Have a overly rich carb, I have a roller cam and a 750 dp holley carb, My question is they have said drill the primarys or secondarys to help get the richness out, I know the transfer slot is uncovered by a lot and there is no adjustment in my mixture screws, What throttle plate do I drill primarys or secondarys? the second question is location to drill , in the center of the butterfly or close to the transfer slot ect, I have never seen a diagram on where to drill the hole or the best location. and is it primary side or secondary what one should I drill out, thanks for the help

Before you drill, make sure it isn't the power valve causing the problem. Check your vacuum at idle and make sure your power valve has a value about 2 in/hg lower.

I checked the power valve and it is fine, the float level is where they recommend and I was to the point of drilling the throttle plates, just didnt know if you should drill primarys or secandarys, I have seen people who say both, is there one to drill for better results and a certain location, like close to the transfer slot ect,

First of all, make sure you really need to do this. 1 2RUN was right on in what he said about the power valve. You said it was "fine" which I assume to mean that it is not blown. You need to make sure that it is the correct one! Measure your manifold vacuum at idle. If you have an automatic transmission, take this reading with the car in gear, brake set, preferably with someone in the car with their foot on the brake. A rule of thumb is a power valve rated 2" less than your vac reading in gear. If you have 8" of vacuum at idle in gear, use a 6.0 power valve. For a manual trans, take the reading in neutral, and divide by 2. If you have 10" of vacuum at idle, use a 5.0 power valve. What you are trying to accomplish here is to make sure that your power valve isn't open at idle, yet opens quickly as vacuum starts to drop. If you already knew all of this I apologize...just wanted to make sure you understood this before you start drilling throttle blades.

As far as drilling the throttle blades, (I assume you have a 4779) you drill the primary throttle blades on the same side as the transfer slot. Start small (1/16th) and work your way up if you need to. If you have one of the four corner idle Holleys, disregard this, because I don't know a thing about them.

Before you drill, I highly recommend you get a book and understand what's going on. I am referncing "Holley Carburetors and Manifolds" by Mike Urich and Bill Fisher. There is a fairly detailed explanation that will take you up to this point.

Very good explanation BPOS graemlins/thumbsup.gif

One more thing to try before you drill your primaries, is switch to a manifold vacuum source for your timing vac can. More timing at idle will raise idle speed which lets you close the throttle blades at idle a little hopefully "un-exposing" the transfer slot.

And one more thing ;) You can also adjust the secondary idle setting (even on a non-four-corner idle carb). There is a set screw that you can access from under the carb that allows you to crack the secondaries open just a little more. That might be enough to make a difference and get that primary idle adjustment back into the "window" it needs to be without drilling.

How much duration at .050? If the power valves are right try like Eric said open the sec throttle plates a little. I have seen that work very well if the cam is not too big.

I had the same problem with my 750 dp on my bb. After trying a few things I drilled out each of the primary blades to .065. If you was looking at the front side of the plate I drilled it in the very middle of the plate. I think holleys web site shows these throttle plates for sell with holes already drilled in them. It helped me get my idle mixture screws to work and to close up my throttle a little more on idle.

I am dealing with this on friends small block right now and today we are going to take the carb off and close the secondaries some. Your problem is the transfer slot is showing to much. You need to close the secondaries more and then the idle mixtures screws will work. Right now with the transfer showing alot it is pulling and air and fuel right into the carb n the secondary. Trust me, I had to do this on my carb because of my cam and I actually opened the secondarys to much and couldn't get it tuned right. I closed the secondaries some and everything is great now.

My motor is a 383 sbc final compression ratio of 10.7 to 1 Balanced rotating assembley, eagle rods ,Je pistons, steel crank, Heads are world products sportsman , they have had the bowls blended,Everything is port matched and a competition valve job.The cam is a comp cams roller cam duration @50 is 242/248 -570/576 lift and 110* lobe seperation, I am running a complete msd setup and mechanical advance distributor setup.The carb is a 750 double pumper, and i am running the hooker super comp headers .Actually the car has a rough idle since it has a large cam but I have a turbo 400 with a new 3000 stall converter,it seems to idle all day at 175-180* so it idles and doesnt seem to want ot get hot, I am running 38* total timing, have set float level where recommended, went to the #6 power valve since vacumn is only 8# at idle. well I hope to go to the drag stip this summer or within a few weeks since test and tune is starting and I am hoping for mid 12,s I changed the front springs in the car to hodchkis to lower it , and have a new 3.73 posi unit going in this week, I am going to put on cal trac traction bars and go with some street /drag radials, the car feel healthy I am guessing 400-425 horsepower ,I just need to get the mixture figuerd out, thanks for the help

I had the same problem with my 750 vacuum secondary Holley, couldnt get it to idle right without opening the primaries too far. I ended up opening the secondaries a little and raising my initial timing to 16 from 12. This worked but made starts a little harder. So I backed my timing down to 12 and used manifold vacuum for my vacuum advance as Eric68 mentioned. Since you have an MSD setup with mechanical only? then forget that.

I took my carb off and watched the primaries and secondaries as I opened them slowly counting the turns to see the maximum I could open them before too much transfer slot is showing. I tried this on 3 different 3310 750 Holleys and all end up about 3/4 turn adjustment being maximum for primary or secondary side. Open them until theres about .040, or a square of transfer slot showing, then put the carb on. Anyway, thats how I fixed it on mine maybe it will work for you, although having a vacuum advance would help alot.