Classic Car Won't Start? How to Tell if It's Fuel or Spark
Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent
If your classic car won’t start but the engine turns over, the cause is almost always one of two things: no spark or no fuel. Check the ignition first, because ignition faults cause most breakdowns on older cars. Then work back along the fuel system from the carburettor to the tank.
This guide is for cars that crank normally. If the engine won’t turn over at all, or only clicks, that is a different problem. Our general guide to why a car won’t start covers that.
Before you start: stay safe
Older cars are simple, but the ignition and fuel systems can still hurt you.
- The ignition coil turns battery voltage into a very high voltage to make the spark. NGK says modern coils produce up to 45,000 volts. Never hold a plug lead or coil lead in your hand while someone cranks the engine.
- Petrol and its vapour are highly flammable. Never let fuel spray onto a hot engine or exhaust.
- Work outside or in a well ventilated space, with no smoking and no naked flames.
- Do not crank the engine for long periods. Give the starter and battery a rest between tries.
If you are not comfortable doing any of this, stop. A wrong guess can turn a small fault into a fire.
Step 1: check for a spark
The usual first test is to remove one spark plug, refit it to its lead, rest the metal body of the plug against bare engine metal, and have a helper crank the engine. You are looking for a clear spark at the plug tip. Use insulated tools and keep your hands clear.
A cheap in-line spark tester that clips between the plug and the lead is safer and easier to read. If you have one, use it.
Spark at the plug? The ignition is probably fine. Go to Step 3.
No spark? Move on to Step 2.
Classic car needs some care?
Servicing, recommissioning and mechanical restoration for older cars across Kent.
WhatsApp for a free quote Call 07563 350335
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
Step 2: find where the spark stops
Work from the coil outwards.
Coil lead. Check for a spark at the main lead from the coil to the distributor cap, using the same careful method. Coils are often blamed, but they are rarely the real cause.
Rotor arm and distributor cap. If the coil lead sparks but the plugs don’t, look at the rotor arm and cap. Check for cracks, burn marks, carbon tracking and damp. Rotor arms fail more often than people expect, so carrying a spare is sensible.
Power to the coil. With the ignition on, a test light should show power at the coil’s low-voltage side. No power points to a wiring, switch or connection fault.
Points or electronic module. Connect a test light across the coil’s low-voltage side. As a helper cranks the engine, the light should flash on and off. This works for both contact breaker points and electronic ignition. If it stays on or stays off, the points or module are not switching. Points can be dirty, burnt or set to the wrong gap. The correct gap varies by car, so check your workshop manual.
If you have an electronic ignition kit, follow the maker’s own test instructions. Our guide to electronic ignition conversion on a classic car explains how those systems differ from points.
Step 3: check the fuel supply
If you have a good spark, the engine is probably not getting fuel, or it is getting far too much.
Throttle linkage. First, check the throttle linkage is connected and moves freely.
Electric pump. Check that the pump runs with the ignition on. If it doesn’t, check its fuse and wiring. SU pumps have their own common faults, covered in our SU fuel pump problems guide.
Mechanical pump. These rarely fail suddenly on the road, so if yours was fine yesterday, look elsewhere first.
Fuel at the carburettor. Fuel reaching the carburettor tells you the pump, lines and filter are working. A mechanic can check this safely by disconnecting the feed into a container while the engine cranks. If fuel arrives, the fault is in the carburettor. If not, check the filter, then the lines, then the pickup in the tank.
Inside the carburettor. A stuck or sunken float or a blocked jet can stop fuel getting through. Some carburettors also have a small filter screen at the inlet that can clog. Our carburettor tuning guide covers how these parts should work.
Is it flooded?
A flooded engine has too much petrol, not too little. The AA lists these signs:
- the engine cranks fast with a whirring sound but doesn’t fire
- a strong smell of petrol, especially around the exhaust
- it starts briefly then stalls, or doesn’t start at all
The AA says flooding often happens when a cold engine is switched off before it has warmed up, for example after moving the car a short distance. On carburettor cars, the choke richens the mixture when cold, so overusing it can make flooding worse.
The AA’s advice is to crank with the accelerator pressed to the floor for no more than 10 seconds. If it still won’t start, stop and get help. Soaked spark plugs can be damaged and may need replacing. Make sure the area is ventilated, as the exhaust may smoke briefly when it fires.
When to stop and call a mechanic
Stop trying if you smell strong petrol and can see a leak, if anything sparks or smokes where it shouldn’t, or if the battery is running down. Keep cranking a car with a fuel leak and you risk a fire.
How we can help
We are mobile, so we come to your home or workplace across Aylesham, Canterbury, Dover, Deal, Sandwich, Folkestone and the villages around them. We can come out, find the cause of a no-start, and tell you what it needs. Fault finding on points, coils, distributors, carburettors and fuel supply is part of our classic car servicing in Kent.
Labour is £30-£50 per hour depending on the job and its difficulty. Parts are extra. We agree the price with you on WhatsApp before any work starts.
Frequently asked questions
Is a classic car that won’t start more likely to be fuel or spark?
Ignition faults are the most common cause of breakdowns, so check for a spark first.
Can I just spray starting fluid into the carburettor?
We don’t advise it. Spraying flammable products near a hot engine or a backfiring carburettor risks a fire. Find the cause instead.
My car won’t start after standing all winter. Where do I begin?
Make sure the battery is charged and the engine cranks well, then check for a spark, then fuel.
Do I need to check the points gap myself?
Only if you are confident doing it. The correct gap varies by car, so check your workshop manual.
Need help? Send a WhatsApp to 07563 350335 with your car’s make and model and we’ll come to you.
Classic car needs some care?
Message us on WhatsApp with your reg and postcode for a price before any work starts.
WhatsApp for a free quote Call 07563 350335
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
