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MG Midget and Sprite Technical - A puff of smoke

After the head gasket, bottom end shells, and sump gasket fixes, my car is sounding better and going well.
I am now running along up to about 4000 rpm or so and 60 to 70mph and there are no worrying noises. :-) MPG is 30 to 33 which is about what it has always done.
However on return today at the end of about 170 miles since last engine oil top up I looked at the oil level and had to add 1/2 a pint. Started engine, looked at exhaust, no discernible smoke but 'black' tail pipe. Revved up to 3000 rpm no smoke, revved up to 4000 rpm plus and there is a puff of smoke on lift off.
My tick over is consistent at 1000 to 800 when engine warm but sounds a bit rough. (doesn't actually sound rough when I am outside the car with the bonnet up but not as nice as I would really like). Like it might not want to pull properly from standing. When driving in gear its fine. Its willing and wants generally to go.

Only other thing is it sounds like it struggles to start when its already warm and up to running temperature.

Anyway, any thoughts?
Dave Squire

Dave. "Black tailpipe" is a sign of rich mixture. Not always excessively rich, not always excessively rich all the time. But, in the normal operating band for the driving you do, there may be some point where the mixture is excessively rich. It may also indicate, unless you have a brand new exhaust system, a previous condition which may not be present now.

The "puff of smoke" most probably is black in color indicating a temporary excessively rich condition. The fuel does not "atomize" perfectly, nor does it flow perfectly from the carbs to the cylinder. There will be some small amount of fuel which drops out of suspension and either lays at the bottom of the intake manifold or clings to the sides of the intake manifold. When the throttle is closed quickly it creates a greater suction than normal which draws all/most of the raw fuel into the cylinders where it causes an excessively rich mixture for a second or two. This, most probably, is the cause of your puff of smoke, especially if it is black. (If the smoke is white or blue you have some other problem.) The reason the "poppet valves" were installed in some S-U throttle plates was to prevent this form of mixture enrichening as an anti-pollution measure.

The other questions you have sound like judgement calls--you "think" something may be present, but are not sure. Document what you have (e.g. "takes 10 seconds of starter cranking to start when hot") and keep track of it. If some problem is present, it will begin to manifest itself in some form of pattern which you can observe and document.

I, too, find that after doing some form of major work on one of my cars there is a period of readjustment when I hear every little noise, notice every little thing which may indicate a problem. Most of the other people I know tell me they do also. Good time to pay attention, but document what you are seeing and hearing. If it is a real problem you will see it develop. If not, you will again become accustomed to what is happening with your car and will no longer notice it.

Les
Les Bengtson

Got to agree with les above

Something else to consider ...when you adjusted the carbs... was it incrediably hot outside/inside the shop area

Is it cool now.... ive hear of this, not seen it, but if the weather is hot the fuel is closer to vaperisation and needing more fuel to counter a lean carbs

When the weather is cooler the carbs are adjusted for a richer carb in the heat of summer, so now that its cooler fall weather the carbs are actually running richer

So apperantly its best to set up carbs in spring and fall

Like I said, its something ive stumbled across, not sure of its accuracy....but it sounds good, and would explain your situation

But the easy and most likely answer is that the dash pot needle, jet and/or carb piston spring is worn

Prop
Prop and the Blackhole Midget

Thank you gents, fits with my best scenario that I need to adjust carbs. :-)

Like you say Les I can't make my mind up re noises but I haven't been in a position to drive this fast for long durations before so need to get used to it.

Also my heater pipe in the inlet manifold no longer holds water so is by passed. I have another so I need to install it to get warmer intake and reduce chances of drop out.

And as you mention it I remember you saying about spring and fall at some point previously now Prop. I will base my servicing around these times, it makes sense.

I keep saying I need to start a diary / service record. I must do it.

So its carburettor time again, it is significantly cooler than when I did them last, (mid summer after head gasket replacement), won't take long I'm getting used to it. And while I am on I will check the parts for wear. I have another set of carbs so can compare among 4 carbs for what is happening if necessasary.

Thanks again each. Much appreciated, Dave
Dave Squire

before just adjusting the carbs you want to consider if you need to go through the full set up chain or any part of it before the carbs otherwise you could be adjusting them to get round a problem rather than resolving things

tappets - CB points – plugs - timing - carb mixture, in that order


Nigel Atkins

Thanks Nigel, wilco. :-)
Dave Squire

Nigel! surely it's:

HANDBOOK - tappets - CB points – plugs - timing - carb mixture, in that order
Graeme Williams

Graeme,
surely that goes without saying!

anyway Dave has already seen the light and is a disciple to the 'good book'
Nigel Atkins

Not sure if it fits a 'good book' definition Nigel but certainly a very useful and almost perfect reference item for my 1978 car.

And btw car going consistently with noises etc. so it is me that thinks its changing. I am sure its OK now. I am sure some of the noises are exhaust noises I havn't heard before but then the exhaust is at least 10 years old so as Les says its my adjustment time as well as the car bedding in.

Now, a 'good book': Greg Bear, Herbert and particularly Gibson now they are what I call 'good book' authors. ha ha.

Thanks every one, Dave
Dave Squire

Dave,
You need to stop stressing about every little noise. Just drive it and stop fretting.
Guy W

"Guy W, Cumbria, United Kingdom

"""Dave,
You need to stop stressing about every little noise. Just drive it and stop fretting."""

Yes, save the fretting for a blown head gasket...hahaha


Prop
Prop and the Blackhole Midget

Perception of Noises/rattles/rough ride is very dependent on you own state of mind. Went out for a short run today and it felt bl**dy awful!" But then I've got flu (or, as my wife puts it, "a cold"). I was hoping the run would make me feel better but just made the car feel worse!
Graeme Williams

This thread was discussed between 18/09/2013 and 23/09/2013

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