65 4GC carb has no fuel after a few days - Impala Tech
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post #1 of 9 (permalink) Old 09-09-2006, 11:02 PM Thread Starter
 
 
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65 4GC carb has no fuel after a few days

I've got a stock 65 SS 327 with a Rochester 4GC carb. It works great, except that whenever the car sits for more than a few days, the fuel "vanishes" from the float bowls and I have to crank it for a minute or two before it fills the bowl again. I've had it apart several times to look for leaks and can't find any. I even let it sit on the workbench with fuel in it to try and find the leak, and it doesn't seem to lose any under those conditions, just when it's on the car and has the fuel line hooked up to it. Anyone have any ideas? Seems like the fuel might be pulled back through the needle and seat, but I'm not sure why or how.
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post #2 of 9 (permalink) Old 09-10-2006, 10:06 AM
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Cool

Heat from engine is boiling the fuel away (gasoline boils at 101° F) so unless you can cork it up some how, you will need to provide a heat shield to isolate the carb from the manifold, or go with an electric fuel pump that will prime the carb before starting.


Big Dave
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post #3 of 9 (permalink) Old 09-10-2006, 10:30 AM Thread Starter
 
 
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Thanks for the reply Dave. After I posted my question, I found another site that offered the same reason that you gave. I'm not sure if I can agree with it, but maybe I'm just not understanding the entire problem. I can believe that gas would boil and evaporate, and I'm sure the underhood temps are much higher than the boiling point of gas. But if that were the case, wouldn't the gas boil away within a few minutes or hours after being shut off (while the engine is hot)? The problem comes when it sits for more than a few days. Since the restarts OK the same day, or within the next few days, I would have to believe that it's not boiling away the fuel (or at least not all of it). It doesn't make sense to me that the fuel would boil away after the car has sat for a few days. Maybe I'm missing something here.
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post #4 of 9 (permalink) Old 09-10-2006, 01:01 PM
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boiling due to heat from the EGR passage in the intake not being properly shielded is an issue but this doesn't sound like what's happening to you. When boiling happens, you would have definate performance issues and it would run poorly (if at all) when engine gets up to temperature. I'm speaking from personal experience.

My first thought was that you had a leaky carb (there's a quick test for this) but if you have let it sit full of fuel overnight, that's probably not it.

I have to ask how it performs otherwise...what are your hot starts like? Performance? does it run rich or lean, stumble, etc? If it is fine otherwise I would suggest starting procedure...we in today's world of computer controlled fuel injection often forget "how it was" with naturally aspirated engines...we can't expect them to fire up like modern technology when sitting a few days!

Consider this:

Your climate/humidity, etc. I can set a carb full of fuel on my bench in a pan with a piece of paper in the bottom of the pan. If any fuel leaks, it will stain the paper. I have found up to 3/4 of the float bowl can vanish into thin air in as little as 3 days sitting, with the float pretty much all the way down..from evaporation, not leaks. Because of carb venting etc, your carb is not a sealed unit. It is open to the air and gas evaporates quickly (hense the paper, drips could drop and dry up and never be seen...the paper is the proof nothing ever dripped). Depending on your climate this could take longer (or less time) in your carb.

Proper starting procedure is critical also (don't laugh, rochester's are fussy and if you don't treat them right, they become tempermental quickly).

On my '69 Impala with a 4MV, cold starting procedure per the owners manual is to depress pedal to floor one time and crank. The purpose of this is to set the manual choke - not to indtroduce fuel into the carb however you may get a small pump shot if there is enough fuel in the bowl. In my case the carb is set up 100% stock, and if I do ANYTHING else but this procedure it will flood, and either start and die, start and run very poorly, or crank forever before starting...and then running poorly. However if I follow this procedure it will start, go immediately to fast idle, warm up quickly and then it's off for great driving, wonderful performance and power. Having said all this, if my car sits for 3 or more days, it will crank for up to a full 6 or 78 seconds before it fires using this starting procedure...and this is 100% normal.

www.1969chevy.com

'69 Impala LM1 F40 J52 M38 C60 N33
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post #5 of 9 (permalink) Old 09-10-2006, 01:06 PM
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Quote:
Originally Posted by 69chevyguy View Post
Having said all this, if my car sits for 3 or more days, it will crank for up to a full 6 or 78 seconds before it fires using this starting procedure...and this is 100% normal.
Sorry for the typo - mine can crank for 6 or 7 seconds...not 78 seconds!!!!

www.1969chevy.com

'69 Impala LM1 F40 J52 M38 C60 N33
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post #6 of 9 (permalink) Old 09-10-2006, 03:08 PM
Al
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That year of carb tends to have problems with the well plugs leaking. Over a day or two the fuel will drain into the intake and evaporate or wash down the cylinders.
A typical fix is to apply an epoxy over these plugs - carb has to be removed to do this as they are underneath.

I tried a search to see if I could spot a picture online to show you and instead found a good article about it.
http://www.circletrack.com/techartic...uadrajet_tech/
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post #7 of 9 (permalink) Old 09-10-2006, 04:04 PM
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Actually well plugs leaking is a bit of a quadrajet myth. It does happen, but not nearly as often as most would think. Many people - even very reputible shops epoxy the plugs, but if in fact they are leaking this is a temporary fix and you will be re-doing it again down the road. The only permanant solution is to drill out the plugs and tap the holes for a 10-32 1/4 set screw and loc-tite them in.

Do a search for Lars Grimsrud on the internet. He is quite literally the king of quadrajets (and other carbs) and can talk you through the facts and also the myths of these carbs. He has several articles to his name and is well respected by the serious restoration experts. If I can find his website in my files I'll post a link to it here!

www.1969chevy.com

'69 Impala LM1 F40 J52 M38 C60 N33
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post #8 of 9 (permalink) Old 09-13-2006, 06:01 PM
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Cool

Boiling is dependant upon two things temperature and the vapor pressure above the fluid.

If the temp is sufficiently high enough to get a substantial number of molecules moving across the fluid air boundary and they do not collide with other molecules of equal mass floating above the boundary then by Newton's third law they keep right on trucking out of there. If there is a lot of molecules above the fluid (vaporized gas smell you notice when you shut down) then the molecule trying to make a get away will collide with one floating around above the gas and be bumped back into the fluid.

This is why gas will evaporate from an uncorked can but not one the kids actually remember to close up after using it.


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post #9 of 9 (permalink) Old 09-15-2006, 11:05 PM Thread Starter
 
 
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69chevyguy-good tip about putting a piece of paper under the carb while it's on the bench. I might have had a small leak and not seen it because it evaporated after it dripped. I'll try that.

I was wondering if, like you said, it could just be evaporating after a few days. Maybe that's all it is. It runs fine otherwise, no stumble or missfire. Doesn't seem to be rich or lean. When it has fuel in the bowls it starts almost immediately. But when it sits for about a week, I have to crank at least 20-30 seconds, sometimes longer, before I get any fuel back up to the carb. If I only had to crank for 6 or 7 seconds, I would be happy. Maybe I have a combination of a small leak, and the rest of it evaporating? When I get a chance, I'll pull the carb off again and let it sit over some paper.

Thanks for the tips!
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