CRG Discussion Forum
Welcome, Guest. Please login or register.
Did you miss your activation email?
April 24, 2014, 09:02:23 AM

Login with username, password and session length
Search:     Advanced search
Welcome to the CRG Discussion Forum!
Forum registration problems: Make sure you enter your email correctly and you check your spam box first. *Then* email KurtS2@gmail for help.
97527 Posts in 11718 Topics by 4581 Members
Latest Member: Cooper48
* Home Help Search Login Register
+  CRG Discussion Forum
|-+  Camaro Research Group Discussion
| |-+  Originality
| | |-+  Fuel Return lines
« previous next »
Pages: [1] 2  All Print
Author Topic: Fuel Return lines  (Read 4049 times)
bc69
Member
***
Posts: 148



View Profile Email
« on: December 13, 2010, 03:52:37 PM »

Engine flooding...
Mark,
I still am interested by one of your earlier thoughts on the purpose for the return line on the Qjets to start with. I think somebody out there should have the answer.
Makes sense to me that it would be a relief, and allow a better fuel flow helping to keep the fuel temp in the line down. No...maybe...?
I have changed the carb since, don't seem to have the same problem, but it has not been a hot as it was. I think that the above question is addressing my problem. It seems as if the fuel line is getting hot enough to overpressure the line or vaporize the fuel. We all can remember the days when "vapor lock" was a common word.
Brad
Logged
Stingr69
Member
***
Posts: 383


View Profile Email
« Reply #1 on: December 13, 2010, 04:03:05 PM »

The return lines are not used on Holley 4 barrell equipped cars. I have to think the Q-jet is not able to handle higher fuel line pressures. The hot fuel being a problem seems to be more of an issue with todays gas in carbureted vehicles, not just for Q-jet cars. The line does not need to be hot if the carb is getting cooked. This is more common than hot lines IMO.

-Mark.
Logged
bc69
Member
***
Posts: 148



View Profile Email
« Reply #2 on: December 13, 2010, 05:15:14 PM »

Correct in the fact that returns were only on the qjets. But I am still curious in GM's reasoning for its' use, might it be the same.
Logged
KurtS
CRG Coordinator
*****
Posts: 3068


View Profile Email
« Reply #3 on: December 14, 2010, 12:40:56 AM »

Just to confuse things, full-size cars were very different. It didn't depend on the car, but the engine.
The 6 cyl, 327, LM1 and L48 were all single lines.
The big-blocks (LS1, L66, L72 and L36) were dual lines.
(Note this is full-size cars only, not Camaro.)
« Last Edit: June 23, 2011, 11:05:06 AM by KurtS » Logged

Kurt S
CRG
william
CRG Member
*****
Posts: 1136


View Profile Email
« Reply #4 on: December 14, 2010, 07:53:36 AM »

An engineer told me that under certain unusual operating conditions a QJ could consume fuel fast enough to drain the bowl. That's another reason for the large filter. It serves as a pressurized fuel reservoir to cover those rare conditions. If the fuel isn't needed it has go somewhere so that's the reason for the return line. Some current EFI systems also use a return line.
Logged
Kelley W King
Member
***
Posts: 258


View Profile Email
« Reply #5 on: December 14, 2010, 08:23:12 AM »

I have a Q jet on my clone SS car that does not have a return fuel line. The return port is open and I have never seen any fuel come out even though I have driven the car under all conditions from normal driving to shifting at 5500 RPM full throttle. I was going to plug the port but a knowlegable friend says I should not because it vents the carb. Should I plug It or plumb it away from the engine or leave it alone? If fuel did come out it would spray right on the intake manifold.
Logged

69 Z28 RS
69 SS L78
67 SS Chevelle
64 Corvette
66 GTO Tiger Gold
77 Trans Am Special Edition
william
CRG Member
*****
Posts: 1136


View Profile Email
« Reply #6 on: December 14, 2010, 09:03:35 AM »

The QJ return line runs from the filter, not the carb, to the tank.
Logged
bc69
Member
***
Posts: 148



View Profile Email
« Reply #7 on: December 14, 2010, 09:34:58 AM »

