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Hopefully after you find out your issue soon. As far as you wanting to know will your carbs work, the answer is yes to both of them but they both would need to be modified in order to get the proper tune on the idle circuit and adjusted to your camshaft. The 650 double pumper would be on the way richer side as it is out of the box. The primary metering block has a .031 to .033 idle feed restrictor size and the rear metering block has around a .039 idle feed restrictor size. This was on a Holley 4777-7 stamped on the main body.

On the number after the dash mark for example the -7, the higher the number after that the more newer the carb is as Holley makes tweaks to there carbs over time and 90 percent of the time is very minor. The 650 double pumper out of the box is setup for more racy motors and big cams and may or may not be too rich for your engine starting off wise but is in the starting ball field but it will still have to be tweaked some to get best possible tune you can get.

The Holley 600 vacuum secondary is a lot leaner out of the box and for mild engines with small camshafts and near stock or rv cam type of deals. Once you get into anything a lot more aggressive after that with cams around [email protected] duration and lower vacuum readings of 17 inches or less then it will always be too lean and will make you end up having the primary idle set open way too much to get a usable idle and too much opening on the transfer slot and cause your idle mixture screw to either not work well or not work at all.

The 600 vacuum secondary comes with .028 idle feed restrictor size in the primary metering block and the secondary metering plate has a .031 idle feed restrictor size. You would have to replace the secondary metering plate with a metering block. It would have to be changed I guarantee it as I use that exact carb on my build and I modified mine to work with whatever I throw it at on my small block builds.

Before you would go going to do these mods I will recommend that you get a Holley tuning book I listed below. It gives great detail on the different circuits on the holley carbs and how they work and stuff and will give you a better idea on tuning them for the best tune you can get.

Now the mods you can do is you will need a 61-80 drill bit gauge set with a pin vice and you can get them off ebay for pretty cheap and less then $20 bucks if you look good. It comes with tiny sized drill bits you will use to make custom sized idle feed restrictors for your carb. Then the next thing you will need is a good drill bit gauge set of different sizes drill bits to carefully drill out the pressed in idle feed restrictors in the metering blocks.

After that you use a 6-32 tap and then tap the holes left over where the pressed in idle feed restrictors were in the metering blocks. Then you would have to get some 6-32 x 3/16 brass allen set screws from as they have them for pretty cheap in boxes of 25 and 50 count and you would use your drill bit pin vice set to make them the size restroctors your carb would need to get the proper and best idle circuit calibration.

There is a bit more detail on doing the tuning with that mod but this is where you would start at.The thing is the idle feed restrictors are way more sensitive to change versus the idle air bleeds and in 95 percent of the time, I only had to change the idle feed restrictors out and not have to touch the idle air bleeds to get a spot on tune. Idle air bleed changes are more for the finest tweak and final overall tune but usually a wide band O2 is needed to really get the effect of what is going on when doing air bleeds.

It takes only a .001 change in idle feed restritor size to get a double digit percentage change in the mixture going through the ifr going up or down in size and its recommended to do a .002 change at a time up or down as needed. On air bleeds you have to have a .004 change and its sometimes not even noticed by feel or performance of it being changed and outside of using a wide band gauge it takes a lot of trial and error to mess with them to get the finest setting on those.

That will give you the best idle and off idle tune your carb needs and the best mileage on the transition circuit. I get all my carb stuff needed from and they are good on prices on things and they have the secondary metering block you would need for your 600 vacuum secondary if you were to want to run it instead of your double pumper.

I took my stock 600 vacuum secoondary holley and modified my metering blocks for adjustable feed restritors using the brass allen set screws and also my idle air bleeds as well and then got a seconary metering block and center hung fuel bowls and installed down leg boosters and added a quick fuel adjustable pod. I have had it on several builds and it runs like a charm once tuned in.

Here is the Holley book I recommend.



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