

Dominic..I have not posted on this site in many years but I threw away the fixed jet holley and fitted a single SU many many moons ago and redesigned the manifold into the Judson to fit. We were in comms at the time I recollect . The current issue has nowt to do with fuel distribution.
Neil Ferguson

Neil

Welcome back. They all seem low or are you running low compression with the supercharger? I always worked on about 140+ psi.

Steve
Steve Gyles

Steve..agree ..but I expected about 120from my engine… with 8.3 ratio. 1 seems ok and 3 are not far off. 2 isa concern but 4 is awful
I will get to the cylinders generally ( prob do a top end and bore check …but right now i need to concentrate on and resolve my oil venting issue through breather ….any comments ?
Neil Ferguson

Neil
Sounds like you've cooked it unfortunately.
If the compression came good with oil in it to seal the rings for a bit it's a sure sign
I was originally thinking you'd popped a headgasket but the comp coming up with oil points to rings and blowing out the breather confirms that---bugger
If it had done a headgasket you'd expect it to be side by side cylinders and headgaskets of valves don't normally cause breather issues

Now my favourite hate---electric fans---you really need to have a thermo switch or preferably two, one up top at just above thermostat temp and one down the bottom at about 20 degrees less---then you're covered---then you can have your override switch if you want but you won't really need it

My choice----Just a good plastic multi blade belt driven fan is foolproof, no electrics to fail

willy
William Revit

What about trying a leak down test?
Mike Ellsmore

Hi Neil, looks like you may have burnt a hole in a piston or rings. The compression stroke blows past the piston and pressurises the engine internally forcing oil out the breather tube. You may also find that oil is forced past the front & rear crank oil seals so check for a fresh oil leak there.
I Hazeldine

Hi Neil,
sorry to hear about your engine, it does seem likely that you have a piston ring/cylinder bore problem.
At least it has happened at the right time to start a Winter garage project.

You ought to look into installing a thermostatic switch for your twin fans, I still have the Revotech switch that I originally bought for the single 10" Revotech electric fan that I tried before I settled on the Davies-Craig twin 9" fans like yours.

It has worked very well since I fitted it in 2014.

I only have the one thermostatic switch fitted and so I have a manual over-ride switch in case the other one ever decides to not work.

I did buy a Davies-Craig thermostatic control unit to try because it does seem to have a narrower range (hysteresis) than the Revotech switch, but unfortunately the adjustment button inside the switch broke off, the first time I tried it and so I have carried on with the Revotech switch for now.

Your engine issue is just down to bad luck, it reminds me though of a close relation of mine who had something similar happen to his engine, but his mishap was more down to a lack of common sense.

He had replaced the water-pump on his MGB after it had started to be a little noisy. I offered to help him with it but he decided to do the job on his own.

Apparently, it all went well and so he took his wife on a 40 mile run to see if it was ok.
At the half way point, his wife pointed out that needle on the temperature gauge had swung way past the "H" mark.

I asked him if he then stopped to check the coolant level? I was absolutely floored when he said "No, I decided to drive home as fast as I could!"
Then he said "I think I may have blown the head gasket".
I expected much worse and so it proved when we removed the head an found all four cylinders badly scored and the engine party seized!

He couldn't believe it because he said that he had been careful to refill the radiator and that he hadn't actually drained the coolant before starting the job. When I asked him how he had refilled the engine he pointed to the overflow bottle lower down by the bottom of the rad, h then said that it would only take a pint!

He was shocked when I told him that water-pump was at the lowest section of the cooling system so had actually completely drained the engine and that you just can't fill the engine from the overflow bottle because it is virtually at the bottom of the system!

In the event, it cost him a complete replacement engine and he still doesn't like to talk about it.

Let us know how you get on with the rebuild Neil.

Cheers
Colyn





Colyn Firth

Guys ...I do fear it is so ! There was a lot of coolant left in so perhaps it did not do too much damage ??
I can but hope when the box is opened it is not a nightmare .
I will do a blowdown test on cylinder(s) before removing head etc .
Neil Ferguson

Leak down tester are cheap.
https://premium-diagnostics.com.au/products/cylinder-leak-down-tester-compression-leak-detector-petrol-engines?
Cheers
Mike
Mike Ellsmore

Mike..they are and I went sniffing a short while ago and i got the same kit for $40 on Ebay..thank ye gods for the net!
Neil Ferguson

Hi Neil.

