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Channing · 03-01-2020, 01:36 PM

How do I determine, or test a carbureted fuel system to determine if it's getting enough fuel?
I have a 1987 Holiday Rambler 7.4 l that will hardly stay running. It has a 4 barrel Rochester Quadrajet. I hear that there's no fuel pressure test for it because it's not a throttle body, but if I was to squirt gasoline into the carburetor while it was running (barley), and it sounded more like it should, and it acted like it was ready to go, for a few seconds, would that be convincing enough to determine that I need to change my fuel pump and fuel filters and possibly fuel lines? And if it didn't make a difference would that tell me that that system is good? Or is that also bad? Is there a better way to test it? It ran great when I bought it, and over a period of about 100 to 150 miles. It started running bad and it's only gotten worse. The choke does not work. Not sure if it was defective when I bought it or not, but judging from all the carbon on the plugs and the black powder coming out of the tailpipe I'm thinking it probably was and it ran fine. The guy I bought it from said he just changed the plugs. I hear that the choke not working won't affect the way it runs only the gas mileage and the carbon build up and it just harder to start when it's cold. Is that true? How can carbon buildup on your plugs not affect the way it runs? I changed the spark plugs and it ran better, not great, but better, and it was a very short period of time, like a few miles. And if I do change the fuel pump, where is it? The manual I have is very contradicting. Is it in the tank? Is it up by the engine? Is there one in the line next to the tank? I do have one up under my engine on the passenger side. Does that mean there's not one in my tank? Or could there still be one in there?

450Donn · 03-01-2020, 01:58 PM

You need to either have it done or buy the tools to do it. A fuel pressure test is actually fairly easy. Add a pressure gage and turn the key to run. Fuel pump should come on and you can see the pressure. A chassis service manual should tell you what pressure range is acceptable.

Skiddy · 03-01-2020, 02:07 PM

an 87 may still have the mechanical fuel pump, you can still check the pressure just have to put a tee in the line. but I would fix the choke first and see how it runs, black plugs is usually too much fuel. choke closing

rodekyll · 03-01-2020, 03:14 PM

It's a long leap from what you describe to a bad fuel pump. To explain -- a mechanical pump lifts or "pulls" fuel, it doesn't pressurize or "push" it. It should give between about two and six pounds of pressure is all -- just enough to overcome the needle valve on the carb float, but not so much that the float rising can't close it. Remember -- the float bowl is vented to atmosphere. That means that downstream of the float there is NO FUEL PUMP OVERPRESSURE AT ALL (there may be accelerator pump pressure but that's different). Fuel through the jets and into the engine is induced by engine vacuum, not fuel line pressure.

A quad should be able to hold together up to about 10#. Overpressurizing the carb doesn't increase the available fuel at WOT, it just blows seals, gaskets, and the float valve, causing air and fuel leaks, and possibly the float bowl overflowing. The significant pump question is flow rate -- how many cc's can it pump per minute, and does that flow rate keep the float bowl at an operational level through WOT?*

It's more likely that if the problem is fuel related, it is in the metering, not the supply. That suggests carburation -- crudded needles or jets, bad accel pump, vacuum leaks, sticking choke, bad adjustments, etc.

But it's no good going after fuel problems until you know your baseline is good. You establish the baseline with a thorough maintenance service -- tune-up, filters, hoses and such. When everything is clean, flowing free, and properly adjusted, then you can look for problems. If you find bad stuff in the course of the service, fix the known offenders before diving into the unknown.

That's all I've got.

*redneck flow test: install 90º ball valve in the fuel line between the pump and the carb. Run WOT. Quickly shut off the valve. Start counting 1-potato, 2-potato . . . until the engine dies.

