[gtobird]

Sad but true................ I bought a new mechanical fuel pump about 2 years ago. After the engine warmed up, it always seemed to overpower the float and the needle / seat let fuel overflow. I swapped out the carb with (another) known good one from another engine and it did the same thing. A couple of days ago, I made a fuel pump test rig with a gauge and deadheaded it. That pump was a couple of PSI over spec and that was stone cold. I suspect it was worse when warm. Just to verify, I did a test on the same rig using the fuel pump I had on my excellent running 400. It came in at around 5 PSI. It quickly led me to the conclusion that the fuel pump was the likely issue.

So, I'm left with finding a new fuel pump that is highly likely to be within spec. As far a I can tell (and from what I've read), pretty much all of these pumps are now made overseas, so thinking I'm buying a reliable brand name doesn't mean much. Have any of you bought one in the last 3 years or so that works well withing spec? If so, which one???

Please don't jump to telling me I should just use a regulator. Yes, I know that I could solve it that way, but I would like to do my best to do it with a decent fuel pump. If it can't be............ I guess a good regulator will be in my next posted question.

Thanks much for your wisdom and contribution.

Old Joe

[will slow gto]

I would probably get a RobbMc mechanical fuel pump if you’re not having luck with OEM.

https://www.robbmcperformance.com/pr...ontiac550.html

[tom s]

I use the Carter street pump with great success.Tom

[b-man]

Originally Posted by tom s

I use the Carter street pump with great success.Tom

Same here, Carter M6907 mechanical.

I have it on my stock restored Tri-Power 421 HO, no regulator.

Actually primes the carbs pretty quickly after sitting a few days, I’m very happy with it.

I did end up reclocking the pump because the inlet fittings were going to interfere with the frame, it was easy to do without having to remove the pump.

[Radman]

It may not be the fuel pump's fault. Todays gas with the ethanol added is much less dense than straight gas. You may just need to set the floate in the carburetor about 116 of an inch lower.

[Formulajones]

You have a carb that doesn't survive anything over 5 psi?

Every mechanical I've had and used will always lose psi as it gets hot, not gain psi. If you're having a psi gain problem with heat or after heat soak, it sounds more like a fuel heating issue somewhere causing it to build pressure. Likely not the pump but something elsewhere in the fuel delivery system.

All the airtechs, AC's and Carter's Ive used (which are all the same pump) all vary a little bit with their psi but I've never had one that was over 8psi and never had a carb that wouldn't work with what ever psi I wound up with. Some were lower psi but what I never wanted was psi dropping under load, that's a worse problem to have.

These days I don't buy the store bought pumps like the wannabe AC's or Carter's because they have become too unreliable. The last Carter's I bought, first one died a block from the house and the second one died 4 miles from the house. The third time Napa just said never mind and refunded my money LOL. I depleted their stock that day.
If I need a mechanical pump these days I use either RobbMC or I'll buy a Holley mechanical from summit or something similar. I'm more curious as to why your carb is so sensitive to pressures above 5 psi.

As an example I run a summit brand Holley style mechanical on the nomad. I believe it's the 90 gph version but doesn't require a regulator. On cold start it has 9psi and it runs perfectly fine. After driving a bit and things get warmed up it drops to 7 psi and stays there. I consider this perfectly normal and the car drives and functions as it should.

[soupman]

I used a new Carter too and it failed within a few months. Switched to the Robbmc pump 6 years ago and never had any issues, plus no regulator to my quadrajet.

[steve25]

I to have seen fuel pressure drop off to some degree with every mechanic fuel pump I have ever had on a motor since I installed a fuel pressure gauge and it’s perfectly normal since the diagram gets softer with heat.

As mentioned in post #6 if your only having an issue when things are hot then I would also say you should investigate if the fuel is boiling or not.

I properly set up float and properly bent and loaded retainer arm ( what the float slides on to ) in a Q-jet can take 7 psi easy from what my playing around with a regulator has shown me.

On the other side of this discussion 3.5 lbs is the flat out minimum to have the motor being able to power a car even on level ground at the average rpm it takes to do 25 mph.

[gtobird]

Originally Posted by Formulajones

You have a carb that doesn't survive anything over 5 psi?

Every mechanical I've had and used will always lose psi as it gets hot, not gain psi. If you're having a psi gain problem with heat or after heat soak, it sounds more like a fuel heating issue somewhere causing it to build pressure. Likely not the pump but something elsewhere in the fuel delivery system.

All the airtechs, AC's and Carter's Ive used (which are all the same pump) all vary a little bit with their psi but I've never had one that was over 8psi and never had a carb that wouldn't work with what ever psi I wound up with. Some were lower psi but what I never wanted was psi dropping under load, that's a worse problem to have.

These days I don't buy the store bought pumps like the wannabe AC's or Carter's because they have become too unreliable. The last Carter's I bought, first one died a block from the house and the second one died 4 miles from the house. The third time Napa just said never mind and refunded my money LOL. I depleted their stock that day.
If I need a mechanical pump these days I use either RobbMC or I'll buy a Holley mechanical from summit or something similar. I'm more curious as to why your carb is so sensitive to pressures above 5 psi.

As an example I run a summit brand Holley style mechanical on the nomad. I believe it's the 90 gph version but doesn't require a regulator. On cold start it has 9psi and it runs perfectly fine. After driving a bit and things get warmed up it drops to 7 psi and stays there. I consider this perfectly normal and the car drives and functions as it should.

I don't know what the cutoff point is where the carb is overwhelmed. Spec is supposed to be around 6 to 7 PSI max on the pump I used. It was actually putting out over 8 PSI and that was too much for both of the quadrajets I have. The other fuel pump I have puts out 5 PSI from the engine that runs well (with either of the quadrajets). I have no doubt some carbs will tolerate higher PSI that others, but from what I've read around 4.5 to 5 PSI is recommended for street use and it seems to work for me. Over 8 PSI is too much.

