Has anyone drilled holes in their throttle plates, what for, and did it have the intended effect?

Yes and yes. Occasionally in order to get a motor to idle you have to open the butterflys past the idle transfer slots so now the motor is running (idling) on the primary circuit instead of the idle circuit and any adjustment to the idle screws is pointless. By drilling holes in the butterflys they allow just enough extra air such that the butterflys can now be closed enough that they cover the transfer slots and allow you to tune the idle circuit. A 1/8" hole in each primary blade is a good starting point and it doesn't matter where you drill them as they're only purpose is to bleed air into the motor. If your car idles on the idle circuit there is absolutely no point to drilling the holes.

Agree with all of the above except the starting drill size. The required hole size will depend on how far above the off idle transfer slots one is. 1/16th to 3/32 is where I usually start. It's easier to make them bigger than smaller is what I was taught

OK, I follow that. I have an engine with a lopey cam and drilled (not by me) throttle plates that will not idle below 2000 rpm even with the plates fully closed. To me this says too much air is getting through.

I figure I can either take some timing out or close the holes with epoxy.

Make sure the idle timing is in a decent range for operation.

Lopey cam, likely at least 18-25* as a guess.

JMO. Get it started, time it at 36* and see if you can get the idle under control. If the blades are all the way shut and you can't get it to idle down, sounds like a leak somewhere.

Agreed on all of the above. The center carb on the 69.5 A12 cars were drill from Holley on the manual trans cars ONLY(4391). They came from the factory with number 225 throttle plates and they are drilled out to .079. I’ve got a cam that only pulls about 12-14” of vacuum in my AAR. I’ve got 225 plates in mine and they work great. In my last tuning job using an AFR meter, you can really see the difference in even a small change in the size hole. In my vast collect of six pack carbs, I’ve got quite of few throttle plates with holes of all sizes in them from “tuners” in the day.

Make sure the secondaries aren't opened to far. Sometimes those get played with also.

Can you temporarily stick a small piece of (duct) tape over the holes or some way of closing them and check the idle difference? No need to drive it, just warm the engine up and then close the throttle plate holes and see if that's your high idle problem. Does it "diesel" sometimes when you shut it off currently with the holes?

Question #1 What carburetor(s) are you working with? some have the adjustment on the secondaries while others do not

Question #2 have you verified the timing mark (TDC) on the dampner is is the correct spot?

Question #3. If so to #2, What is your initial and total timing

Question #4 Are you running a vacuum advance?

Question #5 If yes to #3, is is hooked up to ported or manifold vacuum?

Either way disconnect and plug the port until the idle and timing are sorted out.

Question #6 What size are the existing holes?

Next read my post 392117 above

Might also want to read 392125 AND 392126.

You can back the timing off but if the holes are too large you are band aiding the root cause of the issue which has to isolated.

Closing the secondaries a bit can also help but do not close them too far as their purpose is to (bleed) keep fresh fuel in the secondary bowl(s) should they not be used for an extended time.

As far as timing goes, the initial and total will be determined by your combination. As this is an idle issue I would start at ~ 10-12 initial.
Some may argue with the above but it is a starting point to get the idle under control. once that is accomplished

Do the above and report back

You may also as mentioned in 392134 have a Vacuum leak. I would verify the above first. We can then look at a possible vacuum leak

How about looking for a vacuum leak first instead of going through the ten or eleven steps first?

How would you go about trying to find a vacuum leak? Spraying carburetor cleaner around the manifold and carburetor or spraying propane (not ignited of course) around the same areas?

If it were me I would first figure out a quick, safe and easy way to temporarily close those holes in the plates to determine if they are actually a problem or not? At this point it's only a guess if they are causing the problem,

You may need two beers.

The lopey cam is in a gen 2 Hemi, stock inline manifold with mongrel AFBs, jets and rods stock values, blue metering rod springs. Rear AFB went to carb shop for addition of electric choke and they told me it was from a 413. Anyway it comes back with a choke and the drilled holes. Both carbs have throttles closed, set up in a progressive linkage. At first set up with no vacuum advance and about 8 base, car would idle at 900 rpm and 9 inches vacuum (with throttles closed), but top end wasn't great. Watched junkerup's video about using vacuum advance and a light went on. Advanced base a few degrees, hooked up vacuum advance to manifold vacuum (side port on front carb), and it went to about 1300 rpm at 23 degrees. WOT much improved (honest, officer...) What happened to 2000 rpm, I dunno.

Pulled a plug (only an hour or so of run time) and it was brown. Ecch. A/F says 14.7 at idle, lean while cruising (about 2200 rpm). Don't dare take my eyes off the road at WOT to read the meter. But it's spending a lot of time at idle in the garage while I mess with the settings.

Timing readings a bit erratic as timing light is having problems triggering with this car (ok with others, clip is "pointed" the right way). But right now it looks like 1300 rpm at 25 degrees advance (base 11) and 15 inches of manifold vacuum. If I switch to ported vacuum (port on front of rear AFB), the rpm and advance don't change. I think I should close up the holes on the throttle plates and tweak the mixture screws (all four) for best idle.

