I have a Holley 700 0-4778-2.
These never seem to get a mention on this BB.
Was just curious why - are they rare or rubbish?
Would this have been fitted to anything as original equipment?
Just curious.
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It's a Holley "700cfm double-pumper". It's a good carb, about $380-400 new now.
It worked great but I only got 8mpg on a cruise!
Oddly some of the butterflies have small holes(bit less than a 1mm) drilled in them, two in each if I have remember correctly. I think this is something to do use on motors with very low vacuum at idle (I could be completly wrong here).
Would they ever have been supplied new with holes in the butterflies?
Oddly some of the butterflies have small holes(bit less than a 1mm) drilled in them, two in each if I have remember correctly. I think this is something to do use on motors with very low vacuum at idle (I could be completly wrong here).
Would they ever have been supplied new with holes in the butterflies?
You are right, the holes were put there by somebody after the carb was purchased. The thought is it would increase airflow for low vacuum situtations. I have made that modification myself and it worked quite well.
The holes are probably a modification (a trick) to the carburetor to aid in getting the engine to idle better with a "wild camshaft". (It's in most of the Holley modification books).
I don't want to say that they are definately there for that reason. They may be there for that reason.
Someone might call me a know it all again if I did.
I don't want to say that they are definately there for that reason. They may be there for that reason.
Someone might call me a know it all again if I did.
> It worked great but I only got 8mpg on a cruise!
What power valve and PVCR diameters did it have? Holley double pumpers
take careful tuning for decent fuel economy.
For a typical gasoline engine, maximum power is made with a fuel to air
mixture that is richer (25% or so more fuel) than the cruise mixture. To
provide additional fuel for high power or high load situations, Holley carbs
utilize a power valve circuit. When the vacuum level drops below a preset
level, indicating a high power condition, the power valve opens and additional
fuel flows from the power valve channel restrictions (PVCR's, also known as
PVCO's - power valve channel orifices). The PVCR's are the two small holes
in the metering block which are hidden behind the power valve. The diameter
of these holes determines the amount of additional fuel delivered, while the
trip point of the power valve determines when it is delivered. If you wish
to set a Holley up for best cruise economy and best wide open throttle power,
you're probably going to have to alter the PVCR's. Unfortunately, Holleys do
not provide a convenient means of making this adjustment. Changing the PVCR's
means drilling the orifices larger or epoxying them shut and drilling a smaller
hole. Note that it may be easiest to drill right next to the epoxied hole,
rather than drilling the epoxy itself. Use a small number drill in a pin vise.
I mentioned that for most applications, the difference between lean best
cruise and maximum rich power is on the order of 25%. This requires a PVCR
approximately half the diameter of the main jets, after you are finished
jetting. Holley had a funny habit of putting sewer size holes in for PCVR's
on hipo carbs. I think they figured they were saving hot-rodders from burning
pistons at full power when the got the mains too lean (but assumed that most
hot rodders had no clue what the PCVR's were for). I have a Holley 780 with
primary and secondary side power valves and the orifices are approximately the
same size as the jets and there are some Holleys that have PVCR's that are
actually bigger than the main jets. Also, when the first emissions controls
came online, they tried to lean things out at part throttle, and then use
enormous PCVR's to get some of the fuel back under load. If you set the
mains so the carb gives a decent mixture at light loads, it will be way too
rich at full power. Then there were the two stage power valves. However,
if you look at the OEM application 4150 Holleys from heavier trucks (Ford
391's, 370/429's, and 534's, etc.) you'll find they are sized about 50% of
the diameter of the main jets (50% of diameter = 25% of area which gets us
the desired 25% enrichment). That ought to tell you something.
That's just one area. You also need to dial in the accelerator pump,
get the main and secondary jets right, etc.
Dan Jones
What power valve and PVCR diameters did it have? Holley double pumpers
take careful tuning for decent fuel economy.
For a typical gasoline engine, maximum power is made with a fuel to air
mixture that is richer (25% or so more fuel) than the cruise mixture. To
provide additional fuel for high power or high load situations, Holley carbs
utilize a power valve circuit. When the vacuum level drops below a preset
level, indicating a high power condition, the power valve opens and additional
fuel flows from the power valve channel restrictions (PVCR's, also known as
PVCO's - power valve channel orifices). The PVCR's are the two small holes
in the metering block which are hidden behind the power valve. The diameter
of these holes determines the amount of additional fuel delivered, while the
trip point of the power valve determines when it is delivered. If you wish
to set a Holley up for best cruise economy and best wide open throttle power,
you're probably going to have to alter the PVCR's. Unfortunately, Holleys do
not provide a convenient means of making this adjustment. Changing the PVCR's
means drilling the orifices larger or epoxying them shut and drilling a smaller
hole. Note that it may be easiest to drill right next to the epoxied hole,
rather than drilling the epoxy itself. Use a small number drill in a pin vise.
I mentioned that for most applications, the difference between lean best
cruise and maximum rich power is on the order of 25%. This requires a PVCR
approximately half the diameter of the main jets, after you are finished
jetting. Holley had a funny habit of putting sewer size holes in for PCVR's
on hipo carbs. I think they figured they were saving hot-rodders from burning
pistons at full power when the got the mains too lean (but assumed that most
hot rodders had no clue what the PCVR's were for). I have a Holley 780 with
primary and secondary side power valves and the orifices are approximately the
same size as the jets and there are some Holleys that have PVCR's that are
actually bigger than the main jets. Also, when the first emissions controls
came online, they tried to lean things out at part throttle, and then use
enormous PCVR's to get some of the fuel back under load. If you set the
mains so the carb gives a decent mixture at light loads, it will be way too
rich at full power. Then there were the two stage power valves. However,
if you look at the OEM application 4150 Holleys from heavier trucks (Ford
391's, 370/429's, and 534's, etc.) you'll find they are sized about 50% of
the diameter of the main jets (50% of diameter = 25% of area which gets us
the desired 25% enrichment). That ought to tell you something.
That's just one area. You also need to dial in the accelerator pump,
get the main and secondary jets right, etc.
Dan Jones
To answer the question about the holes, they are typically drilled to allow more air through the primarys without excessively opening the primarys via the idle speed screw. Opening the primarys too much to achieve proper idle will uncover the transfer slots, adding to the idle/mixture/drivability issues. Good luck..