GM Top Engine Cleaner is an excellent chemical to remove deposits from the combustion cham
At first, during the wetting process, the delivered mixture will be weaker (leaner) than created. Once the wetted fuel starts to evaporate into the charge it will be richer. The engine will respond with an idle instability, since the amount of fuel being supplied will constantly be varied. In addition, it is possible that the poor atomization is not occurring in each carburetor bore and due to the intake manifold style (single or dual-plane) will impact only a few cylinders. The easiest way to mask this skewing of the mixture is to raise the idle speed, which most enthusiasts and professionals alike do. The proper approach is to fix the disparity in charge delivery to each bore.
A greater obstacle to uniform delivery of the air/fuel ratio is carbon deposited on the backside of the intake valve, especially with a port EFI engine. Gasoline is a complex chain of hydrocarbons. When it is burned or exposed to heat, carbon will form. There are two regions where the carbon builds in any engine. These are in the combustion chamber and on the intake valve. They are described as CCD for combustion chamber deposits and IVD for intake valve deposits. When it comes to idle speed, IVD is a very large offender.
The spark plugs should be properly gapped and in good shape. If the ignition system is str
When carbon forms on the intake valve two things occur. The first being the obvious, a decrease in airflow into the cylinder bore, since the carbon effectively makes the valve smaller in size. The other being the carbon acts like a sponge, wicking in the fuel from the engine until it becomes saturated. Once this occurs, it then releases the hydrocarbons into the charge stream, artificially richening the mixture. During the wicking process, the delivered mixture is leaner than the created air/fuel ratio. When the carbon starts to give up the fuel, the mixture is now richer than the delivered ratio. One of the major problems is the carbon deposits do not build uniformly on each valve. It is very likely that no matter how well tuned the carburetor is, the mixture distribution in the cylinder bores (not the intake manifold) will be skewed and cause a rough and rolling idle.
Though this impacts all gasoline engines. port EFI systems are more prone to being affected by IVD. The fuel injector is aimed to spray right on the intake valve. This is done to help atomize the fuel and to also have the heat from the valve improve the vaporization rate. For this reason, an EFI Pontiac engine can run leaner during a cold start through warm-up than a carburetor counterpart. Approximately 50 percent of gasoline is vaporized at 60 degree F, but the intake valve with its face in the combustion chamber will build heat very quickly. Thus, the fuel’s rate of vaporization will greatly improve. But if the intake valve is carbon-laden, then the fuel from the injector is wicked into the deposit at a higher rate than it would be with a carburetor, since the spray is directed at the valve.
The distributor cap, advance weights and vacuum canister all need to be in good condition
Ignition timing will also impact the engine’s ability to idle slowly. As with the idle speed specification, the suggested base timing is a good starting point. From there you should advance and then retard the value two to three degrees and then try lowering the idle speed through the carburetor curb idle screw.
The rate of ignition advance is a function of the flame speed across the bore in relation to the piston speed. The slower the flame burns, the more of a head start (advance) is required. Many factors impact this, such as the combustion chamber shape and design, spark plug location in relation to the bore center, charge motion and cam profile.