Fuel pressure vs idle quality

[69gmachine]

This is just an FYI tuning tip for all those running a Holley.

I just got my engine running and the upgraded tranny installed from last year's catastrophe. My engine has always idled rough and rich, fouling spark plugs way too fast. I finally decided to t/s and ultimately what I found was that the fuel pressure was too high. I had read that a Holley would work OK without a pressure regulator up to 7.5 psi, turns out not to be true (at least on a real street car that needs to be able to idle). The carb was a new out of the box Holley, so I didn't think it was likely the culprit although after taking it apart I did discover 2 gaskets on the power valve... it didn't solve my problem. After playing with the idle bleeds to no avail all day Saturday, I decided to investigate the fuel pressure today. I'm using a Carter mechanical pump with 1/2" line from the tank to the pump, and 3/8" from the pump to the filter and then to the carb. I measured 7.2 psi ahead of the fuel filter. I thought this was a little close to the limit, so I put a restrictor at the pump and knocked it down to 6.1 psi and the idle smoothed out and the plugs look good. I should have thought of this a long time ago. I missed a local Mustang show chasing my tail all day yesterday trying to set the idle screws and throttle stop.

 

[2bav8]

7.5 PSI is definitely too much fuel pressure...
Glad you were able to trouble shoot your situation.

 

[D.Hearne]

What a lot of guys who are new to carbs don't realize. is the fuel pressure has to be less than the force the floats exert on the needle/seats. Too much and you sink the floats and overfill the bowls. It takes very little flow and pressure to run a carbed motor, unlike EFI. The force created in the venturis is the only thing that pulls the fuel from the bowls. The fuel pressure has nothing to do with it. Back in the dinosaur days of the Model T, there was no fuel pump to deliver the fuel, it was gravity fed from the tank, mounted in the cowl above the engine, same way a lot of lawn mowers do today..

 

[bnickel]

i have that same carter pump but i have the stock 3/8" line from tank to pump and 5/16" (for now) from pump to carb and i have 625 cfm road demon, for the most part mine idles fine but it does occasionally load up, i figured that one out earlier today myself, the choke linkage was getting hung up on my stock air cleaner base and not letting the choke open fully, it would open enough that it ran pretty good and didn't act like it was the choke, just an intermittent problem with loading up. i beat the lip of the base in a little bit where the choke linkage was catching and problem solved.

i have an old purolator regulator i've been planning on adding to the system once i change over to the 3/8 line as well, just for a little more tuning options.

 

[JDS68Stang]

Good post, allways nice to have an extra option for trouble shooting a carb when all the others failed. I know a guy w/ a TA that will be glad to have this info.

 

[mrmustangman357]

what about the edelbrocks? what pressure would you run there? They say 6 psi but my idle is rough too. Isn't a telltale sign if it is dripping out of the boosters? I can stick my finger down in the boosters with the car running and i can tell no fuel is coming out

 

[69gmachine]

I never saw fuel dripping out of the boosters, wich is part of why it took so long to diagnose this problem. This wasn't supposed to be a high pressure pump. The only modifications I made were to open up both the inlet and outlet passages and smooth the contours inside the housing. I used a 3/8 NPT to -8 inlet fitting and a 3/8 NPT to 3/8 hose fitting for the outlet. I smoothed the contours inside the pump passageways thinking I would improve flow without increasing pressure since I didn't alter the valves or diaphram spring. When I removed the carb from the engine, the entire underside was dripping wet with gas, so that I couldn't even tell where it was coming from. I only reduced it 1.1 psi (was 7.2 not 7.5) and it made all the difference in the world.

 

[mrmustangman357]

so where did the extra fuel pressure go? on a holley, can't you tell if you have excessive pressure if the fuel keeps coming out the sight glass when adjusting floats? I don't quite have this luxury on my edelbrock. . .

 

[69gmachine]

what about the edelbrocks? what pressure would you run there? They say 6 psi but my idle is rough too. Isn't a telltale sign if it is dripping out of the boosters? I can stick my finger down in the boosters with the car running and i can tell no fuel is coming out

Have you put a gauge on it to verify it is only 6 psi? I would think most any carb would behave the same since they all rely on a needle and seat to shut off the fuel, and 6 psi should be OK.

 

[69gmachine]

so where did the extra fuel pressure go? on a holley, can't you tell if you have excessive pressure if the fuel keeps coming out the sight glass when adjusting floats? I don't quite have this luxury on my edelbrock. . .

It must have been coming out of the transition slot which is partly covered by the butterflies (and Mr. D, you oversimplified a bit stating that the force on the venturis is the only place that pulls fuel from the bowls, there's also a power valve, an accelerator pump, a transition slot for part throttle and the curb idle port). I didn't even check the sight plugs becuase I wasn't planning to adjust the floats. They were set at the factory and I was willing to give them the benefit of the doubt.

When I started hot rodding there was no such thing as EFI... I've played with my share of Holley's, Autolite's and Quadrajunks. If there was a such thing as a 750 CFM Autolite 4100 carb I would never even consider a Holley.

 

[70dvr]

so I put a restrictor at the pump

A restrictor that that gives you the right pressure at idle. Will not give enough fuel at speed. You will need a regulator. I had the same problem. I added a Holley pressure regulator. I set it to 6 PSI 28000 miles ago and no problems.

