From 1973 onwards, MGBs have an anti run on valve installed. This stops the engine immediately so the car will not run on, after the car is turned off. Some people call this dieseling. The Anti Run On Valve is a very good thing, there is no reason to remove it. It will not improve the performance of your car if you remove it. For a broader understanding of the emissions control systems refer to this knowledge base article.
Car Running On? #
There may be various causes, such as timing, spark plugs/gaps and type of fuel. If you have the anti run on valve and your car is running on there are some simple checks you can do. This is quite simple and quick and may solve the issue. Whenever I have this issue with my 1977 this is the first check I make.
Electrical Checks #
Remove the 3 hose connections from the valve (numbered 1-3 below). Also remove the two electrical connections from the top (numbered 4 below). Finally rotate the valve anti-clockwise approximately a quarter of a turn. You can now remove the valve from the mount.
Use a 12v power supply and apply and remove power to the valve. You can fix a connection to one terminal and then connect and disconnect the connection to the other terminal. This simulates the ignition on/off. You should hear the valve click open when power is applied (ignition on). It will click again when power is removed (ignition off) and it closes. If you hold the valve upside down you should also be able to see the valve open and close.
If there is no click or spark as you make the power connection it is possible the electrical elements of the valve have failed.
Blockage Check #
The open/close movement of the valve is about a quarter of an inch. You can check this with your finger pressing in the bottom of the valve – the open/close part is spring loaded. If the electrical check passes or if there’s very little movement of the valve open/close mechanism there may be a blockage. In my experience this is usually a small piece of charcoal from the charcoal canister. You may be able to remove it by working the valve mechanism with your finger. Another option is the blow hard through the valve ports (numbers 2 & 3 in the image above). This should get the blockage out. If you remove a blockage, repeat the electrical test. This will ensure the open/close mechanism is responding to the power state change correctly.
How To – Electrical Test and Blockage Check #
Having described the tests above I realized that an video would like help make sense of this. So here’s a video to cover both the electrical test and the simple blockage test.
Reinstall & Test #
If you have the valve working you can reinstall the hoses and electrical connections and see if that solves the run on issue. Hopefully it does. To be sure, check when the engine is completely up to temperature. A cold engine is less likely to run on.
Factory Explanation of the Anti Run-On Valve Function #
The factory issued a Technical Service Bulletin to explain this new feature when it was introduced for 1973 model cars (meaning production dates around August 1972). Specifically it was TSB 72-A-9 that was published to cover this. You can read the full TSB by using the links provided to get to the TSB section of the knowledge base or jump to the 1972 TSB listings.
Other Materials #
I’ve built this website and spent time on the knowledge base to bring together a lot of information I have collected from around the internet. The intent is making it easy to find rather than having to know about and have skills with search engine query terms to get what you were looking for. Please make use of the knowledge base.
