The Stovebolt Site Forums The Shop Area Engine Shop Fuel Pumps and Other Nonsense

Alright, I want to tie some strings on that previous thread about a possible wiped fuel pump cam.

I figured it would be easier to start a new thread and give a recap.

1950 3100 was running fine with a newly rebuilt carburetor and a year old Carquest fuel pump. It ran fine and showed no indications of trouble for a couple hundred miles or so. It then conked out and couldn't be restarted.

Tests showed the carburetor wasn't getting any fuel. A brand new Carquest fuel pump was acquired and it too failed to pump any fuel to the carburetor. I assumed that the fuel pump cam had worn down to the point where it wouldn't operate the pumps.

After all, what are the odds that a brand new pump wouldn't pump a drop right out of the box?

Well, I installed the old original style pump and it at least pumped some gas but was too raggedy to be relied upon so I sent it to Then and Now Automotive for a rebuild.

Notice the photos of the cam and the failed pump. The cam is varnished up on one side and worn on the other. No look at the wear pattern on the pump arm.

The stupid thing was assembled incorrectly. The three pieces of steel that make up the arm were riveted together unevenly so that only one of the ~1/16" wide strips of metal made contact with the cam.

The failed one has the golden lid. The new one looks like a decent unit but wouldn't pump a drop.

The arms of these pumps don't move very much at all to work the diaphragm. I figure if it's off by a sixteenth of an inch or so it won't work the diaphragm enough to pump enough fuel or even pump any at all.

So, the rebuilt unit appears to be an original unit re-manufactured by Airtex a long time ago or possibly a "new" unit made by Airtex a long time ago. In any event, Then & Now did a great job on the rebuild. $52 for a rebuild kit and I do the work, or $63 more for them to do it. Seemed like a bargain to me. It's cleaned, straightened, new pins, valves, filter, and clear coated.

From this experience I'll advise anyone considering purchasing a new fuel pump, particularly a Carquest fuel pump, to forgo that choice and have the fuel pump you already own, rebuilt.

After installing the glass bowl pump, the truck started right up and ran fine, except...

Whenever the throttle is goosed, the engine bogs and will backfire.

The bogging was noticed at the same time the fuel pump began to die. I put it down to lack of fuel from a failing pump. Obviously that's not the case so what the heck is going on now?

Points, cap and rotor are all good, Jon G condenser is installed. No work has been done to alter the timing or anything like that. Vacuum advance works and dizzy isn't loose.

The only thing that comes to mind is a bad accelerator pump. That seems unlikely as the kit came from Jon H and I was meticulous in the carb's re-assembly.

ANy thoughts on what I need to look for now? As we've seen, new parts don't always mean good parts.

Is there no fuel in the carburetor bowl? You can climb up in the engine bay, goose the throttle and look down the throat for a squirt of gas. If it squirts, you have another issue non-fuel related. If it doesn't squirt, pinch your soft fuel line closed were it comes off the hard line at the tank. Remove the soft line from the hard line which heads toward fuel pump, and attach the hose from a small plastic outboard motor tank and use that for your fuel supply. I don't care if you have a brand new tank or not. We are testing, not guessing. (No offense to you, I often find myself trying to take shortcuts when troubleshooting, so I am as much talking to myself as much as I am talking to you. )
We need to absolutely rule out fuel supply before we can move to anything else which may be wrong.

Yes, the bowl has fuel I worked the throttle this morning while looking down the throat (engine off) and it gave healthy squirts.

I had other stuff I had to do today so I'll investigate further tomorrow with the engine running.

The advantage of the original glass bowl pump is that you can see the clean fuel with no contaminants at the pump.

I have no soft lines in the system. It's all steel with threaded fittings from stem to stern.

Just a few degrees late on ignition timing can cause the hesitation/bog/backfire symptom pretty often. Try turning the distributor housing a little bit counterclockwise (opposite the rotation of the rotor). I've solved similar problems by simply advancing the initial timing 2 or 3 degrees.
Jerry

Does the center hash mark on the gauge at the base of the dizzy represent 0* or theoretical TDC of #1 piston (assuming the engine is timed with the marble embedded in the flywheel)?

