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MG MGB Technical - What causes this ?

I have this problem again.
After idling for a while like at trafic lights or at heavy trafic I can see some smoke behind my car. I dont think it is burnt oil and I think it is unburnt gas because when I reverse it smels and I know my carb floats are at the minimum gap allowed (1mm). Can the level in the float chamber be causing this if it stands to high ? Does it make the mixture to rich at idle ?
Thanks in advance for your inputs
Miguel
Miguel

Miguel,

When the carb needles wear, it usually causes a rich condition just at/off idle. Is the smoke black, and is your idle a bit lumpy? My first guess would be worn needles. Take a look at them and see if there is a groove where it contacts the jet.
Paul Konkle

Paul
The smoke is blue smels like gas takes around 30 seconds to show, and this does not hapen all the time, and when it hapens yes I get an irregular idle. When the irregular idle starts I look at my rear mirror and see all that smoke, accelarate once or twice clean the plugs and hopfully the trafic starts moving.
The needles are new and are # 6 because I have the K & N filters and a "fixed" head the P Burgess way. The engine otherwise works very well pulls fine and the spark plugs have the ideal colour after some road driving.
All this makes me think there is something wrong at idle only, and knowing the fuel floats are at there minimum gap making the fuel level be to high in the chambers, the fuel is sucked into the engine making it get to rich. But this is only my theory and IŽm no expert.
If my theory is right then increasing the gap of the floaters (making the the fuel level go down) would not let the fuel be sucked at idle.
All help I can get is welcomed
ThankŽs
Miguel
Miguel

I forgot this
1969 B HS4 SU
Miguel
Miguel

Blue smoke is usually oil, and burning oil when idling is usually caused by worn valve guides.

Rich mixture is usually black smoke. If a float valve is leaking slightly then it's possible that the level is rising slowly when idling hence richening the mixture. A 69 shouldn't have any of the evaporative loss kit so eventually you may see fuel leaking out of the float chamber overflow. What colour are the plugs after a period of idling and smoking? To check the float valves switch the ignition on but don't start the engine, in fact you could pull a coil wire to stop it overheating. If you have an SU pump it should not click more than once every 30 secs, more than this indicates a leaking float valve, or possibly a leaking non-return valve at the pump.
Paul Hunt

...and if you don't have an SU pump, it might do well to check your fuel pressure. If it's just a hair too high, then at cruise the car may be keeping up with the supply, but at idle, the extra pressure may be pushing too much fuel into the bowls, causing your rich mixture condition.

Just a thought.

Matt K.
Matt Kulka

I would check to see if the engine vacuum is pulling oil from the engine's vent tube. If you still have all the emissions equipment attached, try installing a small K&N vent filter on the vent tube from the lifter cover and removing the hose from the vent on the valve cover (the ultimate option is to have a non-vented rocker cover with a vented filler cap.) This will eliminate the opportunity for your manifold vacuum to pull trace amounts of oil from the crankcase. Since the engine runs at higher vacuum levels at idle, that would be the most common time for it to pull oil into the mixture, hence the blue smoke.
At least that's my theory.
Jeff.

Jeff Schlemmer

Thank you all for your kind help.
My engine including the head was overhauled 4.000km ago so please dont tell me I have worn valve guides.
Yesterday ariving home I left the car idling while I opened the garage door and after around 15sec the blueish smoke started again. I let it idle a bit more and looked under the car for some gas overflow wich did not hapen in the next half a minute. I stoped the engine (Žcause I hate that smoke) but IŽll do the same thing again tomorow. But first IŽll do the trick with the ignition on without starting the engine to check for any leaking float valve. Never had ocured to me before this test. I had some problems some time ago with stuck valves causing all the fuel to overflow. I still have the floters gap at the minimum gap. I have the non adjustable all nylon floaters.
After this test IŽll start the engine let it idle for a while and then check the the colour of the spark plugs plugs as you sugest.
The head was fixed acording to P Burgess book, has K & N filters and I had to install the richer #6 needle to compensate for the better breathing so the needles are also new. Apart from that it is a standart 1969B engine european spec ( I think at this time they where still the same in the US). It does not have any emissions equipement as far as I know. Mine has the crankase ventilation and that makes me think Jeff might have a point there. IŽve never heard of those K & N vent filters.
I am more puzeld now than I was before.
I will update again tomorow please keep on helping.
ThankŽs
Miguel
Miguel

