Carburetors! What do you know about them?

[Soppy]

You could possibly have a pin hole in your fuel line from the tank to the fuel pump.It would cause fuel starvation if your electric fuel pump is not working.

[LETMGROW]

Originally Posted by clifford j

Starving for fuel, are there fuel filters in the supply line that may have blockage. Is there a metal case fuel filter screwed directly to the carb. Is there an inline filter back at the tank. A fuel pressure gauge at the carb might prove lack of fuel pressure when the engine stumbles.

CLIFFORD

Clifford is on the right track IMO. I believe you have a fuel delivery issue to the carburetor rather than a carburetor problem. I'd hate to see you buy another carburetor and everything which would go with it then end up with the same problem.
First off would be changing the filters. The filter in the carburetor inlet can be cut with a sharp knife. I would split it and look for dirt or debris. The inline filter may be metal cased or plastic. If it's metal take the filter off carefully trying not to spill fuel out of it as much as possible. Then drain the filter from the inlet end into a clean bowl and look for dirt, debris and water droplets. There may be another filter you don't know about. This one is inside the gas tank and attaches to the fuel feed line. I have seen these plugged nearly solid to the point an engine would run at low speed but the engine would starve for fuel at higher RPM's causing it to run much the way you describe.
Another possibility is you may have a boost pump in the gas tank which is powered by 12V. when the engine is starting and or running. If you do have a boost pump it may be working intermittently or even not at all. You will have to crawl under the rig and listen for the pump or check the wiring harness leading to the fuel tank sending unit. If there is a pump in the tank there will be several (3 or 4) wires on the sending unit as opposed to just a ground and 12V. wire. Also check the gas line from the tank for a fuel pump inline attached to the chassis.
Rubber fuel lines can become deteriorated and collapse when a vacuum is present from the mechanical fuel pump trying to draw fuel from the tank. Replacing any rubber line would be good maintenance of an older vehicle anyway and would eliminate any possibility a line is causing your problem.
Many auto supply stores have loaner tools you could borrow. Try to find a fuel pressure gauge. If you can find one it would be a good idea (this was mentioned before) to attach it to the carb. feed line and monitor the fuel pressure. Be sure you use the correct adapters and you have no leakage when using this gauge. I'm not sure what actual the specs are for your vehicle but I would like to see pressure around 5-6 PSI with the engine running.
Anyone is welcome to chime in and add to or correct any of this information or ask questions.
Lynn

[arcaguy]

Most if not all Quadrajets have a couple of things that should be done to them to make them work properly if they are old. The first is to put some gray epoxy over the I think it's2 but may be 4 stake in plugs in the carb body. These loosen up and leak over time. The other is the throttle plate shaft bores in the base plate wear and let air leak past them. There are kits available to bore this out and put in brass bushings. I'm sure it you look on youtube someone has a video on this. Properly tuned and upgraded Quadrajets are fine carburetors, better than Holley for the street. Quadrajet carburetors are a spread bore carburetor with small primary large secondary bores type of carburetor where Holley's are a square bore, meaning all 4 bores are the same size. That said there are a bunch of different models of Quadrajet carbs and you need to know which one you have in order to work on it correctly. There is a book on Rochester Quadrajets carbs ISBN 1-932494-18-9. You might want to pick up a copy. Good luck.

[Chieftain27]

