Constant misfire ??????

[John Mason]

If the misfire only starts after a few miles it suggests it happens once the engine gets hot. By all means check the coil as these can break down when hot. Also check your tappets because these will alter when the engine is hot and if not allowed to close fully will cause misfires.

John Mason.

[JohnD]

Never forget...if you think it is ignition it is probably fuel - and vice versa.

 I had a misfire that plagued the mag engined cup model for a few years, i was convinved it was the ignition, i had the mag rebuilt, and changed plugs for warmer, colder etc etc and it made no real difference, i was truly at my wits end with it.

Mr Dunford from Alba Austins offered to have a looked, and with a fresh set of eyes, within half an hour it was disagnosed that a very small twig had found its way into the carb needle and was restricting flow - removed it was back to normal. 

 I was convinced it was electrical as anytime you took the carb off, it was full of fuel, it would start and rev ok and was only when driven hard it would misfire, which was then when it was warm and so i was convinced somthing electrical had broken down!

I would speak with the builder as its more than likley be somthing small and trivial like my issue

[Colin Morgan]

My Chummy used to get hot and bothered and misfire after about five miles (when I mentioned this, one of the other club members said it reminded him of his wife). Turned out to be an air leak into the manifold - so it was running lean. It proved very hard to spot.

[AustinWood]

If it only happens with a hot engine I would suspect the tappet clearances may be too tight. as the valves heat up and expand the clearance reduces.

[John Mason]

I am with you Jim. Hence my comment to check the tappers.

John Mason.

[bob46320]

And of course, the points are clean, set right and the top distributer bearing isn't worn (which could upset the points gap)

[Charles Levien]

My Chummy used to get hot and bothered and misfire after about five miles (when I mentioned this, one of the other club members said it reminded him of his wife).  Turned out to be an air leak into the manifold - so it was running lean.  It proved very hard to spot.

I’ve had some experience with manifold problems ( no pun intended) climbing up the Gross Glockner pass in our Opal. A tell tale is loss of power, running hot and plugs burning white ( ish). Not much to do with misfire. 
I remember trying to find a misfire a few years ago — it turned out to be a fractured connection in the distributor — that also got worse as the engine heated up.

[AustinWood]

It's worth checking the plus. If they are white it's running weak & I would suspect an air leak. If one or more are black I would suspect tappet clearance or a sticking exhaust valave,

[Howard Wright]

Hi

Long shot, but it has happened to me recently. Check the battery terminals and voltage at the coil. I had a very slight short on a battery lead that led to the voltage dropping to below 3 Volts. Caused a misfire after a short while of driving.

Cheers

Howard

[Bluebird7]

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 ???