Spark plugs revisited | The Back Room Archive | Back Room Forum

Fri 28 Sep 2001 16:11

Spark plugs revisited - Andrew

Thanks to all those who commented to my initial query (26 Feb). Before the engine on the 1500TC was rebuilt and then the twin carbs overhauled , there was terrible misfiring, over choking when the choke was pulled out and the 3rd cylinder plug, counting from the front, sooted up almost immediately. When the engine was warmed up, however, everything improved considerably.

The carbs are SUs and I do keep them topped up with the "seasonal grade" of oil, although the engineer says to use lubricating oil eg 3in1. (ps how far up the cylinder should the oil go?) The distributor is not an original but off a Triumph Acclaim which seems to perform adequately: the vacuum tube is new, so should be OK. I will ask him whether he replaced the diaphragms.

The car always seems to fire better if newly filled with petrol. Today everything was smooth and seemed ok. I will take advice re the plugs and buy some NGKs.

One last thing, his data for the parts may not be 100% - the firing sequence diagram was evidently printed upside down!

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Fri 28 Sep 2001 17:22

Re: Spark plugs revisited - mike harvey

The oil goes almost up to the top, so it does not overflow when the damper rod is fitted. 3in 1 is fine. The purpose of the oil is to slow or damp the piston movement up on acceleration, so as to enrichen the mixture for max power, as the SU does not have an accelerator pump. If one plug is sooting up I would check the float heights and needle valve. It's possible the carb is flooding, and perhaps No 3 inlet manifold port is a tad lower than 4, giving just one cylinder the excess fuel. The diaphragm, (I assume the vacuum advance in the distributor as SU's do not have them, it's strombergs) is easy to check. Just suck up the pipe, and if you feel resistance it's OK.
Regards
Mike

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Fri 28 Sep 2001 18:09

Re: Spark plugs revisited - Phil Goodacre

I had a similar problem with a Mini years ago. No amount of tuning would get it to run smoothly. Eventually I took it to an old bloke my dad knew and after about 10 minutes of sticking his ear as close to the engine as he dare, anounced that the inlet manifold was sucking air where it mated on to the head. Took it off and after much head scratching discovered quite by accident, that the thing was badly warped and no matter how much goo was lathered on the gaskets, it would not seal on the 2 end joints. 2 pots ran ok, the other 2 were over rich and if you altered the mixture, pots 2 & 3 were ok but 1 & 4 ran over lean. New inlet fitted and the problem disappeared.
Another problem I recall with the SU was a ridged main jet needle.

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