FUEL : SPARK : COMPRESSION

One of the things many of us love most about historic vehicles is their comparative simplicity. No ECUs, no computer diagnostics, no complicated electronics... Heaven!

It's an oft repeated adage that there are three things an internal combustion engine needs to run - fuel, spark and compression. Once you understand the basics of these, figuring out what your engine is up to when it misbehaves (and it will!) is - theoretically - much simpler.

Correctly speaking, there are actually four things your engine needs to make it happy - fuel, air, spark and compression, but who's quibbling? Here's our simple guide to what puts the bang in the buck.

Vinage car engine

FUEL

Let's begin with the obvious - does your old lady have any fuel in the tank? Daft question perhaps, but I've known it to happen! Fuel senders can lie... OK, so you're certain that there's fuel in the tank (presuming the gauge is working, of course), let's move on.

Fuel starvation can be caused by a number of issues: blocked fuel filter, faulty fuel pump, blocked fuel line, carburettor fault/blockage, a fuel leak, or even water contamination. Finding out if you have a fuelling problem is going to be a case of eliminating potential culprits one by one.

Remember when working on your fuel system that petrol and petrol fumes are highly combustible and not particularly nice for your skin, eyes or lungs - use care and appropriate protection please!

CLASSIC TIP

If your fuel tank is old, an in-line fuel filter can help prevent any contaminants getting through into your carburettor. In-line fuel filters are cheap to buy (starting at under half the price of a cup of high street coffee) and simple to install. They are far cheaper than rebuilding a carburettor.

Many fuel filler caps have an internal breather system. There are too many variants to describe here. Take a look at a manual to see what system your old dear has - check it is not blocked. Some fuel tanks also have a separate breather pipe. It is worth checking that it is not blocked. Blockages in either of these will create a vacuum in your fuel tank which make extracting fuel more challenging for your engine. A simple check is to remove the filler cap and try starting the engine again.
 

If you've previously noticed a loss of power under acceleration or load, spluttering at speed, or suddenly  surging, then the fuel pump is definitely a major contender for why it won't start.

Electrical fuel pumps on older car engines tend to be mounted on or in the fuel tank and are usually pretty reliable, but they can be subject to wear, so take a good look for signs of damage or corrosion to the connections and wiring.

 

Mechanical fuel pumps are usually mounted on the engine and driven by the camshaft. A common fault here is a perished and/or perforated diaphragm. They tend to be fairly simple to replace.

The carburettor (carb) can be anouther source of issues. The engine idle speed (controlled by the throttle adjustment screw on the carb) might be set too low. The carb might also have a sticking piston, a blockage or a worn needle.

Simple mechanical fuel pump test

Assuming you have fuel in the tank and no blockage between the tank and the pump, disconnect the pipe from the float bowl of the carburettor and put the open into a jam jar (or similar). Try to start the engine briefly with the ignition. If no fuel comes out into your jar, either the diaphragm is damaged or the internal filter (if fitted) is blocked.

Testing an electrical fuel pump

By their nature, electrical pumps require an electrical connection, so check that your pump has a clean earth connection and a working power feed. If the fuel pump circuit is fitted with a fuse, check it is OK. Most (but not all) electrical pumps rely on internal electrical contacts (points). You will hear these clicking when the ignition is switched on: no clicking could mean corroded/dirty points. These can be cleaned with a little emery paper.

If the pump is clicking, follow the same steps (above) as for testing a mechanicla pump. If no fuel comes out, check the internal filter (if fitted). If the filter is clean, the pump needs either servicing or replacing.

There are simply too many engine variants to cover all of the possible fuel faults in any depth here, so this is where you really need to refer to an owners or workshop manual for details your specific engine, and/or pick the collective brains of your local club.

AIR

Next simplest check - air. This is an uncommon non-starting fault in vehicles that are regularly used, but it is possible, especially if there are other issues making your engine less than happy. Is the engine getting plenty of nice, clean air? Is the filter clean and dry? Is there any debris  or water in the filter housing? Clean? Next...

