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MG MGB Technical - Sounding 'wispy'

Hi there,
I have a 1970 B GT which I am slowly rebuilding. Engine rebuild has been done, new sports exhaust, new needles and K&N filters.
Having set the tappets, timing and attempted to balance the carbs (using a carb balancer kit), the engine sounds nice when idling, but sounds as is it is drawing air when I accelerate... if this inbalance on the carbs?, wrongly set tappets?, or something else... Tappets where set when hot at 15thous..timing at +12BTDC
Thoughts pleae
Mike

Drawing air from where? I'd expect to hear the intake with the throttles opened, especially if the air filters aren't fitted and I'm leaning over the engine. OTOH you could be hearing a vacuum leak. When you were balancing the carbs did you have to move either or both very far from the initial starting position? If you have to richen one more than the other that implies a vacuum leak on that carb. If both possibly other problems.
Paul Hunt 2

With K&Ns fitted you can definitely hear the air intake when opening the throttles as Paul says. I have them on my B and it's quite easily noticeable.
Simon Jansen

Ain't it just the coolest sound?
Tom

Err, an engine does draw air in, then mixes it with fuel. Jap sports bike desingers spend as much time tuning induction noise to give owners pleasure as they do exhausts. On my B the K+Ns are pretty much drowned out by the exhaust, but this will not be the case for all cars.
Stan Best

Thanks all for your comments.
The problem on my "B" is that the noise sounds like it is more then just induction, it is really loud. I have checked the balance on the carbs, and they look OK. The car idles smoothly at about 800rpm and there is little or no tappet noise - which makes the sound even more noticable.
I have started playing with the timing, advancing it appears to improve the situation - but I'm now at about 6-7BTDC, and the standard for this engine is about 12....
I should add that this is a standard head, but that I am using lead free fuel. Should the timing be advanced or retarded for lead free?.....would this make a great deal of difference?..

Any other thoughts would be welcome.

Mike
Mike

How are the K&Ns fitted? Inside the original cans? Or direct to the front of the carbs? I fitted mine directly using the original curved backplates. I originally had aluminium end caps I machined up but then I got hold of a proper cast end plate that I am using now.
Simon Jansen

The K&N's are fitted direct to the carbs, with the backplates provided.
Mike

OK, I was wondering if maybe you had a sharp edge in there somewhere causing a lot of noise.
Simon Jansen

Mike. It seems that you misapprehend the thought of "advancing timing".

Advancing ignition timing means to move it further than top dead center. Thus, your original timing of +12 deg BTDC is more advanced than your later timing of 6-7 deg BTDC. You have retarded your timing. This will result in a lower engine rpm, at the same carb settings. Hence, less noise because the engine is operating less efficiently.

As others have noted, there is a distinct sound of the induction noise when an engine is properly tuned and at idle. At higher engine rpms, the induction noise may, depending on the exhaust system, be drowned out by the exhaust noise. This is a characteristic of the system.

Without more information, it is difficult to tell whether this is "situation normal" or some form of problem.

Les
Les Bengtson

With fuel it is the octane rating that determines how much advance you can run especially on a high-compression engine. Even then the highest UK (I at least don't know where you are) ratings of 98 and 99 octanes don't seem to be as resistant to pinking as the old 4-star leaded in my engine at least. The only thing you can go by, short of much time and money on a rolling road recurving your distributor, is to run the most advance you can without pinking at any combination of throttle opening, rpm and engine load. The more you can run (and you will be lucky to reach the original advance even on the highest grades of unleaded) will give you the best performance, economy and lowest running temperatures. If you have a low compression engine then you may well be able to run the original timings unless you use low grade fuel.
Paul Hunt 2

Never mind what induction noise it makes, what does it go like? We all get a bit "twitchy" when firing up a newly re-built engine. If there's a problem it will show up in poor performance!
Allan

Thanks again for your comments...
Les you are quite right that I had retarded the timing - and the result was not good!. I have tried again and this time based the timing purely on the "best result" - sound, smoothness, pinking, etc.
The engine is a high compression model, Paul, running on unleaded 98. (I live in France BTW)
The timing is now at about 17BTDC, at which point the engine idles smoothly right down to 650/700rpm.
At this setting, the car accelerates smoothly running up to 60mph without issue and when not pushing too hard (it's done 500 miles since the rebuild now, but I'm limiting things to 4000rpm at the moment) will get there in about 15secs.... which is quite acceptable.
Guess that I will have to leave things at that and let it continue to run-in, hopefully it will get a little quieter over time.

Thanks again for your comments and assistance.

Mike


Mike

I can never quite get my head around why a higher octane fuel means you can advance more.

If you increase the advance then you're firing earlier before TDC. Won't that mean the fuel is less compressed when you start burning it? If the fuel is higher octane then can't it be compressed more than a low octane fuel before you get detonation in which case you'd think you can leave it later before igniting it.

I suspect it is something to do with the fact that the burning is just that, burning and not an instantaneous explosion (which you want to avoid of course). Is it something to do with attaining the peak pressure in the cylinder right on TDC so you get most push as the piston heads down?

At what point in the pistons travel do you get the maximum pressure in the cylinder?

Sorry for the slightly off topic question!

Simon

Simon Jansen

You seem to have most of the answers.

Higher octane means it can take more pressure, which causes heat, before spontaneously combusting. When the plug sparks the fuel does burn, not explode, and it does so progressively across the combustion chamber. As it is doing so the pressure is rising all the time, both from the effects of the burn and the still rising piston until it reaches TDC. If the pressure in the cylinder at any time reaches the spontaneous combustion point, the remainder of the fuel explodes, and that is what causes that tinkling sound or pinking. The earlier in the burn phase the remainder explodes the louder the 'tinkle'. Even after TDC if the burn is still occuring the pressure can still be rising, and you can still get pinking on the expansion stroke. It is pinking before or right at TDC which causes the greatest damage, as the vast majority of the energy released in the explosion can only be realised as heat, which can melt the piston.

Timing is a matter of getting as much burn as you can with the piston as high in the cylinder - after TDC - as you can, to get the greatest 'push' of the piston down the bore. Retard the timing and more of the burn or 'push' occurs further down the expansion stroke, when the piston is going down anyway and reducing the pressure anyway, so less of the energy is used for piston movement, and the remainder comes out as waste heat.

Higher octane means the plug can be fired *earlier*, so the maximum pressure occurs with the piston high in the cylinder, to get the maximum push.
Paul Hunt 2

Thanks Paul, after I wrote the question I thought about it some more and figured that's what must be happening. It is realising that the firing isn't an instantaneous thing. It always bugs me when people talk about it being a 'series of explosions' inside the cylinder when I knew that wasn't the case.

This is also why super/turbo charging works. You stuff more mixture into the cylinder and so you get more burn time and hence more push down on the piston for longer.

Simon
Simon Jansen

This thread was discussed between 11/02/2008 and 23/02/2008

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