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MG Midget and Sprite Technical - Carb jets - do the settings move?

As the miles go on the clock, general re adjusting and checking is necessasary. Its not a problem cos its going to be a warmer weekend this weekend so I can do lots of it.

However of concern is my tick over which seems to have moved up from about 600 (which it still does when cold) to sometimes 1500 when warm. (After about 20 mins of running 50 to 60 mph about 60% of the time).
Still lots of things to do so am expecting changes and the need to reset stuff however is there a lock system for the carb jets? Its not mentioned anywhere that I can find. So when adjusted can they be fixed as I am just wondering if the carb jets are moving down (the tube unscrewing and undoing) with use or is it the rest of the engine is freeing up and I need to check and readjust everything else as well as the carb setup anyway? The engine ticking over on its own by the way is beginning to sound good if you ignore the lay shaft in the gear box and the clutch slip (don't worry about that now). The diff has shut up so I am generally happier that all other drive related problems will be at the front :-).

I know I should check everything else and I will be doing that. Its just the carb setup takes a while so I want an idea of what happens to this 'unlocked or unfixed carb jet adjuster' I mean does it just sit there unchanging generally after its been adjusted? Or is it known to move on its own?
Dave Squire (1500)

The jets shouldn't be adjusting themselves, as the coil spring around the adjusting nut should provide enough friction to stop it moving.

You couldn't lock the jets themselves, as they need to move when the choke is operated.
Dave O'Neill2

sounds like a classic case of grimlin mischief...hahaha

if your seeing an uptick of that much from 600 to 1500 rpm as the weather warms up, my static default answer is a vacume leak... unless the carbs are really worn...they.tend to hold there settings..also check the dash pot carb oil... if its a thick oil and thickens more with cold and thins down alot with heat....maybe that can be an issue

but ive heard of loose dissys and worn dissys being the culpret in this situation...if its "self advancing".. it will pick up in rpms

another area that iv given thought to conserning this situation... is the quality of fuel...our fuel is just trash, and here we have seasonal blends... we just got our summer blend at the local pumps and with the I can tell a.huge differance in truck just pings like a bell on steep climbs in the mornings until it warms up...

my thinking is air/fuel ratio is much more dense when its cold and siuld run faster... but with fuel being so low quality.. I think the fuel actual is to rich and keeps the speed down till it warms up... not to mention that im sure you adjusted the carbs when it was warm....this whole section is just prop conjecture... I cant back it up, so I wouldnt cash a check at the bank on it

one last note... id shot for 1000 rpm as your normal idle speed... any lower then 800 rpm and the effeancy of the oil pump comes into question...wnd anything above 1200 is just alot of extra wear and tear
Prop and the Blackhole Midget

I can't remember was the engine rebuilt at some time?

It's difficult to set the car for it's various uses and the changes in British weather, even with the whole car and all the parts and components in good condition and good working order the setting will be a compromise to the overall use and weather

if there are worn parts they can widen the range of compromise

as parts and components bed-in and wear out and react to and with each other they produce variances in the running of the car

you could adjust items at the beginning, middle and end of each journey to be better set for each point in time - or keep things within an acceptable range and only adjust if required at services

keep up with services and remember the setting/checking order of tappets, CB points, plugs, timing, mixture

points can need resetting again after some miles of use after fitting or adjustment
Nigel Atkins

Even if the jets had changed their adjustment (and I have yet to find jets which did this) the result would be a lumpy idle from incorrect mixture, not a faster one.
The most likely cause is the engine freeing up and the resulting reduced internal drag.

In general, if the car idles without any "choke" while it is cold, then the idle is set too high. When I say "choke" I mean the choke control rather than mixture enrichment as the first half of the choke cable's movement simply increases the idle speed without moving the jets. It is only in the second half of the cable's action that the jets are lowered to richen the mixture.
This fast idle control is there specifically to provide for idle while the engine is cold.

