Technical mechanical fuel pump too much pressure

I am still battling my 390 Ford lifter/push rod issue. I have not worked on it since last weekend when a fuel issue kept me from the lifter investigation. I shimmed the rocker stands for a temporary test,,,another story. I wanted to run it for a while to warm it up. I have an Autolite 4100 4 barrel on it with a new mechanical fuel pump. I put a kit in the carb, new brass floats. It ran about 10 seconds and then gas would come out of both top cover vents, flooding bad. The front first, secondary about a second later. I set the floats probably 10 times and had them laying almost on the bottom of the chamber, same flooding. I pulled both needle and seats, nothing going on there. I changed floats back to the plastic ones that were in the carb,,,same flooding no matter where I set them. I thought the fuel pump maybe had a relief valve problem but was told they don't have one. I put on a small electric one this morning that I have used for temp jobs before. Now it will sit and idle no problem at all with the floats or carb. I have bought another mechanical pump, but not sure if that is my issue or what. Am I missing something?

 

I have run into this quite a few times on new and rebuilt mechanical pumps--pressure all over the place. Now I run a regulator on all builds.

 

Reminds me of the new Airtex I stuck on a friend's V8 Chevvy with a Q-Jet years ago. Went thru the carb 3 times & couldn't make it quit flooding ... called our local V8 guy who asks me how's the fuel pressure, which I can't answer because I'm a still learning youngster. Hooking a gauge to it revealed 18+ lbs. At that time, the cure was to use only an AC pump on the Q-Jet engine.

 

What olscrounger said^^^^^^^^^^^^^^^^^pressure problem solved

 

Yep seems like a problem with off shore parts. I stuck my head under the hood on a 235 powered vehicle a couple of years back, the pump kept overriding the carb. The only thing that the guy had changed was the pump. After that he changed one more thing, added a regulator.

 

I just went down this path with my wife's 1960 Thunderbird (352 upgraded to a 390 FE).

I called Carter to see if there was a lower pressure pump. What we discovered is that a Carter M3528 would work as it puts out 4 1/2 to 5 1/2 psi.

I purchased the pump and double-checked the pressure on the Thunderbird with my SUN fuel pressure tester and the pump put out 5 psi.

I purchased my pump through RockAuto for $33 and feel that it may become the "go to" mechanical fuel for all of us running FE and Y block fords with carburetors.

I hope this help.

 

yep, for 390 and others there are different fuel pumps with different pressures - not an obvious problem to locate

 

In all my years and there are many I have never had a mechanical fuel pump cause too much pressure to the carb overloading it.Have always used mechanical pumps in all my applications with no regulator ?? is this maybe just a ford issue ? gene in Mn

 

Phooey. Go get your old pump out of the trash can, remove the big spring under the diaphragm, put it into new pump. Zooom

 

Put a fuel pressure test guage on it.
Quit guessing.
Trouble shooting 101, second day.

 

I agree!

 

I never had an existing fuel pump on my engine to compare with the new one, just assumed it was correct. Turns out it was for a mid 70s Ford Bronco which never had FE 390s. So, it was wrong. Worked for a while, just pumped crazy, then did not pump at all. Removed it and the spring was broke, arm was floppy. So, bought the correct one and it works fine. Crazy issues with this thing. Thanks for all the input/suggestions.

 

Doesn't sound like just a Ford issue, see below.....

 

If at idle and the bowl full with the bowl inlet valve closed with float pressure, i can see the pressure in the line going up---due to resistance against the float etc. BUT shouldn't the pump be putting out less pressure due to TURNING at a lower rpm?

So, how much pressure does, or should, a stock mechanical pump be producing at idle. Say 600rpm?

 

Hydraulics 101.
Pressure is always resistance to flow and it's usually regulated by a spring loaded bypass valve.
A HV pump won't flow anymore fluid to the item than it can consume/use and the excess volume is bypassed at the regulated pressure.

 

This isn't a brand specific thing...this is a parts source specific thing...as in lots of reports on various forums with the same problem caused by the fact that nearly all the mechanical replacement pumps are now being made in cheap third world countries. China, India, etc.

Just the new world order, get caught up and get used to it.