What are you calling the return port? The one on the fuel filter or the tube standing up on top of the qjet.   The one that comes out of the carb is a Bowl vent over the float assy. This should not be sealed.
Logged
Kelley W King
Member
***
Posts: 258


View Profile Email
« Reply #8 on: December 14, 2010, 10:17:15 AM »

Mine ie a Carter Q Jet. It has a line in the front to the right of the fuel inlet. A 3/8 rubber line will slide up on it. It is open now.
Logged

69 Z28 RS
69 SS L78
67 SS Chevelle
64 Corvette
66 GTO Tiger Gold
77 Trans Am Special Edition
Stingr69
Member
***
Posts: 383


View Profile Email
« Reply #9 on: December 14, 2010, 10:55:10 AM »

What are you calling the return port? The one on the fuel filter or the tube standing up on top of the qjet.   The one that comes out of the carb is a Bowl vent over the float assy. This should not be sealed.
There is a third nipple on the in-line fuel filter that is described as a "vapor return line".  

Some applications (such as on the Q-jet equipped Corvette) have a 3rd port on the fuel pump and an in-carb filter rather than the in-line filter with the 3rd nipple.

The steel return line was not free to GM so I doubt it would be a good idea to remove it from service without a real good reason. JMO.

-Mark.
Logged
JohnZ
CRG Member
*****
Posts: 3963


View Profile Email
« Reply #10 on: December 15, 2010, 06:34:07 PM »

I'm a Holley guy and not very familiar with Q-Jets, but I bounced this thread off a friend of mine, Clem Zahrobsky, who has been rebuilding and modifying Q-Jets for 40+ years, and here's his comment:

"The return line is there to reduce the fuel pressure in the carb to zero when you shut off the engine, for emissions reasons. If there's a return line, you should use the check-valve type internal fuel filter to prevent the float bowl from draining down. In a Q-Jet, as the fuel level lowers due to fuel evaporation, it opens the needle and seat and high-performance Q-Jets use a "windowed" needle seat which is open below the fuel level, so with zero pressure in the fuel line the float bowl will drain down, causing hard starting. If you have a return line, never block it off, as it's a calibrated leak so the fuel pump has extra pressure to compensate for the leak; plugging the return line will raise the fuel pressure enough to cause carb flooding at idle."

Hope that's helpful.  Smiley
Logged

'69 Z/28
Fathom Green
CRG
redge
Member
***
Posts: 54


redge


View Profile
« Reply #11 on: December 16, 2010, 12:30:57 AM »

I'm a Holley guy and not very familiar with Q-Jets, but I bounced this thread off a friend of mine, Clem Zahrobsky, who has been rebuilding and modifying Q-Jets for 40+ years, and here's his comment:

"The return line is there to reduce the fuel pressure in the carb to zero when you shut off the engine, for emissions reasons. If there's a return line, you should use the check-valve type internal fuel filter to prevent the float bowl from draining down. In a Q-Jet, as the fuel level lowers due to fuel evaporation, it opens the needle and seat and high-performance Q-Jets use a "windowed" needle seat which is open below the fuel level, so with zero pressure in the fuel line the float bowl will drain down, causing hard starting. If you have a return line, never block it off, as it's a calibrated leak so the fuel pump has extra pressure to compensate for the leak; plugging the return line will raise the fuel pressure enough to cause carb flooding at idle."

Hope that's helpful.  Smiley
  I have a problem wiht hard starting , where can i found a check valve internal fuel filter ?
Logged
Stingr69
Member
***
Posts: 383


View Profile Email
« Reply #12 on: December 16, 2010, 06:31:11 AM »

 
Quote
I have a problem with hard starting , where can i found a check valve internal fuel filter ?

NAPA #23051 for the shorter filter.
NAPA #23052 for the longer style filter.

Make sure the spring is in there when you open up the carb inlet to chage the filter element.

-Mark.
« Last Edit: June 23, 2011, 10:56:40 AM by KurtS » Logged
Stingr69
Member
***
Posts: 383


View Profile Email
« Reply #13 on: December 16, 2010, 06:35:48 AM »

OK - Here is the whole thing with the explanation from Lars Grimsrud.

