Carburetor fan possibly drains battery below 12v - ever happen to anyone else?

tvmaster

True Classic
Took a drive yesterday. Six miles on the freeway, six miles back on the main roads from the pet food store.

At the halfway point, after an hour in the store, the carburetor fan was still running, on and off. Seems like a long time.

When we got home and hit the 20 degree driveway incline in first, the car stalled.

Couldnt be restarted - multimeter showed battery at 11.85 volts! The trusty 2007 Mazda 3 supplied a jump.

Since the alternator is now a rebuilt 61 amp, and the battery can be charged up to 12.85 volts with my charger, the only thing I can think is that the carburetor fan is running too long after minimal use. For an hour? After six miles?

Is there a way to test these fans, or whether the carb is creating too much heat? Of course, the rebuilt alternator could be rubbish, but it doesn’t seem likely.

Last bit of info: this was the first drive since installing what everyone said was the proper, 13 psi radiator cap, up from the 7 psi cap that’s been on since we bought it.
 

tvmaster

True Classic
Yes, much too long and it will drain the battery. Bad temp sensor or bad electrical connection somewhere.
So there’s a temp sensor for the carb fan - you learn something everyday. I’m assuming it’s easy to find in the 300-page service manual? No chance the carb is running hot?
 

tvmaster

True Classic
Maybe the temperature sensor calibration is bad. Before I removed my fan, it would only run for a couple of minutes after the engine was off.
Why did you remove the fan - replaced with a different method, or just to hell with it…
 
Why did you remove the fan - replaced with a different method, or just to hell with it…
I found that it did not add much value to my setup. Maybe if I lived somewhere hot it would make some difference but after I removed it I noticed no difference in the car's behavior. It added weight that I did not need so I removed it. By the time I got the dual DCNFs, I would have had to remove it or redesign it, so just as well it was off, although the DCNFs would probably take more advantage of the fan than the stock carb since they vent directly to the atmosphere and heat causes the fuel in the float bowls to evaporate rapidly.
 

tvmaster

True Classic
Sure is - mounted underneath one of the rear carb-to-manifold bolts. Controls a relay that switches the fan on and off.
Sure is - mounted underneath one of the rear carb-to-manifold bolts. Controls a relay that switches the fan on.
Ok. Honestly, this thing usually runs on and off, for 20 minutes after the engines shut down, and because of the engine noise, it’s sometimes hard to tell if it turns off, if running, when the engine is restarted.
But it’s never run for as short a time as Don stated with his.
I’m tempted to disconnect it as well if the battery is going to get drained.
 
Ok. Honestly, this thing usually runs on and off, for 20 minutes after the engines shut down, and because of the engine noise, it’s sometimes hard to tell if it turns off, if running, when the engine is restarted.
But it’s never run for as short a time as Don stated with his.
I’m tempted to disconnect it as well if the battery is going to get drained.
I guess you will either need to fix it or reevaluate your "as stock as possible" signature line. I assume you already know that you can make that car run a whole lot better for almost no investment if you are open to making it "not quite stock".
 

tvmaster

True Classic
I guess you will either need to fix it or reevaluate your "as stock as possible" signature line. I assume you already know that you can make that car run a whole lot better for almost no investment if you are open to making it "not quite stock".
Yes, better wouldn’t hurt, although how it runs now doesn’t bother us much, other than not being able to star in a ‘Back To The Future’ sequel. Honestly, if we could just rely on it all, or most of the time, that would make me the happiest :)
I‘m going to check the wires, and if they all look good, may order a new switch. At least then If know if the problem was it, or the engine running hot. But my fans rarely turn on, nor does the temp go over 200f, so I’m hopeful that’s not the problem.
 
I tried to find a spec for the thermoswitch but came up empty. If we knew the turn on temperature, you could test it. In any case, it is independent of the engine cooling system so it should not cause a problem there.
 

tvmaster

True Classic
I tried to find a spec for the thermoswitch but came up empty. If we knew the turn on temperature, you could test it. In any case, it is independent of the engine cooling system so it should not cause a problem there.
Here’s what I found: “Maybe either a 65/55 or 60/50 degree celsius cycle.”
As long as the carb isn’t on fire, we’re good, right?
 
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EricH

Eric Hamilton
Moderator
Here’s what I found: “Maybe either a 65/55 or 60/50 degree celsius cycle.”
If you're curious you can take the sensor out, plunk into a pot of water on the stove with a candy thermometer, measure the resistance with a multimeter as the temperature increases. This isn't necessary, it's just the sort of thing that we science nerds do because that's how we are.
As long as the carb isn’t on fire, we’re good, right?
Yes. The cooling fan is mostly intended to prevent hard starting with a warm engine; without it the heat soak after a long hot run can boil away all the fuel in the float bowl.
 

EricH

Eric Hamilton
Moderator
This by itself is a problem. If the battery is showing 11.85V after 6 miles of driving (and clearly, the car was able to start 6 mils ago) there is a problem with your charging system.
Ah - yes, good point. I had missed that the stall happened after a good long run that should have recharged the battery. Check the voltage across the battery terminals when the engine is running and turning 2500 or 3000 RPMs.... should be high enough to charge.
 

tvmaster

True Classic
If you're curious you can take the sensor out, plunk into a pot of water on the stove with a candy thermometer, measure the resistance with a multimeter as the temperature increases. This isn't necessary, it's just the sort of thing that we science nerds do because that's how we are.
Yes. The cooling fan is mostly intended to prevent hard starting with a warm engine; without it the heat soak after a long hot run can boil away all the fuel in the float bowl.
I like the nerd quotient, good idea :)
 

tvmaster

True Classic
Ah - yes, good point. I had missed that the stall happened after a good long run that should have recharged the battery. Check the voltage across the battery terminals when the engine is running and turning 2500 or 3000 RPMs.... should be high enough to charge.
Last time I checked, after the alternator went in, it was good. Time to check again. Weirdly, the car started after the hour break, while the blower was blowing away in the back. Then took the scenic route. I’m wondering if the blower ever stopped. lol.
 

LarryC

Curator of #10105275
I've posted this a few times, but it is always a useful reminder of how the cycle works. This was a series of temperature readings at the base of the carb on a very hot day after the fan started. Incidentally, the starting temperature was well over 200°. And the temperature of a nearby alternator bracket was 240°. This is why we have water cooling jackets inside the engine.
 

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I've posted this a few times, but it is always a useful reminder of how the cycle works. This was a series of temperature readings at the base of the carb on a very hot day after the fan started.
Interesting chart. Judging from the data, it looks like the thermoswitch is set for around 140F.
 
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