Diagnostic help needed

[robinbartle]

Diagnostic help needed please.

The patient is a Pinto fitted in a Tiger that I bought last week.

That probably makes me an interloper on this site but if it makes it any better I do have the chassis of my Haynes Roadster hanging on my workshop wall, it's just that since our first son came along and we moved house and whatnot, progress has been...delayed. The Tiger was to get me going and also to give me some practical experience because I am - to be honest - pretty damned ignorant where engines are concerned.

Enough self justification. The car runs well most of the time but never for more than about twenty miles, after which it starts to backfire, responds erratically to the throttle, and then usually peters out.

Attempts to re-start usually produce more backfiring along with what I assume must be combustion going on in the carburettor - at least smoke and bangs emerge from the air filter, which must be suffering. On another occasion it failed to fire altogether - no spark visible when plug removed, although the plugs look OK.

Given an hour or so's rest it seems to come back to life most of the time, and it has only happened when the car's been running for a while - but presumably overheating wouldn't cause these symptoms, would it? The temperature gauge isn't getting all that high.

I'm thinking ignition problems. It's got a megajolt. The last owner briefly mentioned he'd had problems with it and I'm guessing mistimed sparks would be a likely cause of backfires etc..? My first thought is to try to follow all the wiring for the megajolt and see if anything is dodgy/loose (frankly what I have seen of the wiring so far is mostly a tad dodgy, to me eye), but given my ignorance I am very probably barking up the wrong tree, so what would others try first in this situation?