So far advancing the timing hasn't resulted in the loss of the "bog". If the throttle is increased slowly, it will achieve the higher rev. If floored, the engine nearly conks out and sometimes backfires.

Maybe I missed it, but a Rochester carburetor?

That's great. you can eliminate fuel delivery issues.
Do what HRL suggested with the timing.

No , this is a recently rebuilt Carter from Jon H.

So far no change.

Another cause of a bog and stumble on acceleration is a slightly lean transition when going from the idle to high speed fuel circuit in the carburetor. Try turning the idle mixture screw about 1/8 turn CCW to richen up the part throttle transition. Is there a good reason to suddenly snap the throttle from idle to wide open?
Jerry

My info is based on a GMC engine, but I'm pretty sure they are the same as your Chevy.

TDC on the flywheel is the vertical line between the U|C.

The "ball" is where the manufacturer wants timing to be set at a very low RPM with the vacuum advance disabled.

Loosen the clamp bolt that secures the plate with the "center hash mark", set the hash mark to zero and tighten the clamp bolt.

In the case of my GMC, the ball is 5 degrees before TDC. GMC says to reduce engine idle speed to 350 RPM when setting timing because mechanical (centrifugal) advance begins at 400 RPM. Loosen the clamp bolt that holds the vacuum advance to the distributor and use a timing light to set timing to the ball. When timing is set, tighten the clamp bolt and check that timing is still correct.

Once timing is set, adjust idle speed back to spec and re-connect the vacuum advance.

I have found that my engine performs better with an additional 4 degrees of advance. By using the octane selector (GMC calls it the "Manual Advance") I can dial in a few extra degrees of advance without the use of a timing light. Just loosen the bolt, move the mark towards the "A" and retighten the bolt. The hash marks are actually about 1.5 degrees apart. If you don't like the way the truck runs at the new setting you can always move it back to the zero mark without the need for a timing light and timing will be back at factory.

Its a good possibility your float level is a little low.

George

This happens when trying to accelerate from a dead stop.

Carb was recently meticulously rebuilt and functioned perfectly when installed for several hundred miles.

Unfortunately my flywheel is new and has no ball stuck in it.

How did you clean the carburetor before installing the rebuild kit? The fact that it ran well for a while tells me some crud that was not removed properly prior to reassembly had gotten dislodged and is clogging up a passageway somewhere. Unless every hole, passage, and assembly is pristine clean, installing a kit is futile. Soften the deposits however you choose (Jon recommends boiling in a solution of Dawn dishwashing liquid and water) and then use massive of volumes of high pressure air to blow the passages clean. If you aren't willing to do things that way, you're wasting your time "rebuilding" the carb.
Jerry

Basically as you describe. High pressure air, carb cleaner soak, soapy water, etc. I'd be very surprised if any dirt or crud escaped notice.

So advancing and retarding the timing hasn't made any significant difference.

When the throttle is pressed, the engine nearly dies and backfires. When the pedal is lifted slightly it will catch its breath and speed up but still popping backfire.

Would fuel starvation result in backfires? Wouldn't it just bog and stall out or wait until the fuel supply catches up?

BTW, I just rechecked the points, cap, rotor and everything is perfect. Points look like new and still set at .022

Lean mixtures backfire- - - -usually through the carburetor, but occasionally there will be a window-rattling boom from the tailpipe.
Jerry

Fuel starvation can be a problem. Essentially backfires can be caused by improper fuel/air ratio, bad timing or a spark plug that for one reason or another isn't firing as it should...not in that order nor in any particular order. Bottom line is you're getting unburned fuel in your exhaust manifold (or even in the pipe leading away) and it is being ignited at the wrong time and in the wrong method. I've even seen a vacuum leak cause backfiring.

I once saw the complete explosion of a muffler. Blew the end of it smack off.

I once blew the muffler of a Model A completely apart. I'd been cranking on it without the ignition on and when I turned the key, it split the longitudinal seam, unrolling the can and blowing the baffles to the back of the muffler. The only thing still attached was the front and rear ends at a small part of the perimeter of each end. I threw the baffles away, wired the can back together and fired up the engine.

