Amazon: B20b Engine Run-On

[AdamG87]

Hi

I have a 1970 122s with standard B20b Engine.
I'm having trouble with engine run on. I've tried changing to cooler spark plugs (NGK BP7HS), adjusted the ignition timing, adjusted the idle and mixture settings although a low idle speed does seem to help a little but the engine won't idle properly when hot. I've also adjusted the valve clearances as per the workshop manual. Also tried different grade fuels. I don't really want to open the engine if I can help it.

Does anyone have any other suggestions?

Adam

[tdz840]

Originally Posted by AdamG87

Hi

I have a 1970 122s with standard B20b Engine.
I'm having trouble with engine run on. I've tried changing to cooler spark plugs (NGK BP7HS), adjusted the ignition timing, adjusted the idle and mixture settings although a low idle speed does seem to help a little but the engine won't idle properly when hot. I've also adjusted the valve clearances as per the workshop manual. Also tried different grade fuels. I don't really want to open the engine if I can help it.

Does anyone have any other suggestions?

Adam

In gear, clutch down, key off and at same time raise clutch. Done in one fluid movement it will stop off compression.
Russ

[arcturus]

Do you get any pinking? Sounds as though the ignition is still too far advanced.What is your static timing set at. Retard it a tad more and see what happens. 20B requires 98 octane.

[Clan]

Originally Posted by AdamG87

Hi

I have a 1970 122s with standard B20b Engine.
I'm having trouble with engine run on. I've tried changing to cooler spark plugs (NGK BP7HS), adjusted the ignition timing, adjusted the idle and mixture settings although a low idle speed does seem to help a little but the engine won't idle properly when hot. I've also adjusted the valve clearances as per the workshop manual. Also tried different grade fuels. I don't really want to open the engine if I can help it.

Does anyone have any other suggestions?

Adam

Try the crude method used on MGB 1800 engines ,, There is a solenoid valve which opens when you turn the ignition off letting an inrush of air to the manifold making the mixture too weak to self ignite .

[swedishandgerman]

When you turn the ignition off, there should be no low tension to the coil so there should be no spark. A such, this is very unlikely to be ignition related. UNLESS you've got the thing set up so badly that something is so hot it's glowing enough to ignite mixture inside the chambers (plugs, build up of carbon, valve tips etc) but I think that's unlikely.

Personally, I would be looking for manifold leaks but more likely, worn spindles on those SU's letting a little air in once everything should be closed down. Have you definitely got both carbs balanced in terms of air flow, mixture and lift?

[dcthompson]

Swedishandgerman may be correct about the carbs, so I would check those first. If not the carbs, you may have carbon deposits in the combustion chamber acting as 'hot spots' whiich ignite the fuel/air mixture. First, before removing the head, you might want to give your ride a 'Mexican tune-up.' This procedure, which can be remarkably effective if you use a sedate driving style, is to go to your nearest (empty) motorway, and simply run the car for five or ten minutes at the fastest speed you dare, but certainly more than 80 or 90 mph. If, after completing this procedure, your car still runs on, you may have to de-carbonize the combustion chambers.

I hope you're not buying cheap fuel; that might be the easy answer.

[Derek UK]

Running on due to pre ignition is pretty rare now days. Fuel burns much cleaner now than it when these card were being sold new. Modern fuel produces hardly any carbon and what des is in flakes which tend to go down the exhaust pipe. Back then you had to do a decoke regularly to get rid of all the black stuff. Now when you take the head of it is usually very clean. For the same reason it is now very hard to read plugs when checking mixtures.
The twin carb engines with their 10:1 compression ration were designed to use 100 octane but with a bit of minor timing tweaking are OK with 97 Premium or whatever you can get, Tesco 99? The engines do run sweeter with the best fuel Try a couple of tanks of Shell V-Performance or whatever it's called now or BP Premium if you can find either of them, most garages have only one pump for it, so you can miss it. Might add another £5 to your bill for a tank but for, say, 1500 miles a year, well worth it for the extra smoothness. I don't get run on with 95 though. Timing, mixture, air leaks are things to check.
Some people find the air solenoid works and some don't. Don't have a toggle switch for it as that WILL catch you when trying to start with it open!

[swedishandgerman]

Did you get anywhere with your running on? It's just that I've all but nearly got there with mine

I blame modern fuels and at worst was with no octane boost and timing on factory settings.

Timing can't cause the running on, because the coil is killed when you flick the switch. However, timing can affect spark plug temperature and chamber temperature and my theory is based on fuel getting ignited due to either unburnt deposits or just hot spots, probably on the plugs.

With the BP7HS plugs and octane boost, I am able to run with more advance. With standard plugs and normal unleaded (equivalent to 2-star) I could run it at 34 dogs max advance before there was any sign of pinking. On the cooler plugs plus octane boost, it's at 36 degrees. This has also resulted in achieving a much smoother and slower idle and in particular, much smooth just off closed throttle, like going around town in a 30 in 4th.

Mixture has been set by raising the jets, then incrementally unscrewing them until they just give smooth running. Any more fuel and it runs on. Airflow has been adjusted at idle using an airflow meter and both carbs are within 5% tolerance. Fast speed is also set with 5% tolerance also using an air speed meter. However, air speed balancing didn't affect running on, just smoothness.

Before, it would run on each I switched on and sometimes until I stalled it. Now it occasionally runs on, but easy to stop by giving it full throttle to get so much air in the manifold, it makes it too weak to ignite

One other thing, are you running a 77 degree thermostat or an 82? I recommend the cooler for twin SU cars and the hotter for single Strombergs having experimented with both.

[AdamG87]

I haven't really been able to overcome the run-on problem but it is somewhat better now that I have fitted the cooler spark plugs. I also took the car for a long run this last weekend and that seems to have helped. The thing is, it doesn't do it all the time.

The timing has been set at the point before it just starts to pink as running it on the factory settings causes it to pink a lot. I've also set the carbs using a colortune for the mixture and air flow meter to set the idle. The engine seems to be running smoothly enough. As far as I can tell, there doesn't appear to be any air leaks. The carbs were rebuilt about 2 years ago and were rebushed so I don't think the problem is there. I think it could just be a build up of carbon deposits or hot spots that is causing the running on. I'm using the 82 degree thermostat but the engine doesn't seem to be running hot as the temp needle stays half way on the gauge when driving and will creep up and stay on the last white line when idling but no sign of overheating.

[swedishandgerman]

What distributor are you running there Adam? Is it an 078? Is there a vacuum control?

Adam