(INTAKE) Carburetor Troubleshooting- Flooding Problems
11-02-03, 02:53 PM
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Nikki-Modder Rex-Rodder
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(INTAKE) Carburetor Troubleshooting- Flooding Problems
This is my take on the usual flooding problems that many Rx-7 owners will experience at some point while enjoying these fine cars.
The following is a list of the probable causes and fixes.
If I've left something out, please add to it.
Sterling
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FLOODING PROBLEMS
Carburetor flooding problems can be attributed to just a handful of things. First thing's first-
"Do you have your float bowl vent solenoid hooked up?!"
This little sucker is a tiny unassuming valve located under your oil inlet lines on the driver side of the carb. It is an electromagnetic plunger valve that closes off the vent to the Nikki carburetor when the engine is turned off. This is to prevent fuel vapors from escaping to atmosphere.
When the engine is on, the electromagnet pulls back the plunger, and the vent is opened. Any fuel vapors that would come out of it would be sucked immediately into the engine even at idle.
I've pondered just why a non-vented Nikki behaves the way it does, creating such a fuel hemorrhage, and this is what I've concluded:
When vented, the normal pressure on the floats is atmospheric pressure no matter what position the floats are in. If they have dropped down and fuel is incoming, the pressure is the same as when they are at the top closing off the inlet valves. But when the the carb is unvented, the pressure on top of the floats increases as the air is compressed by the rising fuel. I don't believe that the pressure in the air at the top of the float bowl ever exceeds the incoming fuel pressure, but I believe it is increased enough to cause the fuel to be pushed up through the main circuits into the booster venturis.
Next scenario we see most is that the carb has just been rebuilt but is flooding. Hopefully the rebuilder did not reset the float level adjusting tabs. They never go out of adjustment on their own, and unless you're mixing and matching carb bits during rebuilding, they usually don't ever need to be reset. (The exception being special application carbs where level or pressure setting are changed dramatically.)
With brand new rebuild kits, there is a microscopic matt finish on the brass needles, but usually not in the brass seats. Subsequently, the needles often hang up in the seats at first, until they establish a polished groove in the finish by rubbing together. This can be helped along by gently tapping over top of them on the inlets with the plastic handle of a screw driver. This may have to be done several times, frustrating as that may be, but eventually they will seat themselves.
Another problem with rebuilt carbs is that the needle clip that hooks to the float tab is installed in such a mannor that it is twisted and keeping the needle from closing all the way. Also, if the pin and springs are not installed, the needle may not seat fully.
If the carb is used, and not newly rebuilt, then you have to question its history. "Was it taken apart and messed with internally?" should be the first question. Someone experiencing the same problem beforehand may have reset (and truely messed up!) the float levels in a last ditch effort to get it working.
If the carb sat for any more than a month or two with no fuel in it, then it could have dried out enough for the rubber needles to start their inevitable breakdown. Nothing will bring them back, and a rebuild is in order. When I say "no fuel", I mean off a car. If it was on the engine and sealed up, the shelf life might be a bit longer, but not much.
If it came from a junkyard, then there's just no telling at all. A rebuild is your only "certain" way of ruling out problems. If it's been immersed in water, The few tiny springs used in the carburetor cna be rusting things shut, including the float bowl vent solenoid valve.
An aftermarket fuel pump with no regulator will definitely cause massive flooding to occur, and will present a serious fire hazard. The output of most 7 pound pumps or better will overwhelm the tiny return line orifice located in the inlet-outlet plumbing, and fuel can gush form the carb overflowing it.
----------------------------------------------------------------------------------------------------
The following is a list of the probable causes and fixes.
If I've left something out, please add to it.
Sterling
----------------------------------------------------------------------------------------------------
FLOODING PROBLEMS
Carburetor flooding problems can be attributed to just a handful of things. First thing's first-
"Do you have your float bowl vent solenoid hooked up?!"
This little sucker is a tiny unassuming valve located under your oil inlet lines on the driver side of the carb. It is an electromagnetic plunger valve that closes off the vent to the Nikki carburetor when the engine is turned off. This is to prevent fuel vapors from escaping to atmosphere.
When the engine is on, the electromagnet pulls back the plunger, and the vent is opened. Any fuel vapors that would come out of it would be sucked immediately into the engine even at idle.
I've pondered just why a non-vented Nikki behaves the way it does, creating such a fuel hemorrhage, and this is what I've concluded:
When vented, the normal pressure on the floats is atmospheric pressure no matter what position the floats are in. If they have dropped down and fuel is incoming, the pressure is the same as when they are at the top closing off the inlet valves. But when the the carb is unvented, the pressure on top of the floats increases as the air is compressed by the rising fuel. I don't believe that the pressure in the air at the top of the float bowl ever exceeds the incoming fuel pressure, but I believe it is increased enough to cause the fuel to be pushed up through the main circuits into the booster venturis.
Next scenario we see most is that the carb has just been rebuilt but is flooding. Hopefully the rebuilder did not reset the float level adjusting tabs. They never go out of adjustment on their own, and unless you're mixing and matching carb bits during rebuilding, they usually don't ever need to be reset. (The exception being special application carbs where level or pressure setting are changed dramatically.)
With brand new rebuild kits, there is a microscopic matt finish on the brass needles, but usually not in the brass seats. Subsequently, the needles often hang up in the seats at first, until they establish a polished groove in the finish by rubbing together. This can be helped along by gently tapping over top of them on the inlets with the plastic handle of a screw driver. This may have to be done several times, frustrating as that may be, but eventually they will seat themselves.
Another problem with rebuilt carbs is that the needle clip that hooks to the float tab is installed in such a mannor that it is twisted and keeping the needle from closing all the way. Also, if the pin and springs are not installed, the needle may not seat fully.
If the carb is used, and not newly rebuilt, then you have to question its history. "Was it taken apart and messed with internally?" should be the first question. Someone experiencing the same problem beforehand may have reset (and truely messed up!) the float levels in a last ditch effort to get it working.
If the carb sat for any more than a month or two with no fuel in it, then it could have dried out enough for the rubber needles to start their inevitable breakdown. Nothing will bring them back, and a rebuild is in order. When I say "no fuel", I mean off a car. If it was on the engine and sealed up, the shelf life might be a bit longer, but not much.
If it came from a junkyard, then there's just no telling at all. A rebuild is your only "certain" way of ruling out problems. If it's been immersed in water, The few tiny springs used in the carburetor cna be rusting things shut, including the float bowl vent solenoid valve.
An aftermarket fuel pump with no regulator will definitely cause massive flooding to occur, and will present a serious fire hazard. The output of most 7 pound pumps or better will overwhelm the tiny return line orifice located in the inlet-outlet plumbing, and fuel can gush form the carb overflowing it.
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