MGB Register Forum

I replaced my damaged 45D4 distributor on my 1979 BGT with an Accuspark one and just went to set the timing.. After getting the static timing pointed to No 1 and close enough to get th engine running I timed it at around 12 degrees using a Gunsons advanced timing light with the vacuum advance disconnected. I was very surprised the engine ran on when i turned the ignition off. Ive varied the timing slightly between 12 and 15 degrees but still got the same effect and had to stop the engine by putting it in gear (very glad I decided to sort out the clutch hydraulics as my previous task).

Now there is some very old petrol in the tank which i know wont be helping things but i'd not had any problems at all with the engine running on at all previously. Nothing has changed other than my changing the distributor and try to set the timing to book value.

Typically of course i didnt check the timing before i removed the old distributor so dont have a reference point for the old normal.

Which way would you recommend taking the timing to stop this running on please ?

If the engine is running on it suggests you have created hot spots capable of ignition of fuel which continues to be drawn into the chambers. Ignition which is too advanced or too retarded could do this in my opinion.

I would simply try more retardation and see if that works. I wouldn't advance beyond 15 degrees.

The other contributing factor could be excessive carbon build up, which tends to glow very nicely and is capable of causing the effect you have. Have you decided the engine recently?

Finally, I have noted that many folk here (myself included) have had problems when fitting non standard distributors. I have gone back to a rebuilt original Lucas 25 and never regretted it!!

Ian F

You should try retarding the ignition slightly, something below 12 degrees, perhaps 10 BTDC for example. Going up from 12 to 15 will make it worse. MGB engines are well known for running-on, especially on modern fuels.

Ian Fozzard wrote: ↑Sun May 02, 2021 7:29 am If the engine is running on it suggests you have created hot spots capable of ignition of fuel which continues to be drawn into the chambers. Ignition which is too advanced or too retarded could do this in my opinion.

I would simply try more retardation and see if that works. I wouldn't advance beyond 15 degrees.

The other contributing factor could be excessive carbon build up, which tends to glow very nicely and is capable of causing the effect you have. Have you decided the engine recently?

Finally, I have noted that many folk here (myself included) have had problems when fitting non standard distributors. I have gone back to a rebuilt original Lucas 25 and never regretted it!!

Ian F

That should be "decoked" the engine - of course!

Ian F

Given that your running on problem started when you changed distributors it seem likely that it is timing related. Is your vacuum advance working ? this should help stop running on. From what you say your engine runs on for a long time with the ignition off suggesting you may have a problem with coke. When you have sorted the timing issue you could try Redex treatment.
The problem with aftermarket distributors is that you don't know what advance curve they are using. You don't say whether your engine is an 18G or an 18V. The fact that you have a 45D distributor suggests a later 18V. In his book 'How to Power Tune your B series engine' Peter Burgess says fairly disparaging things about the 45D advance curve 41610 and favours the 25D curve 40897 'except for the highest state of tune' (you might be able to get this chapter on Google books) I always set my timing at 1000 rpm (20deg BTDC for 25D) vacuum disconnected.
I've just read your original post and in it you say that you have a 79B in which case you are setting way too advanced. With the 45D curve 41610, that's if in fact you have this in your aftermarket distributor, you should be aiming for more like 8 deg at 600 rpm or 9 deg at 1000 rpm.
Having looked at the Haines manual I somewhat confused now. Timing depends on your precise engine number and advance curve. Could be set at 1500 rpm.

I recognise that your running on has arisen following a change to your ignition system and therefore people are correct to focus on that.

My car has been prone to run on in he past. One of the things I have found is that idle speed can effect the chance of it running on. If it idles at 650rpm or 700 rpm it is very unlikely to run on. But if the idle speed is up near 1000rpm it is far more likely to run on. Changes to ignition timing or spark strength can impact on your idle speed.

Thanks for the help and ideas. It is an 18V engine that's original to the car. The car has covered only around 1000 miles since the previous owner had the engine rebuilt so hopefully shouldn't have had a lot of opportunity to become coked. It is though running on particularly old fuel and as the car isnt isnt being driven on the road it is often started on choke, run til warm and turned off again.
When setting the timing I did notice the engine idles faster and smoother with a higher advance. On other types of engine (mostly very old Fords) Ive generally found it a tolerable rule of thumb that when the idle speed increases its probably fairly close to being right , so long as there is no discernible pinking. The running on was unexpected though so i need to think differently and instead back it off to a bit below 10 degrees and advance it a degree at a time whilst stopping and restarting to find the point that running on starts to occur and then take it back a degree or 2.

The 45D distributor has a radically different advance curve to the 25D and is not recommended by Peter Burgess. The early 18V engines (72-74) had the 25D. One can only presume that the 45D curves are an attempt to reduce emissions and improve economy. The other advantage of the 25D is that it has a Vernier adjustment. I plotted out the curves from the data on Paul Hunts Bee and Vee website. You can shift the curves around with the static timing but you cant change the shape. (I cant seem to attach to this post). I did this because my friend decided to replace his 45D distributor with a 123 electronic type. We selected curve 2 which is the standard 25D curve 40897. His engine is moderately tuned. He was very pleased with the result.

The original 45D had a damaged screw that retains the distributor cap clip on one side as this was caught in a chain whilst taking the engine out to tidy up the engine bay a couple of years ago. My rationale was just to replace like for like to ensure the distributor cap doesnt work lose and whilst at it I replaced the points with an Accuspark module. In the meantime Im repairing the original 45D to keep as a back up.

Ive read up about the 25D as an an alternative but views dont seem entirely clear cut and its said the vacuum advance would need to be moved to support it. That wasnt something i was ready to do and I havent yet seriously looked at ways of optimising performance. If I do decide to make improvements and opt to keep with the B engine I would probably go for a Megajolt as I had a good experience of fitting and using one before.

For now I just need to stop the engine running on so I can carry on with the other work on the car. It was fine before the distributor swap so i just need to get the timing back to where it was, which i foolishly didnt check with the old distributor.

The rubber bumper B's where the vacuum take off is on the inlet manifold have a distributor peculiar to that set up only. If you've fitted any other distribution except the correct one then there lies your problems. The vacuum and centrifugal advance will be totally wrong and that is what is likely to be causing the problem. Full vacuum advance is when the engine is idling unlike the conventional set up where the vacuum advance is on a light throttle assuming you still have the original carburettor set up that is. I've checked the workshop manual and the correct distributor number is 41610 and this one is fitted only to rubber bumper cars.
So if the distributor isn't 41610 that's your problem.