Since this issue involves several problems and topics, a general understanding of the systems and issues is important rather than simply addressing a single question on fuel pump check valves. I’m going to throw out a few facts and issues here so you can see and understand the multiple problems with the fuel disappearing issue.

The Q-Jet carb is a bit unique in its design in that the needle and seat assembly is located towards the bottom of the float bowl rather than in the top of the bowl or in the airhorn (as is the case with Holley and Carter carbs). In fact, the needle/seat assembly is located on a raised ledge at the same “elevation” as the accelerator pump inlet slot. The key about this fact is that if the fuel level in the float bowl is allowed to drop below this point, the accelerator pump will not fill with fuel, and the accel pump will not function, even if there is fuel in the float bowl below this level.

Holley carbs, as you know, have the accel pump located in a well hanging below the float bowl. If the float bowl has any fuel in it at all, the accel pump will fill and operate.

Although Q-Jets are equipped with chokes, an engine will generally not cold-start on the choke alone: an engine needs a couple of squirts of fuel from the accel pump even if there is fuel in the bowl and the choke is closed. The accel pump shot is essential to engine starting, and the accel pump in a Q-Jet will not function more than 1 stroke if the float bowl fuel level drops more than about 1/8”.

The fuel level in a Q-Jet bowl can drop more than 1/8” over time from 3 separate causes and issues:

1. Although the problem is exaggerated, it is possible for the bowl well plugs to start leaking on a Q-Jet. When the factory drilled the main jet and secondary fuel transfer passages, they drilled them up from the bottom, through the bottom of the carb. Once the drilling was complete, they pressed lead plugs into the resultant holes in the bottom of the carb. It is possible for these plugs to start leaking. If they do, the fuel level in the bowl can easily and quickly not only drop below the accel pump inlet slot, but can actually empty the bowl completely. This will result in very hard cold-starting.

To check for this condition, simply remove the carb from the engine. Dry off any fuel from the bottom of the carb. Place the carb on 3 tall sockets on your workbench so you can see the bottom of the carb, and fill the bowl with fuel by pouring about 1-1/2 ounces of fuel down the vent tube. If the plugs are leaking, you will see immediate dripping or wetting on the bottom of the carb, and fuel will drip down onto your workbench. It’s very obvious. If this happens, you need to seal the plugs. This can be done by cleaning and abraiding the plug areas (the secondary plugs are not visible without removing the throttle plate) and applying a coat of JBWeld. This “fix” tends to last for a short while, because the JBWeld will crack and separate from the lead plugs. A permanent fix is to drill and tap a 10-32 hole into the primary well plugs and install a 10-32 countersunk socket head screw with JBWeld applied to the threads prior to installation. This permanently fixes the problem. The leaky secondary wells can be fixed by using the secondary well plug seal found in good quality carb rebuild kits.

If fuel does not drip out the bottom of your carb during the leak test, you do not have leaky well plugs, and you have one of the other 2 problems.

2. Fuel evaporation is a problem on a Q-Jet due to the fact that the bowl is vented, and you don’t have to lose very much fuel before the level drops enough to cause a problem. The fact that our modern fuels contain ethanol (15% in many locations) increases evaporation rate and exaggerates the problem. Evaporation rate is higher in hot weather conditions (summer). Other carbs, such as Holleys, are not affected by some fuel evaporation, since the accel pump inlet is in the bottom of the bowl. Remember – the Q-Jet float level only has to drop a slight amount in order for problems to occur, and evaporation will induce this situation (although it will normally not occur in an over-night 12-hour period).

3. The only other place the fuel can go on a Q-Jet is out the needle/seat, and back down the fuel line. Since the seat is located towards the bottom of the bowl, and since the carb is at a higher elevation than the fuel tank, you can see the obvious possibility that the fuel will simply siphon out of the bowl and return to the tank if allowed to do so.

