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MG MGB Technical - Valve Noise

I am new to these tread things and asked this question on another thread but now realize I should have started a new thread. Here goes.

My 75 B makes what what I think is a valve noise when under load going up hill. In a previous thread Paul mentioned pinking but I call it a chirping. I have also heard it called chatter. Usually when I give it more throttle the noise will go away. I do not think I am letting the engine lug as I still have power to increase my speed. It also seems to be worse if I use 89 Octane Fuel ( I normally use 87). Do you think this is from using unleaded fuel without adding a lead substitute and is it harmful?

JR Wood

I would agree with Paul that it may be pinking. It may go away when you open the throttle as the drop in vacuum from acceleration will retard the timing some. For a quick check you could drive it with the vacuum advance line disconnected and plugged, or back off the timing afew degrees. You could also try premium. While the higher octane should help many of the mid grade fuels now has methenol in it. unleaded fuel will not cause pinking unless it is lower in octane.
John H

Thanks John I will give that a try. Do you think the pinking harms the engine?

JR Wood


Pinking could damage pistons if it occurs for extended time.
The main cause is too much advance or(and) weak mixture or(and) low quality gaz or (and) too hot plugs. The fist way to solve that is too check timing and retard it a bit and also adjust your carbs.


Jean Guy Catford

Some different info here to the original thread.

I will repeat that valve noise shouldn't be affected by throttle opening, only engine revs. Where throttle opening affects it, it is almost certainly pinking. But it should be significantly *less* likely with 89 octane than with 87.

As I understand it there are two measures of octane: The UK uses RON (Research Octane Number), there is also MON (Motor Octane Number) measured in a different way, and America uses a composite of these two (RON+MON)/2 which is known as PON (Pump Octane Number).

From 87 US (PON) equates to 90 UK (RON) which is much lower than standard unleaded in the UK.

Standard unleaded in the UK is 95 RON which equates to 91 PON (US).

Super unleaded is 97 to 99 RON which equates to about 93 to 95 PON.
Paul Hunt 2

John, Jean & Paul,

Thanks for the input. I think I will try retarding the timing first and see how that works and then fill up with premium fuel and see if there is any additional improvement. I will then put the timing back where it origionaly was and see what happens. This should let me know if I need to do one or the other or both.

JR Wood

Paul, over here Regular is 87 Octain mid grade is 89 and premium is 91 or 92. If you are in an area that requires it all the grades contain up to 10% ethenol. I have noticed lately that even in areas not requiring special fuel mixtures the 89 octane or mid-grade as it's called here, contains 10% ethenol. That may be why the 89 octane isn't working much better than the 87.
John H

This thread was discussed between 23/10/2007 and 24/10/2007

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