pre ignition

[stiky]

Hi every one i am a new member so please bare with me i have just refurbed the carbs on my 3.4 S Type auto and once up to temperature it runs ok ticks over at 650 revs in gear however i am getting what i believe is pre ignition which does not sound very nice the ignition is 123 electronic and has not been touched is this a mixture issue the plugs are light brown.
cheers jim

[Martec]

Hi Jim,

Can I suggest you say what exactly is happening, at what revs, on load or overrun and I assume it is only happening when cold?

When you reassembled the carburettor butterflies into the carb throat did you ensure that it would full close (no light passing the butterfly) and so completely central?

Have you reconnected the vacuum advance pipe?

Does your car have a crankcase vent connected to your inlet manifold and are all the seals OK?

Are you sure the carb flanges to the manifold are air tight?

Also assuming you removed the jets did you ensure they were centralised when refitted ie will the piston clack down onto the bridge?

Best of luck
Brian

PS what type of carbs have got fitted, HD's or HS.

[piman]

Hello Stiky,

is it pinking you are experiencing, i.e. al ight metallic rattle at low speed when you open the throttle. If so the ignition needs retarding a bit until you just lose it. What curve are you using and have you compared it to the original distributor?

Alec

[stiky]

Thanks every one for the help have now sorted the problem it was 2 fold 1 the new inlet manifold gaskets supplied by SU were too big hence letting air in and 2 the distributor clamp was not tight enough and i must have moved it whilst re fitting the carbs hence the timing was out a little.
cheers jim