This car runs great but when it gets hot and you are idling at a stop light it will cut off or appear to be running on 3 cylinders. Do you think this is an ignition problem or maybe a sticking SU carb. I have a great mechanic who can’t seem to figure it out.
There are a number off ways to figure it out.
Run the car to get it hot, then use a simple timing light with an inductive pickup to detect which cylinder isn’t firing.
A mechanic can also put it on a 'scope. That actually detects the pulses and presents them as traces across a screen, showing any anomolies. The mechanic can also checck the integrity of the wire to the distributor and even check the voltage pulses on the primary of he’s a mind to.
Carbon tracks in the distributor can be a good sign of an erratic spark on one cylinder of the problem is originating there.
How do the points look?
Is there distributor shaft wear, excessive lateral play?
If the igniition system is in good order, then yes, it may be a fuel delivery (carb) problem. You may need to find a good carb guy. Basically, the fuel is fed into a float bowl. The float operates an attached needle valve system tha ragulates the fuel flowing into the bowl. From there fuel is drawn through an orafice by low wall pressure in the venturi. But it’s a complicated and delicate system, that includes a high idle cam, vacuum lines, a choke control, and some other paraphinalia. You could have anything from a vacuum leak to a bad float. A good carb guy can find out.
It also might be a vacuum leak. Here’s some hints from a ‘how to tune an SU’ page:
"If you experience uneven idle, hunting, or an idle that changes (rises or falls) as the engine?s temperature climbs or drops, you probably have vacuum leaks. The most serious fault on most old SUs is wear in the throttle shaft area. To test for this, spray some carburetor cleaner on the outside of the throttle shaft; carburetor cleaner is non-combustible, and if the engine speed drops, it means some of this is getting into the air stream from outside the carburetor. You may also have leaks from the manifolds, from tubing such as the vacuum advance line to the distributor (if fitted), or from other places; the carb cleaner trick works well for locating those leaks as well."
Does your car have SUs or Zenith Stromberg carburetors?
That link was really good. If only it had been available 50 years ago.
Yeah, almost makes me want to go get an MGB and mess with the carbs!
That truely is a great link.
I love the old Brit ragtops. When they’re running. Almost bought a '75 (I think) Spitfire last year, restored. I lifted the hood and grinned widely. I forgot just how little there is under there.