Drilling hole in carburetor throttle plate

I have a 66 mustang with a 200 ci six,automatic that has been upgraded with a mild cam from Comp Cams. One step up from stock.

Having trouble getting it to idle smoothly in drive in the 600 to 700 rpm range. Will idle smooth in drive for

10 to 15 seconds,then begins to stumble as though running on 3 cylinders.Recovers when shifted to neutral.

Have read on line that a hole drilled in the throttle plate will fix this problem allowing more air into the engine.

This info was in regards to guys running 4bbls on V8's with cam upgrades.Could find nothing regarding 6 cyl.engines.

Carb is a holley 1940 and has been rebuilt. I can maintain good idle in drive at 800 to 900 rpm,but that just doesn't seem

right to me. I'm pulling 12 to 13 on a vacuum gauge. Timing at 12 BTDC.

Is this something I could do to lower the idle to 600 to 700 rpm and save wear on my transmission?

Drilling a hole in the throttle plate is a last resort.
I believe your distributor in general is not curved for your engine components & transmission converter if a C-4.
What are the specs on your camshaft????
Who ever is selling you guys with cams with these 108 & 110 L/C should have to live with their idiot recommendations.
Before you purchase a camshaft get an opinion from us on the forum.
Most of the problems can be resolved by looser converters & or proper ignition recurving custom tailored to your application.

The cam is from Comp Cams. Here are the specs.
Comp Cams
Grind F65 260h-10
Valve adjust intake exhaust
.000 .000
gross valve lift .440 .440
duration @ 006
tappet lift 260 260
valve timing open close
@.006 int. 22 BTDC 57 ABDC
exh. 62 BBDC 17 ATDC
Specs for cam installed...
@ 106.0 intake center line
INT. EXH.
Duration @ .050 212 212
lobe lift .2930 .2930
lobe seperation 110.0 902-12 valve springs required

What recommendation for a converter would I have to change out to? Trans is a C-4 and has been rebuilt by
previous owner,as was the engine.Distributor is a later dual advance version.Not a load-a-matic.

dRILLING A HOLE IN A THROTTLE PATE OF A CARB IS MOST OFTEN used to correct throttle plate alignment with the idle transfer slot

Most of your problem is in the distributor.
Thats a pretty mild camshaft so the 110 L/C is fine.
As i stated before you only drill an air hole in the throttle plate if at idle the plate is past the 1/3rd position in the transfer slot.

" Comp Cams: F65-260h-10
Gross valve lift .440 .440
260 Intake & exhaust
Duration @ .05 - 212
lobe seperation – 110

Distributor is a later dual advance version. Not a Load-O-Matic ".

Dat’s da important stuff (on cam), dude !
So, -can't get idle lower, C4?
I agree wid Bill & turbo - drill-out is a last resort.
A 1940 - ain't da best but let's lookat dat & ur TV cable to da tranny...

What is your source of vacuum: ported or manifold?

Vacuum is ported.Taken from the side of the carburetor. Do you think
taking vacuum from the manifold would help?

What would I need to do with the throttle cable to the C-4 trans.?
Make it tighter or loosen it up? Trans shifts hard from 1 to 2,smooth 2 to 3 shift.

Go to manifold vacuum & that will help. However you are using the vacuum advance as a band aid fix. Sometimes this creates a hesitation on acceleration.
Face it if you want superior performance & drivability your distributor needs to be recurved.
After that it is still ok to use vacuum advance at idle but the gap between is much less.
Get one of my DS11's or a one wire hookup is a HEI which I also market. Both would be recurved to your engine combination. You may be satisfied with the band aid fix. Keep in touch we are here to help. Bill

"...What would I need to do with the throttle cable to the C-4 trans.?…"
I've seen great vids on Youtube for the C4's TV cable ('kick down' is on che**ys):
how to adjust correctly, proper tool(s), real mounting tips (transmis end), hardware on carb...

Howdy Back Russell:

Sorry to be so late in joining this thread but, First, I'd start by checking the throttle butterfly to make sure it is closing completely. If it has been rebuilt, or not. To do this loosen the set screws on the butterfly shaft, open against the spring and let it snap shut. Do this several times.

Next a slow vacuum leak in the vacuum canister could be responsible for the symptom of going from good idle to poor idle quality. A good check overall of any and all vacuum lines and connectors wouldn't hurt either.

If you're sure of the condition of the distributor advance mechanisms try 15 degrees for initial advance. The extra advance will help offset the low rpm loss due to cam. And, just for kicks, try hooking vacuum advance line to a manifold vacuum source rather than the ported source, just for a comparison. Sometimes that works better on performance cams.

FYI, Holley #1940 carbs are notorious for idle quality problems.

FYI, Getting your distributor recurved (as WSA 111 suggested) is certainly worth the money.

So, get at it and keep us posted on your progress.

Adios, David

To wsa111...What specs will I need to recurve my distributor to work at 15 degrees initial timing?

Will I need some numbers to tell my electrical shop what to curve my distributor to?

Are there numbers for this application? Does my electrical shop know what to set the curve to?

Not familiar at all with the science involved in recurving a distributor. It makes sense to me to have it

done,in order to keep the engine from detonating at high speed.This is a foggy area for me.Can you set me

straight with this process.?

I can't give you #'s without knowing your static compression & would like to know your cranking compression.
15 degrees may not be the correct initial you need for your combination.
You give me all your specs & I will sell you a distributor that will put your engine on the correct track.
After that A/F ratio will have to be checked cause that can ruin a distributor specs especially if its too lean.

"...the science involved…"
I believe it's just the springs (2) on the plate & offered by the Mr. Gasket co.
Not sure an "electric" guy has the knowledge…
on the other hand I'm just comming frm the on-line demonstrations.

russell, the first thing to do before drilling the throttle plate, is to get the ignition system in proper order. that may mean sending your distributor to someone like wsa111 to have it recurved, or it may not. start with setting the initial ignition timing to at least 12 degrees BTDC. the vacuum advance needs to be from a ported source. now this assumes that you have a non load o matic distributor, and a non scv carb. if you have these either together, or a combination of one scv carb and non load o maitc, or vice versa, then you have other issues that need to be taken care of first.

I have found a guy close to me who knows a lot about this subject.(Distributor recurving).

Hope to have my Mustang in his hands by the end of this month.May he be the person to put an

end to this mystery of my bad idle problem. He also does not recommend drilling a hole in the

throttle plate. Waiting.