(INTAKE) Carburetor Troubleshooting- High RPM Power Loss
11-02-03, 02:43 PM
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(INTAKE) Carburetor Troubleshooting- High RPM Power Loss
This is my take on the usual high RPM power loss 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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HIGH RPM POWER LOSS
Carb power loss can be attributed to a few different problems with fuel flow, but the usual one is a dirty fuel filter. This should be replaced on a regular basis anyway, and I replace my fuel filter at the same time as I change my oil and oil filter.
If a new filter doesn't cure the problem, a fuel pump volume test should be done. It's easy and quick. According to the 1985 Mazda Rx-7 workshop manual, the fuel pump should put out 1400 cc or more in one minute. If it's not putting close to that much out, then either there is a restriction in the line at some point, or the pump is shot. This is the same for all years.
The restriction problem could be before or after the fuel pump. A restriction before the pump would most likely be a blockage. A restriction in the line after the pump could be as simple as you having hit a rock and squashing the steel fuel line under the car. Probably not, but the example demonstrates the necessity to inspect everything.
If the car sat for a long period of time, the rubber lines between the tank and the pump may have become gummed up, especially if premix was ever run through the tank. Usually this results in no fuel flow at all.
Another tank related situation that can cause inconsistent flow problems is an inability for the tank to breathe. The tank breathes thru the charcoal canister in engine bay, and if that has been removed, or the PCV system has been removed, condensate can sit in the tank braether line and plug it. A tank breathing problem is evident by a "whooshing" sound when you open the fuel cap after a hard run.
Finally there is the carb- and I put it last on the list because it is the most protected component of all mentioned thus far, and is the least likely to fail abruptly. However, there are a handful of things that can go wrong, but they shouldn't be entertained until the others are checked first.
You may have read about the small screen filters in each of the banjo fittings over each float bowl. There is one on top of each needle seat inlet, as well. It may sound like a logical place to start, but there has to be so much crap in the system for those to be clogged that it is very doubtful that they are suspect. I have rebuilt many Nikki carbs, and have never found internal screens so clogged that fuel flow would've been seriously impeded. I have seen screens partially blocked, but usually in junkyard carburetors that have sat for a long time, and have oxidized from water finding it's way into every orifice.
The float bowls should be checked by looking into the sight glasses on each side. It's difficult to do on the firewall side of the carburetor, but if you plan on doing more carb work in the future, a dental inspection mirror is great for this. The fuel level should be at the halfway point during idle. If it's substantially lower, and the fuel output is the correct volume, then the float levels are being compromised. Someone may have rebuilt it and inadvertently set them incorrectly. They do NOT need to be adjusted if they have not been touched, as they cannot go out of adjustment on their own.
If the float bowl fuel level is too high, then this is an indication of either the needles being worn out, which will eventually progress to consistent flooding, or again, someone has incorrectly set the float level.
There is another reason you could be losing power, especially at high RPMs. The emulsion tubes could be clogged. This would be a rare situation, but perhaps not if the carb was obtained from a yard. If the carb is second hand, and it's dirty and coated with varnish, then this might be the problem. This would be most noticeable if both the primary E-tubes were clogged, however.
Anytime a carburetor appears old, dirty, and/or covered in varnish, chances are a rebuild is in order or will be shortly. Carbs that sit are worse off then older carbs that have been on running engines because gaskets dry out, water associated dry-rot begins, and the gaskets, diaphragms and rubber needle tips won't ever return to the supple sealing softness they once had.
Don't be intimidated by a rebuild. The easy way is to send it to someone. But you can do it yourself if you can put together a model airplane.
----------------------------------------------------------------------------------------------------
The following is a list of the probable causes and fixes.
If I've left something out, please add to it.
Sterling
----------------------------------------------------------------------------------------------------
HIGH RPM POWER LOSS
Carb power loss can be attributed to a few different problems with fuel flow, but the usual one is a dirty fuel filter. This should be replaced on a regular basis anyway, and I replace my fuel filter at the same time as I change my oil and oil filter.
If a new filter doesn't cure the problem, a fuel pump volume test should be done. It's easy and quick. According to the 1985 Mazda Rx-7 workshop manual, the fuel pump should put out 1400 cc or more in one minute. If it's not putting close to that much out, then either there is a restriction in the line at some point, or the pump is shot. This is the same for all years.
The restriction problem could be before or after the fuel pump. A restriction before the pump would most likely be a blockage. A restriction in the line after the pump could be as simple as you having hit a rock and squashing the steel fuel line under the car. Probably not, but the example demonstrates the necessity to inspect everything.
If the car sat for a long period of time, the rubber lines between the tank and the pump may have become gummed up, especially if premix was ever run through the tank. Usually this results in no fuel flow at all.
Another tank related situation that can cause inconsistent flow problems is an inability for the tank to breathe. The tank breathes thru the charcoal canister in engine bay, and if that has been removed, or the PCV system has been removed, condensate can sit in the tank braether line and plug it. A tank breathing problem is evident by a "whooshing" sound when you open the fuel cap after a hard run.
Finally there is the carb- and I put it last on the list because it is the most protected component of all mentioned thus far, and is the least likely to fail abruptly. However, there are a handful of things that can go wrong, but they shouldn't be entertained until the others are checked first.
You may have read about the small screen filters in each of the banjo fittings over each float bowl. There is one on top of each needle seat inlet, as well. It may sound like a logical place to start, but there has to be so much crap in the system for those to be clogged that it is very doubtful that they are suspect. I have rebuilt many Nikki carbs, and have never found internal screens so clogged that fuel flow would've been seriously impeded. I have seen screens partially blocked, but usually in junkyard carburetors that have sat for a long time, and have oxidized from water finding it's way into every orifice.
The float bowls should be checked by looking into the sight glasses on each side. It's difficult to do on the firewall side of the carburetor, but if you plan on doing more carb work in the future, a dental inspection mirror is great for this. The fuel level should be at the halfway point during idle. If it's substantially lower, and the fuel output is the correct volume, then the float levels are being compromised. Someone may have rebuilt it and inadvertently set them incorrectly. They do NOT need to be adjusted if they have not been touched, as they cannot go out of adjustment on their own.
If the float bowl fuel level is too high, then this is an indication of either the needles being worn out, which will eventually progress to consistent flooding, or again, someone has incorrectly set the float level.
There is another reason you could be losing power, especially at high RPMs. The emulsion tubes could be clogged. This would be a rare situation, but perhaps not if the carb was obtained from a yard. If the carb is second hand, and it's dirty and coated with varnish, then this might be the problem. This would be most noticeable if both the primary E-tubes were clogged, however.
Anytime a carburetor appears old, dirty, and/or covered in varnish, chances are a rebuild is in order or will be shortly. Carbs that sit are worse off then older carbs that have been on running engines because gaskets dry out, water associated dry-rot begins, and the gaskets, diaphragms and rubber needle tips won't ever return to the supple sealing softness they once had.
Don't be intimidated by a rebuild. The easy way is to send it to someone. But you can do it yourself if you can put together a model airplane.
----------------------------------------------------------------------------------------------------
Last edited by Sterling; 11-02-03 at 02:50 PM.
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