catchthepigeon
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Mis fire
 My MG Midget starts and runs perfectly for 15-20mins then misfires and dies on me. Will restart after 5mins but then will continue to intermittently misfire before losing power and giving up. When engine is cold it starts first time and again will run perfectly for 15-20mins before misfiring again etc.
Checked usual things - changed coil, checked electrics ,checked spark plug leads, distributor cap clean/dry, rotor arm seems ok, got spark, checked fuel in carburettor when it conks out. , changed out fuel in case contaminated. Engine temp per gauge seems normal Now I’m stuck. Any ideas ?!
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15/May/10, 9:01
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72 roadie
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Re: Mis fire
I am not a mechanical person but have experienced similar symtoms a few times over the years, although not with my MG's. I found that it was usually the condenser - cheap and easy to replace and probably worth a try!
Bill
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15/May/10, 9:57
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m600 grd
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Re: Mis fire
I had a problem with my Midget which sounds similar to yours, mine was due to the wires coming off the back of the contact breaker having been fitted the wrong way round and the bottom wire shorting out on the plate. The way I had fitted the wires seemed to be the right way round but, if I am not mistaken, they should be fitted the way which looks the wrong way round. I found the problem after having replaced the carbs, the coil, the plugs, leads etc etc. Also check the earth strap as this caused me 2 years of grief with my GT.
Hope this helps.
Cheers
G
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15/May/10, 22:04
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Green Midget
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Re: Mis fire
Have something similar with my Midget at the moment but just a slight misfire. Need to check timing etc.
A question spring to mind, purely from my professional capacity, is what has changed since it was ok? Before you started troubleshooting.
Your problem, as it gets worse over time and then almost resets, sounds most likely to be condensor saturation or possibly fuel/temperature related.
I'd go for the condensor 1st as it's cheap and probably off the shelf.
Let me know the outcome, everyday is a schoolday.
Carl
---
'72 Midget
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21/May/10, 14:47
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catchthepigeon
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Re: Mis fire
Thanks for the advise guys. I have only changed one thing at a time as a check. Up til now prob always the same. Drives perfectly for 10 -15 mins then starts misfiring. Wont start straight away but after 5 mins starts and drive fine for about 5 mins misfires then fades away and conks out( technical term!)
Got new condensor will change it out today. Miffed at missing out on the lovely weather.
MG Midget 1967 - Blue
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22/May/10, 8:58
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FLYMO1
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Re: Mis fire
I too had the same problem some time back. I had changed the head for a big valve unit and it raised the compression ratio which ended up being too high for everything else. Are you getting any pinking? Could try harder plugs but you need to be sure that your engine isn't leaning out too much at the top end. If you've put free flowing exhaust or inlet manifold on recently you may need different needles. Mine wasn't running hot but did pink a bit.
Unfortunately I can't tell you what finally fixed it as I bit the bullet in the end and got the engine rebuilt by Oselli with electronic ignition and corrected CR, so many new parts and never had a problem again.
Could your fuel pump or earthing be weak?
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22/May/10, 13:05
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catchthepigeon
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Re: Mis fire
Mmm new condensor did not help...earth strap next ho hum. 
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23/May/10, 11:59
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Drumtochty
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Re: Mis fire
Recent rotor arms and condensors are very poor quality.
They can be crap from new. I will come back with better suppliers.
Midget engine bays can overheat and vapourise fuel in the float chamber. How long have you had the car and does it only happen in summer
eddie
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29/May/10, 23:13
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Drumtochty
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Re: Mis fire
Ignition
Better Rotor Arm
For 25D Distributor DRB101 £6.95
For 45D Distributor DRB104 £6.95
Martin Jay
Distributor Doctor
1 Bovey Cottages
Waterrow
Wiveliscombe
TA4 2BA
01398 361678
martin@distributordoctor.com
http://www.distributordoctor.com
Or get this electronic kit with the updated rotor arm here
http://www.simonbbc.com/automotiveshop/electronic-ignitions
Sparkplugs can break down under heat for no reason even new ones.
Have you watched if the distributor is advancing normally with a stobe both when running well and when misfiring
Fuel
Does the piston in the carbs lift easily and clunk when it is dropped.
Are the needle valves in both float chambers sticking or leaking put in new seals.
The fuel pump is filling the carbs so we assume OK.
Compression
Have you done a compression test both before and when the problem happens.
PM me if you want a hand. I have time on my hands at the moment and sometimes another set of eyes and ears helps.
eddie
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30/May/10, 16:24
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Drumtochty
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Re: Mis fire
If it has the 1500 triumph engine then one of these may help if it is fuel vapourisation.
http://cgi.ebay.co.uk/Triumph-spitfire-MG-midget-stainless-heat-shields-/250639844055?cmd=ViewItem&pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3a5b4c82d7
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30/May/10, 20:00
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