[gtobird]

By the way, I can't say that the fuel pressure is definitely higher or lower when hot. That was a guess on my part. I've never really gotten the engine to run for more than a few minutes without it stumbling.

[rohrt]

I would just try a different cheap fuel pump. Didn't the earlier Q-jet have a poor design with the needle and seat that makes them more sensitive to the pump pressure?

I have the Carter M6907 with a stock 68 Q-jet and it has over powered the float a few times. Seems to be on the edge of what the needle/seat can handle. Still noes dives under hard acceleration. I just live with it to keep the look I want under the hood.
https://www.hotrod.com/how-to/hppp-1...iac-fuel-pumps

[steve25]

Well the bigger your needle and seat are, that in turn means you needle has more surface area, and than in turn means for any given amount of fuel pressure you have that gets applies the more leverage there is to unseat the needle.

When you set the float level in any carb there is always some play between the holes in the float arm and the shaft ( float hinge pin. ) it happens to hang off of.

So if you’re not careful to take this play into account when you set your float level then that could make for a over full float bowl.

And also very important in terms of a Q-jet when you set the float is to have the float hinge pin seated fully and not just that but also when the air horn is placed on that hinge pin gets compressed and never moves up.

[gtobird]

Originally Posted by steve25

Well the bigger you needle and seat are that in turn means you needle has more surface area, and than in turn means for any given amount of fuel pressure you have that applies more leverage to unseat the needle.

When you set the float level in any carb there is always some play between the holes in the float arm and the shaft it happens to hang off of.

So if you’re not careful to take this into account when you set your float level .

And also very important in terms of a Q-jet when you set the float is to have the float hinge pin seated fully and not just that but also when the air horn is placed on that hinge pin gets compressed and never moves up.

Yup,

Done all of that. Again they run they way they should with a pump that is in spec.

[64speed]

I’m running a Holley pump. It says you don’t need a regulator but I have one. So far so good

[mgarblik]

Cliff Ruggles makes a point in his book about vacuum testing the needle and seat when rebuilding a carb. Something I never did until reading the book. It is flat amazing how horrible the needles and seats are at sealing off pump pressure. It is very possible a poor fit between needle and seat are contributing to the problem. On early Q-jets with less lever advantage on the float fulcrum, 6-6.5 PSI is the practical limit for not overpowering the seat.

[gtobird]

Originally Posted by mgarblik

Cliff Ruggles makes a point in his book about vacuum testing the needle and seat when rebuilding a carb. Something I never did until reading the book. It is flat amazing how horrible the needles and seats are at sealing off pump pressure. It is very possible a poor fit between needle and seat are contributing to the problem. On early Q-jets with less lever advantage on the float fulcrum, 6-6.5 PSI is the practical limit for not overpowering the seat.

I'll repeat one more time........ The failures occur with a pump that was over 8 PSI when I tested it. Both of my carbs work fine on my 400 engine with a 5 PSI pump.

I don't know what the actual PSI level is when the needle / seat / float are starting to be overcome. It could very well be at 7 PSI (or higher or lower). I just don't know as I never had a gauge on the car. The info I have on pump PSI is from the deadhead test I ran off the car on both pumps.

All parts purchased from Cliff and consulted with him on the build of both carbs (done a few years apart). Own his book and I did test the needles and seats during the build(s). Yes indeed........ very valuable information.


Thanks for the input.

[Formulajones]

I think you're going to have a tough time finding a pump that will run at the 4.5-5 psi you want. Every mechanical pump I've used worth anything has always had at least 7 sometimes a pinch more. That's more normal and perfectly acceptable.

Me personally I'd consider a pump putting out 4.5 to 5 psi a weak pump from the start and unlikely it would keep up with the fuel demand when at wot.

[gtobird]

Originally Posted by Formulajones

I think you're going to have a tough time finding a pump that will run at the 4.5-5 psi you want. Every mechanical pump I've used worth anything has always had at least 7 sometimes a pinch more. That's more normal and perfectly acceptable.

Me personally I'd consider a pump putting out 4.5 to 5 psi a weak pump from the start and unlikely it would keep up with the fuel demand when at wot.

I never said I needed a 5 PSI pump. They may run fine on a pump that's greater than that. All I know is 8 PSI is too much.

[67Lemons]

I know you don’t want too but it seems like the only way your going to get that PSI is with a regulator, otherwise you’re searching for a needle in the hay stack with today’s parts.

[Formulajones]

Originally Posted by gtobird

I never said I needed a 5 PSI pump. They may run fine on a pump that's greater than that. All I know is 8 PSI is too much.

I guess I misinterpret what your looking for. You said you have a car that only makes 5psi and the carb works with that, but pumps you buy are 7 or 8 psi and won't work. And you said your looking for a pump that is in spec.

If you're getting pumps that have 7-8 psi you're getting pumps that are in spec. That's a normal reading. Quite honestly if you have a car only making 5 psi it's a bit on the weak side. If you're finding that with a dead head test I'd almost guarantee it's lower than that running down the road and that's starting to get a little questionable on a big V8. Even a stock air cooled vw pump designed for a 36 hp engine makes 6-ish psi at idle on a running engine.

I think like mentioned above if you want 5 psi at the carb you'll have to regulate it. Or find out why your carb is so finicky about fuel pressure.
Or keep buying these aftermarket pumps and maybe eventually you'll find a weak one.

I kind of think long term here as well. What are you going to do when the time comes and you need another pump? Go through all this again? I think I'd rather figure out why the carb is finicky and make that work with a normal pressure reading, or regulate it down. So next time you buy a pump you're not in this situation again.