Originally Posted by LETMGROW

Clifford is on the right track IMO. I believe you have a fuel delivery issue to the carburetor rather than a carburetor problem. I'd hate to see you buy another carburetor and everything which would go with it then end up with the same problem.
First off would be changing the filters. The filter in the carburetor inlet can be cut with a sharp knife. I would split it and look for dirt or debris. The inline filter may be metal cased or plastic. If it's metal take the filter off carefully trying not to spill fuel out of it as much as possible. Then drain the filter from the inlet end into a clean bowl and look for dirt, debris and water droplets. There may be another filter you don't know about. This one is inside the gas tank and attaches to the fuel feed line. I have seen these plugged nearly solid to the point an engine would run at low speed but the engine would starve for fuel at higher RPM's causing it to run much the way you describe.
Another possibility is you may have a boost pump in the gas tank which is powered by 12V. when the engine is starting and or running. If you do have a boost pump it may be working intermittently or even not at all. You will have to crawl under the rig and listen for the pump or check the wiring harness leading to the fuel tank sending unit. If there is a pump in the tank there will be several (3 or 4) wires on the sending unit as opposed to just a ground and 12V. wire. Also check the gas line from the tank for a fuel pump inline attached to the chassis.
Rubber fuel lines can become deteriorated and collapse when a vacuum is present from the mechanical fuel pump trying to draw fuel from the tank. Replacing any rubber line would be good maintenance of an older vehicle anyway and would eliminate any possibility a line is causing your problem.
Many auto supply stores have loaner tools you could borrow. Try to find a fuel pressure gauge. If you can find one it would be a good idea (this was mentioned before) to attach it to the carb. feed line and monitor the fuel pressure. Be sure you use the correct adapters and you have no leakage when using this gauge. I'm not sure what actual the specs are for your vehicle but I would like to see pressure around 5-6 PSI with the engine running.
Anyone is welcome to chime in and add to or correct any of this information or ask questions.
Lynn

Okay I’m running down the list...
I found a pretty ugly hose by my fuel filter under the rig. I replaced it with 3/8 vs the 1/2 as that’s all I had on hand. I will be going to the auto parts store for brand new hoses, fuel filter and carb fuel filter. I believe when I had my carb rebuilt they never changed the filter in it because I tried to gain access to it and the fuel line started to bend! So I stoped and checked for leaks and I’m good so I have to get a repair kit for that so I can actually inspect and change the carb fuel filter.
Now I went under the rig searching for a fuel pump and even put my ear to the gas tank with my wife turning it on and I canning locate one or hear one. I even opened the gas cap and put my ear to it and I do not hear one. The rig was starting up no problem every time.
Does this mean I do not have a in tank fuel pump?

[clifford j]

If there is a fuel pump in the tank and it's GOOD you can remove the hose coming from the tank and have your wife try to start the rig, you should see fuel squirt , (be careful) . Not sure about your rig, but a tank that is not vented properly can experience a vacuum which can prevent gas flow. Some fuel caps are vented some not, depends on the environmental system your rig has. Just a thought.

CLIFFORD

[Soppy]

Originally Posted by Chieftain27

Okay I’m running down the list...
I found a pretty ugly hose by my fuel filter under the rig. I replaced it with 3/8 vs the 1/2 as that’s all I had on hand. I will be going to the auto parts store for brand new hoses, fuel filter and carb fuel filter. I believe when I had my carb rebuilt they never changed the filter in it because I tried to gain access to it and the fuel line started to bend! So I stoped and checked for leaks and I’m good so I have to get a repair kit for that so I can actually inspect and change the carb fuel filter.
Now I went under the rig searching for a fuel pump and even put my ear to the gas tank with my wife turning it on and I canning locate one or hear one. I even opened the gas cap and put my ear to it and I do not hear one. The rig was starting up no problem every time.
Does this mean I do not have a in tank fuel pump?

My 1977 did not have a fuel pump in the tank or along the frame. If it sat for a couple of weeks , it would not start due to lack of fuel. If i poured gas in the card it would start and run just fine. I connected an outboard motor tank to the pump and it would work just fine. I never leaves the yard so I didn't bother looking any furthur.

[Jyrocharlie]

It's likely, the pump in your tank has a broken gas line from the pump to the outlet on the tank. The pump will still pump some but without much pressure. When your gas level gets below where the line inside the tank is broken, the fuel pump on the engine quits because it's sucking air from the broken hose inside the tank. you can't fix a carb that doesn't get fuel to it.

As for the engine not shutting off, there is a solenoid at the carb linkage that allows the carb to completely close when you shut the key off. The end of it adjust the idle speed, not the screw on the baseplate. If the solenoid's working and it still runs when you shut it off, listen for that vacuum leak, get a squirt bottle of water and spray it around the hoses, you can hear the engine stumble when it sucks in some water.

[yeloduster]

Originally Posted by Jyrocharlie

It's likely, the pump in your tank has a broken gas line from the pump to the outlet on the tank. The pump will still pump some but without much pressure. When your gas level gets below where the line inside the tank is broken, the fuel pump on the engine quits because it's sucking air from the broken hose inside the tank. you can't fix a carb that doesn't get fuel to it.

