544 B20B help
 

I am trying to fix a couple of problems. My 544 has a B20B engine. It currently diesels(run on) and has hesitation on acceleration. Timing is at 11 and advances to 23 at 3000 rpm. I know it should advance more. It has a vacuum pot that is blocked off and the carb vacuum is blocked off. It has a Weber 2 barrel carb. Will it help to connect the vacuum? I have lowered timing but seems to not solve the run on. I have had the heads off and there is not a carbon build up.

https://i.postimg.cc/y3XZVQf4/2021-07-20-13-02-38.jpg

https://i.postimg.cc/H8CM1D99/2021-07-20-13-02-49.jpg

I'd start by checking the timing but there are some more bits of advice here

https://en.wikipedia.org/wiki/Diesel...tential_causes

Can you clarify the vacuum question? For example, have you got a connection to the distributor for advance / retard? (Doesn't look like it - but there are some electronic devices that might be fitted that do it differently - so tell us more about the ignition system too please)

Right now the vacuum on the pot is sealed off. It has always been that way. The carb port is sealed off as well. I could connect the vacuum from the carb to the pot. However I do not know if this will matter.

I can't answer that for you at the moment.

Have a look in the distributor and have a look in the engine bay for a box of tricks that is after market for running the ignition system.

####

Explanation:-

The original mechanical system would have used vacuum to adjust the timing.

As the engine speed (rpms) increase the spark plugs need to spark sooner because the amount of time taken to burn fuel is essentially constant but the speed at which the piston is compressing air is not.

From your first post it sounds like that something is changing so it is possible that an after market something or other has been fitted to do the job that was originally done by connecting a vacuum pipe between the carb and the dashpot on the side of the distributor.

I think you need to find out what has been fitted. If nothing has been fitted and the vaccum pipe isn't there then your first post will need some different explaining!

B20B ignition timing is 10 degrees advance at idle speed vacuum disconnected ..
you have disabled part of the system which isn.t going to help . The vacuum advance increases the advance when cruising which helps economy and also makes for a quieter smoother engine . You need to start with the system working as factory ...

have you had the head skiimmed over the years ? is the non standard carburettor calibrated properly? has it a heat insulating block between carb and manifold ?

lots to consider !

If ignition isn't causing the run-on, then engine running too hot or mixture too weak are the other common reasons. I take it by 'pot' you mean distributor?

As above and what compression ratio do you have and fuel octane? But as above get the correct distributor or a 123 distributor setup and working properly first, check tappet clearances and then the carb setup. Think I remember a B18B distributor with the vac advance rather than retard is a good option.

Plus 1 to all the above especially a change to the fully mechanical distributor. The one you have is working mechanically but will have different springs and maybe weights to giving you a different ignition curve. These engines were designed to run on 100 octane (Ron) and that is hard to find in the UK. 99 available in many places. 97 the best compromise but a couple degrees of retardation over standard may be needed. 93 is bad for these engines. They will run of course with a bit more retard but not well. 97 is worth the extra for performance and smooth running. Maybe even a bit extra MPG.
97 octane “super unleaded” in Britain is roughly equivalent to 91 octane premium in the United States. If you can find higher than that, go for it.
Of course a nice pair of SU's should give you benefits all round if they are fresh and not worn out.
Anti run on valves are available. These work a solenoid when you turn the ignition off, which opens a valve that floods the inlet with air which kills the engine. Quite expensive!

You have identified a number of problems which may be unrelated.

First off, the ignition system problems will not directly cause dieseling or run-on. Dieseling / run-on is caused by a hot spot in the combustion chamber that is continuing to ignite the fuel mixture (in an untimed manner) after the ignition system is turned off. The hot spot is typically a carbon deposit somewhere in the combustion chamber that is acting as the source. The problem may be exacerbated by a fuel mix or ignition timing (too much advance) that is elevating combustion temperatures. The problem may be precipitated by a fuel mixture that is rich leading to carbon deposits. The carbon deposits may also be caused by excessive oil burning.

Fuel quality / octane rating can play a secondary role. If you are incurring persistent low level detonation that can raise combustion temperatures creating hot spot ignition sources.

The sure fix for decarbonizing is removal of the head and mechanical cleaning of all of the surfaces. A slower solution that may work is to gradually mist water or methanol into the carb intake while the engine is running. The steam reacts with the hot carbon to create a combustible gas gradually resulting in removal of the deposits (turbo charged cars that run water / meth injection for detonation control usually have remarkably clean combustion chambers). You can try the additive in the tank solution; but, I have no faith in those snake oils.

Some later carburetors have an internal run-on control device which shuts off fuel to try and eliminate run-on. Perhaps whatever carb you are using has such a device so you may want to check that out. Pretty hard to retrofit an anti run-on device - shutting off external fuel supply is useless because of the large reservoir in the float bowl. My 142 has D jet fuel injection and the injectors turn off when the ignition turns off - fuel injected cars never suffer from run-on.

As to the ignition timing. Since a B20B in a 544 is a hybrid you need to check the part number for the distributor. Volvo used a wide mix of ignition control strategies on the B 18 and B20 engines. The distributors on B18 engines used static advances around 20 deg. The part number on your distributor will confirm that you are using the correct distributor with the correct advance curve for your B20B's vintage and market (advance curves changed during production and depending on the market - emission requirements). If the distributor is correct then lack of advance may be caused by a sticking / jammed centrifugal advance mechanism.

This is the distributor I have. I know it is not correct but I cannot find the correct one. This one has worked for many years. Also pic of cylinder inside.
https://i.postimg.cc/pXqDBxjj/2016-06-21_14.24.22.jpg
https://i.postimg.cc/SND04XBv/2016-06-21_14.24.06.jpg
https://i.postimg.cc/8Pty8XkW/2016-04-24_14.57.41.jpg