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MG Midget and Sprite Technical - Idling speed

I set my carbs about a month ago mixture and balanced the throttles. The The best idle speed was about 900rpm. Been out for a run this morning and the idle speed has dropped to 800rpm and feel a little rough. I'm going to reset it next weekend, but my question is why has the rpm dropped? is it wear in the throttle spindles?
P Ottewell

different ambient temperature and/or temperature of car at the different times

when first set did you keep and get car to temp and revving to clear out

http://www.youtube.com/watch?v=4nvGLgO6pj0&list=FLr4Udxf9e9Cq7fxH4V23MWQ&index=32&feature=plpp_video

more details in the Driver's Handbook of course :)

ETA: before doing carbs did you set tappets, points and plugs or better still a full service
Nigel Atkins

Nigel, yes and yes....
P Ottewell

might not be carbs at fault and they do tend to wander

was the petrol getting hot perhaps
Nigel Atkins

it's the feels a little rough bit, if you mean as you going along then that could be the other side i.e. electrics - depending on what you mean by running rough - not running lean

did you replace any parts at the same time
Nigel Atkins

Throttle spindle wear is the usual cause of these symptoms, or the wandering Nigel mentions. When spindles are tight, these symptoms don't happen at all.
If there is wear in the spindles, it will show up as play when you wiggle the ends of them in situ and it won't be possible to get an even and consistent idle until it is rectified.

On the other hand, given the cost of doing so and that erratic idle is the most significant downside of wear, some people just wind the idle up a bit more and leave it at that.
Paul Walbran

Common causes are:
points settling in, especially if the dist cam was not lubricated; closes up the gap and retards timing.

Mixture set at some temperature, which has now changed. Especially common when guys work on the car on a nice warm spring day, and the weather then reverts to the more usual cold and wet, maybe 30-40F colder. A flat richer frequently fixes it. I always used to set the carbs for expected normals, rather than the in-shop actuals - made a real difference in customer satisfaction. A late fall tuneup got set a bit rich because winter was coming, and a spring tune on a cold day got set lean because summer was coming soon; I also told customers to feel free to drop by so I could make this change at change of season - free.

FRM
FR Millmore

if it wasn't for the rough running bit I thinking same as Paul has put
>>just wind the idle up a bit more and leave it at that<<
Nigel Atkins

I was assuming the rough running is at idle, which air leaking in though the spindles will cause. If it is rough once off idle, that's another thing altogether.
Paul Walbran

If it's about SU's, a quick fix is to put valve seals on the outer ends. Keeps dirt out as well.
Alex G Matla

It's only a bit rough at idle, once I'm driving along it's fine. I'll quicken the idle speed a bit, however, she was idling nicely at about 900 revs. what should the idle speed be? 1000rpm seems a bit quick to me and I'm thinking of mpg in these times of high fuel prices, (rings his hands).
P Ottewell

depends which engine you've got, it'll be in the Driver's Handbook, I've never achieved it with my present midget, mine seems fast then when fully warmed up and the electric fan cuts in the alternator slows it down to about right

rev counters can be out too so generally if it sounds about right then it probably is but mine sounds fast most of the time, carbs are simply beast but not that accurate or frugal

when you think about it the internal combustion engine is old technology which has altered very little with just some tinkering and bits added it to make it less inefficient

I know what you mean about saving petrol though, I don’t mind using a bit of petrol on a nice run but having it disappear whilst stuck in traffic or urban driving does seem a waste
Nigel Atkins

Good point about rev counter calibration Nigel, especially the early electronic ones which often end up drifting.
Paul Walbran

mine is a later rev counter and the needle bounce is amusing only 1 or 200 out across the ramge though which is good as I use it to read my speed as I've yet to recalibrate my speedo, 25% slow
Nigel Atkins

Gone out tonight and the idle speed is back to 900 revs. I'm going to leave it alone and live with it.
P Ottewell

It also depends on the camshaft. A hotter cam needs a higher idle. Too low and the followers can bounce orso I imagine.
Alex G Matla

This thread was discussed between 22/04/2012 and 24/04/2012

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