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Old 03-01-2006, 08:34 PM
72NOVA454
 
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To Drill or Not To Drill (Carb Question)



Ok, time to vote.

I've got one of those rough idle, stinky smelly, burn your eyes out difficult to idle situations with my BB chevy rebuild. New big cam installed. No vacuum leaks. If I can't get the idle mixture screws to do the trick and refuse to buy a new carb at this time, should I try drilling 1/8" holes in the primary throttle plates to make the engine run leaner?

Please vote yes or no.

thanks.

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Old 03-01-2006, 08:36 PM
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Leejoy,


NO

That's my vote.

scholman
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Old 03-01-2006, 08:39 PM
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Wait... the idle screws do nothing? Same thing happened on my Mustang. Make sure that timing is PERFECT. Run the car awhile, then try again, it might work. My idle is perfect now.
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Old 03-01-2006, 08:55 PM
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How do you have your idle screws (front and back) set? You should have a perfect square on the primary transfer slot. Once that is set, adjust the air mixture screws, it you need a little more idle speed open the secondaries a bit (with the screw you have to get at from the bottom). Does your idle mixture screws have any effect on the idle? If not you probably have your throttle plates open too far (idle screw). If you can't get a decent clean idle, you may have no choice but, to drill the primary throttle plates. Sometimes that is the only (easy) way. You don't have screw in air bleeds right?

Royce
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Old 03-02-2006, 07:00 AM
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hey camaroman help me out

what do you mean by "perfect square on the primary transfer slot". Also, my cheapy holley 650 carb has idle mixture screws on the primaries only - not the secondaries.
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Old 03-02-2006, 07:06 AM
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If you look at the throttle blades from the bottom the transfer slot will make a perfect square. If you have too much transfer slot it will idle rich.

The transfer slot delivers fuel as the carb transitions from the idle circuit to the main circuit.

Some carbs have a stop on the secondaries that can be altered to crack them open to allow a little more air. This is better than drilling the throttle valves.
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Old 03-02-2006, 07:31 AM
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ok - so I have to remove the carb to check that right? I already did something like that last summer before the rebuild. I have the secondaries opened a bit to allow more air into the manifold. that allowed me to close down the primaries back to a position where they are using the idle circuits again. now that I am done with the rebuild and having this start all over again, I guess I will start from scratch and adjust the mixture screws also. are you saying it is basically the same result if I drill the primary blades "or" open the secondaries a little more?
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Old 03-02-2006, 09:30 AM
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Thanks Infomaniac.

Leejoy, Yes you have to take the carb off and turn it upside down (you probably want to drain the float bowls first). Opening the secondaries should be done before drilling. Meaning drilling is the last resort, you don't want the secondaries opened too far either. What the drilling does is lean out the idle a bit. With long duration cams sometimes, that is the only way to go.

Royce
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Old 03-02-2006, 09:32 AM
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carb

What is you initial and total timing????

keith
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Old 03-02-2006, 09:50 AM
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keith

it's a mess right now. the initial timing "for today" is about 35deg BTDC at idle (no vacuum advance) don't know total timing yet because I am still tuning it and selecting which MSD springs I should use on the centrifugal weights. Help................
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Old 03-02-2006, 09:50 AM
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Quote:
Originally Posted by camaroman7d
Thanks Infomaniac.

Leejoy, Yes you have to take the carb off and turn it upside down (you probably want to drain the float bowls first). Opening the secondaries should be done before drilling. Meaning drilling is the last resort, you don't want the secondaries opened too far either. What the drilling does is lean out the idle a bit. With long duration cams sometimes, that is the only way to go.

Royce
royce

I already have the secondary throttle plates cracked open a bit. perhaps I need to do more.
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Old 03-02-2006, 10:08 AM
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Drilling carburetor

Before you make any modifications to your carburetor you need to get everything else set properly first.

First you've got get your ignition situation straightened out. Timing issues will affect how the engine idles.

From there you'll need to go through and baseline your carburetor. Setting the amount of transfer slot that is exposed on the primary, and secondary side.

If you can screw the indle mixture screws all the way in, and the engine still runs the engine has to be getting fuel from some place else. The next logical spot would be that the primary butterflies are open too far, giving the engine fuel.

What kind of RPM are you looking at idling the engine at?
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Old 03-02-2006, 10:16 AM
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Dear BG tech support.

first of all thanks for responding. very nice of you.

I don't know where to set the timing. It's a custom setup of course - not factory. Big hydraulic cam in there. right now the initial timing is approximately 30 deg BTDC at idle (no vacuum advance) If I go less or more than that 30 deg number, the engine starts to have troulble staying running. the primaries are down pretty good drawing from the idle circuits not the transfer circuits (did that last year before the rebuild) the secondaries are cracked open to allow a leaner mixture (but they may have to be cracked more) I will play with the mixture screws next time I run the motor and test them as you say. I am looking to get the normal 600-700 RPM at idle when the car is in gear. Is that ok? Someone mentioned to me to check the power valve on the Holley carb. what are your thoughts?
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Old 03-02-2006, 10:21 AM
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Quote:
Originally Posted by leejoy
keith

it's a mess right now. the initial timing "for today" is about 35deg BTDC at idle (no vacuum advance) don't know total timing yet because I am still tuning it and selecting which MSD springs I should use on the centrifugal weights. Help................

You need to get the base timing down to around 14 to 18 degrees before trying to figure your mechanical advance. Then proceed as all the others have said on the idle slot and secondary throttle plate adjustments. Drill the primaries as a last resort.

Personally, on a 2 mix screw carb, I prefer to drill as apposed to opening up the secondaries. The front of the carb will be richer and the secondary side will be fairly lean drawing in the extra air without the added fuel from 2 add'l screws.
On a 4 screw carb, its easier to open the secondaries some and then compensate on all 4 corners.

My son's 400 did the same thing, cept we had a Qjet on it. I ended up drilling a 5/32 hole in each primary butterfly and it cured the idle mix problem right up, but its a permanent modification too, short of changing out the primary blades if you drill too big.

How big is the cam?
A "big cam" and wanting good idle @700 in gear may not be possible, as we found out. We ended up with 700 in gear but in park it jumped to 1200 and would not shut off. We ended up putting an idle solonoid on it to get it to settle down when we shut the key off.

Mark

Last edited by Jmark; 03-02-2006 at 10:44 AM.
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Old 03-02-2006, 10:41 AM
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Leejoy,

How big is your camshaft? What is the rest of your engine combination?
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