Working with carburetors in racing is almost always a tricky proposition. And let’s face it, tuning for asphalt where speed is about being smooth and slowly rolling into the throttle so you don’t spin the rear wheels is much easier than competing in a dirt environment. Racing on dirt requires lots of throttle input to help steer the car and keep the rear suspension up on the bars. There is no getting off the gas at the end of the straight and staying off it until you are exiting the turn.
One problem that’s plagued dirt racers practically forever is getting a stumble when you blip the throttle at the end of the straights. You need to get the car to roll over on the right side and then get right back on the throttle so you can power through the turn. But a stumble only serves to drop the car back down on all four tires and upset your race car’s delicate balance.
We’ve seen all types of fixes and band-aids tried, but nothing has really worked consistently. And now that more dirt cars are running cable- or belt-driven fuel pumps off the back of the block, the problem has only gotten worse.
Until now. Jimmy Carter is a longtime racer — and not the retired politician that you might be thinking of — who now works with his grandchildren racing Dirt Modifieds. Actually, they race Modifieds a lot.
“Yeah, I have three grandsons who all race Modifieds,” he says. “Two of them run Modified and one runs a Sport Mod. We run all over the United States, mostly around the Midwest. We run 90 nights a year, four nights a week, so we stay pretty busy with the race cars.”
Carter says that his grandsons all struggled with the stumbling problem, and he had looked for quite some time for a way to solve the issue.
“Well, I have been experimenting with fuel pressure regulators for about two years trying to get it right to keep the carburetor from stumbling,” he says. “What I noticed was, you adjust your fuel pressure by having it under load to try to match the rpm’s the engine will be running on the track. Depending on if you are running gas or alcohol, you need different pressures for different applications. So when we are running gas, we’re running about 6 1/2 pounds of fuel pressure.
“When you get out of the throttle at the end of the straight, the fuel pressure at the needle and seat will want to spike. What I mean is when you get out of the throttle that return line cannot allow the fuel to go back quick enough, and it floods into the carburetor. It does it with every pressure regulator I have worked with. No matter what we tried, we still had the problems with the spiking of the fuel when you’re getting out of the throttle causing an overloading condition and the engine stumbles. And everybody has the same problem.
“I’ve got one grandson who is able to keep an eye on the fuel pressure gauge while he is driving,” Carter adds. “I don’t know how he does it, but he can. In that car we are running alcohol, and he says when he gets out of the throttle at the end of the straight the fuel pressure gauge, which was running eight pounds of pressure where we had set it, will spike up to 15.”
Carter says the solution to the problem came to him one day when he was researching fuel pressure regulators and stumbled upon Aeromotive’s double-adjustable bypass fuel pressure regulator. This regulator from Aeromotive is designed for use with carburetors and features two independent pressure adjustment options for the racer. One adjustment is to set the fuel pressure to the carburetor when the engine is at idle, and the second is for when the engine is at speed. This regulator was engineered for the high-flow, variable-volume fuel pumps spun by either a cable or belt that have become very popular in dirt track racing lately. It also is engineered with a large return port to send fuel back to the fuel cell when the pressure is exceeded. What’s unique here is that Aeromotive’s engineers have actually designed the port to take a -10 fuel line so that high volumes of fuel can be pulled out of the system and returned to the cell. Aeromotive claims that this regulator (part number #13214) creates the smoothest and most repeatable fuel curve from idle to wide-open throttle of any bypass system.
“When I saw this double-adjustable pressure regulator, a light went on,” Carter says. “I thought this might do the trick. I’ve tried a lot of different pieces over the years, and it’s hard to believe that this one regulator was finally able to solve so many problems.”
Carter says he now has Aeromotive’s double-adjustable pressure regulator on all of the family’s race cars. He places the pressure regulator on a bracket near the fuel pump, which is a high-volume pump mounted on the back of the block. One line runs from the fuel pump to the regulator and is covered in an insulator to help keep the fuel cooler. Two equal-length fuel lines exit the regulator and feed both bowls on the carburetor. And then there is also a return line from the pressure regulator that dumps back into the fuel cell.
The pressure regulator has one inlet and one return port, so the most sanitary installation is to mount the it at the end of the carburetor fuel log. You can also mount the regulator somewhere on the chassis near the engine and simply run a line from the fuel log to the regulator. When the fuel to the carburetor reaches the set pressure level, the regulator opens the valve to the return line, sending fuel back to the fuel cell and almost instantly bringing the fuel pressure back to desired levels. Carter says they initially ran a -8 return line but upped it to a -10 and saw even better performance as a result.
