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MG Midget and Sprite Technical - Fast idle has changed - why

So, I've put a new headgasket in now. And she started.

But...

The idle is very high and its not running great. Now I do need to adjust the timing, but before I can even begin there I need to the idle down from 2500 revs.

I have had the carbs apart, I've had the jets out etc... but not played with the fast idle screws... how can the idle change so much? Have I done something strange? I don't want to start playing with the fast idle screws just yet... i just can't see how they could have moved on their own.

Any ideas?

1500 with std SUs
C L Carter

The choke is a bit odd... when the choke is pulled, it lowers the jets as it should... but the cam doesn't touch the fast idle screws at all...

its been a long day... can't get my head round this... must be something simple!

C L Carter

You are right to think back on what you have moved and what has not been altered. So if you haven't done anything to alter the timing, then it won't be down to that.
Two possibles spring to mind that you have altered. It may have a manifold air leak as you disconnected that. A weak mixture because of an air leak would raise the tick-over revs. The other possibility is a sticking throtle cable. Or maybe the ferule on the end of the outer cable isn't seated properly, which again would raise the revs.
Guy Weller

you need to start over from scratch...as I recall, you adjusted and fiddled with everything

1st id adjust the valves, then id set the carbs to.basic factory specs, then id adjust the dissy timming, then adjust the carbs..and recheck your timming

btw... run the engine for a half hour with the rad cap off to burp out all the bubbles...otherwise it will turn into valcano nightmare of a mess


a coolant system not burped out can make a high speed idle

prop
Prop and the Blackhole Midget

This is what mine does and I have been chasing down all the things that these fellows on here are saying. Its getting better (my over the top tick over is now 1500 or so although 750 when cold). So I reckon the improvement is in direct proportion to my improving ability to:
Fix and fill the cooling system. (could be fixed but heater still not very warm) hmmmm.
Adjust the valve clearance. (Still too large) Practice Practice.
Set the carbs. (settings affected by everything else) Sort of the easiest bit for me now but takes the longest.

In that order.

Half a day, no interuptions, keep notes, correct tools and books to hand and open to correct pages before start.

Remember valve clearance engine cold, carbs adjust engine hot. (I just love that one) :-)
Dave Squire (1500)

Had just the same problem after changing a gasket, it turned out to be the throttle cable, it was not located correctly in its seat in the pedal lever, as Guy said above. Once it was popped back, revs went down from 2500 to 800.
Tony
Tony G

O/T

Dave,
the fix and fill cooling system but heater still not very warm - which, how much work did you do on the cooling and heater system?
Nigel Atkins

Not as much as I would like yet. I am going to have to remove the heater I think and have a go at it off the car at some point.
The cooling system has been:
1) Flushed, and lots of crud came out (brown and black stuff)
2) Hose failed on heater tap, back flushed and system flush using flush stuff, and lots of crud came out again.
3) Run at high speed for 45 mins on A1, thermostat gasket fail, and yes you got it lots more brown water.
I have been replacing weeping hose clips and taking off and reseating hoses as necessasary. I don't have any real water loss now (Only put an egg cup full of water in after 3 days today).
Interesting point is I measured the water in so I know the heater was empty. Hmmmmm.
At no time have I had real heat (just warmnes yes) out of the heater so I recons the heater is just blocked up. Or I have a cool 1500 (Joke?) Its a matter of time and testing.
The good news is that the engine sounds if not great (I don't know what great for a 1500 is yet) then very good now (the AA man said it sounded good and he runs classics). I have the oil and filter ready for another oil change soon. The gearbox and diff have shut up and sound quiet after the A1 blasting so no fixing needed there. But the petrol pump (spare arrived today) needs swapping before I do any real great distance again.
Did valve clearance again today and I got it better again.
I am smiling because it is getting there.
Dave Squire (1500)

