Constant misfire ??????

[Charles Levien]

Cheers guys, just back from a break, so will investigate all the above suggestions over the next few days, and report my findings.
Charles L, what did you mean by "fractured connection in the distributor " do you mean a crack in the Dizzy Cap ???

Not the cap — the bit that runs round and connects to the condenser

[Tony Griffiths]

Cheers guys, just back from a break, so will investigate all the above suggestions over the next few days, and report my findings.
Charles L, what did you mean by "fractured connection in the distributor " do you mean a crack in the Dizzy Cap ???

Not the cap — the bit that runs round and connects to the condenser

If the strip is broken (and I've come across one or two in the past) just replace it with a short length of wire.

[Bluebird7]

In the peace and quiet of next week I'll go through all the suggestions, but as I was taught when I was a 'race engineer'
only change one element at any one time !!!!
No1 on the list is; check the valve gaps, but compression is good and it starts ok, its under load that it misbehaves .
then dizzy, coil, plugs,leads etc.

[Ian Williams]

"its under load that it misbehaves" sounds more like weak mixture than ignition.

[John Cornforth]

Hi

At risk of adding yet another voice.  You describe good starting and running OK for several miles before getting a misfire under load.  This suggests something changing when it gets thoroughly warm, and presumably coming good again when it cools.  The condenser is Number 1 suspect for this sort of malarky, but you have already changed that. I think the coil is the next suspect, as others have suggested. The standard coil primary is about 1.5 ohms, and I suspect there are many other 6 volt coils that would work, at least for elimination purposes. Plug firing voltage requirement goes up with cylinder pressure, so if the ignition system is weak the voltage can be OK at idle but insufficient under load.

Also, I have sometimes found that dirty points work initially but become worse when warm - I have no idea what the mechanism is, this is just from experience. When I say dirty, I mean there is a thin invisible film of tarnish/corrosion on the contact faces.  A strip of thin business card soaked in switch cleaner and drawn between the faces will help.

[Bob Kneale]

Hi

At risk of adding yet another voice.  You describe good starting and running OK for several miles before getting a misfire under load.  This suggests something changing when it gets thoroughly warm, and presumably coming good again when it cools............

John's sound advice reminded me of a similar malady on my wedding day! Very similar symptoms which turned out (2 weeks later when I investigated in more detail) to be a cracked tappet block which expanded when hot to seize in its guide, causing the problem! Replacement of the item restored performance instantly. Moral- don't over tighten your locknuts!
As you say, systematically check one element at a time and don't tackle this one till you have eliminated the simpler  ones, especially as this will involve removing the head!
Good luck with your investigations .
Bob

[PeterN]

See my today’s post re plugs. You don’t mention if you’ve replaced them

[Bluebird7]

UPDATE !!!!
Having changed, in order, the condenser, points, rotor arm, plug leads & king lead, coil, and plugs + timing, and 'test drove' after each change,
It still hops and bangs down the road like a demented kangaroo with flatulence !!!!!!
I've now completely run out of ideas, back over to you chaps before a gallon of 5 star is poured over the car !!!!
forgot to mention, running a 1 1/8th SU side draft & plugs Champion D16 looked a bit sooty,

[Tony Griffiths]

It might be worth looking at two forum discussions about distributors and their earth. Both reached the same conclusion and it might be the cause of your problem? https://www.austinsevenfriends.co.uk/for...p?tid=8427 and https://www.austinsevenfriends.co.uk/for...p?tid=9526

[Nick Salmon]

Sorry you are still having trouble. At the start of this thread you wrote 'now been fitted with a SU, and after the rebuild started 'on the button' & ran OK for 60/70 miles.'

I would point the finger of suspicion at the carb. I had a terrible misfire on my MGB which turned out to be caused by a seal in the carb breaking up and starving the engine of fuel. It would run ok until put under load  when it would snap, crackle and pop all over the place.

If you have a spare carb, try it.