Never underestimate the power of the mechanical fuel pump

[Bt Doctur]

I was chasing a sometimes overflowing carb since last year. Rochester was original (89) so I figured it might need a rebuild. So in the meantime I replaced it with a Edelbrock that was used by a customer for 3 weeks until he got his back. No issues running . One day I could run the bay for 1-2 hours and get back to my slip ok, sometimes with a little black exaust smoke . Other time it would flood out almost immediately. So I rebuilt it again with a new needle and seat, and it still flooded out. Disgusted I went home for the night and pondered how a mechanical pump could ever increase its output pressure . Never had or seen it happen in over 55 years on any motor.
Rigged up a pressure gauge and low and behold 14 1/2 pounds of pressure . Could not believe it.
So if your have a carb overflow using a mechanical after you rebuilt or checked everything else
pop a guage on the pumps output , you just might fine your problem too

 

[poconojoe]

Wow! That's a lot of pressure! No wonder it was flooding the carb.

 

[flashback]

Did you figure out what caused the increase?

 

[Bt Doctur]

No, replaced it with the Carter style electric one

 

[Scott Danforth]

BT, did you potentially not have the 1/4" spacer under the fuelpump? that would over-stroke the diaphragm.

was this an airtex pump? (potentially off-shore)

 

[Bt Doctur]

this boat was running this same pump for years before i got the boat. Used it with a dealer reg for short test runs and occassionally the rochester would flood. Thought it needed a rebuild so installed a like new 1409. Worked ok for a while and then it started the same thing so figured it was also due for a build. Beginning this year I did a quick rebuild on the rochester and took the boat out for some runs. Worked perfect for a bit then did the same thing.

Ordered a official rebuild kit and overflowed immediately. Install the electric pump and an still running the rochester with no issues. When I plug in a gauge it will read 5 psi or 14.5 psi when ever it wants to. Never had a mechanical do this on its own.
A Poltergeist because I`m a new owner????????????

 

[Lou C]

That is very odd. I had the OE pump on mine for about 30 years and then replaced it with a Sierra. Both worked fine, never had a flooding problem. One thing with the Q-Jet is that there is a hanger for the needle valve, and the bracket for the float has a couple of small holes, that hanger just clips on it does not go in the holes, if put in one of the holes that could cause it to stick. Which could concievably cause flooding.
How a pump could work fine and then increase pressure is really a mystery. But I guess the pressure gauge doesn't lie. I guess it is possible it could have been OK till the needle valve got old, or got affected by E10 gas and could not seal against the high pressure of the pump.
Here's a pic of what I'm talking about. Look closely at the bottom of the pic where the needle valve is.

 

[matt167]

The way you bump pressure on a mech fuel pump for forced induction was to boost reference the weep hole, which puts pressure on the diaphram and causes the pressure to rise 1:1... I wonder if the weep hole got plugged by insects. If it couldn't vent, it could hold pressure ontop of the diaphram and cause high pressure. That's a Wild guess but as good of a guess as any

 

[Lou C]

hmm now that is interesting.

 

[Grub54891]

I've rebuilt many of these, the only issue I've ever had is if the float was not replaced it would flood. Now the breather in the fuel pump could be an issue, but I haven't looked at one for awhile. It should have the hose coming from it to the carb, so it vent's that way instead of leaking fuel to the bilge.

 

[Lou C]

Ok this is interesting because I tried to use a brand new Holley on my old engine after I rebuilt the top end. Had a lot of trouble with rich running that I could never solve. I actually pulled out the old Quadrajet and rebuilt it and it ran much better, then decided to replace the original fuel pump and found that the breather was clogged with sludge….so I wonder did my old pump have this problem and maybe it affected the Holley more than the Quadrajet for some reason…. I never tried the Holley with the new pump so who knows but that really makes me wonder!