FIRE UP · 03-01-2020, 03:53 PM

Quote:

Originally Posted by Channing

How do I determine, or test a carbureted fuel system to determine if it's getting enough fuel?
I have a 1987 Holiday Rambler 7.4 l that will hardly stay running. It has a 4 barrel Rochester Quadrajet. I hear that there's no fuel pressure test for it because it's not a throttle body, but if I was to squirt gasoline into the carburetor while it was running (barley), and it sounded more like it should, and it acted like it was ready to go, for a few seconds, would that be convincing enough to determine that I need to change my fuel pump and fuel filters and possibly fuel lines? And if it didn't make a difference would that tell me that that system is good? Or is that also bad? Is there a better way to test it? It ran great when I bought it, and over a period of about 100 to 150 miles. It started running bad and it's only gotten worse. The choke does not work. Not sure if it was defective when I bought it or not, but judging from all the carbon on the plugs and the black powder coming out of the tailpipe I'm thinking it probably was and it ran fine. The guy I bought it from said he just changed the plugs. I hear that the choke not working won't affect the way it runs only the gas mileage and the carbon build up and it just harder to start when it's cold. Is that true? How can carbon buildup on your plugs not affect the way it runs? I changed the spark plugs and it ran better, not great, but better, and it was a very short period of time, like a few miles. And if I do change the fuel pump, where is it? The manual I have is very contradicting. Is it in the tank? Is it up by the engine? Is there one in the line next to the tank? I do have one up under my engine on the passenger side. Does that mean there's not one in my tank? Or could there still be one in there?

Well,
Like Rodekyll says, you need to do some preliminary checks before going crazing on buying fuel pumps etc. Being it's an '87, my memories fading by the minute as I age but, is the 7.4L a 454 GM or, is the 460 Ford? I think the 7.4L was the 454 and the 7.5L was the 460. But again, no memory. Both are great motors with lots of torque and get-up and go. Neither one of them passes by a fuel station though, they automatically just turn in without input from the driver.

Anyway, it sounds as if you have a mechanical fuel pump on or near the engine. A Quadrajet carburetor, contrary to some beliefs, is a great carburetor, as long as it's been gone through, by someone who knows them. They meter fuel just fine on both low and high rpms. They're like a toilet flush when you need the secondary's.

But, if you've just acquired this rig, I'd do what's been suggested and that is, get the basics down first. Find out if the spark plug wires are *leaking* and or need replacement. The distributor cap (I think it still has one) and anything else in the ignition side, may need changing. While you're at it, get a compression test to determine what kind of guts this big boy's putting out. That will also tell you if you've got any one cylinder that's not keeping up its end of the bargain. And, it will tell you the BASIC condition of the engine in question. If you've got good, even compression, for all 8 cylinders, that's a great starting point for efficiency building.

And, as far as the infamous CHOKE is concerned, it's a VITAL part in proper cold engine starting and running. Again, it's one of those things that one needs to know Quads to make sure it's operating correctly. In the old days, when carbs were the masters of the universe before fuel injection was born, naturally asperated engines used carburetors to provide a fuel/air mixture to engines. And, a cold engine does not have what's needed for proper mixing of that fuel/air ratio. So, A CHOKE is needed for a given amount of time, during that *warm up* phase.

There are governing factors that cause the choke to close, and governing factors that cause it to OPEN too. And those factors are COLD and HEAT. Now, not only does that choke provide increased venturi action which, provides a richer mixture for a cold engine but, the mechanism's attached to the choke plate are what's called a *fast idle cam*. During a dead cold start, the correct procedure for STARTING a carbureted engine is, get in, sit down, turn the key on, push your gas pedal to the floor ONE TIME and let up.

What that does is, first, if the *accelerator pump* is working as it should, it will jettison some fuel straight down the carb throat and into the intake manifold, and second, it allows for linkage on the carb to allow for the bi-metal spring in the choke housing to spring the linkage involved, to close the choke plate. And, at the same time, the FAST IDLE CAM will be placed into a proper setting, to allow for a faster idle under choke conditions. The choke will close solid for initial start up. But, upon the engine actually starting, the choke will actually SLIGHTLY open at a pre-set, given amount. It's called the choke *stand-off setting*. Other wise, if the choke stayed fully closed while the engine was running, you'd have an over fuel condition and you'd get that BLUB-BLUB-BLUB-BLUB and a bunch of black smoke. NO GOOD FOR SPARK PLUGS AND YOU'RE WASHING THE CYLNDER WALLS WITH FUEL that will also wash away the oil needed for lubing.

Now, once that cold engine is running and warming up, the bi-metal spring that was used to spring that choke into action, will also reverse its operation, to cause that choke to back off some, and, along with that, the fast idle cam too. And that will lead to a lower engine idle, 'cause the engine is now warm enough to sustain its own, without additional fuel.

