drill holes in throttle plates?

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.

Results: with ported vacuum driving the vacuum advance, throttle modulation off idle is MUCH improved, and the need to brake torque at stops is reduced (though not eliminated). Manifold vacuum is 15" at idle, ported vacuum is 12" (this is with the throttle blades completely closed). When the throttle is opened, both drop, though manifold vacuum drops much more than ported does, probably because the ported side has all of Earth's atmosphere to suck on. Both vacuum levels then increase as engine RPM increases (all this is with no load). The spark advance is thus not being retarded so much off idle when using ported vacuum, and so it makes sense that power comes on more gradually. That was a very insightful suggestion, thanks!

I have also noticed a "chug-chug" mode when starting warm without opening the throttle- there isn't enough vacuum to advance timing, so the engine chugs along like a steam engine at low rpm until it either dies or you open the throttle a little, which develops vacuum, which advances the timing, and then the engine idles smoothly with the throttle closed. Is this why they had throttle stop solenoids "back in the day"?

Carb numbers are 3437S front and 3858S rear. The dies used to make the stampings are very different, so I suspect they are not of the same vintage.

Now if I can make it smoke a little less...

3437 is for a 383, the other is 413 but they are small secondary carbs and both are rear carbs. Not sure how you have progressive linkage with them. All extra vacuum ports plugged? Which carb has distributor connected? You would be better off with some clone carbs or real hemi carbs in my opinion.

Vac advance should be on primary carb, which is the rear one, and ported vacuum.
Smoking ? Oil or fuel ?
On mine, we had to stagger-jet, change the metering springs, and use silicone umbrella valve seals from Silver Seal; cleaned it right up.

After all this fiddling I don't doubt it. Vacuum advance is now connected to the ported vacuum port on the 413 carb (rear), all other ports blocked or capped except the big one on the back of the rear carb for the brake booster.

I made the throttle linkage with spare parts and stuff from Ace Hardware. It seems to work fine. The photo was taken before the rear carb went for refurb; it has a choke and a fast-idle linkage now.

Something is not right as you should have NO vacuum at the front (PORTED) opening at idle. If you look at the two pictures below, In the 1st pic you will note the side port is much lower than the front one. That is because the side port sees manifold vacuum for an external use such as a choke pull off. The front port is higher as its opening is ABOVE the butterflies and NOT exposed to manifold vacuum. The vacuum at that port is created by a "Venturi" effect. IE: as more air passes by the opening it begins to try and draw air in creating vacuum.

The second pic shows the path for the lower port. The Front port cannot be seen as it is ABOVE the butterflies. If yo have the carb off, and open the primary butterflies you should see a small round hole that feeds the front port. Blow air or carb clean through it
It cannot have any vacuum being drawn at idle unless, A. the butterflies are too far open or, B. the butterflly for that side is not closing in conjunction with the other side

Your chugging when starting is likely due to your not opening the throttle a bit when starting. These cars are not fuel injected and need to be dealt with differently than today's vehicles. The other possibility is that fuel is percolating in one or both of the carbs

I am not trying to be offensive but you have a 25K motor with mismatched carbs and are asking for more expensive problems if you don't either A. get a pair of carbs meant for a dual 4 street setup. Or B. Drop back to a single 4 intake. Most will not notice with the air cleaner your running. Either way you will need to spend some $$ to do it properly. If you happen to have a nice wife🙄🙄🙄 PSST, Santa's coming Actually you might need this one

BTW, I have pre made factory appearing throttle cable brackets and kick down adapters for the single 4 setup

Here is a video showing an alternative to drilling throttle plates. DISCLAIMER: This is an Uncle Tony video. He seems to be controversial. None the less- here it is:
https://www.youtube.com/watch?v=JvL0-pBWb44

I'm just having a hard time trying to understand how a multi carb setup would ever need drilled plates. There's at least one extra idle circuit that can be used.

Somewhere you are getting extra air (I still say there are a pair of butterflies not closed somewhere) Two holes in the butterfly don't cause it to idle at 2000.

Vacuum at a part-time port means that those primaries are open.

Maybe you think your closing the butterflies but your linkage is hanging them up.

Your linkage is attached wrong. The slotted side goes to rear carb. Rear throttle can open without pulling on front. At WOT you adjust that linkage that both carbs full open. It looks like your linkage is too long and already out of adjustment and the front may not be opening soon enough or go full. But that is just a guess from the picture.

The choke pull off tube should be plugged on both carbs but I am not sure what type of choke and how they attain pull off. On the hemi integral choke, there is no tube and the housing mates to the pulloff port. You can then add the electric to the housing. I attached a 70 hemi rear that has the electric inplace of a heat tube type.

I think they ruined that rear carb for you. You can't get a decent balance anymore. They should be able to replace the primary blades for you.

Everything about hemi had to do with proper distribution with the hemi manifold. Staggered jetting and rods for some. Long down stream venturis and venturi tabs on some bores.

The linkage works as you describe. The slot is at that end because it made the hardware work out. I also made sure neither accelerator pump either held a throttle open or kept it from opening fully. Installed choke is electric and covers a vacuum port behind the housing. Seems to work OK. Rods and jets are staggered but that's just copying the values from a book, not experience or tuning.

What carbs would you recommend?

Edelbrock makes a pair that are for a dual 4 setup But I don't have any experience with them.
The bore spacing may be an issue as well as the air cleaner opening size. The early AFB's had a smaller opening than the carterbrocks and holleys. I believe the bore spacing is also different but not 100% on either. I have both 5/16 and 3/8 inverted flare fittings for the edelbrocks to retain a stock appearing fuel line.

What year is your car? You can pick up early version Hemi carbs at a more reasonable price then later carbs. I have a set of 66 carbs but they are a little pricey as they are cadmium plated so would be more for a points car. I do have a 67 set but have not finished them yet.

If you went the Edelbrock or early carter 9625 types, I could provide you with the correct early tops that are 4 1/8" opening like original.

Hemi had 1 7/16" primary with 1 11/16" secondary. 4139/4140 for the 66 cars. 4139/4343 for 67 cars. These would be the pre emission carbs and air bypass type.

I think Mancini sells the hemi clones with reproduction levers and jetting for hemi. I also think someone had a set on ebay for sale recently.

More about your motor and I am sure folks could help steer you.

Engine is a '66 out of a drag car and rebuilt years ago. Carbs I have been messing with came with it. Has lopey cam and headers, stock intake. About 7-9" of vacuum at 900 rpm, which can only be reached with no vacuum advance attached. Current rear carb (rebuilt 413) shows about 12" ported vacuum at 1150 rpm, which is where it idles if vacuum advance is hooked up. Jetting is as close to stock as I can get it, runs OK on 91 CA gas plus octane booster.

Anyone get the clones from Harms Auto? Engine is not in an original Hemi car, so functional equivalents are OK.

Since you have a bumpy cam, stock jetting would only be a starting place, at best.

Those tiny 413 carbs will hold you back a bunch
I would sell them and either find a set of Carters for dual in line carbs or a set of the Edelbrock's and set them up like the OEM Carters were with that weird stagger jetting that works really good