Spark Plugs and Gaps wrong?

[Stags Antlers]

engine rebuilt 5000miles ago and running well and amazingly economical ive had 30.5mpg on a 1700mile round trip to scotland ! (and thats with undersized 185 70 tyres!) however it has a misfire between 1700 to 3000 thats getting worse and also now starting to creep in after a period of idling. Getting worse as ive started to fiddle wth the distributor and carb damper dashpots. Lumenition is fittedand so far ive checked timing, vacuum advance, low tension circuit, cap, rotor etc and was starting to move onto the Carbs for a closer look - but then id thought id check the plugs - theyre Autolite Optima 64 (does that mean a 64 (0.6mm) thou plug when new?) they were no doubt new with the engine rebuild but all of them have gaps of around 0.95mm! according to Halfords manual they look to be a healthy colour with virtually no deposits . Question is this gap seems wrong and is this the misfire issue? and also that fact that it never starts on first churn of starter and the distributor rotor contacts dull off fairly quickly - are the large gaps straining the rest of the system but conversely are they contributing to the excellent economy! (25 to 30.5 mpg average)
Is it worth resetting the gaps or should i go for a correct plug
Finally could this contribute to valve wear? as ive had to re shim the tappets in some cases since the rebuild

[Stagdad]

Is that 1700 miles read from the speedo or off a satnav? The mpg may not be quite as good as that if it's off the speedo with smaller tyres.64thou seems a very large gap,25-30 thou is more the usuall for normal plugs maybe 35thou with a performance coil.

Cheers Steve

[nib]

engine rebuilt 5000miles ago and running well and amazingly economical ive had 30.5mpg on a 1700mile round trip to scotland ! (and thats with undersized 185 70 tyres!) however it has a misfire between 1700 to 3000 thats getting worse and also now starting to creep in after a period of idling. Getting worse as ive started to fiddle wth the distributor and carb damper dashpots. Lumenition is fittedand so far ive checked timing, vacuum advance, low tension circuit, cap, rotor etc and was starting to move onto the Carbs for a closer look - but then id thought id check the plugs - theyre Autolite Optima 64 (does that mean a 64 (0.6mm) thou plug when new?) they were no doubt new with the engine rebuild but all of them have gaps of around 0.95mm! according to Halfords manual they look to be a healthy colour with virtually no deposits . Question is this gap seems wrong and is this the misfire issue? and also that fact that it never starts on first churn of starter and the distributor rotor contacts dull off fairly quickly - are the large gaps straining the rest of the system but conversely are they contributing to the excellent economy! (25 to 30.5 mpg average)
Is it worth resetting the gaps or should i go for a correct plug
Finally could this contribute to valve wear? as ive had to re shim the tappets in some cases since the rebuild

I had to re-shim mine after 1000 miles after head rebuild,regarding the plugs stags can be very temperamental with certain brands,my car had a bad misfire on NGKs but I resolved the problem by using Champions I do believe its vice versa with some other owners.
Mark

[KansasCityMilkman]

I've just changed from NGK to Champion at 30 thou and it's running much better. LD Parts cheapest I've found for these plugs.

[silverfox]

Hi, Is the spark gap you mention in inches or millimetres ? Mine are NGK set at 30 thousands of an inch. Terry

[martin]

Hi Antlers (?) We're used to dealing in old money, the metric figures don't mean much ! Change your plugs for NGK, Bosche or similar, and set them at 30 THOUSANDTHS OF AN INCH ! Get a set of imperial feeler gauges. Martin.

[Andy Rundell]

go for a set of known plugs - NGK BP5E/BP5ES or Champion N12Y's
30 thou gap seems a good choice.

30thou is appox 0.762mm

You gap of 0.95mm is 37.4thou - which is bigger than usually recommended as the larger gap to bridge will require absolutely tip-tip ignition components.

bringing your gaps back to closer to 30 thou may well sort things - if not you could be suffering early stages of Lumenition failure. Change and check the cheaper items first (plugs, gaps, rotor arm, dizzy cap, plug leads, coil) - at least if there is no improvementyou'll know they are in good shape before moving onto the more expensive components - but it would not surprise me if the optical switch or power module were on their way out.

........Andy