And that's WHY you need to get this Quadrajet carb all in sync and working CORRECTLY! If you're gonna own a carb'd gas motorhome, it pays to get that engine working at top efficiency since they eat gas like it's free anyways. A carbureted engine is a very poorly governed engine but, it's what the world had before fuel injection was born. Good luck.
Scott

Ray,IN · 03-03-2020, 09:37 PM

Quote:

Originally Posted by Skiddy

an 87 may still have the mechanical fuel pump, you can still check the pressure just have to put a tee in the line. but I would fix the choke first and see how it runs, black plugs is usually too much fuel. choke closing

My uncle had a fuel pressure gauge for mechanical pumps at his filling station. You removed the line from the carb., pushed in the rubber stopper, pulled the distributor wire. Then another cranked the engine for a minute.
Of course that was measuring static pressure, not while the engine was under load.

Brad247 · 03-03-2020, 10:12 PM

Don't forget to do a flow test to I have had pump pass the pressure test and not the flow test

Dav L · 03-04-2020, 05:50 AM

If the motor starts then it has fuel. If it then runs bad, and the plugs are clean, then the carb is likely partially plugged up or the choke isn't opening up.

If it runs good until you start climbing a hill and pushing the gas pedal down far, then that is indicative that the fuel system isn't keeping up. First place to look is replace the filter.

Then do a fuel pressure test. Put a T in the line at the inlet to the carb and a guage at the other end. You can usually borrow a fuel gauge from most chain auto parts stores. You will want to keep the gauge in sight of you while driving. See if the fuel pressure drops when pulling a hill. If yes, then you have a pump that needs attention.

But, sound more like a dirty carb than a fuel pump from quick read.

alank · 03-04-2020, 09:40 AM

All the advice given is goo sound advice. From what you posted, the “black suet build up” would come from a carb that is letting too much fuel enter the cylinders. Last engine I had that did this had a cracked diaphragm inside the carb that allowed excess fuel to be drawn into the engine. The more it cracked, the more fuel went through and the worse the engine ran. I would be looking at a carb overhaul kit, or rebuilt exchange carb.

FIRE UP · 03-04-2020, 09:48 AM

Hmmmm, no answer from the OP. Are you interested in answers?
Scott

Bob_C · 03-04-2020, 09:57 AM

Sounds to me like the balsa wood float covering has become non existent.....also the 2 soft plugs in the bottom of the bowl are leaking.......have rebuilt so many of those thru the years.......good carb when they're right....

NITEHAWK · 03-04-2020, 09:58 AM

The old Quadrajets had a center threaded rod that holds the air cleaner on to the carb. Tighten the wing nut too much and you distort the aluminum around the choke so much that it will stick closed, usually until warmed up. In the meantime the carb is dumping a lot of raw fuel down into the engine.
Take off the air cleaner, see if the carb sticks when actuated. If so, "percussion tune" the threaded rod just enough to "un"distort the carb throats. In other words, tap the end of the threaded rod with a hammer. Do NOT overtighten the wing nut after doing this.

Roy1 · 03-04-2020, 10:18 AM

You didn’t say how old the gas is? If it is a few years old you may need to replace it with fresh fuel. You also may want to disconnect the fuel line where it connects to the quadrojet using 2 wrenches so you don’t damage the fuel line and replace the filter while the line is disconnected crank the engine with the line in a jar it should fill the jar with gas rapidly.

shootist · 03-06-2020, 05:15 PM

Carb sounds way rich. Maybe now the fuel pump went. Disconnect the line at the carb, hold your finger over it and crank the motor a few seconds. After 5 seconds lift your finger off the line, it should still have pressure . don't squirt yourself in the face. Thats for the mechanical pumps bolt to the front rt side of the block. Electrical low pressure pumps seem to bleed off quickly so just turn on the key and see if you get a decent stream. Black exhaust means the carb is way rich and it will wash out the piston rings in a very short while. You'll have lower compression and burn oil. Much of it may go away after rebuilding the carb. New float, epoxy the mainwell plugs on the quadrajets, no exceptions. Smell the oil dipstick for a gas smell.