403 Sparking plugs
 

The plugs on my 403 are prone to "coaking" up after a few months after service. Fuel mixture has been checked and ok. I always put the choke in as soon as possible and make sure the engine is warmed with oil hot before parking. Plugs suffer from carbon deposits that cause misfiring and loss of power.
The plugs are standard NKG.
I believe that it is possible to obtain 10mm plugs that run at higher temperatures that may resolve the problem. Current plugs are I think grade 5 or 6.
I also believe that choice of petrol my help. I do not nornally seek out particular brand but perhaps I should.

I have fitted a Winterburn capacitor discharge ignition system to several of my cars including my 401 and plug problems are a thing of the past, I am not keen on working on car electrics but have had no problems installing the systems myself. The control unit allows the option to run in standard or cd mode as well as switching the system off which can be used as an immobiliser.
Also no adjustments to timing are needed, no awkward modifications to your distributor which could leave you stranded by the side of the road if alternative systems fail though you may have to reduce idle speed in CD mode due to the more efficient fuel burn, on an S2 Bentley I got about an extra 2 mpg , significant on a car doing about 14/15 mpg.
Geoff.
http://www.capacitordischargeignition.com/

P.S. NGK recommend the CR7 HS plugs for the 401 and 403 which is what I have in my 401.

403 sparking plugs
 

If I may put in my ha'p'orth - in the early 2000's I used to run my 403 on the NGK C7HSA plugs which are recommended for ' touring ' engines, and suffered occasional plug fouling. I switched to the recommended hotter running NGK C6HSA for ' sports ' engines with great success. The critical thing is to make sure that the hotter plugs do not cause pre-ignition ( not always audible ). Quite how how I was supposed to detect non-audible pinking without taking the head off was something that I never found out!

For what it is worth, may I also share the remainder of my meagre knowledge. The NGK CR6HS and the C6HSA plugs run at the same temperature - the 'R' denotes a 5 k/ohm built in resistor to suppress ignition noise on radios, televisions etc. The 'A' in C6HSA denotes a difference in length of the cylindrical collar between the hexagon and the gasket, which is not important in Bristol 2-litre engines.

If you fancy trying N.O.S plugs ( if you can find them ), the original plugs for the 403 were KLG P.TEN L80 or KLG PEN L70 at first. Later, Champion Z 10 plugs were used.

To the best of my knowledge the above is accurate but please don't rely on it!
Dave Dale.

403 Sparking Plugs
 

I will seek out the plugs mentioned and look into the discharge ignition system.
Will give feed back upon further experimentation. Hopefully to report sucess!

Graham ,
The details of the NGK Plugs given by Dave Dale are the wrong way around .
The C6HSA is suitable for Touring engines and hotter than

C7HSA which is suitable for Sports engines and are a colder plug.

How have you checked that the mixture is correct??

With old original Solex carbs , often the float level is too high which can flood the idle circuit and foul the plugs.

Many Electronic ignition systems only take the place of the points and condensor .
A points distributor in excellent condition should not cause any problems .
Many ignition problems with Lucas distributors are in the bob weight system that advances the ignition when the engine speed increases .

If you have previously needed to fit C5HSA (Hot) plugs to your engine , I'd imagine there is some other problem which is causing the fouling.

Geoff

403 sparking plugs
 

Geoff, you are correct. I got it the wrong way around. Apologies to all ( absolutely nothing to do with me getting old, of course! ). The hotter C6HSA are recommended for the touring engines. There is an interesting article on the subject of engine damage, pre-ignition, sparking plugs etc. in the BOC Bulletin no. 129 ( Summer 2004 ) by Spencer Lane Jones.

403 Sparking Plugs
 

I am using the "hotter" plugs for touring.

Fuel mixture recently checked was correct although exhaust has been high in HC. Oil consumption is minimal.

The Solex carbs were re-built by Carburetter Exchange of Leighton Buzzard in 2014 so should still be in good condition. I did have a subsequent problem of flooding due to dirty fuel (grit under needle valve) but this was resolved by an in-line fuel filter. Having said that, I did have a temporary unexplained reoccurance which seems to have cleared but may well have contributed to the fouling.

I will follow up with a check on fuel mixture and ignition health. For the moment, whilst the plugs are still clean, we are running well.

Graham
Although the carbs have been rebuilt by a specialist , the only way to check the float level is at 16mm down from the top of the main carby body ,is by an external sight tube with the engine running .

Geoff

Thanks. Given the history of flooding and the fact that petrol consumption is high at 15 - 18mpg I will certainly look at the float levels. Previously mixture has only been checked by gas analyser on idle.
I recall an article in the Bulletin some time ago, I think by Alan Blackwood, where the art of checking the float chamber levels by external tube was described in detail. Unfortunately I cannot find my copy of the relevant Bulletin.
Geoff, can you or other members point me in the right direction or recall the bulletin number?

Graham,
15 - 18 mpg is not very impressive , most 6 cylinder Bristols should achieve 26 - 30 mpg on a steady trip at 70 mph and about 20 in city driving .

Attached is a simple float level sight tube similar to what I use .
It is made by drilling a hole in the centre of the hex of the main jet holder. Then solder a small piece of copper tube into it .

I suggest you check the diameters of the main & idle/pilot jet to see if an earlier owner has drilled them out . this was a common practice in the past.

I have found the best way to check the mixture is with the use of a mobile gas analyser, it can give the mixtures right through the rev range under various conditions .

Geoff

403 Sparking plugs and float level
 

Thanks again, both for advice and photograph.

Will take on board and get the float levels sorted, along with the fuel consumption and the plug fouling. May take me a little while but will report back.