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Rough Idle, RPM drop and stalling at intersections


jalexquijano

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fuelreturn002Large.jpg

 

 

Fuel travels to the carburetor where there is a Y in the pipe and returns. This is the return to the tank pipe with the very small restriction in the end. Without this restriction the fuel would just return to the tank. With the restriction pressure builds up on the carb side. Cool fuel is constantly circulated and filtered. Any vapor bubbles will be pushed back into the tank.

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Don't know what the local rates are now. Don't even know for sure what's wrong with it. Often posting pictures shows something un-thought of or assumed. In some cases it's a simple thing to fix.

 

The 240 uses a mechanical fuel pump but there is an optional electric pump as well. It's not clear to me if both or one or the other are used.

 

Fuel pressure may be too high causing flooding. Easy fix but has to have the pressure tested. You can have this done in Panama. A $50 fuel pressure regulator will fix this.

 

You may have a fuel return line.... or not. If so, it might be blocked causing vapor lock. Again easy fix.

 

What about that temp sender in the air filter? The one that opens an air bleed to the intake to correct for hot idle richness??? Has that been checked???

 

You can duct cool air from in front of the rad to the air filter with $5 worth of drier vent pipe.

 

Does the temperature gauge run in the 'normal' range? If slightly high, immediately replace the thermostat. Cost under $20 probably a good idea anyway, to eliminate it as a cause. Suggest a 170F or tropical range. Don't know the history of the car... maybe from up north and has a 180F ???? in it???.

 

Does your rad have a shroud around the fan? This increases the fan's efficiency at low speeds and keeps the engine cooler.

 

 

These are all easy things to check or change or have someone do... even in Panama.

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we tested with a gauge and it is pumping at 4psi. Electric Fuel Pump (Airtex 8012S). PO removed mechanical pump! Any better option for an electrical pump other than this Brand? THis one pumps good and i know the pump is not the problem. No overflow at the bowls and the glass filter is always full. I will ask the mechanic to check on the return line.

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Okay, finally decided to pick up the car from the mechanic. Charged me US$160 for changing the fan clutch, thermostat, wrapping heat shield on the fuel lines and replacing wáter pump. He also changed the exhaust manifold pipe gasket. Car timing was set at 10BTDC. Car ran fine. No backfire either from intake or exhaust. However, once i reached home while playing with the car stereo temperature started to rise until reached 3/4. RPM dropped from 1000 to 500 and then raised to 1000 again. A few seconds later it stalled and i had to crank it again. Took out all plugs and cleaned them. I guess i will need to take the radiator out and flush it completely. The problem clearly has to do with heat. What a waste of money and time.

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These...the fan clutch, thermostat, wrapping heat shield on the fuel lines and replacing wáter pump. He also changed the exhaust manifold pipe gasket. Car timing was set at 10BTDC...are now assumed to be eliminated. Most vintage car owners do almost all their own work. It's part necessity for some and part enjoyment for others. If you make $20 an hour, you have to work 3 hours or more to pay a mechanic for an hour of his time to learn how to fix your car. If you learn what works and what doesn't and narrow down your problems yourself you get to keep this knowledge. 

 

I assume the mechanic put a 170F thermostat in?

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 Always replace a thermostat. You can try a 170F thermostat. This is the tropical range. I wouldn't run without a thermostat.

 

 

185F isn't really high enough to boil gas but under hoot temps from exhaust heat may be higher.

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Always replace a thermostat. You can try a 170F thermostat. This is the tropical range. I wouldn't run without a thermostat.

 

 

185F isn't really high enough to boil gas but under hoot temps from exhaust heat may be higher.

This is a problem with SU carburetors. They get very hot from radiated heat from the hot exhaust. Under hood temperatures can get very high. Worse in the tropics. Is the stock heat shield in place?

 

Take the hood off and drive around. If the problem goes away than that's the cause. Just too hot.

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Which is why the 280Zs had to add the hood vents.  Mind you that was EFI, but it's the design of the Z engine bay that causes high temps.   Very poor air flow due to the low profile grille.  The ZX even had to add a fan to blow air from the opposite side of the engine over to the injector rail.  And my 310 has a small fan (looks like a mini desk fan) to cool the carb due to the same issue- a very crowded engine bay.

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Okay! I see the problem is heat! I have removed the thermostat to see if the rough idle and stalling is eliminated. I have the front carb idle mixture knob set at 4 turns clockwise and the rear one at 3 turns clockwise. If i set them both at 2.5 turn as the Ztherapy video i will start hearing some intake backfire. The car does have the carb heat shield installed. Will test the car this weekend and if rough idle and stalling continues will take the car to a radiator shop for complete flush and repair.  If not i will have to remove the freeze plugs and clean the wáter passages!!

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The engine may be at the proper temperature so flushing the rad may be a waste of time. ... it's the trapped hot air caught under the hood around the carbs. Take the hood off and drive it. If it goes away then this is it.

 

The later 280z and 280zx had electric fans and plastic duct work to blow cooler air on the intake.

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You're confusing engine temperature with underhood temperature.  While the 2 are related, if the ENGINE is not overheating (and if your temperature gauge works, you should know if it is) then pulling the thermostat or flushing the system is not going to solve the issue of you having too much heat under the hood.   In fact if you make the cooling system more efficient you'll end up with even MORE heat under the hood, as it has to go somewhere.

 

The heat that (might) be causing fuel boiling would be coming from the exhaust manifold.  If you have headers it'd be even worse. 

 

As Mike has said repeatedly, take the hood off and see if the problem goes away.  You seemed to indicate you couldn't make it happen in the driveway (or garage).  Was the hood open when it wouldn't happen?

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Already found a Manifold Heat Control Thermostat which is in working order. I will replace it and at the same time take the radiator for a flush and repair if needed tomorrow. I will test the car with the lid open today and perform a vaccuum test. I have not opened the float valves on any of the 3 screws SU carbs Ztherapy refurbished for me. I read that i need to check if the float level could have possibly been affected during shipping? Ztherapy says not to open the float valves. Any opinions on this?

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The manifold heat control valve is in working order but you are going to replace it????

 

If your heat gauge is in the normal range there's no need to flush the rad.

 

Eliminate the under hood temperature as a cause first. Costs you zero.

 

 

 

Take the hood off and drive around. Does it work? Yes or No

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Did radiator flush. Radiator tech says radiator is fine! When returning home and got stuck on traffic the idle became rough and the car stalled 2 times. Reached to my garage and all the plugs where black. Idle mixture is set to 4 turns clockwise at Front Carb and 3.25 turns clockwise rear carb. Im getting really fed up! If i lower this settings you will start hearing backfire at intake. Is a leak down test the same thing as a compression test?

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Two Questions:

 

1. Could my Brake Booster have an internal vacuum leak causing this problem? or could the master cylinder be faulty?

 

2. I have also noticed my Petronix 40511 3 0hms coil gets very hot after 15 minutes of driving! I am using it through the ballast resistor for protection. It came connected this way when i bought the car from previous owner. How hot should it get? Not as hot as been unable to hold it with your hands?

 

I just want your opinión on any of these two probable causes and how to i test them to be sure they are not the cause of the rough idle and stalling at intersections or traffic jams.

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Okay. Suppose the problem is underhood heat. How can i fix this? Can you show me how to i check that the fuel return line is not blocked? Where exactly shall i check do you have a picture? I have one mechanic that says the problem is the Master Vac Booster Brake and Master Cylinder. He tested the Master Vac Booster Brake with a manual gauge and diagnosed it had an internal vacuum leak as it was loosing vacuum.

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