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MG Midget and Sprite Technical - tickover speed?

Hi All
Back from my 750 mile journey - mostly motorways at 70mph.
One thing I have noticed is that the tickover is now sitting at around 1100 - 1200 rpm. It has gradually been creeping up over the last 5 or 6 weeks to be honest. I had it set to about 800 previously. I have tried the slow running screws on the carb, but they had no effect whatsoever. Another thing I have noticed is that when hot, it runs on for a second or two after switching the ignition off.
It was also reluctant to turn over on the starter the other day, so could this be the timing too advanced? It has electronic ignition that the PO fitted, and which was once giving a good steady tickover, down to about 500rpm if so desired!
It is definitely the engine speed, one can hear it running a lot faster than it should, and not a mis-reading tachometer, though the tach has also started playing up - reading about 3000rpm at 65mph then dropping to 2500 when speeding up to 70mph.
Any ideas would be gratefully received! Thanks, Dom.
Dominic Excell

It sounds like a vacume leak

Id give the intake manifold and carb nuts a good twist with a wrench, it certianly cant hurt to stick a timing light on the marks and see where it sits

But im thinking vacume leak, is the top of the list

Still ... a differance of 300 to 400 rpm increase isnt exactly chicken feed...I think thats enough to say something is off kilter

Prop
Prop and the Blackhole Midget

It can also be the throttle cable sticking a bit. Maybe the return spring has gone weak or missing.
Sandy
DARNOC31

Could be a very common SU carb fault usually caused by the needle not seating properly - try taking off the dashpots and carefully wiggling the pistons and/or taking them out and cleaning them, the jets and the needles.
Nick Nakorn

Thanks chaps. I know the spindles were a bit worn, so I sealed the ends with caps I made from "chemical metal" which helped enormously (5000 miles ago). The needles and jets were replaced at the same time and still fall with a satisfying "clunk". I'll recheck the linkages and jet return springs again, but you're right, it does point to a leakage....
Dominic Excell

Dom,
I think the high idle could cause run on

if you've not already done so during your servicing then oiling the organ pedal plate hinge and linkage plus the the points on the connecting arm (all as per the good book) might help

perhaps remove the inner accelerater cable and inspect it for snags and then clean and oil it

if not already done clean and oil carb linkages

as suggested check return spring strength

as the tach is also playing up I'd check all its associated up and downstream wires and connections are clean, secure and protected

same for igniter head and just check inside and outside dissy cap for scuffying on the delicate wires

if you adjust the timing you'll need to also check the carbs but bear in mind carbs are an item that you have compromise on setting up so if you get it spot on now you'll be readjusting it perhaps next spring or next service
Nigel Atkins

Dom

The cause of your run on is engine running hot. The root cause of this could be any number of things, but for me it was a combination of slightly incorrect mix - it was running slightly weak - and ignition timing slightly out. I fiddled about somewhat and it has improved, but not quite been eliminated. Of course, this assumes all other things are equal - correct grade of plug, leads OK, no leaks which would generate a weak mix, etc etc.

On other matters, what may have happened during your long run is that your jets may have become free of crud, and you are now obtaining a more correct set up from which to set mix and ignition. If you cannot now reduce engine speed by idle screw alone, that in itself is suggestive that previous mix/ignition setting was incorrect.

Tachometer is probably your drive cable on the way out.
Mark O

I don't think Dom's tach is cable driven

although I think he keeps his speedo reading high to improve his MPG and keep his cap on his head :)

Dom does a few long runs and drives at reasonable speeds reasonably regularly but if crud clearing is needed I have offered to give the car a proper blow out run for him and get the needles to the South East region of the gauges, I'm too helpful sometimes :)
Nigel Atkins

1. Running on is caused by hot spots in the combustion chamber, e.g., glowing carbon deposits, the edges of the chamber, plug electrode, burrs and rough edges. These can overheat when everything is set up correctly. Some makes of fuel seem more prone causing R O. As said above, over advanced ignition and weak mixture do lead to hotter combustion chambers as does high C R and too hat a plug grade.
2 Also, as regards speed of idle, check the butterfly discs are central and have not slipped.
3 If the butterflies are indeed working properly then backing off the throttle screws should stall the engine. So air must be by-passing the discs, the usual culprits are worn spindles together with leaking carb or manifold gaskets.
4 The tacho gets it's signal from the coil, which, in turn, is controlled by the ignition module. An erratic tacho and idle speeds, running on and poor starting could all point to the electronic module, it's connections or interference from HT leads or arcing. Hall effect can suffer from stray electro-magnetic signals.
Allan Reeling

It could be related to your other thread of the broken radiator...it may have leaked out a little and been dryed by the air and engine heat.and.making the engine run hotter because of a lack of fluid

Prop
Prop and the Blackhole Midget

This thread was discussed between 25/11/2013 and 26/11/2013

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