“A lot of guys are weight conscious, and they will try to run a -6 or a -8 return line, and we found out that it’s just not enough, especially with the high-capacity fuel pumps we are running today like a Waterman or a KSE. You need that return line capacity to get back and dump it into the fuel cell so that you don’t have a pressure spike from the unused fuel. And the other thing we like,” he continues, “is they’ve got the pressure outlet right on the front of the regulator. So you can run it right to your pressure gauge and you get a real accurate fuel pressure reading to help you make your settings and adjustments.”
Carter says he has found the best performance when the regulator is set to hold the fuel pressure at 3 1/2 pounds at idle, and then when the engine is throttled up to allow the fuel pressure to increase to 5 1/2 to 6 1/2 pounds of fuel pressure.
“We were actually surprised by how much we were able to lower the fuel pressure as we started experimenting with the Aeromotive regulator,” he says. “I believe lowering the fuel pressure helps keep the fuel from foaming, but we just weren’t able to do it before with a dead-head regulator.
“Imagine pouring water from a garden hose into a bucket. If you get a slow stream, you just get a nice stream of water coming in at filling the bucket up. But if you turn that water spigot wide-open and hold the end of the nozzle against the bucket, you get a foaming action. It’s the same with fuel. When you have foam inside the carburetor and the carburetor is trying to add that fuel to the airstream, it just doesn’t work as well so you cannot get a consistent throttle.”
To his credit, Carter hasn’t kept his discovery to himself but has instead been very willing to share and help other racers. “I happened to run into Randy Sweet when he was working with Waterman (Racing Components), and I told him, ‘Man, you’ve got to try one of these regulators.’ He was working with Scott Bloomquist and they were running at higher RPM than us but having the same problem. Scott had noticed the same issues we were. He said that when he got out of the throttle the carburetor was loading up, and they had tried everything. If you back the fuel pressure down enough so that it doesn’t stumble, you don’t get enough fuel to the carb and wind up with a lean condition. So they have switched to the Aeromotive pressure regulator too and, as far as I know, have been happy with it.”
The problem with the loading up and causing a stumble is it normally causes a tight condition on the race car. The car either wants to shove the nose, or it stumbles and loses grip. When you are running a four-bar car, it comes off the bars. And if the car comes off the bars, the suspension unloads and you lose forward bite. When that happens other cars are going by you.
We are looking forward to trying the system ourselves, but time didn’t allow before this issue of OneDirt magazine was due at the printers. But we were able to find another reputable voice in the racing industry that doesn’t have a dog in the fight. Jack Hickman works with Waterman Racing Components, a leading manufacturer of fuel pumps for many types of racing, and he says the company now recommends Aeromotive’s double-adjustable fuel pressure regulators to be paired with its fuel pumps.
“The problem we were having came from using a pump that runs off the back of the power steering pump,” he explains. “That is a high-flowing pump because it can also work with alcohol, so not only does it flow much more, but the amount of volume also goes up with engine rpm. What we were seeing is most all the regulators out there are too small, so our pump keeps going up on the volume and the fuel starts building up behind regulator. At the end of the straightaway when the driver jumps out of the throttle, that big amount fuel behind the regulator can’t get through and it will actually flood the regulator and then flood the carburetor.”
Hickman says that a high quality pump like the one Waterman recommends for Dirt Late Model and Dirt Modified racing really does benefit from a fuel pressure regulator with a high volume return line. Being able to rapidly dump the excess fuel back to the fuel cell and drop the fuel pressure in the system helps keep the carburetor functioning optimally.
Plus, he has also found the benefit of running even lower fuel pressures. “We have found that we can run our fuel pressures about a half pound less than before, and some engine builders are telling us they can run as much as a full pound less than they were,” he says. “That helps a lot because the lower you can get the pressure going from the regulator into the needles and seats and the float bowl, the less aeration you’re going to get. That’s a big thing for us, you get a lot more “solid” fuel without any air bubbles going into the float bowl. We’ve seen racers being able to get down to four or five pounds at idle, and then on the top in they are trying to say around 6 1/2 to seven pounds of fuel pressure. There really are lots of benefits for this setup all the way around.”