Motorbike slogan on rear mudguard
Vintage retro fue pump

SPARK

Another obvious question: how's your battery? If your engine doesn't need manual cranking, does the battery have enough charge to turn the engine over? Check battery.

 

Sparking problems can be caused by HT leads, spark plugs, distributor, coil or alternator. If it's a motorbike engine, it could also be caused by an ignitor fault.

First: are all of the leads from the distributor to the spark plugs and alternator on tight and not showing  signs of damage or terminal corrosion?

If the cables are OK but she still won't start, let's move on to the coil. The coil takes the six or twelve volts from your battery (not enough to start an engine) and transforms it to the high voltage your spark plugs just love.

There are two ways to test the coil - on or off the engine. There's a great article on bench testing a coil here and here (multimeter required). No multimeter? That's not a problem - you can check if there is sufficient voltage coming from the coil by doing a spark test on the spark plugs.

Old spark plugs

One at a time (to ensure everything goes back in the correct order!) take out each spark plug and check for damage, and check too that the threaded end isn't wet with fuel, oil or carbon, and that the terminals at each end are clean. Don't let anything (including dirt) fall inside the hole the spark plug came out of. Clean gently with sandpaper if necessary. Using a feeler gauge, check that the spark plugs have the correct gap.

To test for spark, take out one of the spark plugs and then plug it back into the boot of its HT (high tension) lead. Making sure there's no spilt fuel hanging around, lay the tip of the spark plug against the metal body of the engine. Keep well clear of all moving parts, and do not hold the body of the plug. Get someone to turn on the ignition and then briefly start the engine. If you have sufficient voltage for ignition, you'll see a nice, bright blue spark arcing from the tip of the plug. You should be able to readily see the spark even in daylight.

 

If the plug isn't firing, then it isn't getting any current so now you'll need to get dizzy (sorry - irresitable pun - dizzy is a common nickname for the distributor).
 

Your distributor, simply put, takes high voltage from the coil and distributes it to the spark plugs. 

Beneath the distributor cap is the rotor arm that rotates past a series of contacts, each of which connects to a specific spark plug. All of that rotation and sparking results in gradual wear to the arm and contacts. The regular heating and cooling, and vibration from the engine, can also cause the plastic housing to degenerate.

So, let's start with a visual inspection of the outside - any fractures or serious cracking? Yes? It could be time for a new one. No? Open it up and look at the rotor arm and contacts.

Check for obstructions to current flow - are the metal surfaces of the rotor arm and contacts free of oil, dirt or corrosion? Lightly clean the contacts with sandpaper. Whilst you're in there, check for any build-up of dirt or oil preventing the easy movement of the rotor arm. Rotor arm restriction can cause stalling or backfiring when the engine is running.

Once the contacts are clean, it's a good idea to check that the contact gap is set correctly for your car. There's a good article here on how to set up and adjust your contacts, and another comprehensive article on distributor operation and set-up from the Cornwall Austin Seven Club here.

Troubleshooters - engine won't run

COMPRESSION

The internal combustion engine works by compressing a mixture of fuel and air to create a controlled explosion. Ergo, if your compression isn't good, the performance of the engine will be compromised.

A compression test will tell you about the condition of your piston rings, valves and valve seats. Compression values (listed in p.s.i.) vary considerably from engine to engine, so your first step is to find out what it should be. A 10% variance is usually considered acceptable.

If you want to do a compression test yourself rather than going to a local garage, you will need one bit of specialised equipment - a compression tester. A basic one can be purchased for as little as a tenner, but do bear in mind that you pays your money and takes your chances with anything really cheap...

Owners World provides a detailed guide to using a compression tester for wet and dry testing on classic engines here. Popular Mechanics also has a straightforward how-to guide here.

Spark plug hedgehog sculpture

It's My Classic - the free online classic car and motorcycle magazine for people with a passion for heritage vehicles.