I had some backfires when I got my 38 on the road. It split the muffler wide open. I squeezed it back together and used old license plates screwed down to secure it.
The cause was the wire lead to the condenser was sometimes shorting out on the distributor.

I don't know if this will do you any good but we test a fuel pump with the engine running and the carb hooked to an auxillary fuel supply. Ours is a gallon can with an electric pump attached. A fuel pressure gauge is attached to the pump output line and the auxillary tank to the carb. If pressure okay the outlet line is held over a catch can to check fuel flow. We also have a "limp home" set up with an air tank, regulator, and small fuel tank. The fuel tank can be pressurized from 2.5# to 150#s. This will cover most carb and fuel injected cars. If it messes up while on this set up it IS the carb.

Try putting a vacuum gauge on it and see how where it's at. A vacuum leak could cause a lean condition.

Hi again everybody. I've been busy with non-car related things so haven't had a chance to do any more investigating until today.

Coilover, your suggestion makes sense and I may have to buy an electric pump and make a setup like that.

Anyway, the fuel pump appears to be providing enough fuel. Acc pump gives healthy squirts when goosed.

Whne the engine starts and idles normally, goosing the throttle induces a bog to near stalling until the throttle is let off. The engine then resumes "normal" operation and begins to accelerate if the throttle is gradually pressed.

While peering into the carburetor while running it looks like the engine is getting plenty of fuel. in fact, when the bog/near stall/backfire occurs, there seems to be plenty of fuel spraying into the throat from the jets.

Can the engine be getting too much fuel to cause a flood? Would a flood situation also cause the problem I'm experiencing?

In case you're wondering, the bog situation occurs during normal acceleration while driving, not just in the garage when the throttle is opened up.

FYI, newly rebuilt mechanical glass bowl fuel pump is installed.

Make sure your vacuum advance is working properly.

Ditto on the vacuum advance.

Also, how far out is the idle mixture screw?

Adjusting for too rich an idle mixture will cause this exact symptom. If your engine is not broken in, try 1 1/2 turns, engine with 1000 + miles but not using oil, 1 to 1 1/4 turns, 500,000 miles or more and burning more oil than fuel a maximum of 2 turns.

Jon.

Vac advance works - or at least the dizzy rotates CCW when engine rpm is increased.

Engine has not been rebuilt so break-in isn't a factor. I adjusted mixture screw according to manual after installing rebuilt carb. Engine ran fine for many months and miles before developing this problem which coincided with the fuel pump failure.

I thought both problems were symptoms of the same disease, i.e Fuel Pumpis Chinensis.

It seems there were two problems manifesting themselves simultaneously.

Remove a spark plug or two to see if they are covered in soot. If so, you have a carburetor issue.

Yes, the plugs are all covered in soot (not oil).

Your problem has just been solved. Congratulations. You have a bad carburetor.
You can either try to figure out what is wrong with your current carburetor of find another one.
I am guessing that what you have been dealing with is the most challenging and frustrating of all diagnoses. Two unrelated issues at the same time.
Fuel delivery and a malfunctioning carburetor.

Yes. After I installed the rebuilt original style fuel pump I was certain everything would be fine.

As for the carb, it too is a recently rebuilt unit from a good original. I'll take it off and pop it open but unless I see something ridiculously obvious, I'm not sure where to start.

Start by checking the fuel pressure, with the engine running on the fuel in the float bowl and the line to the carb deadheaded into a vacuum/pressure gauge. It should be no more than 4 PSI. Some rebuilders use a spring that's way too strong, and the fuel pressure overpowers the needle valve in the carb, causing a high fuel level and a flooding condition. The fuel pump am does not control the output pressure- - -the spring does that. Opening up the carb shows exactly nothing, because it's not the problem.
Jerry

The pump was rebuilt by Then & Now whom I trust to do a proper job.

I will test the pressure, however.


It could show whether or not the brass float is hung up or even filled with gas and sitting in the bottom of the bowl.

I submerged it for 4 hours prior to rebuild and it came out dry but who knows?

Take a picture of your distributer ,cap off showing the points.