Carburetors! What do you know about them?

[Chieftain27]

Originally Posted by yeloduster

I think you might be looking in the wrong direction. As a suggestion, you could get a fuel pressure gauge and mount it using a tee where the gauge is on one leg of the tee the the other goes to the carburetor between the mechanical fuel pump and the carburetor. Leave the engine cover off and go for a drive. Your pressure should be above 5 lbs and less than about 12 lbs.
See what your fuel pressure is when problems occur. If you still have fuel pressure it indicates a problem in the carburetor. If you have no fuel pressure it is a fuel delivery problem.

It is possible that the needle valve in the carburetor is hanging up shutting off the fuel INSIDE the carburetor. The procedure described above should at least separate the carburetor from the fuel delivery system. I would start there.

If you have no pressure then before you try anything else you need to assure that you indeed have a pump in the tank. If there is a pump in the tank there will be at least three wires going into the tank. One wire is for the fuel gauge. The other two will be a hot wire and a ground wire for the pump.

Look close. Somewhere near the tank there may be a connector from the motorhome to the wire(s) going to the tank. Locate the wires.

When the key is turned on the pump should run for about 30 seconds even if the engine is not running. There is circuitry that sends power to the pump. This circuitry monitors the engine oil pressure. After about 30 seconds if the oil pressure is not present or the signal from the sensor is not present the pump shuts down. If this wire is broken, disconnected or the switch is bad the pump will shut down. If you start the engine, go back to listen for the pump it may have shut down before you get there. Get a helper, get under the motorhome and have the helper turn on the ignition but not start the engine. Without engine noise you may be able to hear the pump. Another indicator of an in-tank pump will be a pressure regulator in the fuel line near the tank. If you have inspected all those fuel lines and didn't see a pressure regulator back there you may not have an electric pump.

If no pump noise you have to figure out if you ever get power to the pump. Fixing no power to the pump might be easier than dropping the tank!

See THIS link to Vintage RV forum. It shows a drawing with the location of the fuel pressure sensor. You might join this forum. It has lots of info that might be helpful for your older motorhome including many threads about the electric fuel pump.

The electric pump has 2 functions. It reduces the possibility of vapor lock and it provides enough fuel during long steep grades where the engine is working hard for an extended length of time. For the vapor lock function to work there is (on my 1988 anyway) a fuel return line from the mechanical fuel pump to the fuel tank. The correct fuel pump will have a small hose connection that is connected to the return line. Vapor lock may still occur if you have the wrong fuel pump that does not have this connection or the return line is plugged.

Sorry for the long post!

For starters i appreciate your time in writing an informative post…no need for apologies.
I will run the test as suggested and attach a fuel pressure gauge to ensure it is a faulty In tank fuel pump.
While under the rig today I was able to use my phone and record a video of what it looks like above the tank and there I was able to see 3 wires going directly into the center of the tank. This leads me to believe I do have an in tank fuel pump. Also right by the rear right wheel is my pressure regulator. I just did a quick google search and confirmed that I do have one. I thought that’s what that thing was but I wasn’t sure of it.
My question is if I hear no pump noise does that mean the pump is not working? How is my rig able to start if the in tank fuel pump is not working? As you said the in tank fuel pump is to prevent vapor lock and assist with steep hills. Is it really just air flow that pulls the gas from the tank all the way to the carb?
I will check my mechanical fuel pump also for the little return hose. I know it has three connections but I never paid the third connection any mind.

I will run another test tomorrow just to be SURE I can or cannot hear it. This will be my third attempt. My first attempt I had my wife assist and we turned the rig on almost right away(could be why I didn’t hear anything) and my second attempt I was running from key to where my fuel cap was with the cap off. Tomorrow I will work with assistance.

Last question: what do you mean when you say needle valve in the carburetor is hanging up shutting off the fuel inside the carb? What is this needle? I’m a learning beginner with carburetors and haven’t heard about this.

[Jyrocharlie]

https://www.youtube.com/watch?v=NQuaayJNhew

[yeloduster]

My name is Larry by the way. If we are going to keep meeting like this it would be nice to be on a first name basis.

I will tackle your questions one by one.

Question 1:

My question is if I hear no pump noise does that mean the pump is not working?

