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MG Midget and Sprite Technical - !275 Misfire when hot - How to diagnose

I have a problem with my 1275 midget which has just appeared.

When at normal engine temperature (temperature gauge below "N") it runs well, smooth at all revs up to 5500rpm and lots of power.

However, when really hot (Gauge above "N") it still drives well but misfires above about 4500rpm.

My question is, how do I diagnose or fix this problem? Most components are relatively new (engine, carbs and distributor were new 30000 miles ago)

Rotor arm and plugs are new. Distributor has been converted to electronic (Magnetronic).

Any ideas on diagnosis, or do I just have to replace likely suspects until I find the problem one?

Mike Parris

M Parris

When it cools down a bit its fine ? if so try changing the coil. (30000 is a while ago lol)
K Harris

Try changing the rotor arm for a non black one.

P Burgess

Id get it up to temp where its mis firing then pull in a super dark garage "total pitch black...darker the better" pop the bonnet and have a look for an electric blue light show with the engine running, if its sparking anywhere then you will see and know what to do or replace

if nothing there run to miss and try another good coil thats cool...or if you got a multi gauge and know how to test and know the specs. you can test it that way...I think les has a great website for testing electrics....yepp here it is....

if not there

run to miss, then wrap the carbs, carb floats, intake manifold and fuel line about to with in 3 feet the of the back side of the engine with aluminum foil to block heat soak...(dont trap the heat coming from the header, but deflect it)

Plus maybe?? (My idea) mist down the carbs and intake manifold with ice cold water thur a spray bottle to help cool the intake system down ...just dont get crazy and spray down the head block and header...this could be a bad idea as you can crack some expensive parts

if not there run to miss, pull valve cover and look for a sticky valve...most likely an exhauste valve...and most likely #4 due to the water dosnt cirrculate well back there in the head and gets a bit hotter in that area of the head.

If still nothing above......Heck, Why are you asking me, Do I look like the man that shot liberty valance? LOL


oh absolutly replace the black rotor for the red if not already having done so thats the 1st thing, if not for the soul purpose of preventive maintance

they just dont make good quality chinese auto parts like they used to...but the boxes sure are prettier



This thread was discussed on 25/07/2010

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