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Author Topic: Location of the Distributor  (Read 3435 times)
davidewanprice
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« on: October 25, 2009, 05:21:35 pm »

On the side of the distributor is a pressed metal dome with a pipe protruding, On to this is a rubber pipe which runs to the carburettor, well on my car the pipe coming from this dome is located so close to the wall of the engine way that it creates a very tight bend for the rubber pipe, so tight that over time it seems to damage the rubber pipe causing it to tear. I guess having a rip in this pipe doesnít help the running of the engine. As far as i'm aware this has been like this since i've had the car abd Iíve noticed that on other cars this pipe isnít situated so close. Is it just the case that the distributor needs to be taken off and turned 90 degrees and re-fitted? If so, exactly how is the distributor held in place and by moving it, will this affect the set up of the car? I have a spare distributor and have taken a look at it but it doesnít give me any idea how itís held in place to the engine. I guess there is some form of locking screw that holds it?
« Last Edit: October 25, 2009, 05:23:12 pm by davidewanprice » Logged
Anders Dinsen
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« Reply #1 on: October 25, 2009, 07:10:53 pm »

The metal thing with the hose to the carb is the vacuum advance. It's designed to advance the ignition a bit more when you go off the throttle. It's an "economy device" because extra advance essentially means more mechanical energy out of the engine (until a certain point) and less fuel consumption. The connection on the carburettor is carefully located to give the right amount of vacuum at the correct throttle openings.

About the distributor, you shouldn't try to move it! Turning it around adjusts the ignition advance. If you turn it a quator turn or so it will probably not run at all.

But you can fit a new hose. If the hose is broken, best advice is to block it off on the carburettor and run without vacuum advance until you find a new piece of 5 mm hose.

By the way, there's a bolt under the distributor on the cylinder head holding it in place. Or rather, holding a clamp in place, which keeps the dizzy from rotating. That bolt is very hard to access Sad

/Anders
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davidewanprice
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« Reply #2 on: October 25, 2009, 07:22:29 pm »

Thanks for the advice, for now ive shortend and re-fit the pipe. Maybe a change of type of piping may be the best answer, the one fitted is very soft rubber. The main problem is the lack of space and the tightness of the angle. It may even be that due to the angle its pinching the pipe and blocking the flow..
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JL
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« Reply #3 on: October 25, 2009, 08:53:11 pm »

Hi David
If you really need to move your distributor 90 degrees it is possible; depending on the way you need to move it say 90 degrees anti clockwise then you will have to move the ignition leads 90 degrees clockwise, if you have to move the distributor clockwise then move the leads anti clockwise. If you are accurate with your 90 degrees the car should run OK but it would be worth checking the timing with a strobe light when you have finished.
Regards
John
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davidewanprice
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« Reply #4 on: October 26, 2009, 12:23:35 am »

I don't think its worth doing at the moment but maybe at somepoint in the future, its just one of the many things I've noticed was not quite right on my car, and when you look close, Oh theres a hole in the rubber pipe... Its something I can keep an eye on but I dont have the technical know how to be messing with the timing.. Why make a small job in to a large job..
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roy4matra
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« Reply #5 on: October 26, 2009, 09:32:39 am »

The metal thing with the hose to the carb is the vacuum advance. It's designed to advance the ignition a bit more when you go off the throttle. It's an "economy device" because extra advance essentially means more mechanical energy out of the engine ...

Not quite right Anders I'm afraid.  The way the vacuum advance works is this: The mixture is fired just before top dead centre because it takes a small amount of time to burn and produce the power and if it is fired at just the right point then the power is produced just after the piston has gone past top dead centre, and therefore it pushes the piston down rapidly to give that power to the crankshaft.  If the mixture is fired too early it tries to stop the piston reaching top dead centre and thus stop the engine; and if it is fired too late the burst of energy is wasted as the piston has already gone down.

Now this is the critical point, a weaker mixture takes longer to burn, so you need to fire it earlier to produce the power at the same correct point just after top dead centre.  So as you lift the throttle slightly to stop accelerating and start cruising the mixture gets weaker and you need to advance the ignition point.  Since the vacuum increases as you lift the throttle, this is used to pull the timing into advance to fire the mixture earlier.  The combination of the smaller throttle opening, higher vacuum, and extra timing advance add up to a more economical cruise condition.

