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MG Midget and Sprite Technical - Confused idle

Its me thats confused.

There are idle screws on my twin carbs and I understand them no problem. However every now and then I hear the term fast idle and I thought it refered to the stop screws on the carb linkage that interact with the camms when the choke is used. (there are some labells on twin SU carb diagrams that confirm this and I have found them on the car). However every now and then someone will mention fast idle like its something else.
I was reading my Haynes manual last night and came across fast idle in the carburetter and fuel section data. Apparently not measured for UK cars but different settings for the USA cars depending on destination and emmissions.
Anyway if someone could just describe 'fast idle' and help me be straight for a 1500 UK car it would put my understanding straight.
Dave Squire (1500)

Dave,
I wont give you my simplified version as it'll probably confuse you more, instead I've asked Guy to pop across and explain but I think he's now off testing contact areas so you might have to wait

even after learning all about fast idle don't get too hung up on figures (especially in Haynes) they refer to a new factory standard car 30+ years ago, they a good guide starting point only

switching topics so as not to drift on CL's thread - taking the matrix out means you can fully check it and fully clean it inside and out it also means you can check the seal around it and the seal for the heater box, if both of these are fitted correctly and in good condition it will mean you get more of the matrix heat into the cabin - you'll only have to do this work once if you do a good job of it and in future keep up with coolant changes

same with taking the radiator out, shaking it and turning it upside down on the back flush can help to get out those last bits of crud

I know what you mean about every time you do some work on the coolant system another part plays up which is why I suggest doing the whole lot at once and replacing any suspect parts or components that way your not retracing your steps and you know that's the cooling/heating system dealt with (allowing for wear and tear)

as for the thermostat, even if it was stuck open after a reasonable warm up you should have the same reading on the gauge when the engine is running but the car is stationary for a while

a lower reading could be because of the time of year and the fact you have an oil cooler, the oil cooler can be covered to prevent over cooling in cold weather or an oil thermostat fitted

I may be wrong but I'm not aware that UK cars were fitted with oil coolers as standard
Nigel Atkins

Nigel,

I definately don't have an oil cooler in this car.

The problem is definately either more crud just happening to loosen, the heater matrix blocked, or after thinking about it, (you know I focus while typing),...... the only pipe for the heater I have forgot about which is the heater outlet back to the block. In fact I think this is probably the only pipe I have not had off the engine now I am reviewing things with you. The radiator and engine cooling side is working well as I have gone miles (40 miles one day at a stretch) without it overheating (I reckon the thermostat gasket blow was caused by some crud just happening to come loose after 30 miles and a rest on a car park that then blocked the system somehow like in the overflow tank pipe). So hopefully I will find time to have a go at the outlet pipe later today. If the matrix is blocked or even shot I have another one thats new for the other car anyway.

So, there is no problem on the keeping at it front, I am smiling and I mean it. Its just great to feel the car getting better with use. Feels like its coming alive again.
Dave Squire (1500)

Dave

Basically the fast idle is the idle speed the engine runs at when on choke. Not to be confused with warm idle, which is the idle speed the engine runs at when off choke.

There is a set screw which basically acts off the cam of the choke opening mechanism which in turn opens the throttle butterfly valve slightly and proportional to the amount of choke applied. This is to open the throttle for cold start. Further rotation of this cam also pulls on a jet link which pulls down on the needle seat to enrich the mix. Both actions are required to enable the thing to start. The rpm setting at full choke should be in the book. It is essential that when off choke, the set screw is clear of the cam and this not actuating the throttle. - see pic. Again there is a clearance measurement twixt screw tip and cam in the book somewhere.

Throttle idle or warm idle is set similarly with a wholly different stop screw. This sets the idle speed when the engine is off choke. rpm setting again in the book. This acts entirely independently of the choke mechanism.

Pic should make things clearer.





Mark O

Thanks Mark, thats sorted it in my head now, the decription 'warm tickover' should be my 750 ish rpm when at running / hot temp and the fast idle needs looking at through the screws and camms.

Excellent pic by the way. Better than the one I have in my SU book. Printed it for my workshop paperwork.

Thanks again, Dave
Dave Squire (1500)

Dave

Nae bother....for me, fast idle is set at smoothest yet lowest revs possible, which is about 2000 or so. I generally knock choke down about 5-10s after start. 50% choke being about 1200. Mind I have a nice single HIF44 which seems well suited to the slightly higher than normal compression head.
Mark O

no oil cooler, sorry Dave I’m confusing your car with another then

if you’ve not taken the matrix out you don’t know if it even has a seal round it or any seal left, same for heater box

cooling can be below par and still not cause overheating especially this time of the year

to get the full results from clean the cooling/heating systems you might have to put in the full work

sorry but thermostat gasket blow was more likely to be a poor gasket or the area not being clear or clean

there’s a JT vid about matrix and how to do a test on it

be careful taking heater pipes off, clips fully loosened and use a twisting action, I added grommets to the holes in the heater box were the inlet and outlet pipes go through, it locates the pipes better and makes them a little more secure when taking the hoses off or on

just a thought – the inlet and out let are the right way round, if they’re like the A-series they’re counter intuitive

as for fast idle 2,000 would be very high for Dave, as I put don’t get too hung up on number and allow for inaccuracies of the rev counter, I’d have thought the fast idle would be on the cam at around 1,200+ on new car run in and new/carbs/dissy/etc./ect

and for the need for choke for more than 10 seconds
Nigel Atkins

This thread was discussed on 15/03/2013

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