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Running problems!

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AJW17/03/2019 21:30:30
278 forum posts
117 photos

Having stripped and painted my newly made Centaur it has been assembled paying careful attention to valve timing.

Original spark plug gap was 0.012 which I have increased to 0.025 to give a meatier spark.

Running doesn't seem to be very good? Not smooth or that regular.

I think my main problems are the spring on the disc valve in the carburettor. Westbury suggests a 'light spring' but in fitting the lightest spring I could find results were very poor, and no firing at all with no spring.

This has led me to find a stronger spring which is giving the results as in the YouTube link. However although it runs as can be seen there is little in the way of speed variation with throttle/ignition adjustment.

Not sure if it can be heard on the link but the disc valve can be heard being lifted from its seat not as a click but as a low frequency sort of buzz upon each induction stroke. Should I try and find a stronger spring?

Mixture screw is actually unscrewed out past its thread! but exhaust doesn't appear smokey as in over rich.

Any guidance on improving the running would be most welcome!


Howard Lewis17/03/2019 22:32:21
2757 forum posts
2 photos

Suggestions, based on the assumption that reassembly has not resulted in any air leaks.

1 Reduce spark plug gap. The ignition system may not be able to provide a reliable spark with a 0.025" gap.

This may account for the uneven running. The spark that you see with the plug out, may not be available under compression conditions inside the cylinder.

2 The stronger spring for the disc valve may be reducing the period for induction. The fact that you cannot get the mixture to an over rich state, suggests that it may in fact be too weak, which, again will cause uneven running.

BUT Only change one thing at a time. Revert the spark plug gap to 0.012"

If that improves running; with the timing optimised, Change back to the weaker the spring on the disc valve.. If that improves matters, and allows the mixture to be richened, to provide best running; then re optimise the timing. The two settings affect each other. A weak mixture burns more slowly, so requires more advance, but may require a stronger spark to start combustion.

It is process of re iteration, until you get the best result. You will eventually reach a point where small adjustments have little or no effect. Those are the optimum settings.


MichaelR18/03/2019 09:25:23
351 forum posts
64 photos

I have a light spring on my Centaur disc valve and it gives even running, my theory is if the spring is to strong the engine suction will struggle to lift the valve and give intermittent fuel supply, if you watch the disc valve spindle which protrudes through the throttle lever arm you should see the spindle moving up and down as the valve opens and closes with a nice even movement.

My fuel needle is only open about half a turn when running, and I have full throttle range, having said that it is all down to good luck on my part as I am no expert on engine management.

Minimag Ignition CM6 Plug

First Run Link before final adjustments.


JasonB18/03/2019 11:18:25
17098 forum posts
1843 photos
1 articles

I've done a few engines with copies of Lunkenheimer carbs and they have a similar valve all with a very light spring, infact the smaller scale ones often leave the spring off and just let gravity close the valve. my Robinson has a flat disc valve and that makes quite a marked "farting" sound as you can hear about half way through this video.

As Michael mentions most model engines only need the fuel valve open a small amount, try the flame method I mentioned in one of your other threads to see what your mixture is like.

AJW18/03/2019 12:29:05
278 forum posts
117 photos

Thanks for all sugessugges, plug re gapped, hopefully have some play time later!

Michael, I believe the spring strength is quite critical, I have quite a selection of springs as I save them from everything and anything going but wish I had more light varieties!

Jason, that's one chunky flywheel! I can definitely hear the valve lifting and think mine is sort of sounding similar? which makes me think my spring tension can't be a million miles out?

In this link of engine running quite erratically the valve can be heard.



Edited By JasonB on 18/03/2019 13:11:37

AJW19/03/2019 18:32:42
278 forum posts
117 photos

Armed with a selection of potential springs the second one I tried seemed quite promising.

Continuing to work through my spring selection I came to the opinion that my second choice was worthy of retrying but this time it didn't seem to run anywhere near as good as the first time?

The only thing that changed during these trials was engine temperature, in particular the carburettor which by now was untouchable.

On this particular design the exhaust gases are ported through the carburettor body I presume to prevent icing, as a result I am thinking that the vapour mix is being affected, unfortunately I didn't measure the temperature it had reached but will try again when cold - this time armed with a thermometer!


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