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MG MGB Technical - Missing badly under load

Hi, My 71B has been running as sweet as a nut - up until yesterday. Started up fine and ticked over no problem. Got about 200 yards up the street, which has a slight incline and she started missing badly with the occasional slight backfire. I managed to get her around the block and back up the drive where she ticked over as normal. I revved the engine out of gear - no problem. In gear, missing and nearly stalled. Checked plugs this morning, a little lean but all about the same colour .025 gap. Points .015, no cracks in the distributor cap all looks clean. Could it be the coil or capacitor? Any suggestions?
Thanks
Ken
Ken Martin

Hi, Ok, I have now changed the distributor cap, new plug leads and plugs (NKG) The car is running smoother on idle, but still missing (a sort of fluttering noise from the exhaust) under acceleration. Any ideas anyone?
Ken
Ken Martin

Have you changed the condenser and regapped the points at .015-.016" yet? That's where I'd start. Closing gaps on points also change your ignition timing significantly, and that sounds like what you're experiencing. Exhaust backfiring is a sign of retarded timing, which would happen if your points open late.
Jeff Schlemmer

I'm prety sure the missing was either the plugs, cap or leads as that has now gone. I had already checked the points and they were right on .015. I checked the dash pot oil levels on the carburators and they both seemed a bit low. Topped them up and the fluttering also seems to have disappeared. However, I will be taking her around the block a few times before I drive anywhere decent. If I continue to get any symptoms I will let you know.
Thanks
Ken

Ken Martin

Yes, missing under load is usually weak spark hopefully capacitor. Last time I had this it was the Pirhana electronic ignition on the way out. Several years ago but I remmeber it well as it was on a parade lap at Silverstone and I had to let a Metro Turbo get away, and a good spark would have seen him off.
Stan Best

Everyting seems to have cleared up so keep your fingers crossed. I need to drive her all I can in our short summers.
Stan, I used to live in Aston Clinton, Bucks many moons ago. Great roads up in the Chilterns.
Thanks
Ken
Ken Martin

You wil remember The Bell I'm sure :-) The Aston in Aston Martin is there because Squadron Leader Martin competed at a hill climb there, amd Aston is higher up the alphabet than Martin.
Stan Best

I have had identical symptoms - missing on load but OK when reving. I found it to be the rotar arm tracking.
With electronic ignition and plugs gapped at 28 thou I get through a rotar arm a year so always carry a spare. Talking to Luminition last year they were thinking of developing an uprated item.
Bill

Hi again
Well I thought the problem had gone away, but it returned just as bad. So, I'm in the process of installing new points and coil to see if that works. Problem is I can't open the points anymore than about 010". The slider end is then stopped by the low tension connection to the baseplate.
To me that would signify either a worn cam or a worn cam rider. But the points are new and I cannot see any wear on the cam. I noticed the base plate can be rocked slightly from side to side on the shaft which does cause the points to open and close a bit more, but when everything is seated, only 010" max. Please don't tell me I need a new distributor.

Stan, definately remember the Bell, although I never had the opportunity to try it. I understand it's closed now.
Re Aston Martin, the following is and extract from the Aston Martin Web site
"
It was back before the Great War that Robert Bamford and Lionel Martin joined forces to sell Singer cars, and to prepare them for hill climbing and racing. Successfully too: it was Martin’s performances with these cars at the hillclimb course in Aston Clinton, Buckinghamshire that was to provide the inspiration for a name when the pair started making their own car".

I once thought of phoning up Astons and with my surname and the fact that I lived in Aston Clinton - "can I have a ride please".

Anyway, to business, anybody with ideas?
Ken

Ken Martin

This thread was discussed between 27/07/2008 and 30/07/2008

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