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MG MGB Technical - Crankcase smoke

Finally got my car back from the body shop and will start on the other work needed, tune up etc. I'm concerned by the vapour/smoke from the crankcase breather. In an earlier post I questioned the way that the breather went out under the engine with the carb connections blanked off. I wanted to trace the reason for the missing over 3000rpm and was going to return the breather back to the carbs as a starting point before all the other checks of timing, plugs leads and tappets. When driving the car I can smell the crankcase emissions from inside the car. With the fresh air vent closed it's a lot lower. I've looked at the vent pipe with the engine idling and you can see faint smoke from it but it remains the same if the engine is revved. On the back of my hand you can feel a faint puff of the engine. So what causes this? the car does not smoke from the exhaust but I've not had it long enough to see if any oil is being used. Could it be the valve guides, a head change would not be a problem as the head is not an unleaded one, but should this be a last resort.
Roy Soper

Roy,

Absolutely normal - there is always a little blow by the piston rings and this escapes through the tappet cover vent carrying a little oil mist with it.
Normally this is vented into the carbs and is consumed by the engine so you don't smell it.
Chris at Octarine Services

Thanks Chris, think I'll stick with the original plan then and pipe this back into the carbs for now. At least this will cut down the smell.
Roy Soper

Chris, Roy
I have had the opposite problem with respect to missing above 3000rpm sometimes.
I have added K&Ns along with a stg 2 head and 715 cam during the rebuild >2k miles ago. It was reasonable on oil consumption on running in, but since the first 1000 miles it has taken to consuming a lot of oil (1L per 300-400 miles). I am told by friends that there is a small amount of smoke under acceleration and some oil slugs are eveident in the distr. vac. pipe. The missing is not constant, just occasionally which I suspect is oil fouling the plugs?
My c/c breather is vented to the twin HIFs as per original. (it's a 1980 GT)
I shall be looking at the compression, as well as the plugs, and maybe trying to vent the c/c via a small K&N breather. But it sounds as if you are having the problem the otherway round.

Martin
Martin ZT

Martin
I have had a similar experience. I have K&N filters on my 77BGT and if I leave the crankcase vent pipe connected to the carbs I use lots of oil so I vent it to the air. I haven't worried to much about this until I saw a recent article that said that having a slight vacumn in the crankcase helped oil mist circulation which helped lubrication of gudgeons, lower piston skirts and even help oil to flow into the cam followers.
So I am now a bit stumped. Has the better airflow through the K&Ns resulted excessive oil mist in the intake? If so, how do I reduce this to an acceptable level so I can get back the benefits of the oil mist lubrication without the excessive oil burning?

Any ideas from any experts out there.

Cheers
Ray
RM Taylor

Use a swirl pot to separate the oil from the fumes - used on engines like the Rover M16/T16.

Or fit a restrictor in the vent pipe to reduce the volume of fumes sucked through.
Chris at Octarine Services

Chris
Thanks for your quick response.
I have tried a sidecover from an Austin 1800. It has a large pot. This didn't seem to help. I will try the restricter. I just need to figure out how to make one.
CHeers
Ray
RM Taylor

Thanks, Chris. Interesting theory, Ray, which considering there was a recent advert for a baffle plate the sits between the block and the sump with cut-outs for the cranks throw to reduce 'windage losses', it does beg the question which works best.

I shall attempt to create a swirl pot, my Austin 1300 had one coming off teh clutch housing/c/case to feed the carb.

Martin
Martin ZT

Martin When a B or any other engine is at road speed there is a lot of oil sloshing and beeing thrown around inside. This lubes the parts that are not pressure fed. This is usually not a problem unless you pass too much air through the breather system.The idea of the breather systen is to work at SLIGHT vacuum to releave load on seals and avoid oil leaks.If you pass gobbs of air through the crankcase you must suck this oil in suspension into the intake. The secret is to restrict the air flow to just do the job by using as others have said .125 restricters in the breather to carb pipe or better still only let a little air into the motor from the atmosphere (OE oil filler cap) Lets face it how do you get the last bit out of your milk shake(you suck like hell) If you do this with a motor you will suck the oil that is floating in the air out. Your oil seperator in the side plate must be clean and working but if you need to start to put on a lot of add ons that the car did not need in the first place I'd bee looking at the pistons and rings. Try a PCV valve in the suction to control the air flow. I think what I just said is sort of a long winded version of what Chris said
DENIS

Well the crank breather pipe is now back and connected to the carbs and the smell has gone from inside the car. Reading the other comments about increased oil use is interesting. I'm going to change the oil today so it will give me a good starting point to measure oil use of the engine. Did the test with removing the filler cap at idle and it makes no difference to the idle speed so think this will need to be replaced as I read that the idle speed should change as the crankcase vacuum is altered. Also my misfire over 3000rpm is still present so that needs to be looked at. I'm convinced it's electrical as I noticed when it was missing the rev counter bounced about wildly, as that is electrical feel this is the side to look at first.
Roy Soper