You should be able to hear the fuel pump. A better test as someone else suggested is to disconnect the fuel line, turn on the key and see if it pumps fuel. Caution, if it is pumping it will pump a LOT of fuel.. When I have done this I used gloves and had the fuel line directed into a container to catch the fuel. Use your helper for this test!

Question 2:

How is my rig able to start if the in tank fuel pump is not working?

In the carburetor is a cavity called a float bowl. This cavity contains gasoline the engine uses to get started.

Question 3:

Is it really just air flow that pulls the gas from the tank all the way to the carb?

In the absence of a working fuel pump in the tank the mechanical fuel pump causes a vacuum in the fuel line from the tank sucking fuel from the tank and pushing it to the carburetor. As a percentage very few carbureted vehicles has an electric fuel pump in the tank. They rely solely on the mechanical pump on the engine. If the vehicle has set for an extended period of time, weeks not days, eventually the fuel in the carburetor will evaporate leaving the float bowl empty. If you start the engine when the float bowl is empty you must crank the engine for a long time as the mechanical pump works at cranking speed drawing fuel all the way from the tank 20' or more away. Eventually the float bowl is refilled and the engine starts. If you have an in-tank electric pump that is working fuel is pumped to the engine much faster when starting after a prolonged idle time.

Question 4:

What do you mean when you say needle valve in the carburetor is hanging up shutting off the fuel inside the carb? What is this needle?

That is 2 questions but one answer suffices. The float bowl has floating on the fuel surface a float. The float is attached to a valve that keeps the gasoline in the bowl at the proper level. Too much fuel and the bowl over flows flooding the engine and maybe pouring gasoline out of the carburetor onto the engine. Too little fuel and the engine starves for fuel and doesn't run or maybe doesn't run well.

This valve is called a needle valve. As the float rises with the fuel it presses a tapered pin (needle) up into an orifice (seat). This arrangement is commonly called a "needle and seat" or a "needle valve". The pin and orifice are matched so that when the taper of the pin seats against the orifice it shuts off fuel flow even though there is pressure from the pumps.

Sometimes there is malfunction in the carburetor that causes the needle to get stuck in the seat. If this occurs the fuel to the float bowl is shut off or severely limited even if the fuel pump(s) are working. This condition can be intermittent. Sometimes working OK and other times not.

[Jyrocharlie]

Originally Posted by Chieftain27

Makes sense. I’ve thrown away the ideal of replacing the carb already. Thanks to everyone here.

I’m trying to figure out what you exactly mean with the next part you mentioned...
What will close on the carb or what is suppose to close on the carb when I shut the engine off? Quote: “ The end of it adjust the idle speed, not the screw on the baseplate.” <— what do you mean here?
I understand everything else for checking for vac leaks. I actually found a giant hole in one of the 3/4 inch vac hose. Its the biggest hose on the carb at the top, any idea what it’s for?

Look at this picture, it should be your carb. the solenoid is attached. that is where you adjust the idle. it has a few names, anti diesel solenoid, Idle stop solenoid, ect. When you turn the key on, this solenoid extends and touches the carb linkage, sometimes you have to hit the gas once for it to extend. on the end is the idle adjustment once it's extended. it retracts when you turn off the ignition, closing the carb butterflys completely.

If that hose with the hole in it is on the side of the air cleaner, it goes to the inlet of one of your EGR pumps that inject air into the exhaust manifolds, some 454's have 2 air pumps to burn left over gas in the exhaust. Those are very sensitive to belt tension and you can ruin them if the belts are too tight.

The video I posted shows how to make the pump in the tank run anytime you have the ignition on, there is no timer on your 86 to make the pump run when you turn the ignition on, it's strictly off a oil pressure switch.

[Jyrocharlie]

on a quadrajet without the solenoid, you adjust the idle as it shows in this picture: With the solenoid, this screw is adjusted to where the butterflys barely close but don't close far enough that they hang a bit when you touch the throttle linkage to open it.

[T12]

The fuel pump is separate from the carb. It's sole purpose is to pump gas to the carb. The carb has a pump built into it. As you push on the gas pedal the linkage on the carb pushes this pump to inject gas directly into the throat of the carb. This compensates for the fact that the gas and air are sucked into the engine as opposed to being injected. You can tell when they go bad you can get hesitation or your engine can stall when cold. Usually if you run your engine often these pumps stay lubricated and will not dry out. But all gaskets and rubber can dry out when gas evaporates from carb when it is not used. A rebuild is usually cheaper than a new carb. I have one on my 84 Pace Arrow on a P30 with thr 454

[Ryqster]

"Makes sense. I’ve thrown away the ideal of replacing the carb already. Thanks to everyone here."