I checked the clicking of the fuel pump this morning. It stoped after the normal initial clicking and then clicked every 25sec. I then started the engine warmed it up and no smoke for a long time. With the engine warm I let it idle for a short while and there was the smoke again. It is blue and smels like oil. I know I had told it smeled like gas but I also had the previous problem of the floater wich caused the overflow (that is solved). I removed the spark plugs and the colour is tan beige and completly dry.
What do I have now ? Is Jeff right ? When the engine was rebuilt the side valve cover with the crankase ventilation filter and pipe was washed and looked good. The valve guides in the head where machined out of some very special metal acording to P Burgesss. So please please dont give me bad news.
ThankŽs again
Miguel
Miguel

Jeff mentioned the crankase ventilation system pulling oil up from the rocker cover and pumping it in through the carbs. Disconnect the hose to the carbs and plug the carb end. If it still smokes it isn't that.

Let it idle again, then *before* it starts smoking on its own give the throttle a prod. If that makes smoke then it really does look like guides or hopefully just valve stem oil seals. Did Burgess supply the valves too? If they did use a special material for the valve guides then maybe they needed special valves too.

Paul Hunt

Miguel,

Did you (or your builder) install Valve stem seals? They come with the Head Gasket kits.

My head was rebuilt and ported by Mike Brown (an excellent job) and the valve seals were deliberately left out to increase valve stem lubrication (I think that Peter Burgess suggests this in his book). Result is that I get exactly the same condition that you have.

I dont mind living with the smoke, might make the guides live longer :)
Nigel Spratling

Miguel,
To fix the urburn gas smell and blue smoke problem try this:
1) reinstall the old needles and install a fuel pressure regulator ,set it at 2 psi
2)install MGC valve stem seals on the intake ,and MGB valve stem seals on the exhaust
Jose Santos

I disconected the hose from the carbs and the smoke stoped.
What am I looking at now ?
When reassembling the head I also instaled the valve stem seals.
Miguel
Miguel

Miguel,
How many miles/kms do you have on the last oil change? I would try a oil and filter change. You might be experiencing thinning of the oil, causing it to vaporize easier. The high vacuum at idle then is pulling the vaporized oil into the carbs.

Ray
Ray Ammeter

I found that the tappet cover with the breather in it tends to get clogged up with oil when it's old, and if your crank case pressure is a bit high, it tends to blow oil up the breather pipe and dump it into the inlet manifold. I created several smoke screens in this manner before I did the following:

Remove the tappet cover, and cut a nice big hole on the inside with a grinder (I removed about 1/3 of the inside area, all at one end) being careful not to damage the gasket face. Pull all the wire wool out of the inside (use pliers, it's sharp), clean it up and reinstall. No more smoke for me!

Hope this helps,
--
Oliver Stephenson
Oliver Stephenson

Miguel - remove the oil filler cap once the exhaust has started smoking. Do you get loads of oil smoke out of the rocker cover? If so there is an engine problem which is causing oil to be burnt in the crankcase. Hopefully this will not be the case, so the most likely cause is the breather drawing oil into the carbs and burning it that way (are the breather pipes you pulled off very oily?), in which case you need to look carefully at the crankcase breathing system. The air inlet should act as a restrictor which means there should be some vacuum in the crankcase, this can be felt by putting your palm over the oil filler when the cap is removed and the engine running, and removing the cap should make the engine run slightly rougher or slower. So you don't want something that is completely open to the air, but neither do you want it completely blocked either.
Paul Hunt

I changed the oil and oilfilter 2.000km after the engine overhaul and now the car has 2.000km on this oil wich is ELF Prestigrade 15W40. Can it be that the oil is to thin for this engine or to modern ? Knowing I should use 20W40 there is a small diference.
The tappet cover was cleaned 4.000km ago during the engine overhaul. Can it be that it was not properly cleaned ?
Thanks again for all the help I can get to solve this problem
Miguel
Miguel

15W/40 is a 'normal' oil, Castrol label theirs for 'basic' and for 'modern' engines, you should be using 'basic'. If the tappet-chest cover breather was blocked during the cleaning process it could cause the problem.
Paul Hunt

Heya,

I would normally expect that with an engine this new, using a modern oil wouldn't be a problem. I use Duckhams Hypergrade in my now ~8 thousand mile engine and still get well over 1000 miles to the pint of oil. Unless of course, someone has installed your piston rings upside down...

