Mechanical fuel pump pressure test
 

I'd like to pressure test a new mechanical fuel pump as I keep reading stories about how they are often over or under the PSI specs. This is a standard Carter fuel pump for a vanilla 68 350 carbureted engine with the single input fuel line.

Any suggestions of how to test it off or on the car? I don't want to permanently change the fuel line between the pump and carb by adding a pressure gauge. I've heard of people doing a quick test by connecting the output line of the pump directly to a pressure gauge and just cranking over the engine. Does that provide enough rpm to give a good reading? Open for thoughts or other methods.

Thanks
OJ

What your proposing is fine it will work you can even let it run on fuel in the bowl with pressure gauge on the disconnected line jump out of drivers seat observe pressure until it quits running

I have a permanent pressure gauge on a certain engine during cranking is gets up to 6psi and sits there a bit after cranking showing the valves are still good in it

You can test it off the car by simply figuring out how to hold the pump and operating the arm enough

My first thought is to find out if there's a problem before you go look for one. If your pump delivers fuel and the carb needle and seat hold the pressure, you might not need to worry about the pressure being too high.

If you are committed to measuring, though, my thought is to identify your carb inlet or pump outlet fitting and get creative to add a "tee" using rubber fuel line and barbed fittings.

Here's just one example:

Link to One Example Online

It might even be easier to do this at the fuel pump.

The hard fuel line will move enough to let you do this, whether at the fuel pump or at the carb.

When I chased a flooding issue, I went through this with a couple pumps so hang on to your test rig... you may need it again.

Mike

So that test only tells you deadhead pressure, at zero flow. OK if that's all your interested in. To check pressure vs flow at all operating conditions, you would have to instrument both a pressure and flow gauge, then capture data at various combinations of situations

The primary problem nowadays with new mechanical pumps is incorrect pressure, ... To high, ... Some reportedly around 9psi if i remember right, .... You would hope a true blue Carter pump is correct

That's consistent with my experience and understanding.

If gtobird wants to fully characterize his pump, then agree he needs a flow vs pressure plot across the full range. But as Formulas says, and what I've seen on this forum, the typical concern for a stock carb application is overpressure, which is likely highest with zero flow. The typical concern for a performance application is sufficient flow.

gtobird - What carb are you feeding?

Are you trying to solve a problem or avoid one?

I looked up a bunch of aftermarket mechanical pump listings a while back and found some of the "tall can" pumps are actually "spec'd" (at least listed) to deliver 9 or 10 psi. The "short can" pumps are typically spec'd lower, around 6 psi max. And neither of these "spec" values guarantee that's what you actually get. It wouldn't surprise us to see a pump spec'd 6 psi max delivering 8 or more psi.... which may be why gtobird is trying to avoid a rat-hole ???

Forgive me if I'm wrong, but my notes say a Carter M6122 (tall can) was spec'd at 9 psi max. Carter's short can pumps for similar vintage (M4868, M4690, M4885) were spec'd for 6.5 psi max (again, what I find in my old notes). I have no idea what they actually deliver.

Mike

I have a new short can pump and it appears that it might be blowing the needle off the seat on my quadrajet at idle (maybe). At times, it pushes gas out of the carb at the gasket between the base and airhorn. For now, I just want to confirm whether or not the pump is the issue. If it is and I replace the pump again, I'm going to run the same test right away. If it isn't the problem, I can cross it off the list as being the cause of my problem. And...... yes, I have done all sorts of other testing, tried other known good carbs that run perfectly on other engines, changed carb power piston springs, run vacuum tests, prayed to Jesus and said naughty words which made me pray more to Jesus. Yes, the other carbs were flowing gas out at the airhorn gasket too and worked fine on my other engine.

Thanks to everyone for the input. It's one of the few common things I haven't tried to test yet. The engine is being yanked and parted shortly anyway, but I'd like to see if I can drive it a bit for the remainder of summer.

OJ

Q Jets are also known for defective foam floats, they become partially waterlogged with gas. They will hold some pressure at the needle and seat, but will sink prematurely, opening the needle too early.

If the carb sits around for periods of time dry, it will act fine at first, then as the float takes on more fuel it will flood.

Not a common problem, but I have seen it a few times over the years.

Not the case here. All carbs had new guts and all ran fine. In fact, one of them was an NOS carb with the only changes being the upgraded float, etc for modern gas which came from Cliff. Ran fantastic on my rebuilt 400. At this point, I'm not looking to examine other possible issues until I rule out the fuel pump. If that isn't it, I'm probably going to just move on from this engine as I never intended it to be in the car for long and I have other fun mobiles for the rest of this summer.

I went down a similar path when battling a flooding carb.

Here's how I gained confidence in the carb by directly measuring the max pressure the needle and seat could hold, as shared by Cliff:

https://forums.maxperformanceinc.com...errerid=160974

My carb was good for 12 psi (static on the bench).

I replaced my pump but honestly, I don't know that was the issue because I shipped my carb to Shaker455 and he found a few issues that might have contributed.

Mike

You shouldn't really have to permanently change the fuel line to run a test gauge on the car. On cars with factory lines I simply unhook the metal line at the carb. They usually have some wiggle room to pull it away from the carb or to the side a bit. Slide the nut back and push some rubber hose over the flare and install a clamp. I install a short piece of flare steel line to the carb and install another rubber hose and clamp. Between the rubber hose I install a "T" fitting and run a rubber line up to the windshield with a fuel pressure gauge and tape it down to the windshield.

I do it this way temporarily so I can go for a drive and watch fuel pressure for cars that owners complain of laying over. I've been doing this for decades and it quickly helps to nail down fuel delivery issues. When done I remove it all and the factory metal line is reinstalled back on the carb.

It takes a little time rigging but it's what you have to do on some of these cars when you don't want to modify things.

Q jets are a little different in what they tolerate depending on the float design so you really need to know what your pump is putting out. Best way to do that with a mechanical pump is on the car running with the setup I described above. You can bench test the carb with an electric pump if you wish to find it's limit that way but it won't answer your mechanical pump questions.

What he said!