Weird "A "series crankcase compression problem in MG Midget
Discussion
I have owned my '72 MG midget for 30 years and apart from the usual adventures and mishaps, the original engine has been the reliable and strong beating heart, even after running the last 50,000 miles of its 160,000 mile life pretty hard in lightly modified tune. Roughly 3,000 miles ago after a fast run back to Birmingham from the Isle if Wight, it suddenly developed a large oil leak from the rear of the engine. The local MG garage removed and repaired the engine. Whilst out of the car, the cam was changed to an MG metro one, along with new timing chain and cam followers- no other change. Ever since then it has emitted one large cloud of smoke every time it reaches running temperature; usually next to the same bus stop at exactly the same point on the route to work! From the oily mess in the crankcase breather pipes at the carbs this seems to be a crankcase compression issue. The engine has been rebuilt again since (rebore, new pistons, balanced crank- the whole thing runs beautifully and sounds great), but this single but large cloud of smoke occurs every time the car is driven from cold.I wonder if it coincides with the thermostat opeing? The crankcase breathing arrangements have not changed. Garage man, who knows his stuff, is stumped and says he has only met one other such example (also on an A series midget), which he only cured by routing the crankcase breather pipe into a Coke can. A long thread on this site suggests crankcase compression and breathing arrangements are pretty important, but make no mention of Coke cans, so I am not sure I want to go down that route. What's going on, and what is the cure?
I've worked on quite a few 'A' and 'B' series engines in various states of tune and have had odd things happening with crankcase compression. Sometimes if there's a leak/loss of compression the car loses power, sometimes the opposite. 
If the engine is otherwise running well why not just use an oil catcher? (I suspect this is what the coke-can was for)
It may be worth running a compression test on all cylinders just to make sure there are no head gasket problems, also worth looking at and smelling oil/water to make sure there is no evidence of cross-contamination or combustion products.
If the engine is otherwise running well why not just use an oil catcher? (I suspect this is what the coke-can was for)
It may be worth running a compression test on all cylinders just to make sure there are no head gasket problems, also worth looking at and smelling oil/water to make sure there is no evidence of cross-contamination or combustion products.
Thanks for replies. The odd thing here is that it never did it in 30 years, and now it always does; but only once per day, at the end of the warm up- not at any other time. Yes there has been work on the head/valves and a rebore and rebuild, but either this was long before the problem started or after.I'd have thought that it would either do it until it got warm and then stop, or once it had got hot, but then all the time, or on acceleration/ deceleration. I have wondered if it happens as the thermostat opens though I have no idea what connection there is. The issue started after the cam was changed, but no other changes were made at that time I don't think, and I don't know how it could be connected to that.I have been keeping a look out for cross contamination but found it. Also replaced the rocker box oil filler cap. Not sure how to fit an oil catcher without disabling the breather arrangements...
Sounds like a good idea to do a compression test,but that should be prefect given the work that was done 1,500 miles ago
Sounds like a good idea to do a compression test,but that should be prefect given the work that was done 1,500 miles ago
The A series was in its own heyday, in the years before, the closed breathing systems were developed. The A+ engine was the BMC/BLMC/Rover attempt to meet the more critical emission requirements, that eventually lead to closed breathing system. The A+ did enable the A series to continue in production for some years and achieve acceptable emissions for a while longer.
From your description I do think your diagnosis of primary crankcase compression must be right. I do think an external oil catcher (Coke Can) would work and I have used such systems several times on Midgets, Minis, and various other A series mounts successfully over 50 years of A series ownership. Still have 6 A series cars. Love them all.
Still my favourite engine although, a supercharged Buick 4.9 V8 does make cars shift and howl. It does NOT achieve 40+MPG. Or 10MPG actually.
The pressurisation causing oil blowing out of the crank oil seals is the classic sign of excessive crankcase pressure. Why this is happening is anybodies guess. My suggestion is some damage, wear or aging has caused the seal between the pistons and cylinders to deteriorate significantly.
The compression tests should give some idea of the cause, I would be interested to hear the results.