I tried cleaning the tappet cover once, but it didn't help. Bear in mind that on early engines the cover was empty anyway, so you're just reverting to an earlier standard that I believe works better. I see no need for all the wire filling. This is just my opinion of course.

Btw, I'm sure the answer will be yes, but did you run the engine in carefully? Going straight to wide open throttle with new parts means they ain't going to work very well :o)

Hope this helps,
--
Olly S
Oliver Stephenson

Paul we must have been tiping at the same time this morning as I had not read your last post and was still repliyng to the previous posts.
So IŽve removed the filler after the engine was warm and there is some suction from the engine did the same with the tube from the crankase and same efect, light suction at idle. The breather pipes have no oil.
Yes I did a carefull runing in. Yes asking the obvious can sometimes help.

But now let me explain this.
This is not my only car, but it is the one I enjoy driving and I could almost call it my dayli driver. So I also check tyre pressure regularly, water and oil level. Yesterday I had to pick another car so the MG stayed in the garage at work. This afternoon before doing what I just did and reported above I did the obvious again .
I checked the oil level. ... it was -+ 4mm above MAX.
Now call me whatever you want I am shure last time I checked it it was al the MAX level. I did not ad any oil. The oil looks clean ( no water ).
Can it be that because I drive the car four times a day (short trips) it does not run down and level in those 3 to 4 hours before going home ? I dont usualy check the oil level at home, mostly at work where it stayed tonight. I have the original inverted oil filter and oil cooler. Have I been driving with to much oil ? How can someone be so Dum ?
I am removing some oil, taking the car home and tomorow morning IŽll check the oil level after one night rest. Is one night enough ?
If this was the reason for the smoking I am sorry I gave everyone so much trouble.
IŽwill post the result tomorow morning.
A very ashamed Miguel
Miguel

Your oil level was probably not the reason for your smoke problem. I had the same thing occur until I removed the vent pipe hose from the lifter cover/oil separator and installed a small round vent filter. If you go to the K&N website you can look up the proper size. It may also be in the archives. I assumed my problem was because of the new performance cam, aluminum head, and a freshly rebuilt engine. Could it possibly be from excessive oil pressure?
Jeff.

Jeff Schlemmer

Back to the drawing board.
The level this morning was corect (I removed some oil last night) and when I arived at work I let it idle and after 1 minute there came the smoke again. Like Paul sugested I disconected the hose from the crankase and the smoke still keeps on. In this case the K & N vent filter wont help I guess ?( Or am I wrong ?)
I think we are closing in on the problem and it looks like either burning oil thru the valve guides or piston rings.
I dont think I have to much oil pressure at least the pressure readings are fine acording to the pressure gauge.
I spoke to the tecnhical director of ELF oils yesterday and he couldŽnt see anything wrong with the oil I was using. Only thing he said was I could use an even cheaper ELF oil than the one I am using. A "normal" mineral 20W50.
I would like to get to the end of this
Thanks Miguel
Miguel

It is looking more like a mechanical problem, although I would have said that if it were rings/bores you would probably get pressure in the crankcase, at least some of the time, rather than a smooth suction. If that is true it is pointing to valves/guides which at least isn't an engine-out job. But remember that Nigel said his engine smokes like you describe, and that is because he deliberately left the valve stem oil seals out to ensure lubrication. One thing I can't find is whether this problem started suddenly, or you noticed it gradually building up. The former would indicate something has failed, the latter increasing wear.
Paul Hunt

I dont know how and when it started. The engine got finished end of September and I was so glad that I even posted here about the joys of "no smoke". I was still runing in when I posted and I could not see any smoke at any time. One day in traffic I noticed there was some smoke but of course it couldŽnt be me with my newly run in engine. Thinking back I canŽt be shure wether the problem has allways been here since the overhaul.
Nigel left the valve seals out but mine where instaled.
I guess IŽll have to live with it and investigate further.
If something develops I will post it and I will also keep on reading this BBS where I have learned alot in the last few years.
Thank you all for your help
Miguel
Miguel

This thread was discussed between 14/02/2002 and 20/02/2002

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