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MG Midget and Sprite Technical - overheated - any suggestions

I stupidly put a new thermostat in my 1500 just because I had it - the damn thing didn't work and I cooked the engine. It now runs like a bag of sh*te above idle speed.
I've checked as much as I can to find the problem but all seems ok - good spark, ok and even compression on all cylinders, fuel getting through fine. Are there any signs that could point me in a definite direction - I was thinking burnt or sticking valves but I don't know what the symptoms of this would be. Any thoughts out there??
Andrew Dunn

Do you mean you done a compression test? Burnt or sticking valves would have shown up on this.
Guy

I don't know if you're already beyond this but in case not -

do you think you might have blown the head gasket - look up head gasket failure for some of the symptoms of HGF but be aware then you may have some all or none and still have HGF
Nigel Atkins

I did a compression test and all 4 cylinders were >110psi on my cheapo gauge. I don't believe the actual values because the gauge is cheap and nasty and always reads low but they were all virtually the same so I was moving away from head gasket. I'd normally expect one or two to be lower than the others if it was the gasket.
I wasn't sure if burnt/sticking valves became more obvious at higher revs. The compression test can obviously only shows what's happening at starter motor revs.
I'm just a bit confused as to why all seems well except it's running like a load os sh*te.
I've had a good look around the manifolds and no obvious cracks or leaks on either induction or exhaust side.
My brother has kindly been volunteered to change the head for a spare one so that will at least rule out the valves and gasket.
I'll try a spare coil to see if that makes any difference.
It's just odd that it was running 110% prior to overheating and then 25% immediately afterwards.
Andrew Dunn

if you could swap out for a loan coil

when you put cooked I thought you meant roasted :)

on my previous Spridget I had a heater hose go and I risked running to a place to find they hadn't any so i though I'd go home via the daul-carraigeway to keep the airflow up forgetting it was via a 30 mph long hil with a speed camera on it - when I finally got home I lifted the bonnet and daren't touch the rad cap but lifted off the oil cap to see the oil bubbling so I thought that's it but I got away with just an oil and filter change

having said that I've had a few HGFs and they're always easier to spot the start of with hindsight

your problem could be something stupid unrelated or related to your overheat, I'd run through a checklist of service things, main HT lead, dissy cap, oil in carb dampers, heatsheild, electic connection, etc, ect, - could be something stupid so doing a systematic check could pick it up
Nigel Atkins

Andrew,
When it is "running like sh*te", how does the rev counter behave? If you have a LT electrical problem, it will likely flicker and twitch around, irrespective of the actual revs. It can be a useful start point in tracking down rough running. You could be on the wrong track thinking that it is to do with the cylinder head.

The other thought is that if it overheated, pre-detonation could have damaged the plugs. They can then spark OK at low revs but not when you rev up. And are you sure you haven't muddled up the HT lead sequence?
Guy

Swap out??? Nigel???

Are you turning Yankee?
LOL :)
Robert (Bob) Midget Turbo

Bob,
I did used to watch a few repeats of American Hotrods and Orange County Cyles(?) but thinking about it I probably picked it up from my mate who loves USA and takes an extended family over there every year for a 4 week+ holiday - he'll never be allowed to set up business there but everyone can dream

yeah, it's his fault "if it don't work swap it out"
Nigel Atkins

I like the tacho idea I'll look at that tomorrow.
The plugs are all fine - took them out for the compression test, a quick brush up and back in. Swapped the leads for a different set but still the same problem. Cap and rotor look fine and are only 6 months old. It's got a 12 volt electronic ignition system fitted which has been faultless since fitted a couple of years ago.
Still confused but it must be something simple:o(
Andrew Dunn

Andrew,
you know about faulty new rotors?

the electric head leads tend to be delicate and easily nicked or scuffed inside and outside the dissy and can then play up in odd ways

how badly did you overheat?
Nigel Atkins

Double checked your firing order? 1-3-4-2.
If you had inadvertently switched 2 cables, you may then have copied the mistake when you fitted a new set of leads.

1-3-4-2
Guy

Nigel, wait, you did what? Why didn't you just loop the other radiator hose to bypass the heater??
Andrew F

Andrew,
arh, well, the leak wasn't that bad and I didn't have any tools I was hoping to sort it where I was going for hose

then I checked the hose and made it worse so after being let down I thought I'd nip home as it wasn't far but forgot the hill

I think it could be described as a bad decision

I was also p*ssed off that the place gave me a flat reply and obvious could have helped and sold me something as a get me home, nowadays I'd have just have ignored him and asked someone in the workshop but I was younger and even more stupid than now and it put me in a bit of a black mood my mind was on getting away from the guy for my own benefit that was a better decision
Nigel Atkins

This thread was discussed between 21/02/2012 and 22/02/2012

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