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Wales
by Joske Vermeule, September 1
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Jun 14th, 2026
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Joined: Sep 2014
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L - Learner Plates On
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OP
L - Learner Plates On
Joined: Sep 2014
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Good evening chaps
My 2000 3.9 Plus 8 has developed a misfire under load, which is making pulling away from rest and overtaking a little difficult. I've replaced the plugs, pulled off and refitted every connection I can find to no avail - any ideas for narrowing down the problem?
Ta
Mike H
Mike H
Bourgogne Franche Comté
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Joined: May 2007
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Part of the Furniture
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Part of the Furniture
Joined: May 2007
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When I had this problem it was the plug leads. Could also be a faulty distributor cap.
Steve A11OGE Red 1989 4/4 4 seater
'A Morgan is for life, not just for Sundays'
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Tricky Dicky Member of the Inner Circle
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Tricky Dicky Member of the Inner Circle
Joined: Dec 2009
Posts: 37,435 Likes: 980 |
Worth removing the distributor cap and cleaning all the 8 contacts and rotor arm - check tension on the carbon brush for resilience. Good visual on the cap will tell you if it has been tracking under load.
Otherwise, leads and all the other faston's associated with ignition underbonnet, could even be the ignition coil breaking down.
If in doubt swap them out one by one these items are cheap compared to a moments hesitancy at the "wrong" moment.
2009 4/4 Sport Henrietta
1999 Indigo Blue +8
2009 4/4 Sport Green prev
1993 Con Green +8 prev
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Charter Member
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Charter Member
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Have a look on a dark damp night, shows the tracking much more.
.+8 Now gone for a 1800 4/4. Duratec in bright yellow.
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Just Getting Started
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Just Getting Started
Joined: Apr 2013
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If your car is a late Hot-Wire, it still has a distributor, and replacing all the plug-leads, then the rotor arm and distributor cap, and then the ignition amplifier (small black box) and then the coil might be the route to tracking the problem. All are pretty routine and inexpensive service parts. Do check all connections are sound as you do so.
If the car has the later Gems injection system, then after plug-leads, you are only left with the coil pack to check/replace at the rear of the engine.
All this assumes that it is ignition, not fuel, that is at the root of your symptoms.
Do let us know how you get on.
Andrew Green
Glorious Devon
2002 Plus 8
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L - Learner Plates On
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Thanks chaps, will work my way through the list. Managed to get it to go away for a bit yesterday after pulling and refitting the king lead, so may be getting close
Mike H
Bourgogne Franche Comté
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Roadster Guru Member of the Inner Circle
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Roadster Guru Member of the Inner Circle
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It's been pretty well covered above, but the short will be a breakdown of insulation somewhere. The king lead is an obvious candidate as it's taking 8 pulses for a single pulse on each plug lead. A wide plug gap can force a miss, but you've already changed the plugs so they should be OK. There was a workshop note for MGBV8's which advised not having two specific plug leads next to each other on the offside bank. I don't remember now which these were - but it was because the induction from one could upset the other if they were side by side. Might have been 4 & 6 because only 3 fires between them, or 4 & 8. (1,8,4,3,6,5,7,2).
Follow Ray's advice and look in the dark.
DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4
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Tricky Dicky Member of the Inner Circle
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Tricky Dicky Member of the Inner Circle
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Another common fault is the proximity of the leads to the alternator (slack loop) as it's another known with this power unit. Caused by induction from the alternator playing games with a lead/leads and effecting a miss under load conditions.
2009 4/4 Sport Henrietta
1999 Indigo Blue +8
2009 4/4 Sport Green prev
1993 Con Green +8 prev
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Joined: Mar 2009
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Talk Morgan Expert
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Talk Morgan Expert
Joined: Mar 2009
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Coil packs do do this occasionally and they are cheap enough from a breakers yard to try. Mine turned out to be standard issue from a Pug 206 but of course is a four cylinder unit.
Last edited by meabh; 29/09/15 09:12 AM.
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Joined: Sep 2014
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L - Learner Plates On
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OP
L - Learner Plates On
Joined: Sep 2014
Posts: 201 Likes: 8 |
New distributor cap and rotor arm ordered, will look at plug lead routing at the weekend...
Mike H
Bourgogne Franche Comté
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