Mercruiser Fuel Pump Pressure?

rossjo · Post by **rossjo** » Thu Sep 09, 2010 9:36 pm

Does anyone know how much pressure the stock Mercruiser mechanical fuel pump puts out (7.4L)?

Its overwhelming the carb float on my Qjet (and flooding after running at high RPM) ... may need a fuel pressure regulator.

LandVF36 · Post by **LandVF36** » Thu Sep 09, 2010 11:25 pm

I'm pretty sure its 4-5 PSI. At 7PSI, the float in the carb can't hold back the pressure.

Big D · Post by **Big D** » Thu Sep 09, 2010 11:30 pm

I thought it was around 7 PSI. I'll check and get back to you. Ross, check your fuel line clamps. If it sucks back air, it'll airate the fuel and the float won't rise properly but you'll have too much fuel in there and cause the same symptom. Have had this happen.

rossjo · Post by **rossjo** » Fri Sep 10, 2010 11:52 am

thanks for the tips. The engine/carb have run for several years, so its hard to imagine I need a pressure regulator now.

The Qjet is finicky - I'll check the line for air leaks Big D ... good tip. There really aren;t any fuel line clamps, as the engine is about 2.5 feet fromthe tank, and all that I have in line (in order) are:

1) tank,
2) fuel shutoff valve,
3) flow-scan pickup,
4) fuel/water sep/filter,
5) fuel pump,
6) Qjet carb (with its little filter)

Need to read up on the Power Valve as well ...

alexander38 · Post by **alexander38** » Fri Sep 10, 2010 4:46 pm

Are you sure its not a fuel pump going bad and pump it back up the overflow line and filling the carb up. my next shot would thrash in the float bowl spray some carb cleaner down the tube worked for me once. and doesn't q-jet have accelerator pump instead of a power valve ?

prowlersfish · Post by **prowlersfish** » Fri Sep 10, 2010 5:45 pm

No power valve on the Q-jet

rossjo · Post by **rossjo** » Fri Sep 10, 2010 9:47 pm

Sorry - I meant Power Piston, not Power Valve (like the Holley on my Tiger has).

I can pull the overflow line off and no fuel (or effect at all).

The Power Piston spring may be too tight - or leaking ... it uses vacuum to shut it off until higher speed when it opens to provide more fuel for the primaries.

Per the Corvette Forums I'm in (still have a Qjet on my Vette):

"Tech Tip #4
How can you tell if your power piston spring is too stiff and not allowing the power piston to â€œseatâ€ at idle?
If your engine does not produce enough manifold vacuum at idle and/or cruise (due to a lumpy cam or other engine parameters), it is possible that the power piston is not being pulled all the way down to its seated position due to the power piston spring being too stiff. The result is that the car will run very rich at idle, and the idle mixture screws will have little effect or response. Idle speed may also â€œfloat,â€ with idle speed starting high and gradually decreasing until the engine stalls due to the engine getting â€œloaded up.â€ There will typically be a puff of black smoke out the tailpipes when you â€œflickâ€ the throttle.

To test for this, pop the top off the carb, remove the power piston/rod assembly, and remove the power piston spring from its bore. Re-install the rod/piston assembly without the spring and put the carb back together. The carb will now run in the full-lean condition all the time. If this clears up the idle, improves idle mixture screw response, and eliminates the black smoke when you flick the throttle, you need to install a softer spring. Edelbrock has a complete power piston spring assortment available. You can also get many of the springs from GMPartsDirect using the GM part number shown in the carb listing chart."

rossjo · Post by **rossjo** » Fri Sep 10, 2010 9:48 pm

alexander38 wrote:my next shot would thrash in the float bowl spray some carb cleaner down the tube worked for me once.

Not sure I'm following you here ... ?

alexander38 · Post by **alexander38** » Sat Sep 11, 2010 8:11 am

I had some junk small mind make it to the bowl. Sprayed a little cleaner down it and it cleared up. But it sounds like u have a different issue .

rossjo · Post by **rossjo** » Sat Sep 11, 2010 12:42 pm

Still not following you, but I think I may have fixed it.

I pulled the carb apart again (3rd time this week), and found the power piston sticking. I removed it, cleaned it well, and trimmed 1/8" off of the spring (lighter spring = leaner). Runs great now - will allow it to cool and then restart this evening and run to full temp at various speeds. If it runs OK, we'll drop it in tomorrow.

Too hot to boat today ...

alexander38 · Post by **alexander38** » Sat Sep 11, 2010 4:12 pm

Ok I'll try , had junk in my float needle I took the overflow tube off it and spared carb cleaner in it and it cleared it. You have or had if your fix works ( and please fallow up with us) a whole different thing. Sorry for the confusion on the subject.

Big D · Post by **Big D** » Sat Sep 11, 2010 11:52 pm

Ross, for future reference:

At 1800 RPM:
Airtex pump = 5.5 - 7 PSI
Carter pump = 5.25 - 6.5 PSI

rossjo · Post by **rossjo** » Sun Sep 12, 2010 8:14 am

Not sure which mechanical pump I have - it fits on the top of the water pump bracket.

Anything over 5psi is really too much for the Qjet, but it used to run fine.

This pump came off of my old engine, which had a Carter AFB on it.

Big D - thanks for the info!

rossjo · Post by **rossjo** » Sun Sep 12, 2010 9:21 pm

Well, that wasn't it - back to square one.

Runs on the hill ... runs at ide an dlow speeds in the water.

When I run it up over 1500 RPM (22 knots) for 2-3 minutes, it dumps fuel down the carb. When I stop, and shut everything down - fuel is STILL pouring into the carb (you can even HEAR it).

I've done a lot, but haven't found the problem ...

rossjo · Post by **rossjo** » Mon Sep 13, 2010 1:58 pm

Big D wrote:Ross, for future reference:

At 1800 RPM:
Airtex pump = 5.5 - 7 PSI
Carter pump = 5.25 - 6.5 PSI

I wonder if these fuel pumps have an internal regulator that could be destroyed with ethanol?