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MG Midget and Sprite Technical - 1275 misfire. Underheating?

Having a problem with my 1275.
Firstly, the car was running very well late last year before they started gritting and I hid her in the garage.
I've filled up with super unleaded plus Castrol valvemaster for the start of the season - tank was approx. 1/4 full beforehand.

The car will start up fine and once warmed through will idle quite happily at 1000 rpm. Accelerates well at a standstill too.

For the first ten minutes or so of a drive it performs perfectly, accelerating smoothly from slow speeds up to 60 or so across the gear range.

After this time, the slow speed range seems to attract a misfire which sounds and feels like a fuel shortage.
- Keeping the revs high gets over the problem.
- when I get home and the car has sat idling for a while it will behave itself.

Throughout the trip, the temp gauge never gets beyond halfway between C and N - I put this down to having an oil cooler with straight through pipes - plus it's still quite cold in Lanarkshire.

So far, I've reset the tappet clearances, swapped the accuspark unit and reset the timing and replaced the HIF44 float needle and jet. I've also tried tuning the carb with a colourtune - spark is blue with orange flecks at idle changing to a steady light blue at higher revs.

There's no sign of any water / mayonnaise in the oil so I'm hoping it's not HGF although I'd have expected the temp to go through the roof anyway.

plugs look the way I'd expect them to.

I plan to cover the oil cooler with card before the next run as I'm wondering if the misfire could be brought on by the engine being too cold but I'd be interested to hear any other opinions?

the stats -
1293. high lift cam. roller rockers. big valves, LCB. Hif 44 with K&N . Accuspark kit.

graeme jackson

does your rotor arm have a rivet holding it together? If yes that may be the culprit and suggest it's replaced by a genuine red non rivet one.
Jeremy Tickle

Thanks Jeremy.
The rotor is a red one that came with the accuspark kit but it is a couple of years old so I'll get a new one ordered tonight.
The coil is the same age - is that likely to be suspect too?
graeme jackson

coil? only if the rotor arm doesn't change anything, even then diagnosis is better than just changing out relatively new parts. Coils ex-factory used to last several decades.
Which cam did you install, and what did you set the timing to?
David Smith

David. It's a piper magnum 270 and it's been in there since 1994 :-) The problem only surfaced last weekend after sitting in the garage for a couple of months. before being laid up it was running fine so whatever has gone wrong has been recent.
graeme jackson

This is a long shot but if you look at my thread "Annoying short-term breakdown" you will see that I recently had very much the same symptoms you describe. I did all the things you did to track it down but it turned out it was due to a non venting petrol cap. I think your journeys are a bit short to create this problem but you never know (I came to a compete stop after around 60 miles). Check the cap is venting properly. Good luck.

Chris Hasluck

"Temp gauge never gets beyond mid way between C and N" - I think this is pretty normal and unlikely to be the cause of your problem.
Have you checked the oil in the carb dashpot? Low oil here might effect the running once it has warmed up and the choke is in. Have you noticed how the rev counter needle behaves when it begins to misfire ? If it flickers wildly it is likely to be am electrical, and not a fuelling problem.

Could be condensation in your dizzy cap. I had this in the past due to a crack. Moisture gets in then as the engine heats up it condensates in the cap causing a track. Not saying that it is that but give you cap a spray with WD.
Peter Ottewell

Do you have a mechanical fuel pump?
Alan Anstead

nope - standard Su in the axle well
graeme jackson

I know it doesn't solve your problem but cases like this are why I always recommend people use their classics year round. They never laid the cars up over winter when they made them :)
J White

Put some isopropyl (dry gas) in your gas tank. Condensation builds up in your gas tank. The water contaminates the fuel. A small amount of water will make your car perform just as you describe. Here is the states it goes for a buck bottle. Put two in and at least you can rule fuel contamination out.

Next I would do a compression test just to make sure you have a good base line on your cylinders.


Maybe your dashpot has got sticky and not moving freely
This would possibly go unnoticed while cold and warming up but would certainly cause a drop off in performance when warmed up
A clean up and free movement check wouldn't hurt and if it isn't that, it's one less thing that it could be,

On the cold running - Thermostats seem to fail in the cool direction nowadays. -It's weird really, years ago they used to fail shut and cause overheating but that is rare now as they seem to fail by gradually opening earlier and earlier causing cold running-like yours

William Revit

X2 steven... esp if you have ethinol fuel... that stuff is only good for around 3 weeks before it goes off

a couple other ideas...

check the points in the fuel pump for a corrosian build up... sitting unused can put a film on the breaker contacts

you have roller rockers... remember to set them with a feeler gauge from the side... not the front...specing from the front will give a false reading as the feeler gauge will roll the wheel up .... specing from the side the wheel stays in place

try running a shot of compressed air 30 psi ? Thur the fuel line before the fuel pump towards the fuel tank... you maybe suffering what im dealing with... lots of crud in the fuel tank clogging the little filter just inside the tank ...remove the cap before hitting the line with compressed air or more damage could happen

also ... next time feel up the coil when the problem happens and see if you get a 2nd degree burn on your hand.

Prop and the Blackhole Midget

Also check the fuel pump ground wire for early stages of corrosion and dirt build up
Prop and the Blackhole Midget

This thread was discussed between 29/03/2016 and 30/03/2016

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