Hi, my name is Holley...Blow Me!

(WARNING: long post.) My Holley 600 Double Pumper got the the blow-thru treatment today. I'm not really a carb wizard this is based on the hanger 18 mods (which has a much more detailed explaination of what I'm doing here) I started with a stock 600DP that I bought on ebay.

Disassembly. Removed the float bowls, removed the metering blocks, separated the body, removed the sqirters, and turned it upside down to get put the small check valve. Removed the accelerator pumps and springs. This has the old style with the check ball in the acc pump so I left that alone. Removed the needle and seats as well as the jets and the brass floats. This has vent extensions so I removed them as well. I also removed the idle adjustment screws and the cork gaskets. I then ran a file over the main body where the metering blocks mount to knock down any high spots. I went through with a razor blade and scraped down all the gasket surfaces clean.

Not necessary for blow-thru but helpful for tuning I took a file and knocked off the ridge on the vacuum port. This allows the bowl to be removed more easily for jet changes.

This is one of the big mods with a blow-thru...I drilled the PVCR (Power Valve Channel Restrictors) to .0650 The idea is this...when the power valve opens it allows more fuel into the main well...increasing the size of the PVCR allows you to richen up only under boost (when the power valve is open) and lets you run a smaller jet for cruise conditions. .0650 is only slighty larger than stock which is about .0590...but I can always go up if need be. The tape around the drill bit is so I accidentally do drill too far

...Here is the finished product. The metering blocks, float bowls, and other misc small parts then went for a soak in carb cleaner...The main body and base plate got a good scrub down with a soft brush and carb cleaner ...and then all got a blow dry with compressed air.

Next up Nitrophyl floats and jet extensions. Because with a blow-thru you are pressurizing the float bowls you cannot run standard brass floats...they will crush under the pressure. So, I added Nitrophyl floats which are solid and will not crush. The rear also got jet extensions as well...they come with a special notched float to fit over the extensions. These are so that under acceleration when the fuel sloshes to the back of the bowl it doesn't uncover the jets leaving you lean...not a good thing in a turbo car.

Bigger needle and seats. A blow thru can go through a lot of fuel...and the fuel system must keep up...this includes filling the float bowls...I added .120 needle and seats to allow more fuel into the bowls keeping them full under high demand conditions.

Power valve. Nothing fancy here...just a standard 6.5 power valve. We'll see how this works come tuning time I know a lot of blow thrus use something lower like a 4.5. [Edit: I ended up going with 4.5 to start with...]

The rest of the reassembly. Here are the metering blocks all ready to go back in...new jets, PV, needle and seats, and gaskets. I took a shot in the dark for jetting...I put in 74/84 as a baseline...I figure at least it should not be lean with this to start.

Next since the choke horn has been removed from this carb I have to block off the hole where the rod used to come through as this would cause a boost leak. I small chunk of the JB Weld Stick worked great.

That is it. Here it is reassembled. I'm still gonna run some epoxy over the tops of the metering bocks where the plugs are just for extra insurance so that the boost won't blow them out...but otherwise I'm ready for a blow.