GLE220J - 1970 18/85.

Just wondered - run on valve any good?

The first thing to try is to reduce the idle speed, 100 rpm can make a difference. Even in the days of high octance leaded fuel a high idle could lead to running on with these engines especially after a long high speed run. Not so easy for Andrew with his hotter cam that probably needs a higher idle to run smoothly. To obtain a smooth lower idle requires there are no air leaks through the carb throttle spindle etc and that the crankcase emission system (including PCV where fitted) is working properly. 

Maybe worth investing <£10 in an EBAY boroscope camera to see whether there is any carbon build up which can cause running on especially is the mixture is weak or the timing is out.

Plenty of discussion on the MGB sites as this is a common problem with the twin carbs and hotter cam requiring a higher idle setting than the single carb crab. I wonder if stale modern fuel may also be a factor?

Engaging 4th gear and applying the brakes as you turn off the engine works of course but isn't very elegant a solution!

regards

David

Mark, my apologies, I should have recalled that you modified your engine with a fast road Kent cam, upgraded pistons etc. Having refreshed my memory, needed at my age , I see the cylinder head combustion chamber appears to have rounded off the sharp edges that can cause localised heating and running on as well as other issues.

However, I see you had thought about rebuilding the carb but not sure if you got around to this. Also I see before the engine was rebuilt you had pinking and timing issues and were considering an Accuspark ignition unit or similar. Note the latter won't address timing issues but may be more reliable than points (or not!).

With your engine's higher compression and hotter camshaft it would be even more critical to ensure the carb is in good condition and a revised needle used matched to the changed fuelling requirements.

The ignition timing will also need to be revised, typically (often quite a bit) more advance at the lower end to suit the cam and get a decent idle, and almost certainly retarded at the mid to upper rpm range with less maximum advance. (If you haven't limited the maximum advance you may run the risk of serious engine damage). As each engine is different it is impossible to know by how much, but depending on which distributor you are using it could be 6 degrees or more, so quite considerable. Maybe Andrew can give you an indication of the settings on his programmable unit as some sort of an indicator. 

The distributor advance can be achieved by modifying the current distributor stop, springs and weights and it may also be necessary to change the vacuuum advance mechanism to avoid over advancing under less than wot. The alternative is to use a programmable unit such as Andrew's 123. Really the only way of getting it right is on a rolling road where they are familiar with these engines, such as offered by Peter Burgess.


regards

David

Edit:

I should have added that by increasing the advance at idle not only can you achieve a smoother tickover at a lower RPM, but this allows the butterfly to close further and that helps prevent run on by limiting the fuel. However, the downside is that you do need to modify the advance curve to ensure correct timing above idle and it can make the car harder to start. (I believe 123 and Alson Amethyst programmable units do provide for less advance when cranking to compensate).   

That's very interesting, some running on! I hope the head gasket isn't damaged by the overheating from the weak mixture. In my opinion it shows that the scientific approach is the only real solution for a reliable and effective tune for a modified engine.

You are very lucky to have found a rolling road tuner with the level of knowledge to be able not only to select a needle but to modify it as well, it takes a lot of skill.

Did you find the cause for the valve timing changes you noted in an earlier post?

regards

David

He wants the twin carbs off my old engine and is talking to someone about fuel injection. Go figure... Might be interesting!

Chris.

Now where would those twin carbs have come from

If you go for fuel injection that would make an interesting thread here but what would then happen to the twin carbs, a certain Penguin may try and recover them for his Princess maybe?. Lots of challenges ahead.

regards


David

Oh no! Penguins and ignition not a happy mix! If you look at the original Motor article back in 1964 the Mk1 1800 specific fuel consumptiono was some 20 to 40% better than the MGB from 2000 to 4000 rpm.
David