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The anti-run-on 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 later the Metro when the compression got up to 10.5:1 std.
Most cases of run-on in B's are down to wear in the carbs causing the mixture to lean off when the engine speed cuts after the key is turned off.
There are other factors as well associated with the tuning: incorrect ignition timing, idle speed too high (should be 700RPM but worn carbs usually mean it can't idle that slow, producing a double whammee) or mixture too lean. 96 octane petrol doesn't help, the engine works much better with 98. Marginally cooler running N8Y spark plugs help offset the difference in petrol as compared with the N9Y without any adverse effects on other aspects of performance.
In 95+% of cases, if the carbs are in good order and the engine is tuned correctly with the correct idle speed, 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 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, the engine doesn't run-on when first started up and then turned off immediately, but reverts to run-on if run 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 occurence seems related to the head - swap the head and the problem goes. Put the head on another engine and that engine acquires run-on.
A further 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 restrict cooling around the plugs.)
In summary: B engines are susceptible to run-on as we all know, but if everything is right, most engines won't. A few engines defy all attempts but are nowhere near as bad if all the above are attended to.
If run-on does occure, it is always better for the engine to put it in gear and stall it with the clutch than allow it to continue. An anti run-on valve solves persistent cases and make all others more tolerant to incorrect set-up.
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