Trouble Shooting Aug 1, 2015 11:23:36 GMT -5
Post by Scoutpilot on Aug 1, 2015 11:23:36 GMT -5
So. It won’t start…
The Sun is up. It’s beautiful weather outside. Let’s go for a ride in the Jeep. Everyone piles in the old CJ. If you’ve been performing regular preventive maintenance, this should be a great day. But, you haven’t. Been too busy. You turn the key, press the starter and nothing happens. One more time. Still nothing. O.K. Now what?
Since this site is about Carter WO, YF and YS carburetors and since, as a friend of mine once said, “90% of carburetor problems are electrical in nature.” I’m going to defer the electrical trouble shooting to other experts and their knowledge.
No start at all?
A motor needs fuel, air, and spark to run. The carburetor supplies the fuel and air. The carburetor uses intake vacuum to supply fuel to the engine. As air is pulled down through the throat of the carburetor by intake vacuum, created by the pistons moving in the cylinders acting as pumps, fuel is pulled from the carburetor's fuel bowl and mixed with the incoming air to form the combustible mixture. At idle, the fuel enters the carburetor throat through the idle port just above the throttle plate. At higher engine speeds, fuel is pulled through the main metering jet into the venturi (the narrowest part of the carburetor throat). The air/fuel mixture then flows down through the intake manifold and into the cylinders where it is burned to produce power.
Though the basic operation of a carburetor is fairly simple, it also relies on a number of add-on devices for cold starting and warm-up idle control.
When a carburetor is clean and working properly, the engine should start easily (hot or cold), idle smoothly, and accelerate without stumbling.
Problems that are often blamed on a "bad" or "dirty" carburetor include hard starting, hesitation, stalling, rough idle, flooding, idling too fast and poor fuel economy. Sometimes it is the carburetor, but do not rule out possible electrical/ignition issues (weak starter or battery, worn or dirty spark plugs, bad plug wires, cap, rotor, etc.). In fact you should check them first. Carburetors can be tricky to rebuild, and expensive to replace, so you want to be sure of your diagnosis before you touch this critical part.
Cold Starting Problems
Hard starting can be caused by a choke that fails to close and causes a rich fuel mixture when the engine is cold. But there's no need to rebuild or replace the carburetor if all that's needed is a simple adjustment or cleaning of the choke mechanism and linkage.
If the shaft, linkage, or the cable that opens and closes the choke is dirty, one or more of these may cause the choke to stick.
Other causes of hard starting include, separately or perhaps in combination, other than the previously mentioned ignition problems, vacuum leaks or low compression (worn rings, valve adjustment, etc.)
Hot Starting Problems
As for hot starting problems, the carburetor is seldom to blame. A hot start condition is usually the result of too much heat in the vicinity of the coil, carburetor, fuel lines or fuel pump. It also could be a poorly functioning Heat Riser Valve.
Heat is the enemy of the coil. The oil inside overheats and breakdown of the circuit begins. The failure is gradual and seldom noticed until failure. The owner’s failure to deal with it at that point will lead to reaching for the cell phone when stranded on the side of the road.
Heat causes the fuel in the fuel lines, carburetor bowl or pump to boil. This creates a "vapor lock" condition which can make a hot engine hard to start. The fuel can become over pressurized and overwhelm the float and thereby affect the needle and seat. Replacing or rebuilding the carburetor wouldn't solve anything because the real culprit is heat. What needs to be done here is to reroute the fuel line away from sources of heat (like the exhaust manifold and pipe), and/or to insulate the fuel line by fabricating a heat shield or wrapping the fuel line with insulation. You may need to install a heat shield between the carb base and manifold throwing a barrier between the carb and the manifold, head and block.(See also, FLOODING, below.)
A Heat Riser Valve that is stuck in the fully-open position will cause hard warm-restarts because the excessive heat from the exhaust basically cooks the fuel in the bowl to the boiling point (depending on the blend, as low as 100ºF). This leads to fuel pressures in the bowl that can force fuel past the metering rod/jet and out the main nozzle. This will flood the intake and cylinders with gas. This is also bad from the standpoint of lubrication of the cylinders as the film of oil on the cylinder walls will wash away and allow excessive wear to the rings and walls at restart.
Hot start problems can also be caused by excessive resistance in a starter or poor battery cable connections. Or, for ya’ll with Pertronix, et al., a faulty ignition module that acts up when it overheats or gets less or more than the recommended voltage.
Hesitation or Stumble When Accelerating
Hesitation is a classic symptom of a lean fuel mixture (too much air, not enough fuel) and can be caused by a dirty or poorly adjusted carburetor (metering rod set too low, idle RPM too low), a weak accelerator pump or a worn throttle shaft/bore.