In the meantime, as a temporary measure, an external oil breather with catcher does work, I have used them and with reasonable maintenance, these will enable decent journeys to be made in the car.
Good luck to you in your efforts to identify the problem. I think blow past the pistons, for whatever reasons, is the probable cause.
From your description I do think your diagnosis of primary crankcase compression must be right. I do think an external oil catcher (Coke Can) would work and I have used such systems several times on Midgets, Minis, and various other A series mounts successfully over 50 years of A series ownership. Still have 6 A series cars. Love them all.
Still my favourite engine although, a supercharged Buick 4.9 V8 does make cars shift and howl. It does NOT achieve 40+MPG. Or 10MPG actually.
The pressurisation causing oil blowing out of the crank oil seals is the classic sign of excessive crankcase pressure. Why this is happening is anybodies guess. My suggestion is some damage, wear or aging has caused the seal between the pistons and cylinders to deteriorate significantly.
The compression tests should give some idea of the cause, I would be interested to hear the results.
In the meantime, as a temporary measure, an external oil breather with catcher does work, I have used them and with reasonable maintenance, these will enable decent journeys to be made in the car.
Good luck to you in your efforts to identify the problem. I think blow past the pistons, for whatever reasons, is the probable cause.
http://www.pistonheads.com/gassing/topic.asp?h=1&a...
This thread points at something.
http://www.minimania.com/web/startreturn/1/MaxRetu...
This thread points at something.
this thread said:
Unfortunately, the front timing cover has the standard vent filled with steel wool.
Which led me tohttp://www.minimania.com/web/startreturn/1/MaxRetu...
that thread said:
I did upgrade to a tensioned timing cover from a Metro
So it appears that the timing gear from a metro engine will have an effect on your crankcase breathing. I reckon I would start there.Thanks.
The threads i have read on this suggest that having the correct vacuum is important; routing to a coke can removes the connection to the vacuum at the carbs; so whilst that might collect the oil (or maybe without the induction effect it might not occur??), would it not remove the vacuum from the carbs that is there for a reason, leading to another problem?
That it might be linked to the metro cam sounds a possibility (although I don't know enough to understand how) because the start of the problem coincided exactly with it being fitted. I wonder what CC breathing arrangements the MG metro had?
Not sure about the bores and compression issues there, since the problem was there before and after a rebore/ new pistons/valves,(although worse before, and it is only now just run in at 1,500 miles - it is sadly seldom used at present). Garage man has suggested that with more running in it may go away, but i suspect it is more than that. He also said that on another Midget with similar issues he'd tried adding the barrier from a TR6 in the pipe from the timing cover to carbs, but this did not work.
The threads i have read on this suggest that having the correct vacuum is important; routing to a coke can removes the connection to the vacuum at the carbs; so whilst that might collect the oil (or maybe without the induction effect it might not occur??), would it not remove the vacuum from the carbs that is there for a reason, leading to another problem?
That it might be linked to the metro cam sounds a possibility (although I don't know enough to understand how) because the start of the problem coincided exactly with it being fitted. I wonder what CC breathing arrangements the MG metro had?
Not sure about the bores and compression issues there, since the problem was there before and after a rebore/ new pistons/valves,(although worse before, and it is only now just run in at 1,500 miles - it is sadly seldom used at present). Garage man has suggested that with more running in it may go away, but i suspect it is more than that. He also said that on another Midget with similar issues he'd tried adding the barrier from a TR6 in the pipe from the timing cover to carbs, but this did not work.
Check the breathers. They tend to get clogged by old oil mud (maybe when the engine was put aside when dismantled?
The metal wool can be totaly removed in order to make the breating ability better.
Removing the vacuum pipe is not a perfect solution. It may eliminate the smoke, but can lead to oil leaks. It is good for a check though - You can go for sure that the oil smoke is connected to the crank gasses.
The metal wool can be totaly removed in order to make the breating ability better.
Removing the vacuum pipe is not a perfect solution. It may eliminate the smoke, but can lead to oil leaks. It is good for a check though - You can go for sure that the oil smoke is connected to the crank gasses.
Edited by camelotr on Saturday 16th June 15:10
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