Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Carburetor fuel pressure issue

General Tuning Discussion

Forum Posts



Tech Articles

Discuss all things tuning in this section. News, products, problems and results. 

= Resolved threads


Full story for context, 1969 chevelle with reasonably built 454 (600whp on motor built to handle 500 shot of NOS) car/engine ran without issue for 14 years driving and dragracing, car isnt caged and runs mid to low 11s on motor so NOS is only used on occasion for last pass as tech marshals will park me. One even looking to run a little better turn on NOS and tried 150 instead of the usual 100, car cracked the frame in the rear and broke driveshaft/trans tail shaft. Frame was fixed and boxed for more strength and in doing so body ended up 1/4-1/2” higher, as there is no areo i wasnt concerned, car now runs a little warmer but still dosent overheat at all. Now for the issue, intermittently fuel pressure will climb and push the needles off the seat. Since issue began have replaced fuel pressure regulator, rebuilt carburetor twice, replaced fuel pump with no luck, and lowered base fuel pressure (went as low as 4.5psi base and it climbs to over 9psi) to help but no luck. All parts are quality (holly regulator, mallory pump, holly carb) no routing of the fuel lines has changed in 14 years and all the same parts and power levels remain the same. Any further suggestions or tests i can try?

You've eliminated pretty much anything I would suggest - primarily the fuel pressure regulator - but you don't mention if you're using a fuel filter, as there may be tiny bits of crop loosened in the 'mishap' that are holding the needle slightly off the seat at times. Viton coated needles, if they're an option, seal better than simple steel/brass needles.

Do you have any logging for the fuel pressure, or a gauge that's showing it being raised, or is this your best guess?

There are usually different sized needles and seats for carb's, have you tried different ones and/or did this problem start after a carb' rebuild that may have a larger orifice?

Different carby' manufacturers, and models, may have different recommended fuel pressures - have you checked the manufacturer's for your carb', and does it have a sight glass or are you using float position to set fuel height?

The greater the info' you can supply, the greater the chance of figuring this out.

There is a in line fuel filter that has also been replaced since it has started, the carburetor has the good needles, and i have a manuel fuel gauge that is showing the pressure increase. Nothing was changed from 14 years of perfect running to now aside from the 1/4-1/2” in ride hight, then after the issue started have rebuilt/replaced parts but kept the same spec. Did notice the modulator valve in the transmission has gone bad so just replaced that, it is vacuum operated so unsure if it could of caused the increase in fuel pressure?

I would focus on the fuel pressure regulator. It may be that it fails under different temperature conditions, or today's fuels have caused corrosion that prevents diaphragm seal from functioning properly. I have always had good fuel pressure control (EFI), using Aeromotive regulators. Here is there offerings for carbs:


BTW, Aeromotive have a few interesting tech bulletins that might be helpful:



The regulator was the suspected culprit, ut was rebuilt twice replaced with a new regulator and then a known good regulator with no change to the issue.

Read the PDF about temperature sensitivity for pressure gauges located in the engine compartment.

That was a very interesting point, thank you for that, David.

It was 30 years ago, so things may have slipped, but in the principles, design, and calibrating of these type gauges, I don't recall this problem being mentioned - but IIRC, it was all 'conventional' gauges either open and so relative, or vacuum filled and so absolute, we were working on.

Something the OP may consider is one of the options available that uses an electronic sender and gauge inside the vehicle to monitor the pressure - some still use a machanical type, but that has a much higher risk of causing a fire if the capilliary/pressure line leaks or breaks. I don't know if they're still available, but atleast one company used to have a sort of hybrid system, with a line from the rail/carb' to a remote electrical sender.

A little surprised they didn't mention the problem of centre hung/mounted, as illustrated, floats for drag racing, as the fuel level gradient, and the way the float reacts,can apply much more closing force on the front needle, and unload the rear - I think it's that way round? Side hung are much less susceptible to this. Centre hung are MUCH better for vehicles that are intended for lateral forces, though.

The gauge is located in the driver compartment (should have mentioned that) the pressure builds on the street aswell as the track

Re-reading it, David, he says "Now for the issue, intermittently fuel pressure will climb and push the needles off the seat."

This suggests it isn't just a gauge issue, but an actual over-pressure condition - if the OP will confirm this is his actual problem, with the engine cutting out from the excess fuel spewing out of the vent tubes, and copious amounts of black smoke from the exhaust?

Kent, it isn't clear if you have an actual problem with a pressure rise, a problem with the pressure gauge, the pressure regulator, or a combination of two or more?

If the carb's is flooding, there is an actual pressure problem, if it isn't you may just have a pressure gauge problem. I the former, are you sure it's a genuine pressure regulator - that's the only part of the system that will affect the potential over-pressure at the carb'. If the gauge reads high, have you tried cooling it by spraying it with water to cool it - assuming the variation seems to be temperature related, of course? And on that, you can actually get higher under-bonnet (hood) temperatures with low speed driving, because there isn't the airflow carrying away the heat, that higher speed running provides.

Issues is over pressure and fuel coming out the vent tube. Nothing was changed before the problem started aside from the slight 1/4-1/2” the car was raised, and that seems to make the engine bay warmer. Since the pressure issue started i have replaced pump, filter, regulator, and rebuilt carb with no change

Good, thank you, that clarifies things nicely, and running points through my mind as I type...

Because it's a physical problem, manifesting as fuel discharge, the gauge reading isn't the primary issue. Regardless of the potential output of the fuel pump, the regulator should be limiting it. Whatever the line pressure is in the fuel line, between the regulator and needle assembly/ies, as soon as the float needle(s) lift(s) off the seat, that should bleed off the fuel and limit the pressure rise. The filter should be stopping the crop in the fuel line, but you didn't mention where it is - if between the regulator and carby, it won't prevent the crap passing into the regulator and possibly causing that to stick and/or leak internally, but if before the regulator it should be keeping that clean.

Everything seems to point to a compromised pressure regulator.

You still haven't given the brands and models for the regulator and carb' - it's possible you have a counterfeit, or badly designed regulator, and carb' pressure requirements/recommendations vary a lot between manufacturers, models, and uses - as I think I mentioned, larger orifice needle valve assemblies are more sensitive to pressure, and may work better with it lowered - the manufacturer should have recommendations.

The VERY slight difference in rake should have SFA affect on the pressures and underbonnet/hood temperatures - I'd be more concerned if, if you're using the same fuel tank, the mis-hap hadn't stirred things up and some of the crap was still being picked up and causing problems.

Fuel filter is pre pump; pump is mallory; regulator is holly, was rebuilt after issue started, then replaced with a new one, then that one was swapped for one off another one of our cars that is known to be working; carb is holly and has been rebuilt twice trying to resolve the issue; pressure was set to 7psi then dropped a few times as the issue continued and was set as low as 4.5 but will still rise over 9 and blow out the vent tube. Everything on the car was the same as it had been for 14 years with no issue and has been rebuilt or replaced with the same spec with nothing seeming to make a difference.

I'm sorry if this doesn't help, but I would check for a potential restriction in the return hose from FPR to tank, confirm tank can vent well and if the tank has a rollover vent that it hasn't become stuck shut. If the fuel can't easily get back to the tank, it can cause an increase in pressure despite the FPRs best efforts.

There is no return, hasnt been for 14 years and none of the other carbureted drag cars in the our shop (including 2000hp 7.17s 1/4mile car) have return

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?