Drilling the throttle plates

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nothingbutdarts

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Need experiances with drilling the throttle plates.

OK , it's a "DORF:" It's a 77 3/4 ton F250. Worked over 351 with a Edelbrock Performer.

Problem: If the timing is set where it should be, the throttle plates are open so far the ported vacuum to the dist. is active causing the vacuum advance to be in at idle, I have to advance the timing so far to get it to idle and not have the ported vacuum active that it's not good for the engine. It runs very good advanced and equally good retarded, however it idles irratically when the dist. vacuum advance is active at idle.

Would it help to drill a hole in each primary plate so that I might be able to close the throttle plated more and get the ported vacuum port so it's not active there for being able to set the timing back down?
If so, where do you drill the hole. Towards the front of the plate, or the rear of the plate or does it matter? How big?

Any help and advice would be appreciated.
 
You normally drill the secondaries about 1/8 hole on the backside, to keep the primaries on the idle circuits. You can also crack the secondaries by that little set screw on the bottom of the carb. There is a kit somewhere that you can install at the base of the carb stud that gives a jetted bleed of air, same as the holes but its adjustable.

OOPS, just saw it is an Edelbrock...disregard.
 
never had to do that with a carter/eddy... have had to do that with a Holley though. I believe it was a 3/32 hole on the same side as the vacuum slots (front). Is the carb all clean inside and all the ports clear? I have had 4 or 5 of those carbs on different engines and never had to do that.
 
How about a carb jet epoxied into a vacuum cap on one of the vacuum ports? or even better, a little ball valve on a vacuum line into the cab, you could "dial-a-leak"....
 
when I drilled the holleys (had to do 2 of them, same engine though, different times though) I got the trick right out of a Holley tuning book. Come to think of it, they were both on my wife's car which happens to be a ford as well...
 
If you advance the timing at idle does it pick up RPM?

If it does, the engine wants the timing. If it's "warmed over" there isn't a book or set initial timing number, it's what the engine wants.

Probably going to have to set initial, then shorten up mechanical advance in the distributor.

My opinion on drilling throttle plates... don't do it unless every other option is exhausted.
 
Just for ***** N giggles how about you try the other vacuum nipple on the carb? I read somewhere that one comes in at idle and one doesn't. might not do the trick but worth a shot.
 
find a way to open up the secondary butterflies...even on a holly..if ya cant get it with that then you can resort to drilling but I'm stickin to my story....don't drill till ya tried other avenues......
 
I drilled my primaries 1/8" on my 850cfm TQ to help idle quality on a 340-4spd with a MP508 cam. It helped me idle around 900rpm with that cam.
 
MY340 said:
I drilled my primaries 1/8" on my 850cfm TQ to help idle quality on a 340-4spd with a MP508 cam. It helped me idle around 900rpm with that cam.
Click to expand...

mine are also drilled as it had a much larger cam in it before going back to a closer to stock grind.
 
Here's another vote for Crackedback's advice. Where did you get the idea that advancing the timing at idle is "not good for the engine"?
 
slantsixdan said:
Here's another vote for Crackedback's advice. Where did you get the idea that advancing the timing at idle is "not good for the engine"?
Click to expand...

Somewhere around the time when the starter wants to jump out of the engine because it's having a hard time cranking it.
 
Well, sure, but do you have to advance the timing that far to get the right idle speed? If so then yes, probably gotta drill the plates, but I'd try everything else, first. What's your PCV setup look like? A PCV valve with higher flow at idle will increase idle speed without having to drill the plates.
 
Put as much on it as the starter will take when warm, at least you know the limit. It may not want that much initial timing. Start around 15 btdc and see how it goes.

I believe some of the ford distributor vacuum cans work on manifold, not ported vacuum.

Unless it has a monster camshaft in it, drilling the plates is usually not necessary if you have the other systems dialed in properly.
 
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