So it is likely (and usual) that the idle needs adjusting downwards as the engine beds in.

However, don't ignore the golden rule before doing so: Check ignition condition and timing BEFORE going anywhere near the carb adjustments!
Paul Walbran

Check that your throttle cable isn't sticking, it may be something as simple as that.

I Ball

good call there, especially on a 1500 I believe

ragged cable or dirty or dry cable or connections are usually picked up on the cleaning and lubing during a service (details in usual book)
Nigel Atkins

Cheers all,

Your confirmations and advice are much appreciated. You have between you addressed a lot of engine noises and niggles I was wondering about as well as my basic question. Thats great.

Will check spring is there on jet adjusters and not broken. Its so mucky under there it needs a clean anyway.
1000 rpm tick over sounds like it will solve my 'dry engine' start up noise. (I am in let it settle to low revs before turning off to save the turbo bearings habit - doh).
Fuel is best we can do here, shell V power or similar.
Will be doing all in order Nigel. No engine rebuild so yes lots of things need attention but getting there.
That was one of the things I was wondering as well Paul so will get that in as part of the adjustments as well somewhere.
And yes Ian you got it and now you mention it the throttle cable hasn't had any attention since first start in the summer. Simples but essential check that I have not given any attention to since.

The Met office reckons it will be warmer again this weekend. Have most bits I will need and more. SWMBO is at work both days so 'tally ho' over the weekend as they say around here.

Thanks everyone the list is ready.
Dave Squire (1500)

good habit not to switch the engine off as soon as you've stopped as it also means you can listen to check all sounds and gauge readings are as usual - same for when you start don't just pull straight off have a listen and look

in the same vein switch all electric off before switching off engine and don't run any electrics until the engine is running

and remember of lot of servicing work is just basically cleaning and lucubrating but it does need to be done regularly

I use Tesco 99, good stuff and readily available most places

don't forget gauges are only gauges so your rev counter might be spot on or 1-200rpm out either way, when the tickover sounds good then it probably is despite what the gauge says

the figures for setting up refer to a new car so make allowance for the fact that was over 30 years ago
Nigel Atkins

It seems to me that most so-called carb faults are caused by the ignition system. Check that first.
Mike Howlett

Amen to that Mike, that's why carbs have such a fearsome reputation to tune:

New owner hears the myth, car out of tune so tries to tune carbs. Car runs no better because the fault was in the ignition all along. Having had no success at "tuning the carbs" the new owner confirms the myth in his head and spreads the word!

I've seen it so many times, and had to put it right.

The real situation is quite the contrary. On examples which I had serviced exclusively over 2-3 decades I set up the carbsinitially and at each subsequent tune-up once the ignition system was serviced first, when the carbs were checked nothing needed changing apart from the very occasional small adjustment to compensate for high mileage jet wear.

If the spindles are worn they can become harder to get a consistent idle, but the simple solution is to overhaul them.
Paul Walbran

Paul x2 - rebuild the carbs when you get the car, never touch them again, except to clean the vac chambers. If used in winter, richen a flat or two in fall, lean out the same in spring.
Early cars with fixed needles don;t even wear the needle/jet if centered correctly.

FR Millmore

oil and filter - change (if not already done)

tappets - check/set (if not already done)

check, clean and/or adjust as required points, check fitted correctly (Handbook) only replace if really need then get NOS ones

plugs - check, clean and adjust as required, replace if more than around 2+ years old

timings - check set as required

cables, choke and accelerator - check for fraying and lubricate cables and connections (see Handbook)

carbs - replace air filters, clean inside and out and linkages, lubricate linkages, check dashpot oil, check/adjust mixture as required

fuel filter - personally I'd fit a disposalable filter just before the carbs if only that shows you the fuel has made it that far or if it's clogged

coolant - check/top up as you're doing a running clean
Nigel Atkins

This thread was discussed between 27/02/2013 and 01/03/2013

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