Run On

 In Engine - Tuning, MGA-MGB, Midget

Run-on is a term used to describe what happens when the key is turned off and the engine continues to run somewhat erratically for a second or two, or even quite some time.  Run-on causes high loadings on piston rings and has the potential to damage the engine, so should be addressed as soon as possible, even if it is simply by stalling the engine with the clutch until a more permanent fix is sorted.

Run-on is caused by the fuel spontaneously igniting in the absence of a spark from the plugs.   In turn, there are a number of factors which can cause this spontaneous combustion.  These causes are covered in a general way on my page combustion problems and detonation while the specific means of addressing it are outlined below.

In summary:  B series engines – particularly the higher compression ratio version – are susceptible to run-on as we all know, but if everything is right most engines won’t do this . A few engines defy all attempts but are nowhere near as bad if all the items set out below are attended to.

No 1 cause:

Most cases of run-on in B’s are down to wear in the carburettor throttle spindles, which allow air to leak in in varying quantities at idle and small throttle openings.  This causes the mixture to lean off when the engine speed cuts after the key is turned off … and lean mixture is a key pre-condition for spontaneous combustion.

Worn spindles can be easily detected by wiggling the throttle shaft to see if there is any movement between the shaft and carb body – there should be none.   There is limited value in trying to address the run-on by other means while wear is present in the spindles.  Renewing the spindles and (if required) their bushes is relatively simple and inespensive, will fix the problem completely in 90% of cases, and will also considerably improve the smoothness and consistency of the car’s idle.

Other factors associated with the tuning:

  • Idle speed too high (should be 700RPM but worn carbs usually mean it can’t idle that slow, producing a double whammee).  This is the next most important factor after the condition of the throttle spindles.
  • Mixture too lean.
  • Incorrect ignition timing
  • Marginally cooler running N8Y spark plugs help offset the difference in petrol as compared with the specified N9Y without any adverse effects on other aspects of performance.

Cooling system

A relatively unusual thing to watch for is that sometimes there are cases of mismatch between the cooling holes on the head.  Some of the very late B series heads had a different water jacket hole pattern on the spark plug side. Fitting this to an earlier block will considerably restrict cooling around the plugs and results in run-on, pinking and even burnt plugs.


Octane rating is important for combustion stability so has a big effect on run-on.  Any B series engines over 8:1 compression should not be running 91 octane fuel.  Even 95 octane petrol is marginal, the engine works much better with 98.

Problem cases

In 95+% of cases, if the factors above are addressed the engine won’t run-on. Some of the rest are simply coped with by depressing the clutch when turning off the engine (the extra drag from the release bearing is enough to make the difference).

However, a small number defy every attempt to eliminate run-on and I’ve formed the view that this is due to the nature/placement of the cores which formed the cooling jacket in the head affecting the efficiency of cooling around the spark plugs and thus causing them to run a bit hot. I’ve come to this conclusion based on the following observations:

  • Some heads won’t run-on at all even under adverse conditions such as a modified head with very high compression and std cam timing.
  • Working very quickly, a fresh set of plugs is fitted to a warm engine, if the engine is started and turned off immediately it doesn’t run-on, but reverts to run-on if it runs for any longer than that.
  • Refitting the old plugs after they have cooled has the same result, so it appears down to the temperature of the plugs.
  • The occurrence seems related to the head – swap the head and the problem goes. Put the head on another engine and that engine acquires run-on.

Anti-run-on valves

An anti run-on valve solves persistent cases and make all others more tolerant to incorrect set-up.   This valve was never std for a B as far as I know (I may be wrong but I’ve not seen it in any parts books) but it was used in other vehicles like the Marina, and then later the Metro when the compression got up to 10.5:1 std.

The valves are available, but quite expensive – certainly more expensive than any of the above.

If all else fails

As noted above, if run-on does occur, it is always better for the engine to put it in gear and stall it with the clutch than allow it to continue.

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