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MG MGA - Goodwood and plug fouling

I went down to Goodwood last weekend and saw LBL101 and SRX210 race in the first event on the Sunday. A Mercedes 300SL disappeared into the distance. There was also a nice MGA Police car which did the honours between races.

Before I went I had set the mixture to give a decent colour. My car drove very nicely down there except after idling for a few minutes in the queue to get in when it started to misfire. I checked the plugs when we arrived and they were really black. The idle speed did drop to 500rpm in the queue and I raised it to about 1000 for the run home.
On the way home the same thing happened, in a queue it went all lumpy and then cleared when we got going. For the last 120 miles there were no queues.

I have just taken the plugs out and they don't look too black. No1 is on the left. So not quite sure whether its still too rich or something is happening at idle. Any thoughts?

This has all started since I started adjusting the mixture and carb settings a few weeks ago. I have not touched the ignition.




John Francis

Photo of Police MGA


John Francis

Here's a suggestion....
Check your vacuum connection to the rear carb (distributor advance)...That little devil can cause all kinds of problems....Even if the pipe is in the fitting, it may be loose in the hole....
The car will run fine at speed, but be really rough at idle....
While your at it, check to see if the vacuum advance is not functioning.
Also, take a propane torch (unlighted!), turn on the gas , and wave it around the engine while it is running....Wave it over all the connections at the carb and manifold...If engine smoothes out, you have a
vacuum leak....
This can also be done with carb cleaner, but the propane is less messy.
Edward
Edward Wesson 60MGA

John

The plugs look spot-on to me

Good ideas from Edward

I think I'd need to know a bit more about the history of the engine and carburettors to comment constructively.
Standard engine? Standard camshaft?
Carburettors recently rebuilt?

"This has all started since I started adjusting the mixture and carb settings a few weeks ago" ... what prompted you to adjust the settings?

David
D Brown

Hi David

The engine was rebuilt 2000 miles ago, carburettors rebuilt by Burlen and it was tuned up by Peter Burgess on his rolling road about 2 years ago, 1000 miles.The engine has a Piper camshaft. Since then I did change the cylinder head because Peter noticed a crack in the original one. Peter rebuilt the replacement head to a fast road specification.

I am just wondering if the head change might have altered things and whether I need different needles in the carburettors. The plugs definitely go really sooty when idling but seem fine after a run and the car performs fine in normal traffic. I haven't tested in for leaks though but did set the carbs up with a balancer.

The engine actually runs quite smoothly at idle and it just soots the plugs.
John Francis

John

Well, the engine is clearly not an old smoker, and the carbs should be OK. Mind you, that supposes that the jets are correctly centred. If not (and you can check this with the lifting pins - see the workshop manual) then the carburettor pistons will prevented from dropping completely, and the mixture will be rich at idle. Also, you will eventually have jets with oval orifices.

Camshafts designed to provide a bit more overlap between inlets closing and exhausts opening will make for a lumpier idle. But I guess we're not talking about a racing cam here.

Your view that the needles, or the needle/spring combination isn't right for the rest of the engine also bears consideration. If that's the case, and if the rolling road tune up was focussed on getting the best combination of power and low emissions, then you might well have ended up with a weak-ish mixture at operating revs, and a smooth idling speed.

Your thoughts on the above? Then we can start researching the correct needles/springs (I would hope that Piper might help with that)

David
D Brown

If this only seems to occur during tick-over when stationary it sounds very much like fuel vapourisation in a very hot engine bay. This problem will be made worse by the added ethanol in 95 octane petrol. I always got this problem in a jam until I fitted a 3" bilge blower in the carb air inlet duct. There should be plenty of info in the archives here and Barney's website.
Pete
P N Tipping

I'm by no means an SU expert, so take my suggestion with a "grain of salt"....But, if you look at most of the replacement jet needles available, you will see that the idle-diameter of the needle (the needle at or near the base), is about the same....It is only as you accelerate , that the taper is different...
This means that a leaner needle will only be leaner at speed, not at idle....
Your plugs look fine to me (as mentioned above)....Almost all of these engines will build carbon on the plugs if they idle for long periods (like sitting in traffic)....
This can also result in hot-spots on the plugs,(burned carbon), that only burn off after getting under way.
Next time you drive the car, run up to traveling speed, and then shut off your engine and coast to a stop....Then pull your plugs...If they look pretty much like your picture, then you are fine. Just my 2 cents.
Edward
Edward Wesson 60MGA

This thread was discussed between 21/09/2013 and 28/09/2013

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