I respectively disagree. The OP hasn't affirmed that he has an in-tank fuel pump but if he has an in-tank fuel pump the pump is mounted on the bottom end of the tube. If the tube is broken it will pump no gas. The engine would never run.

[Jyrocharlie]

I had a 86 pace arrow. I chased a problem for weeks, I removed the tank to replace the pump. the pump was working, (you could hear it) but the line was rotted and split about 6" above the pump. The line wasn't broken into, it was split from age and ethanol. Most of the longer gas rv's have that pump. it's a throttle body injection pump ( 18lb) with a holley regulator mounted next to the tank. That pump runs when the oil pressure switch has oil pressure. it's a hose, a 34 year old hose soaked in gasoline.

[Jyrocharlie]

From another source: https://www.classicwinnebagos.com/fo...p?topic=8041.0 As indicated in the Chevy MH Service Guide (available here in the free manuals section), Chevy did not start installing an electric fuel pump and frame rail regulator until 1985 1/2. That was an in tank version. They used a 1/2" fuel line. The electric pump was used to push the fuel up to the mechanical pump on the engine. The regulator insured the pressure was low enough to not damage the mechanical pump. The mechanical pump was retained until 1990 when they converted to a TBI system which only uses a electric pump.

Chevy did a study that resulted in the 1985 1/2 change. They also put together recommendations on how to add an external pump and regulator for pre 1985 1/2 rigs. All of this information and how to data is contained in Appendix 7-7 of the Chevy MH Service Guide.

Dave

[yeloduster]

Quote:
Originally Posted by Jyrocharlie View Post
It's likely, the pump in your tank has a broken gas line from the pump to the outlet on the tank. The pump will still pump some but without much pressure. When your gas level gets below where the line inside the tank is broken, the fuel pump on the engine quits because it's sucking air from the broken hose inside the tank. you can't fix a carb that doesn't get fuel to it.

Sorry Dave, I misread your post. I thought you were saying that the pump was at the top of the tank. I agree that if there is a break in the line between the pump and the outlet to the tank that the pump would run but with reduced pressure. In the scenario you propose, when the fuel get below the break the mechanical pump could suck air.

[Chieftain27]

Originally Posted by yeloduster

Quote:
Originally Posted by Jyrocharlie View Post
It's likely, the pump in your tank has a broken gas line from the pump to the outlet on the tank. The pump will still pump some but without much pressure. When your gas level gets below where the line inside the tank is broken, the fuel pump on the engine quits because it's sucking air from the broken hose inside the tank. you can't fix a carb that doesn't get fuel to it.

Sorry Dave, I misread your post. I thought you were saying that the pump was at the top of the tank. I agree that if there is a break in the line between the pump and the outlet to the tank that the pump would run but with reduced pressure. In the scenario you propose, when the fuel get below the break the mechanical pump could suck air.

Question: why can I not hear my pump in my tank turn on or make any kind of noise? It fires up every time first try. I even made it to the parts store yesterday and a few other stores with no problem.
I replaced my fuel filter in between engine and gas tank. I ripped apart the filter to inspect the insides and it had a bit of debris in it.
I inspected my lines and every inch of them...they are good.
My next step is the filter in the carb and then the fuel tank pump...
Also has anyone ever just cut a hole in their floor vs dropping the tank? I’m considering this option

[Chieftain27]

Originally Posted by Jyrocharlie

It's likely, the pump in your tank has a broken gas line from the pump to the outlet on the tank. The pump will still pump some but without much pressure. When your gas level gets below where the line inside the tank is broken, the fuel pump on the engine quits because it's sucking air from the broken hose inside the tank. you can't fix a carb that doesn't get fuel to it.

As for the engine not shutting off, there is a solenoid at the carb linkage that allows the carb to completely close when you shut the key off. The end of it adjust the idle speed, not the screw on the baseplate. If the solenoid's working and it still runs when you shut it off, listen for that vacuum leak, get a squirt bottle of water and spray it around the hoses, you can hear the engine stumble when it sucks in some water.

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?

[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!