 

[D.Hearne]

It must have been coming out of the transition slot which is partly covered by the butterflies (and Mr. D, you oversimplified a bit stating that the force on the venturis is the only place that pulls fuel from the bowls, there's also a power valve, an accelerator pump, a transition slot for part throttle and the curb idle port). I didn't even check the sight plugs becuase I wasn't planning to adjust the floats. They were set at the factory and I was willing to give them the benefit of the doubt.

When I started hot rodding there was no such thing as EFI... I've played with my share of Holley's, Autolite's and Quadrajunks. If there was a such thing as a 750 CFM Autolite 4100 carb I would never even consider a Holley.

What I meant by that statement was the venturis pull the fuel once the carb is in the main circuit, fuel pressure has nothing to do with how the engine draws fuel from the carb. The pressure is only there to supply fuel to the carb, not to introduce it into the engine. If you like the Autolite so much, you should be equally at home with a Holley. Both are similar in construction and function, the Holley simply is more adjustable than the Autolite. But if you do want an Autolite 750, just buy a Holley model 4010 750, it's basically a Holley made Autolite.

 

[D.Hearne]

Have you put a gauge on it to verify it is only 6 psi? I would think most any carb would behave the same since they all rely on a needle and seat to shut off the fuel, and 6 psi should be OK.

Not all carbs use the same float to shut off the needle/seat, hence, not all will be OK with the same fuel pressure.

 

[jeffnoel]

I also recently got my 302 running and have the smallest Holley mech pump rated at 80 GPH (free flowing). My stock fuel lines from the tank to pump are 5/16" I think and -6 from the pump to the carb (Holley 600) my fuel pressure gauge shows 8psi at idle. It idles fine and I don't have enough time on the motor to accurately read the plugs but should I look into a regulator?

 

[10secgoal]

so where did the extra fuel pressure go? on a holley, can't you tell if you have excessive pressure if the fuel keeps coming out the sight glass when adjusting floats? I don't quite have this luxury on my edelbrock. . .

Pinch the fuel line between the pump and carb. If it idles better, your pressure is too high. If nothing changes, you are fine. It will run for a while with no fuel entering the carb. Long enough for you to tell how it runs.

 

[69gmachine]

What I meant by that statement was the venturis pull the fuel once the carb is in the main circuit, fuel pressure has nothing to do with how the engine draws fuel from the carb. The pressure is only there to supply fuel to the carb, not to introduce it into the engine. If you like the Autolite so much, you should be equally at home with a Holley. Both are similar in construction and function, the Holley simply is more adjustable than the Autolite. But if you do want an Autolite 750, just buy a Holley model 4010 750, it's basically a Holley made Autolite.

I actually did look for a Holley 4010. They've been out of production for a while now. There are two main things about the Autolite that I like: the top comes off instead of the bowls on the ends leaking gas everywhere, and the annular boosters get great gas mileage, provide excellent cold starts, and great throttle response. I will say that the advent of the blue gaskets has made a huge improvement in a Holley's reliability. In the old days you may recall that we used to put cherry chapstick on the brown gaskets in order to be able to reuse them while tuning a Holley. Even so, if you ever let them dry out from infrequent use, you had to replace them all because they'd shrink and leak everywhere. That's when I developed my aversion to Holleys. The main reason I paid the bucks for a re-issue 428 CJ Ford spec Holley is because it has annular boosters on the primaries, and no unnecessary vacuum ports. With the modern blue gaskets, my car sat idle for a year, I filled it up and had no leaks!

 

[69gmachine]

A restrictor that that gives you the right pressure at idle. Will not give enough fuel at speed. You will need a regulator. I had the same problem. I added a Holley pressure regulator. I set it to 6 PSI 28000 miles ago and no problems.

That may be true, but I had enlarged the output opening from the stock size, so I just put it back closer to the out of the box size. At any rate, I'll dyno tune it this winter and if I do indeed starve it for fuel at the top end, I will address it then. For the moment at least I can drive around town and on cruises (maybe join the pony drive as it comes through Virginia?)

 

[D.Hearne]

Holley 4010's come up on Ebay all the time. Check em out. My local speed shop also had one on the used parts shelf awhile back. Damn thing looked almost new, if not new.

 

[ron67fb]

When I still had the 2V I liked the design of my junkyard Autolite 2V so much that after the 4V conversion I bought a 4010 as soon as they came out. They just didn't seem too popular and were eventually phased out. Aside from the channel-riffic base that was prone to vacuum leaks, it's a nice carb and I still drive with it.

 

[69gmachine]

When I still had the 2V I liked the design of my junkyard Autolite 2V so much that after the 4V conversion I bought a 4010 as soon as they came out. They just didn't seem too popular and were eventually phased out. Aside from the channel-riffic base that was prone to vacuum leaks, it's a nice carb and I still drive with it.

It's good to hear from someone with 1st hand experience. What do you mean by "channel-riffic" base? I didn't know they were prone to vacuum leaks.

I'm actually pretty satisfied with my 735 re-issue Holley. It looks cool, is very tunable and has the annular boosters I wanted. For some crazy reason I didn't even think about trying e-bay for a 4010, and I've bought a lot of stuff on there.