Well had a play with the HT & LT side of the electrics today after changing the oil. Plugs were fairly new and light brown in colour so just a wire brush up for them. Relocated the HT from the coil as all the leads were tied together in a bunch and a friend mentioned that this can sometimes cause interference between leads if the coil HT is held tight to other plug leads. At this point I noticed the LT wire to the coil seemed to be a little on the weak side so cut this back and fitted a new crimp. Also checked the other LT side that goes to the distributor and this needed a little clean up. Quick blast down the bypass and it now revs better made it up to 4000 rpm!!!! before I cut the OD in. Still feels a little hesitant or maybe it's me but I'll check the timing next. Previous owner fitted one of the sensor type points and there is a black disk with 4 bumps on so nothing I can check here, not even sure what make it is. Was going to set the timing with a 12v lamp but will have to use a timing light now.
Roy Soper

If removing the oil filler cap doesn't change the idle revs then it almost certainly isn't the cap that is the problem but no 'suck' from the carbs. With the engine idling and cap removed you should be able to feel suction with the palm of your hand or a sheet of paper over the filler hole. If there is none the breather circuit is blocked somewhere. If you have smoke issuing from the pipe connected to the engine then that side must be OK, so check for vacuum at the carb ports and Y-pipe. However if the cap makes no change to the idle but your hand does, then that implies the cap *is* faulty in that it is not restricting the flow through it, which will be weakening the mixture, which could be contributing to a missfire.

If the tach is flipping about with the mixture then that indicates an ignition LT problem. Either points bounce, very much less dwell/too big a gap, loose connections, missing distributor ground strap etc. However with some kind of electronic trigger it could be an electronic problem, not unknown, which only substitution will prove. Some of these don't require the distributor ground wire, some do.
Paul Hunt

Just to report an update.
I fitted a K&N breather and blanked off the carbs. I also refitted theoriginal air cleaners and new paper elements.
After a 200 mile run this afternoon, virtually zero oil consumption, no hesitation, and a very good fuel economy.
Plugs are also the correct shade of brown.

I think I had two parrallel issues: Too much vacuum, and duff 2nd hand K&N elements enrichening the mixture contrary to normal expectations. I did clean them but perhaps these are best fitted with the right needles.

With respect to the front tappet chest, these do have a wire mesh insert, or should have, I thought, which should act as an oil separator. Now I may have been stupid and removed my mesh when cleaning the parts so I will check this out at some stage when I can pluck up the energy to try and access it.
Also, a colleague reminded me that the Austin 1800 and IIRC on my 1965 Roadster there was a valve on the c/c breather which was meant to shut off at idle and decceleration when the highest vacuum occurs to avoid sucking oil out of the engine and causing poor emissions. However, it often stuck, either closed causing rich running, or open causing weak mixture and stalling. I guess this is why they went to an oil separator and straight through tube to the carbs.

HTH
Martin
Martin ZT

My 79 b runs pretty darn well, but has moderate oil consumption. The dipstick is very loose. How important is it to have a tight seal around it? Thanks.
Paul

Did a round trip of 60 mile and noticed the misfire is still present over 3000rpm. Strange thing was that on the return trip I pushed the engine through the misfire to about 4000rpm and no missing. I'm now thinking the distributor is playing up or at least something on the ignition side. Had the dist/cap off and managed to get this black disk off the shaft to see the sensor had a luminition label, no condenser fitted. Has anyone got this fitted and know if the condensor should indeed be removed with this system. I've not got any information on this from the previous owner. Checked the base plate could move then tried sucking on the vacuum pipe and found to even get some slight movement you had to suck real hard, is this normal for the 45D4 type?. I'm borowing a timing light next week and wil be able to see if the distributor advances the timing smoothly when the engine revs. Tempted to just get a secondhand distributor with normal points to see if this solves the problem.
Roy Soper

Roy,
you probably have the Magnetronic ignition from Lumenition. The black disk should have 4 magnets inserted into the "bumps".
FWIW, I have had the Magnetronic on my C for 3 years now without problems. It doesn't require a condenser.
I think you should check your distributor advances correctly, if you move the rotor against the springs and let go it should return to its stop, and the movement should be smooth.
Also, to be sure, check the connectors on the magnetronic wiring. The PO may not have done a good jobat crimping them on.
A timing check should be next on your list.
HTH,
Rufus
Rufus Pool

Roy,

You should not have to suck "real hard" to get the plate to move. Also the suck should hold the plate in position - if it returns before you let the suck go then the diaphragm is split and the vacuum unit needs replacing.

Check that any securing screws for the ignition pickup are not too long and are binding on the base plate.

Chris at Octarine Services

Thanks for the replies, I think that maybe the electronic side of it will be OK and it's the mechanics of the distributor that are letting it down. I'll look to get a secondhand 45D distributor with standard points I can then check this replacement distributor first clean/oil etc then fit and test. If this cures the problem I'll swap over the luminition sensor for the points and remove the condensor. I'd be happier without the condensor ever since I had one fail on a cortina, caused the engine to backfire and split the exhaust silencer...ouch
Roy Soper

This thread was discussed between 23/09/2004 and 03/10/2004

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