THE Holley HP Ultra range of carburettors are terrific for high-performance applications, and just recently I was asked to troubleshoot a HP 650 Ultra XP carb. It was on a 304 Brock Commodore that wouldnít idle with a 244@.050 solid cam. If youíve grown up with EFI, chances are tuning any carb will be a daunting task, let alone these XP Ultras with their tiny jets and air bleeds everywhere.
So I thought Iíd try and explain one aspect of the process: the power valve. The purpose of the power valve is to open under acceleration and make the fuel/air mixture richer for more power. Itís controlled by engine vacuum; when you push down on the accelerator pedal, the carburettor butterflies open up, the manifold vacuum drops and the power valve opens.
The numbers on the side of the power valve indicate the amount of vacuum needed (in inches) to open the valve. So a 65 power valve doesnít open until the manifold vacuum falls below 6.5 inches; a 105 opens when vacuum falls below 10.5 inches; and so on. The number has nothing to do with fuel flow rate.
A small mild cam in a well set up engine can idle at, say, 18 inches of vacuum. The greater the camshaft duration and overlap, the less vacuum there is at idle. And when the vacuum at idle falls below the power valveís vacuum rating, the valve opens too far and makes the idle mixture too rich, causing the motor to die. So the power valve must be rated lower than the amount of motor vacuum at idle. When a power valve opens it is the same as increasing the main jet size by eight.
So hereís how the five-litre Holden was set up to idle. The 650 Ultra XP had power valves in both metering blocks, so we removed the float bowls and metering blocks. For most applications a power valve is not required in the secondary metering block, and itís best to block it off with a Holley PV plug and increase the primary jet sizes by 8-10 above what are fitted standard. I replaced the 70s with 80 jets at the time. To determine the primary power valve number we needed, we did the same to the primaries, blocking the power valve off and increasing the jet size. The tapered idle mixture screws were wound in gently against the seat, then backed out one-and-a-half turns.
These carbs have four corner idle screws and they need doing all íround.
The assembled HP 650 was then bolted back on, and when the owner hit the ignition key the engine sat there and idled. I put a VDO vacuum gauge on the intake and it was sitting on 5.5 inches of vacuum at idle. A 35 power valve was then fitted to the primary metering block, the 70 jets were refitted, and the 304 idled like it was supposed to.
Something really important for automatic applications is to check the vacuum at idle with the car in gear. An engine can be idling nicely at, say, 1000rpm in neutral, but with the transmission in gear the converter can pull the engine down
enough to activate the power valve, go rich and die. Make sure someone is sitting behind the wheel with the brakes on hard when setting the idle with the auto in gear.
When a small power valve is used, such as the 35 used here, there can be a lack of response at part-throttle, such as when you put the accelerator down a little bit to go from 60km/h to 100km/h. The motor will seem to hesitate and die. What is happening is that the car is cruising on the transition or main circuit at maybe 12 inches of vacuum, and when you put your foot down the vacuum doesnít drop enough to make the power valve open, so the engine runs lean when it needs a bit more fuel to accelerate. If this is happening, a quick Band Aid cure is to go up two jet sizes, for example 70 to 72. Normally the Holley carbsí mains are jetted so well out of the box they rarely need changing. If 70 jets are fitted, on a dyno the carb will have nearly perfect air/fuel ratio for power.
Itís been my experience that when a car is cruising and bogs during part-throttle application it is often the idle feed restrictions that need to be bigger, not the main jets. The idle feed restrictions meter the fuel to the transition slots, and in many cases a lot of street driving is done on these transition slots. The Holley XP Ultras have removable restrictions so itís very easy to tune them by swapping the restriction jets. On early-model Holley carbs I used to drill the metering block idle feed restrictions out to 40thou or so. Or if too big, I would use 10A fuse wire to reduce the area.
I hope this helps some of you if you encounter an idle problem with a new Holley carb. Many times a Holley will bolt on and work right out of the box, but sometimes with big cams and little engines they need a little tweaking to get right. s