403 Sparking plugs

[Geoff Dowdle]

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

[dave dale]

Originally Posted by Geoff Dowdle

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

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.

[GrahamH]

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.

[Geoff Dowdle]

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

[GrahamH]

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?

[Geoff Dowdle]

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

[GrahamH]

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.