The Mechanic – Issue 152 – Spark plugs

Spring has sprung and vans are starting to come out of hibernation. After months in the garage with the occasional start up to keep it ticking over, your engine can suffer. I have personally experienced this after months of an engine sitting during restoration work; work complete, driving away from the workshop, van wouldn’t accelerate. Reason – fouled spark plugs.

I have also had a spark plug with a closed gap (don’t even ask how that happened, but involved losing part of the carburettor through the engine… lucky!)

The condition of your spark plugs can make a massive difference to the running of your engine, so it’s worth checking them every so often, especially after a period of time unused.

Hopefully the following information will help to make you a spark plug expert.

Before starting work on checking your plugs, it is helpful to have the right tools to hand as accessing the rear two spark plugs at cylinders 1 and 3 can be a real fiddle, especially on later twin-port engines where access is further compromised by the inlet manifolds. A short 21mm socket and universal joint may give you a bit more flexibility.

When checking the plugs, it can help to remove each lead and plug individually so that you don’t get them mixed up. This will cause an incorrect firing order and your engine will not run.

When removing the ignition lead from the plug, be sure to pull it off by the connector, not the lead itself, as you’ll run the risk of pulling the lead off the connector (trust me!)

If you notice any damage to a connector or if a lead is a lose fit, it is best to go out and buy a new HT lead set.

Make sure you have the socket on the plug properly when you’re undoing them and it’s also best to do all this while the engine is cold to avoid burning yourself!

Once the plug is out, take a good look. Is it brown, grey, sooty or oily? If the engine is running right, it should be light brown or grey. If it is sooty but dry, your engine is running rich and not burning all the fuel. If the insulator is white and flaky then your engine is running too lean. Either way, you’ll need to tune your carb to adjust the fuel/air mixture.

If the plug is wet and oily, there are a couple of possibilities. The first is that you’re not getting a spark, in which case you may have noticed a misfire. If this is the case, check the HT lead connection at the plug and also where it pushes into the top of the distributor cap.

A worse scenario is that your engine has worn piston rings and/or valve guides, which means a rebuild is on the cards. If there is serious carbon build up on the plug, or what looks like molten bits of metal, chances are your ignition timing is out.

Whatever their condition, while the plugs are out of the engine they will benefit from a good clean up using a brass wire brush.

While you are at it, check the spark plug gaps using a feeler gauge. For most air cooled engines the gap should be 0.024″ or 0.6mm, however check your workshop manual because the gap will be different on some engines. If the gap is correct, the gauge should slip in and out without much resistance. If it is too loose, you can adjust it with a gentle tap on the workshop floor to close it slightly, or if the gap is too tight, carefully prise open the contact with a flat bladed screwdriver.

Spark plugs should be checked every 3000 miles and replaced every 10,000 miles as part of your service routine. If you suspect a poor running engine there is no harm fitting new ones sooner, they are relatively cheap for a set.

When refitting, always start screwing the plug back in by hand, only using the socket for the final tightening, otherwise you risk forcing a cross thread. If you feel any resistance early on, unscrew and carefully try again.