Ok, so I am going to do this without stepping on...well you know. I do not claim to be an expert on many things however, QuadraJet carbs are and have always been problematic, for all of the reasons described here in this thread. Yes I used to race a vehicle that had one and yes I modified this piece of junk until i took it off and beat it with a hammer till no part was larger than a nickel. I immediately discarded the manifold so I would never be tempted to use it again. Purchased a replacement Edelbrock manifold and carb (back in the day they were Carter Carb's) and never looked back. Tuning was a snap, jet kits were available and the car never ran better. There is so many problems that can pop up with the Q-Jet they are hard to count. Fuel Injection will ultimately cost you more although once you learn the programming its pretty easy. I think you said it was a 1987? most states probably have exempted this from testing. Get rid of the air pumps, cat, if so equipped...some weren't keep the crankcase vent stuff (PCV) and you will find this will solve any carb problems you have. If cost is a factor, no easy fix for a bad Q-Jet. Holley/Dominator makes a direct replacement but these are not the best replacement carb for a motorhome IMHO. Dual exhaust and GOOD headers can help on the fuel economy side. BTW the intake manifold replacement on a 454 is pretty easy if you have mechanical skills. If not try to get some friendly help. Good luck and now I will shut up!

[NOTAGT]

"Carburetors man, thats what life is all about" Phanton of the Paradise

[RM Art]

I've got eight great running engines:

- Two 1977, 255 HP, 350 cid marine engines with QJ carbs

- One 1985, 325 HP 350 cid truck engine with QJ carb

- One 1989, 260 hp, 350 cid marine engine with QJ carb

- One 1967, 400 HP, 430 cid muscle car engine with a QJ carb

- Two newer V6 engines in RV's with FI

- One 1995, 280 HP, 454 with TBI

[LargeMarge]

Your engine has two 'fuel pumps'.

One, integral to the carburetor, is the ACCELERATOR pump.
It dumps extra fuel into the engine during... acceleration.

The other fuel pump is usually on your engine block, a mechanical fuel transfer designed to... TRANSFER fuel from the fuel tank to the carburetor.
A mechanical fuel pump operates only during the spinning of major components such as the crankshaft and camshaft.
Any time your engine is running, fuel moves.
Any time the engine stops, the fuel transfer stops.

On your vehicle, pressing the throttle pedal moves a mechanical link -- a cable -- to the carburetor.
At the carburetor, a tiny reservoir called a 'bowl' holds a couple-three spoonsful of fuel.

Pressing the throttle pedal causes the tiny carburetor-mounted ACCELERATOR pump to squirt a measured amount of fuel into the carburetor throat.
On a non-running engine:
* Repeated pressing of the throttle pedal drains the bowl.
* Repeated pressing of the throttle pedal pumps unneeded fuel into the intake manifold.

CAUTION:
* Dumping fuel into a non-running engine can cause:
a -- a fireball out the carburetor next time the engine cranks during a start.
b -- a backfire through the exhaust system... potentially rupturing the muffler.

* If the engine is not started, the excess fuel can drain past open intake valves into the engine cylinders, and can ooze past the piston rings into the main oil reservoir, diluting the engine oil and reducing its lubrication ability.
The engine oil lubricates the bearings on the crankshaft, rods, and camshaft.
The engine oil lubricates the valves.
The engine oil also transfers heat.
Reducing its efficiency is a bad idea.

[NeilV]

Yes Indeed, be mindful that pumping the gas pedal without starting the engine can result in dilution of your motor oil from the accelerator pump squirting gasoline down the intake mainifold which will drip down through any open or leaky intake valve and then wash down through the piston rings into the oil pan. Also watch out for ethanol deteriorating the rubber diaphragm on any mechanical fuel pumps run off a cam on the engine since once the diaphragm starts to leak, gasoline can end up pouring into the crankcase again diluting the motor oil and causing extreme engine wear.


I still have my exhaust gas analyzer from when I used to fine tune Rochester Quadrajets.