Early Q-Jet fuel systems, generally, did not use fuel return lines. Although return lines have been used by GM in some performance and air conditioned applications since the mid-60s, the common use of return lines to reduce vapor lock issues did not become prevalent until the mid-70s. In the early fuel systems used with the Q-Jet, the non-return fuel pumps relied on the fuel pump’s internal check valve to prevent the column of fuel between the pump and the carb from draining back through the pump. As you can imagine, if the column of fuel between the pump and the carb is allowed to drop down, there will be a suction created at the carb by the retreating fuel in the line. The needle/seat, although tight-fitting, is not designed to positively stop fuel flow in the opposite direction. So the fuel draining back through the leaking fuel pump check valve can actually draw the fuel out of the float bowl down to the level of the needle/seat elevation. This drop in fuel level is enough to prevent operation of the accel pump, and the car will not cold start easily. On cars without return lines, a good quality AC/Delco pump, with a check valve intended to eliminate fuel drainback, is essential to good cold-start characteristics. Most aftermarket pumps do not have check valves that will prevent drainback over a period of time.

With the advent of fuel return lines on later Q-Jet systems, other safety issues were also addressed in the GM fuel systems. The most notable of these is the “check valve inlet filter,” often thought to be a fuel return checkvalve. This is not the case: it is coincidental.

The check valve inlet filter is actually a roll-over fuel spillage prevention device. GM found that in the case of a vehicle rolling over, the fuel tank ends up being higher than the carb. Depending on the attitude of the tank and the fuel level in the tank, it is possible for fuel to flow from the tank, through the lines, through the pump, and right through the carb. You can imagine the possible consequences of being trapped in a rolled-over vehicle with fuel pouring out of the carb and onto the ground… So GM installed a filter in the carb with a spring-loaded check ball. This check ball requires a certain pressure to unseat and allow fuel into the carb – easy for a functional fuel pump to accomplish, but the pressure of gravity-fed fuel from a roll-over cannot unseat the valve and allow fuel flow. Roll-over problem solved.

But the roll-over valves also assisted in solving another problem, as you can now see. The return-style fuel pumps that came into use at the same time as the roll-over issue could not positively prevent the fuel from draining back from the carb due to the internal bleed orifice for the return fuel. The roll-over valves seemed to solve the problem, even though they were not designed to do so.

Yet problems persisted, because many people removed the “restrictive” check valve filters and installed the early-style filters without the valve. Also, if the filter pre-load spring is missing or weak, the check valve filter will not seal and prevent reverse fuel flow.

So what should you do about your hard-starting Q-Jet? Here’s the list:

Check for fuel leakage out the bottom of the carb and repair it if needed. My money is on that your carb won’t leak.

If you have a car without a return fuel line, use a good quality fuel pump with a good check valve. The pump should not allow fuel drainback.

And here is a problem that nothing to do with a fuel level drop. It has to do with accelerator pump compatibility with modern fuels. Many accelerator pumps in the older cars, and a bunch of pumps being sold in brand new rebuild kits, are not compatible with the ethanol in today’s fuel. The result is that the accelerator pump will swell in the pump bore and seize. The outer spring on the pump rod will allow the pump rod to move and appear to function, but the pump is not working. A hot engine will start without the pump working, but when cold, the engine will require excessive cranking to start. The engine has to have a pump shot to start cold. So make sure your accel pump is working and not seized in the bore. I see this commonly.

Use the roll-over valve inlet filters. NAPA part number for the early (short) filter is 23051. The long filters are 23052. Make sure your filter spring is installed.

If these measures fail, you can do 2 more things:

First you can use a needle/seat assembly that does not have the “windows” in the bottom of the seat. This will block the fuel from being able to enter the seat and flow back.

The other thing you can do is to remove the clip that attaches the needle to the float. This will allow the needle to drop down into the seat and seal off the seat any time there is no fuel pressure. Although it does not produce an absolutely positive seal, it does slow down the drainback enough to allow easy starting of the engine.
Logged
bc69
Member
***
Posts: 148



View Profile Email
« Reply #14 on: December 16, 2010, 09:57:43 PM »

Mark,
This was what I was fishing for!  A great thread and one that will be helpful to many!

 Thanks. Brad
Logged
Pages: [1] 2  All Print 
« previous next »
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.19 | SMF © 2013, Simple Machines Valid XHTML 1.0! Valid CSS!
Page created in 0.091 seconds with 17 queries.