You may well need two beers. The idle and off idle on AFBs can be a royal pain in the a$$ to tune. If you eliminate the other possible problem areas and have isolated the problem to the tunes in the carbs, the AFBs can be much more difficult to dial in than the Holley platform carbs which can actually have adjustable idles and transitions. Unlike the CarterBrocks.

Timing is CRITICAL with cams that have a good amount of overlap. And it can be hard to stabilize lots of initial with the idle bouncing around. Often 25* at idle and all in by 2,000 is necessary to get some combos happy. Then you have to worry about kick back when starting, especially hot. And if the engine likes the timing all in that early when in high gear. Lots of things to consider. And everything affects everything else.

Good luck.

P.S. I have 26* at idle and all in by 1800 in my 540 Hemi. It does hit the starter some when hot, but not enough to back it off and suffer the other consequences. It also took some tweaking of the Dominator carb to get the best idle and transition. That would have been much more difficult with CarterBrock(s).

Switching to ported vacuum with no change indicates and issue with that port as there should be NO VACUUM there UNLESS the primary butterflies are too far open. Also note your manifold vacuum at WOT is likely pretty low ( meaning no advance) as using manifold vacuum typically is only beneficial for cruising ( here come the argments ) if you happen to pull the carbs, measure the hole size, and hold them up to a bright light to see if the plates are centered as best as possible in the bores. If not slightly lossen the screw and center them. An off center plate can hold the throttle open increasing the RPM at idle. You can also check the opening for the ported vacuum which should be closed off and fully above the blades

Without actual carb numbers this is weird diagnostics. Hemi did not need holes in throttle blades. 66-67 cars where air bypass, so you had an air valve. Venturi in Hemi are different type than most AFB non hemi. If you have odd carbs you probably also have a distribution problem. Is the manifold Vanke mod? Ignition type? 68-71 carbs where emission carbs and leaned out, but still no holes required for stock car.

Even folks that use Edelbrock clones don't seem to have issues when jetted correctly.

Did the carb shop have your car when they drilled holes in carb blades? Otherwise confused why a choke swap got you a modified carb.

Agreed 100%. The addition of a choke should not require adding holes to the primary butterflies. One could go so far as to ask them about it but be prepared for a bull💩 reason

I can't really tell what the vacuum is doing under load since all these measurements take place in my garage. With no load the vacuum will drop when the throttle is first opened, then quickly increase again as the engine speeds up and sucks more air through the carburetor. I have to imagine that under partial and light loads on the road the timing contribution from vacuum is significant. I road test the car to see if the total timing is too much, and so far it isn't.

The similarity of the vacuum level at the manifold and ported ports I chalk up to the high idle speed. I'm sticking with manifold vacuum for now.

The additional advance at idle makes it idle nicer, just faster than I'd like, so I want to stick with vacuum advance. I plugged the holes in the throttle plates and got the idle down a little. Idle mixture screws still effective so I'm not sure the holes were ever needed.

Carb shop only had the rear carb. I told them it was a non-original carb from a dual-quad Hemi. I don't know why they drilled the plates, I hadn't asked for it, just the choke. They said the carb was from a 413- maybe the other one is, too. The Hemi manifold is stock AFAIK.

The car ('70 Charger auto) is OK on the open road but difficult in around-town traffic because it stalls unless you brake-torque it. It's clearly a work in progress. I suppose the real fix for stalling is either a different cam or a different torque converter.

Certainly off-idle and light loads are where the car is not well behaved- power does not come on gradually. If my tuning difficulties stem from having the wrong type of carbs instead of my ineptitude, I'll buy into that explanation. I'd need to know what to replace them with.



I am getting REALLY good at taking this air cleaner off and putting it back on.

Run the vacuum guage inside the car or duct taped it to the windshield The Vacuum is doing excatly what i tis supposed to


Your comment on the two ports is likely correct. With the holes plugged and the butterflies lowered you should no longer be seeing any vacuum at the ported opening

Try bumping it into neutral when stopping and then back into gear when taking off (Did it for years for the same reason IE: converter to tight for the camshaft

using manifold vacuum for the advance will aggravate the low speed drivability as the advance drops out when accelerating and does not return until cruise. The ported vacuum should climb and add more advance until the throttle is opened beyond a given point. by that time your mechanical should be taking over. Hope this makes sense

Sad part is I used to live just up the road from you off Bernal and 101

Try bumping it into neutral when stopping and then back into gear when taking off (Did it for years for the same reason IE: converter to tight for the camshaft)

Yeah, doing that too. Not the most elegant, but it gets you there.

using manifold vacuum for the advance will aggravate the low speed drivability as the advance drops out when accelerating and does not return until cruise. The ported vacuum should climb and add more advance until the throttle is opened beyond a given point. by that time your mechanical should be taking over. Hope this makes sense

Now THAT is an interesting idea. I'll try it tomorrow.

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What numbers are on the mount flange of the carb? Passenger side front.