The accelerator pump squirts fuel into the throat of the carburetor when the throttle opens. This helps offset the extra gulp of air that is sucked in until fuel flow through the metering circuits can catch up to the change in air velocity through the venturi (the narrow part of the carburetor throat). The accelerator pump use either a leather skirt (WO series) or a Viton™ diaphragm (YF and YS) on a piston to pump fuel through its discharge nozzle. If the diaphragm is torn or the piston skirt is worn, the accelerator pump may not deliver the correct volume of fuel. Or, if the discharge nozzle is plugged with dirt or fuel varnish deposits, it can restrict fuel flow.
The operation of the accelerator pump can be checked, in all but the YS, by removing the air horn, looking down into the carburetor, and pumping the throttle. You should see a jet of fuel squirt into the venturi(barrel) of the carburetor. If no fuel squirts out, or the stream is very weak, the accelerator pump circuit has a problem.
If the carburetor jets are coated with fuel varnish or mineral deposits, or there is dirt inside the fuel bowl, this can restrict the flow of fuel causing a lean condition. Cleaning the carburetor with carburetor cleaner can get rid of the dirt and varnish deposits to restore normal operation.
Air leaks elsewhere on the engine can also lean out the fuel mixture. Air can enter the intake manifold through loose or cracked vacuum hoses or the PCV system. Vacuum leaks in the carburetor base gasket or insulator, intake manifold gaskets, or other vacuum accessories can admit unwanted air. Air can even get into the manifold past badly worn valve guides and seals.
A defective PCV Valve that fails to close at idle or when the engine is cold can be another cause of hesitation.
Other causes may include a defective distributor advance mechanism, a weak ignition coil, carbon tracks in the distributor cap, bad plug wires, worn or dirty spark plugs that misfire when the engine is under load, or even an exhaust restriction. Even bad gas can cause hesitation problems. So before the carburetor is rebuilt or replaced, these other possibilities need to be investigated and eliminated.
Hesitation Under Load
A hesitation, stumble or misfire that occurs when the engine is under load can be caused by a weak ignition coil, or cracks in the coil or distributor cap, or bad spark plug wires or loose ignition electrical connectors.
An engine can stall if the idle speed is too low, the fuel mixture is too lean, won't burn, stops flowing or the ignition system runs out of spark. Rebuilding or replacing the carburetor won't eliminate this problem if stalling is ignition related or due to a weak fuel pump, plugged fuel filter or fuel line, bad gas or a faulty vented gas cap.
A simple adjustment may be all that's needed to increase the idle speed or richen the idle mixture. But if the engine is sucking air through a vacuum leak somewhere, no amount of adjustment may totally eliminate the tendency to stall. The vacuum leak must be found and fixed before accurate idle speed and mixture adjustments will be possible.
The carburetor may have to be rebuilt or replaced if there are internal air leaks in the carburetor itself, a sticky needle valve is starving the carburetor for fuel, or the jets, air bleeds or metering passageways in the carburetor are dirty or plugged. Reaming and installing bushings or replacing the Throttle Shaft would be required if the throttle is badly worn, or the carburetor housing is warped or damaged.
A rough idle condition is usually caused by an overly lean fuel mixture that results in lean misfire. A common cause of idle problems is air leaks between the carburetor and intake manifold (tighten the carburetor base bolts or replace the gasket under the carburetor), air leaks in vacuum lines or the PCV system. Other carburetor-related causes include an idle mixture adjustment set too lean (back out the idle mixture adjustment screw one quarter of a turn at a time until the idle quality improves), or a dirty idle mixture circuit (which may require cleaning and rebuilding the carburetor).
Other possible causes of a rough idle include excessive compression blowby (worn rings or cylinders), weak or broken valve springs, or ignition misfiring due to worn or dirty spark plugs, bad plug wires or a weak ignition coil.
This is a problem that is usually (but not always) the carburetor's fault. The carburetor may flood if dirt enters the needle valve and prevents it from closing. Or it will flood if there is a gouge or deep scratches in the height adjustment tang of the float which grabs the needle and forces it in at other than a zero angle. With no way to shut off the flow of fuel, the bowl overflows and spills fuel into the carburetor throat or out the bowl vents. A flooded engine may not start because the plugs are wet with fuel.
WARNING: Flooding can be a very dangerous situation because it creates a serious fire hazard if fuel spills out of the carburetor onto a hot exhaust manifold/pipe.
A carburetor can also flood if the float inside the fuel bowl is set too high or develops a leak and sinks. If all that is needed is a new float, there is no real need to replace the entire carburetor. Floats are not part of a rebuild kit and must be purchased separately, so if new gaskets are also needed, a rebuild kit will have to be purchased, too.
Flooding can also be caused by excessive fuel pressure from the fuel pump, forcing fuel past the closed needle valve. Flooding may also be caused by excessive heat in some instances. A heat riser valve that sticks open will create a hot spot under the intake manifold that causes the fuel in the carburetor bowl to boil over and flood the engine through the metering rod/jet and out the main nozzle.