Why would fuel be backing up in a carburetor on a 1985 Chevy blazer?

Answer 1

Carburator Leaks / "Fuel Back Up"Although I'm only a "shade tree mechanic," I'll try to get this answer started. Maybe someone who knows "better" will pick up the thread and provide a more complete and accurate answer.

I'm not quite sure what you mean by "backing up," but I suspect that excess fuel is being delivered to the carb, resulting in fuel leaking out of the carb, and/or "flooding" the engine.

I am not personnaly aquainted with the 85 Blazer engine or carb, but if it is the normal carb and hook up, then the problem you describe may be caused by a defective or "sticking" float valve.

A carburetor has a chamber, sometimes called the "float bowl," which holds maybe a cup or two of fuel. For the carburetor to function properly, the top level of the fuel in the bowl MUST be maintained at a SPECIFIC LEVEL.

As the engine uses the fuel, a fuel pump [separate from the carb] replaces the used fuel, but it is designed to pump MORE than the engine would ever need.

To prevent too much fuel from being pumped into the carb float chamber, there is a float inside the chamber which is attached to a small valve which, depending of the rate of fuel use by the engine, may close [at low fuel use], or open wide[during heavy fuel use], and may hover, partially open [during intermediate consumption].

Fuel pumps usually operate continuously while the engine is running, regardless of the throttle setting and the related amount of fuel consumption. Therefore, there is the tendency to always pump more fuel than the carb needs, AND the float valve which is controlled by the float, often closes entirely [as when the engine is idling].

IF THERE IS SOMETHING WRONG with the float or the valve, then the valve may not be fully closing which allows way too much more fuel into the carb bowl which will result in flooding of the engine, and possibly fuel leaking out of the carb on to the manifold or engine.

By the way, fuel leaking out of any part of the fuel system, ESPECIALLY IN THE ENGINE COMPARTMENT,can be a VERY DANGEROUS FIRE HAZARD!!!

No offense is intended, but based on the form of your question, I suspect that this serious problem is beyond your ability to properly fix, and I strongly suggest that you not drive the vehicle in this condition, AND that you enlist the services of a PROFESSIONAL MECHANIC to troubleshoot the problem and to make proper repair(s).