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MG MGA - Tapping Valves

When I am putting a little strain on the motor in 4th up a hill I experience quite a bit of valve tapping. I changed to a higher octane fuel and it reduced the tapping quite a bit but it still occurs. My timing is set at about 13 degrees BTDC. I only have a old cheep timing light. Would it make a difference if I picked up one that has the dwell feature on it?
WMR Bill

Sounds like detonation to me. I would use the best octane fuel available and then retard the ignition until it stops. Detonation is very destructive to engines.
If however the noise does not stop it may be excessive valve clearance noise, but these two noises are quite different.


Mick
M F Anderson

Bill. I agree with Mick. The sound you heard, under the classic situation where audible detonation would be found, was pinging (called pinking by the Brits). The fact that you mention, "I changed to a higher octane fuel and it reduced the tapping quite a bit but it still occurs." is further support for the concept of detonation being what you are hearing. The higher octane rating would, under the same circumstances, reduce or eliminate the pinging which was present with a lower octane fuel. The octane ratings of modern fuels has dropped several points over the last ten years. This includes the so called "premium" grade of fuel which has been reduced two to four points over the years. Under these conditions, some vehicles will require the use of an octane booster, even when a premium grade of fuel is used.

As to the timing situation, you may be over advanced which would be one of the causes of detonation. When this happens, the fuel/air mixture is fired too soon and the piston is, by the movement of the other pistons and the inertia of the rotating parts, being forced upwards against the downwards force of the burning fuel/air mixture. But, there is only one way to check this out and that is to use a timing light having an adjustment feature that lets you read what is the actual advance at any engine speed. With that form of timing light, you can check the maximum mechanical advance at 3,000 rpm and see if it is in the 30 degrees of advance range. If it is more than that, the timing is part of the problem.

Such a timing light also allows you to check the advance at lower engine rpms and to make up a graph showing engine rpms and degrees of mechanical advance. Old, worn distributors will be erratic in how they advance, not showing the designed in mechanical advance curve. Weak mechanical advance springs allow the mechanical advance to begin too soon and reach the maximum desirable advance (about 30 degrees at 3K rpm) too soon. What is ideal at 3K is excessive at 2K. You can also have a non-operable mechanical advance which can affect engine performance, but does not, commonly, allow pinging. A timing light having the ability to read the actual timing advance by "dialing the flash back to the TDC line" is an excellent tool to have.

Having a timing light which provides a dwell or tach feature is not, in my opinion, as useful as having a separate dwell/tachometer. I like to be able to check the dwell, switch over to the tachometer function, then, check out the ignition advance, at various engine rpms, without having to switch features on the timing light.

Les
Les Bengtson

Bill you say your timing is set at 13 degrees BTDC

Unfortunately that means nothing in terms of whether your engine is pinking or not due to poor ignition timing (I assume Les was saying the following)

All that means is that if, the distributor has a 10 or 11 degree mechanical cam in it to allow 20 to 22 degrees mech advance giving a totalof 33 to 35 then it will be fine. However you probably do not know how much mech advance the dissy has and I would expect that it could have about a 13 degree cam in it which would give 39 total all in which would make it too far advanced.

The ONLY sure way to set ignition timing is to measure the total advance at about 4.5KRPM vac disconnected and set that to 34 degrees for a mildly tuned engine. At this setting irrespective of which dissy we have installed the ignition timing will be optimised.
Robert (Bob) Midget Turbo

Robert. Yes, I was saying the following, but in a more comprehensive manner. The common advice to set the timing at 30 to 34 degrees of advance at somewhere between 3,000 and 4,500 rpm is only part of the equation. It makes the assumption that the mechanical advance is in good working condition with a mechanical advance curve suited to the engine. Implicit in this assumption is the fact that the distributor is the original, factory distributor or a replacement which exactly duplicates the factory distributor.

I have seen too many distributor problems to be comfortable with only advising that one set the overall maximum advance, then proclaim that the timing is set.

The three most commonly encountered problems are:

Mechanical advance springs have weakened over the years, allowing the weights to swing outwards sooner than they should. This puts in too much mechanical advance too early. This problem can only be determined by taking a series on readings between the point of max advance and idle and plotting the mechanical advance curve to compare it with the factory curve.

Second, the distributor has either been replaced or rebuilt, either resulting in a different advance curve than the original specification. I found one "rebuilt" distributor which had a mechanical advance cam which allowed 80% more mechanical advance than specified for that application. Until you do an actual test, either on the engine or on a distributor test machine, you do not know what you have in either of these cases.

Third, frozen mechanical advance. Poorly maintained distributors can have the advance weights frozen in place due to rusting and the points cam frozen onto the distributor shaft due to rust. They can, also be frozen due to build up of old oil and dust/dirt. The net result of either is that there is no mechanical advance possible. Under such a condition, when you set the overall advance at 32 degree at 3,000 rpm, you have 32 degrees at all other engine speeds also.

You and I, Robert, have been doing this stuff long enough that we know these things without thinking about them. But, we do not have to ask these questions. People who do have to ask, may not have as extensive a knowledge base as we do and may require more of an expanded, detailed, problem description and solution that someone who has done this for 40 years.

Les
Les Bengtson

May I just make one observation please Les.

No matter what the advance curve of the distributor you have fitted, the only way you can time it in to optimum is to set it at 4.5K RPM and 32/34 degrees advance. Any other method relies upon the advance characteristics being known.

I do agree that it is fairly important to know the advance curve of your distributor although once the max advance setting is set figures for the rest of the rev range are not absolutely important and have more effect upon fuel economy than they do on performance.
Robert (Bob) Midget Turbo

This thread was discussed between 27/04/2009 and 28/04/2009

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