Very high performance engines do not use this system since the timing advance on the overrun can over advance the ignition causing engine damage, so cars like the Mini-Cooper S and Lotus twin cam for instance had distributors without any vacuum advance mechanisms.  A more economical cruise condition was not really a requirement with these cars! :-)

David, as Anders pointed out, you must not rotate the distributor otherwise you will alter the ignition, and that is a fixed requirement.  There is a way to alter it, but it is more involved.  You remove the distributor AND the drive gear, and rotate the drive gear meshing one or two teeth, then refit the distributor and re-time the engine with a strobe light.

The distributor is held by a 'Y' shaped clamp with a bolt which is very difficult to get at unfortunately, so for the moment you should leave it alone.  I can do the modification for you some time if you wish.

Roy
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davidewanprice
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« Reply #6 on: October 26, 2009, 06:51:31 pm »

Thats a very nice offer and at some point i'd like you to check my car over but we'll discuss that some time soon (PS I have not forgotten about the badge I promised you..) I hope to be going to the NEC so i'll drop it off then.
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Anders Dinsen
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« Reply #7 on: October 26, 2009, 07:29:31 pm »

The metal thing with the hose to the carb is the vacuum advance. It's designed to advance the ignition a bit more when you go off the throttle. It's an "economy device" because extra advance essentially means more mechanical energy out of the engine ...

Not quite right Anders I'm afraid.

Didn't I write "to a certain extent", Roy? Smiley

But you are right, of course, about the leaner mixture requiring more advance. As a practical example of what you are describing, I can mention my own car, which runs quite lean with the sidedraughts, but only does so with 15 degrees static advance. That's 5 more than standard. At one point in my project, the distributor had rotated a little and advance was only about 6 degrees static. With that, the car could only run well with much richer jets and fuel consumption was horrible.

Will the higher amount of ignition advance not cause pinking? It could very well do with lower octane fuels, but I always run 98 or 99 RON fuel, so it's not a problem and has been verified on the rollers during tune up.

/Anders
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roy4matra
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« Reply #8 on: November 07, 2009, 04:31:08 pm »

Didn't I write "to a certain extent", Roy? Smiley

Yes but you stated more advance means more mechanical energy out of the engine, which taken on its own is not correct.  The important point is to always achieve the power 'push' just after top dead centre, and this is done by a combination of mechanical and vacuum advance, and throttle position, etc.  More advance 'on its own' could produce less power if the advance is too much for the conditions, because the push comes before top dead centre!  So it has to be the right amount for the conditions.


Quote
Will the higher amount of ignition advance not cause pinking?

Not necessarily.  'Pinking' is pre-ignition.  This is a situation where the air-fuel mixture is igniting ahead of the actual spark.  That is not directly related to ignition advance and if you think about it, firing the mixture earlier can actually stop pre-ignition!  There can be a number of reasons for pinking, including fuel that is too low an octane, but it could be caused by a hot point in the combustion chamber, or a too rapid a rise in temperature, even when the correct octane is used.  And detonation, which many would find hard to tell from pinking, is even worse and far more destructive.  It can blow holes in pistons!  Pinking can often produce detonation too.  This is why you should not allow pinking to continue for long.

What is detonation, and how is it different to pinking?  Detonation can and usually does take place after ignition.  Since the mixture burns and is not all 'exploded' at one instance, you have a flame front working away from the initial burn point - which should be the spark plug, but could be the hot spot that has caused pre-ignition; and if the remaining air-fuel mixture ignites before the flame front reaches it, you have detonation.  Then you get a shock wave caused by the two flame fronts colliding.  It is this shock wave that is so destructive.

Roy
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Anders Dinsen
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« Reply #9 on: November 15, 2009, 08:12:01 am »

Hi Roy

Typical me, I read your reply and started thinking. Then forgot to reply to thank you for your explanation about the difference between pinking, pre-ignition and detonation! Sorry about that!

Your explanation about the ignition advance needing to occur just after TDC was not new to me however, and was in essence what I was thinking about. Thanks for sharing the precise explanation.

/Anders





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'82 Murena 2.2 prep 142
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