Page created by T June 10th 2009:
The Need For Cap Priming:
When you try to start a Carburetor Engine fitted with a Mechanical Fuel Pump that's been standing for longer than 1 month the Carburettor will be empty because all the Fuel will have evaporated away.A Carburettor Engine that has run out of Fuel will have the same problem. The Float Bowl will have emptied and need the Fuel Pump to refill it before the Engine will run properly. Simply cranking the Engine to make it start can drain most of the Battery's power before a Mechanical Fuel Pump has time to fill the Carburettor enough to start. You need to Prime the Carburettor by tipping a small amount of Fuel down the Throat. Cap Priming spins the Engine over long enough to make the Mechanical Fuel Pump fill the Carburettor Float Bowl. The Float Bowl must be full before the Engine will run without needing further Cap Priming.If the Battery is fully charged go ahead and Cap Prime.If the Battery is flat and needs a Jump Start from another Car follow the Jump Starting link at the bottom of the page.You will need to Cap Prime as well as Jump Start.
Electrical Fuel Pumps:
If the Car has an Electric Fuel Pump there will be no need to Cap Prime because the Electric Fuel Pump can fill the Carburettor Float Bowl rapidly.
Fuel Injected Engines:
Because most Fuel Injected Engines use Electric Fuel Pumps there is no need to Cap Prime them. The Fuel Pump will fill the Fuel Rail rapidly during the first Start as long as the Battery has enough charge.
In the '50's, British Cars had a small Lever on the side of their Mechanical Fuel Pumps that allowed you to easily fill the Float Bowl. You simply wobbled the Lever until the Float Bowl was full and Fuel Pressure was up. You could tell this because the Lever no longer pumped. Some cars, like the Rover 3.5 had both Mechanical and Electric Pumps.
Note that there is potential for Fire so follow the steps closely to Cap Prime and if you're Jump Starting too, follow the Jump Starting Link in the Links section below.Only allow a small amount of Fuel near the Car. Keep the Fuel Can a safe distance away. Screw the Cap back on the Can before you attempt each Start. Avoid the temptation to look down the Carburettor Throat. Delayed Ignition or Combustion can throw Flame into your Face and cause loss of Eyesight.Note that it's important to screw the Air Cleaner back on before Cranking the Engine. The Air Cleaner prevents any backfire flames that may occur from spreading thereby minimising the risk of fire .
Cap Priming Method:
1. If possible push the Car out into the open. If not possible a Fire Extinguisher is a good investment.2. Make sure the Handbrake is on and the Car is in Neutral if a Manual or Park if an Automatic.3. Cap Prime the Engine by removing the Air Cleaner and pouring a Capful of Petrol down the Carburettor Throat. 4. Refit the Air Cleaner screwing it back on. 5. Crank the Engine. It will fire and run briefly then stop again.6. Inspect the outside of the Carburettor each time the Engine stops to see if Fuel has begun dripping out. If it has the Needle and Seat and Float will need to be inspected. Don't Crank the Engine again until the Needle and Seat and Float have been fixed. 7. Remove the Air Cleaner again and put another Capful of Fuel down the Carburettor, then refit the Air Cleaner making sure to screw it back on again. 8. Start the Engine again. It may only run for a few seconds again. If so 9. Remove the Air Cleaner again and put another Capful of Fuel down the Carburettor, then refit the Air Cleaner making sure to screw it back on again.10.Continue the process until the Engine keeps running without stopping. It should only take about 3 primes altogether. Once the Carburettor is full the Engine will continue to run without needing any more Cap Priming.Delayed Ignition or Combustion is very likely in an Engine that hasn't run for a long time since the Valves may not seal properlyand the Ignition may not operate properly until after the Engine has run for a while.
Practical Cap Priming:
If you watch this Link you'll see the first step taken is to Cap Prime the Engine .... 202 First Start.They place the Fuel Can away from the Engine which is very advisable since Exhaust Flame soon becomes evident. The Engine's problem is likely Plug Leads in the wrong location because you see fire come out the Carburettor.At 10 seconds they retard the Spark Timing by rotating the Distributor clockwise, but rotate it too far then try to compensate by rotating anticlockwise, but too far. On the Next Start the Engine kicks back under Cranking because it is too far advanced.They address the Timing issue but by then there is no Fuel left in the Inlet Manifold and another Cap Prime should be done. You'll note they are running out of Battery which is why Cap Priming is so important. Finally heavy Backfire from the Exhaust occurs when they adjust the Distributor with the Engine running. This is because one or more Plugs is sparking while its Cylinders Exhaust Valve is open.