[Chieftain27]

Originally Posted by yeloduster

My name is Larry by the way. If we are going to keep meeting like this it would be nice to be on a first name basis.

I will tackle your questions one by one.

Question 1:

My question is if I hear no pump noise does that mean the pump is not working?

You should be able to hear the fuel pump. A better test as someone else suggested is to disconnect the fuel line, turn on the key and see if it pumps fuel. Caution, if it is pumping it will pump a LOT of fuel.. When I have done this I used gloves and had the fuel line directed into a container to catch the fuel. Use your helper for this test!

Question 2:

How is my rig able to start if the in tank fuel pump is not working?

In the carburetor is a cavity called a float bowl. This cavity contains gasoline the engine uses to get started.

Question 3:

Is it really just air flow that pulls the gas from the tank all the way to the carb?

In the absence of a working fuel pump in the tank the mechanical fuel pump causes a vacuum in the fuel line from the tank sucking fuel from the tank and pushing it to the carburetor. As a percentage very few carbureted vehicles has an electric fuel pump in the tank. They rely solely on the mechanical pump on the engine. If the vehicle has set for an extended period of time, weeks not days, eventually the fuel in the carburetor will evaporate leaving the float bowl empty. If you start the engine when the float bowl is empty you must crank the engine for a long time as the mechanical pump works at cranking speed drawing fuel all the way from the tank 20' or more away. Eventually the float bowl is refilled and the engine starts. If you have an in-tank electric pump that is working fuel is pumped to the engine much faster when starting after a prolonged idle time.

Question 4:

What do you mean when you say needle valve in the carburetor is hanging up shutting off the fuel inside the carb? What is this needle?

That is 2 questions but one answer suffices. The float bowl has floating on the fuel surface a float. The float is attached to a valve that keeps the gasoline in the bowl at the proper level. Too much fuel and the bowl over flows flooding the engine and maybe pouring gasoline out of the carburetor onto the engine. Too little fuel and the engine starves for fuel and doesn't run or maybe doesn't run well.

This valve is called a needle valve. As the float rises with the fuel it presses a tapered pin (needle) up into an orifice (seat). This arrangement is commonly called a "needle and seat" or a "needle valve". The pin and orifice are matched so that when the taper of the pin seats against the orifice it shuts off fuel flow even though there is pressure from the pumps.

Sometimes there is malfunction in the carburetor that causes the needle to get stuck in the seat. If this occurs the fuel to the float bowl is shut off or severely limited even if the fuel pump(s) are working. This condition can be intermittent. Sometimes working OK and other times not.

Thanks Larry.
The names Preston.
I swapped out the in tank fuel pump today.
I just decided to start there as since having the rig I’ve always wanted to.
The screen/filter was torn so good thing I did but now I still got no power to the pump.

[yeloduster]

Hi Preston,

Glad you changed the pump. The strainer on the end of the pump is pretty important. It is possible that a piece of the strainer gets sucked onto the end of the pump inlet restricting flow sometimes and not other times. You may have fixed it.

If it isn't fixed....

Originally Posted by Jyrocharlie

there is no timer on your 86 to make the pump run when you turn the ignition on, it's strictly off a oil pressure switch.

Jyrocharlie says that there is no fuel pump timer on your '86. He may be right. If so the electric fuel pump won't start until you have enough RPM in the engine to build oil pressure. That probably means the electric fuel pump starts right after the engine starts.

You probably should check for power at the pump with a voltmeter with the engine running. Don't depend on your ears.

If you have assured that the pump has power I think you should install a fuel pressure gauge between the mechanical fuel pump and the carburetor. Drive it without the engine cover. Make note of the pressure while idling, while driving when things seem to work OK and when things go bonkers. It will help separate the fuel delivery system from the carburetor. If the pressure remains the same or nearly the same it is in the carburetor. If the pressure goes down a lot (50%) or so when driving and/or goes really low or near zero when things act up you it is fuel delivery system.

I'm waiting for your report!

[clifford j]

Note,,,Last thing Preston said he has NO ! power at the pump. Now he has to chase the electrical feed circuit, Good place to start is the Pressure switch in the oil galley. Someone mentioned he has a switch that operated and sends power to the gas tank pump soon as oil pressure builds. The picture supplied shows the switch is next to the sender for the oil pressure indicator.

CLIFFORD