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MG MGB GT V8 Factory Originals Technical - V8 Misfire

I did my own conversion four years ago using a standard SD1 V8 complete with its Lucas distributor Opus ignition/coil ballast resistor etc. I fitted RV8 style through the wing manifolds and it runs on standard SU carbs/manifolds as the factory GT V8.

The car has been tuned/set up by my local garage several times, mainly to get it past the emissions part of the mot test.

It starts and runs fine from cold but once it gets fully warmed up it misfires badly when you try to accelerate to the point where sometimes it feels like a V5! The car has new plug leads which are not tracking, cap and rotor arm. It will now misfire to the point where a real backfire occurs scaring passers by.

Gut feeling is of an ignition problem, I'm wondering whether to replace the Opus with a Luminition kit, any thoughts, or should I be looking elsewhere.

Glyn
Glyn Parker

double check that the ignition wires are in the proper order on the cap 18436572. It could be wrong wire on a sparkplug.
Richard Morris

What carburetion and inlet manifold?

What petrol pump and pressure at carburettors?

Could be fuel vaporisation?
Nigel Steward

Glyn,

had a comparable problem with the Opus 35DE distributer and changed to the Newtronics setup some years ago. Working fine since then.

Another problem might be the setting of the floats and the capacity of the fuelpump, as, with the SD1 a normal SU pump in good condition is at it's limit, so check fuel pressure too.

Ralph
Ralph

Mines an EFI but a while ago I was playing around with putting a variable resistor on the temperature sensor. The idea was to make the mix richer. I found that after the car warmed up and I had the resistor turned way up (mixture too rich) it would do exactly as you've described.
You might be too rich, try leaning it out a bit.
Peter

Sounds identical to the problems a bad coil gave me

Bruce
Bruce Mills

Hi Glynn,
My experience from a similar problem on my Rover SD1 V8S was the well known Opus failing of the flex wires between the pickup and the amplifier in the distributor. This usually causes a misfire or total cut of spark once the distributor warms up.I replaced all the elecronic internals with an adapted Pertonix Ignitor kit (LU181)and have run like this for about 5 yrs.My RB V8 conversion uses a Mallory Dual Point distributor, which has better calibration potential on a modified engine.
Regards,
Tony.
TONY FREW

This thread was discussed between 26/06/2006 and 07/07/2006

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