Adjusting the fast idle, regular idle speeds and idle mixture adjustments can usually fixa hot or cold stalling problem. A rough idle state is most of the time due to an excessively lean fuel mixture that results in lean misfires. The common cause of idle problems is air leaks between the carburetor and intake manifold, air leaks in vacuum lines or the EGR valve or the PCV system.
Other carburetor related causes include an idle mixture adjustment set too lean which can be resolved by backing out the idle mixture adjustment screw one quarter of a turn at a time until the idle quality gets better. Another carburetor related cause is a dirty idle mixture circuit which may need cleaning and rebuilding of the carburetor. Other possible causes of a rough idle, although not that common, include a malfunctioning charcoal canister purge control valve that is not closing and leaking fuel vapors back into the carburetor.
An excessive compression blowby, weak or broken valve springs, or ignition misfiring because of dirty or worn spark plugs, bad plug wires or a weak ignition coil are also considered as causes of a trough idle. This is a type of idle issue that is usually due to automatic choke. If the choke is only sticking slightly, and it is not taken care of right away, the dirt can build up until the choke is stuck solid. It is a good idea to check the choke and the choke linkage, and clean or repair as needed.
There is a separate fast idle adjustment screw on the choke linkage that regulates engine speed while the engine is warming up. The tip of the screw rests against a cam that gradually rotates as the choke opens during engine warm up.
Turn this screw counterclockwise to lessen the fast idle speed. However, to increase the fast idle speed, just turn the screw clockwise. A high idle speed can also be due to vacuum leaks that allow air to go into the manifold. Another cause can be a malfunctioning idle speed control valve which is stuck in the extended or high idle speed position.
This problem is an issue that most of the time the carburetor is at fault but it is not always the case. If dirt goes into the needle valve and prevents it from closing, it may cause the carburetor to flood. If an engine is flooded, it may not start due to the plugs being wet with fuel. Flooding can also occur if the float inside the fuel bowl is set excessively high or develops a leak and sinks.
In this case, all that is needed is a new float — no need to replace the entire carburetor. Rebuild kits do not include floats, so if new gaskets are also required, a rebuild kit has to be purchased too. Excessive fuel pressure can also cause flooding since it forces the fuel past the needle valve.
In some instances, flooding may also be due to excessive heat. A hotspot under the intake manifold, created by a heat riser on a V6 or V8 engine that is stuck solid, can cause the fuel in the carburetor bowl to boil over and flood the engine.
Caution: Flooding can be very hazardous since it can create a serious fire problem if the fuel spills out of the carburetor on a very hot engine. If you are under the impression that your carburetor needs work, attend to it as soon as possible. It can be rebuilt with a kit or replaced with a new or refurbished carburetor. It is a relatively simple job to clean and rebuild an older single or two barrel carburetor. However, it can be really difficult to clean and rebuild a four-barrel type.
More difficult carburetors may require the skills of an expert or even a professional mechanic. Most of the time, it is easier and less risky to replace a more complicated carburetor than to try rebuilding it.
If your carburetor is either of these, variable-venturi or electronic feedback controls and tamper-resistant adjustments, might as well consult an expert. If your carburetor has worn throttle shafts that are leaking air, damaged, or any if not all of the castings are cracked, the carburetor must be replaced since it cannot be rebuilt. If this is the case, the only remedy here is if you have a second carburetor that you can salvage and dismantle for parts to use and repair the first one.
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Thanks for that. And thanks for reading Autoblog. Reduced engine performance One of the first symptoms commonly associated with a bad or failing carburetor is a reduced engine performance. Black smoke from exhaust Another symptom commonly associated with a problematic carburetor is black smoke coming from the exhaust. Backfiring or overheating Engine backfiring and overheating are other common symptoms of a potential problem with the carburetor. Hard starting Hard starting is another symptom commonly associated with a bad or failing carburetor.
View More. Thank You Thanks for subscribing. Check your in-box to get started. Sign Up More Info. We notice you're using an ad blocker. Please consider allowing Autoblog. Allow Us! Usually, this manifests itself in the idle jet, and since the idle circuit is active throughout the throttle range, a lean idle circuit will cause sneezing or popping in the intake, as well as overheating and backfiring.
Air leaks in the intake after the fuel ports in the venturi can also cause a lean condition, so be sure to check your intake manifold gaskets, and any shaft seals within the carburetor body. Restrictions in the fuel filter or fuel lines can also mimic the symptoms of a clogged carburetor. Black, puffy exhaust and carbon-fouled plugs are the hallmarks of a rich running condition.
Failures in the carburetor float, needle valve or seat can cause unrestricted fuel to enter the bowl. This will cause more fuel to enter the venturi, causing the rich condition, and may be accompanied by fuel leaking from the carburetor atmospheric bowl vent. Stumbles and intakes sneezes that only occur when accelerating aggressively are indicative of a damaged accelerator pump diaphragm or clogged accelerator pump passages.
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