 

Remember what ass u me means don't guess check.

 

Look carefully at your vacuum gauge dial, this very useful tool also tests mechanical type fuel pump pressure output.

Takes only a couple minutes, don't even have to start engine, just crank it over for a few seconds. Like said above "don't guess, measure" saves lots of aggravation and it will always run better when everything in the carb is setup according to Hoyle.

 

I'm guessing there are a whole bunch of people who have no idea what " Hoyle" is. LOL

 

When I bought my Olds, I knew that it had a over fueling problem. It had a rough idle and puked black smoke, I figured the carb needed a little fine tuning or at the very worst a rebuild. Wound up taking the carb off and took it in for a complete rebuild. After getting it back and installing it, the over fueling was worse. Now it would only idle for a few seconds before fuel started running out and it would die, pulled it off several times to reset the floats but it still was over fueling. I could see that the fuel pump had recently been replaced so I checked the fuel pressure, it was putting out over 8 pounds of pressure. I bought one of those Mr. gasket inline regulators and set it at 4 pounds, that helped big time. Still had a little black smoke coming out so I set the regulator on 3. That seemed to cure all the problems, it worked for a little while but then, all of a sudden, it went right back to the way it was before the regulator.
I was going to recheck the fuel pressure but before I could get a reading the starter hung up and wouldn't release, just keeps spinning! That was several weeks ago, I was so disgusted with whole damn thing that I still haven't messed with it since!!

 

Putting a regulator on what is basically a defective fuel pump sounds like a kluge to me, maybe OK temporarily in an emergency. Give the carburetor what it wants, and it will give you what you want.

 

1stGrumpy...that Mr Gasket dial adjust pressure regulator is a known piece of shit...high failure rate or just decides to change pressure on a day to day basis.

Want simple and low cost..Holley #12-803 or the Quickfuel, Summit, etc clone....it is a time proven design.

 

Guess you never ran a bunch of 97's on an OHV V-8.

 

as above for sure, some good advice here. Just don't run a after market electric one and a mech fuel pump together as it may end up filling your sump full of gas by blowing through the diaphram, wont it?

 

TRENDZ WROTE:

Rpm has no influence on pump output. The pump lever only “cocks” the pump. The spring is what moves the diaphragm. The diaphragm and fuel only move as fast as the carb can consume. Every stroke of the lever does not translate to fuel volume or pressure. The spring in the pump determines pressure.



Thanks. Good to know; Now that i think about it, most gm service manuals that i've seen call for testing pump pressure at idle. And they call for anywhere from 4lbs to up to 9lbs on later smog engines (Q-jet).

Maybe i got it confused with the belt-driven rotary type fuel pumps used with, for example, mechanical FI? Those DO increase pressure with increased rpm (i think?)

 

This is just in the last several years it seems a lot of the mechanical fuel pump manufacturing shifted overseas. Carburetors, distributors, points, round ignition coils etc are obsolete in the modern supply chain and mucho crap parts have taken up the slack.

 

I had a mechanical pump putting out 10+psi randomly. New Airtex pump.

Chased idle and flooding problems for a year.

I changed to a carter and no problems so far but I probably would have been better off rebuilding the original pump.



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Just a thought, isn't there a spring gizmo the latches the float down on those carbs? It a goofy thing that can easily not be installed proper and the float won't be able to shut the fuel off from the mechanical pump but will work well enough for those small pulse type electric.

 

Came to this thread late, but lots of possible "causes":

(A) Newly made fuel valve where the manufacturer of the valve did not "stake" the seat
(B) Fuel valve seat with too large an orifice
(C) Float with incorrect buoyancy for the specific carburetor
(D) Newly made fuel pump with too much pressure
(E) Faulty (or missing) fuel valve seat gasket
(F) Incorrect float setting (probably not this issue, as OP has continued to set lower)
(G) RARE - cracked carburetor housing
(H) Motorcraft carb with float hold down spring not correctly installed.

Several have commented on the less than predictable pressure from newly made fuel pumps. Fuel pressure gauges are inexpensive. One test is worth 1000 opinions. We tested one in a Delco box that tested better than 12 psi.

Jon.