Actually I suppose - I am getting there. :-)
Dave Squire (1500)

valve clearances could mean checking/changing timing ans mixture too

so far just a one way flush only on whole system post change of clips and hoses, no seal replacement or cleaning of matrix yet

if you do the more thorough system clean and overall as previously detailed I'd be surprised if you didn't get some improvement on your heater and cooling systems and even more crud out of the engine block and even possibly some out of the radiator and perhaps matrix

renewing or fixing the seals around the matrix and heater box, and perhaps cleaning the matrix fins too, will help with getting the heat into the cabin

I hope you saw my post to use a klingsxxxx? thermostat gasket if it wasn't too late
Nigel Atkins

I have and am Nigel. Each time: -
I have back flushed etc. And then each time something has happened I have reflushed and backflushed until its all coming out clean again before the next run.
If it happens again I have another can of cooling system flush waiting.
I think the main problem is that the car had very little water left in it after standing for all those years in the brother in laws garage and my workshop before I got it back on its wheels so stuff was probably dried inside over all those years is now coming loose in dribs and drabs under increasingly more strenuous conditions as I push it harder and longer.
There can't be much more to come out and the radiator (a crossflow) I know is now very efficient. (A lot cooler on the outlet side than the inlet even when the engine is hot and car standing).
So plan is to swap leaking fuel pump tomorrow for new one (assuming it fits) and give it a long blast on Saturday. See what happens again. Will carry ... erm ... kitchen sink instead of just half the garage tools in the boot with lots of water this time to do running repairs and see if that deters the gremlins :-)
Oh and by the way I got 30 to the gallon even with over 100 miles of over 60mph driving from the last tank. So as the car improves I wonder what the final figure will be? I don't think thats bad for a car like mine is at the moment and my rather naturally heavy foot.
Dave Squire (1500)

is the temp gauge showing proper temp?

if not... and its cold ...you could have a clogged thermostate, in the open postion

prop
Prop and the Blackhole Midget

Thanks Prop didn't think of that. It fills easily so could well be. I'l take a look.
Dave Squire (1500)

Simple test for a faulty or otherwise thermostat.
Start the engine from cold and allow it to warm up gently. As it does so, grasp the top hose and wait, feeling for the temperature change. With a properly operating thermostat the top hose will remain cool, or only very slightly warming, for a period of time and will then quite suddenly warm up as the thermostat opens.

If the thermostat is failing to open the top hose will remain cool with little or no heated water passing through it, or will only slowly warm up as heat is conducted through to it. If the thermostat is stuck open, then the top hose will warm progressively and steadily right from the start, with no appreciable sudden step change.

This test is pretty reliable, simple to do and doesn't involve disturbing any watertight gaskets or part draining the system. Only if the test indicates a suspect thermostat,do you need to then remove it and do the saucepan and thermometer test to see if it visibly opens and at what temperature
Guy Weller

I had a problem with my cooling and it turned out to be the T piece on the end of the inlet manifold it was blocked solid had to drill it out



mark (1977 1500 Midget) Preston Lancs

Thanks for the comments, sorry its taken so long to respond.

I have had to alter all the things you have said, so a full service is due this weekend anyway, and will include timing and carb service.

The problem, and I don't know how it happened... is the throttle cable was too short... god knows what happened, i don't know how it can have shortend itself... obviously something funny is going on and I can't work out what... it definitely is seated properly in the pedal, I thought that was going to be the first problem... anyway, one or 2 turns on the throttle stop screw and its back down to where it should be.and I don't have to stretch it to fit it onto the throttle lever. Strange. Very strange
C L Carter

If the throttle cable were genuinely "too short", then altering the throttle stop screws wouldn't make any difference.
But l wouldn't worry about it. Lots of slight differences from the work done in changing the head gasket could effect the idle speed. And the job isn't finished until you have gone through the adjustments , in the proper order.

No point in expecting it to be right until the work is finished !
Guy Weller

This thread was discussed between 13/03/2013 and 17/03/2013

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