I couldn't remember if you had done the SU swap, which is why I started my reply with "IF"!

But it is still probable that you have damaged the piston. That, in a SC engine, comes from predetonation, and points to lean running or too much advance. It's better to run a little rich to avoid this.

If you are looking at an engine rebuild, I can strongly recommend a 1950cc 5 bearing MGB engine - this drives even better than the SC engine, and avoids many of the issues with supercharging. Adding the Judson to the setup produced such a wild ride that I decided it was too much for me, and I removed and sold the Judson and stayed with 1950cc until I sold my car and moved to Thailand
Dominic Clancy

Ye Gods Dominic ..you have truly changed yr habitat.are you now a Thai permanent resident ?
Do I gather you are not the proud owner of an MGA any longer. I thought once in the bloodstream then ....and you obviously still follow the BBS ..
My Judson has performed well over the years with its single SU .My big events have been a divorce and a house move ( only 6 months ago so still setting up).
I am awaiting a leak tester for the cylinders before I start pulling engine apart so all will be revealed over coming weeks...UGH!!
Neil Ferguson

Emphasising a point Dominic made it could have been caused by timing being over advanced.

I had a somewhat similar experience a wee while ago where I blew the gasket between 2 adjacent cylinders and blew the middle core plug. When I told Cameron Gilmour who had built my engine, and also Colyn’s super one, he immediately asked was the engine going really well which it was and then he said I mean really really well and it was he said this combination of symptom is well known in B series racing engines circles and is caused by over advanced ignition. My head had to be flattened and all valves re ground although my compression is slightly down on one cylinder which is almost certainly a ring problem it is going fairly well with a lot less ignition advance. He was also concerned for my pistons and asked me to check the oil for bits of piston but this was fine.

Clearly your issue is not identical but cause could well be the same.

Good luck.

Paul
Paul Dean

Yes I sold the MG in August 2021 and moved to Thailand in October 2021. I live 500m from the beach in a place called Mae Rumphueng.


Dominic Clancy

Paul,
I think you may well be correct about the timing, my 1950cc engine blew a head-gasket after a fast run over the brilliant Cat and Fiddle pass near Buxton a few years ago.

The engine was pretty new then and had been dyno'd at just below 140 bhp, it was running really well but suddenly lost power and there was a loud tapping noise from one of the cylinders.
When we took the head off, I expected to see some valve or valve gear damage which would explain the tapping noise, but the valves and bores were fine.

It was obviously the head gasket between 3 and 4 which is very narrow indeed on a 1950cc engine, the tapping noise was the sound of the combustion gasses leaking between cylinders 3 and 4.
I replaced the gasket, which was originally a Cometic multi layer type, with the Payen equivalent and I retarded the ignition by a couple of degrees to see if this helped.

I had the engine on the dyno soon after that and Peter Burgess discovered that with the timing reset to the original 31 degrees, there were what he called "some pre-ignition events", he retarded it to 29 degrees which stopped the pre-ignition and found that the bhp at max revs (6500 rpm) had dropped by nearly 4 bhp. However, the good news was that below 3000 rpm, the bhp had actually increased by the same amount!

Having more power below 3000 rpm means that the car actually feels quicker to drive than it did before and so I am more than happy with it.
The Payen head-gasket has survived for 6 more years so far and the engine is still running very well.

I haven't actually noticed the slight drop in power at 6500 rpm, with the gearing of 20 mph per 1000 rpm in 5th gear, an indicated 6500 rpm would mean than we would be travelling at 130 mph! This is a scary prospect in an MGA with all the buffeting you experience with the top down.

I once took the car up to a whisker under 6000 rpm in 5th gear with the soft top up (not on a public road), which is close to 120 mph, but the car felt very light at the front end and so I don't plan to repeat that experience.

Let us know how you get on Neil,

Cheers
Colyn

Colyn Firth

This thread was discussed between 24/04/2023 and 01/05/2023

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