1093cc doesn't like long journeys

I've got a 1093cc mk1 from jan 1984 (one of the very last mk1s!!!) it seems to run perfectly around town for short journeys but dies everytime I try to take it a decent distance (40+ miles). I've had a look at a few things and can't figure it out, does anyone have any suggestions???

It doesn't overheat, the oil temp seems to be spot on.
It isn't a blockage in the fuel filler cap causing a vacum (I have tried a different filler cap).
It had new leads, plugs and dizzy cap and they all seem to be right.
Its had a new fuel filter and the air filter seems fine aswell.
The carb (piersburg) has been striped cleaned and rebuilt etc etc.

I had been told it could be rust in the tank clogging up the system but don't think this is the case for a few reasons, firstly there is no rust anywhere on the car, including the visible parts of the fuel filler neck and fuel tank, so having excessive rust on the inside would seem odd. Also rust clogging up the system would leave the system clogged and stop it running permenantly, where as this engine is able to start again some time after it dies (normally 5-10 mins, but the last time it took a couple of days before it was willing to start).

This is the really confusing part, the fact that it seems to start fine after it has been left for a short while with nothing being adjusted on the engine.

Anyone have any ideas????

Cheers
Seb

ive got 1 1300cc driver an it does ecaxtly the same thing, it scared the hell out of me the first time i did it on the way to telford.
was drivin along happily an it suddenly started loosing power so i pulled over and opened the bonnet an noticed the crankcase breather was full of a thin oil that smelt of petrol, same as you i turned it off sat there for 5 mins an it was okay again for another 10 miles.

asked a mate about it an he said the reason it was breathing so heavy was because the piston rings arnt sealing in the cylinders an the engine oil was going above the piston an not burning.

sorry to be the burden of bad news (if this is whats the matter with yours)

sam

I'm not convinced that that is the likely problem - usually worn rings/bores result in blue smoke as the excess oil is burnt.

I'd suggest carb icing is more likely - check that all the ducting is in place and use the original air filter etc.

Another thing might be to check the tank vacuum - when it conks out release the fuel cap and see if it is OK straight away.

Cheers

Rich

i had the same problem on my series 1 recently replaced the carb for a weber and didnt have ducting to the base of carb and wud drive fine then all of a sudden it wud start dieing and loosing power till it wud finally die competly!!! but wud leave it for a min  then re start and wud be fine again sorted the ducting its fine now  

Carb icing is likely then