Question For Carb Guys - Drilling Hole In Carb Throttle Blades

[enigma57]

Have a question regarding drilling a hole in carburretor throttle blades. I realize that Holley drills their holes on same side of throttle blades as transfer slot.

However, I have always done it on the opposite side of throttle blade from the transfer slot. Figured this would maintain same flow pattern in area of transfer slot as undrilled throttle blade.

So is one way better than the other? And if so, why?

Thanks,

Harry

[Tuner]

Locating the hole near the curb idle and transfer slot discharge encourages vaporization by impinging the high velocity air coming through the bypass holes directly on the fuel coming from the idle discharge and T-slot orifices. If the idle vacuum is above about 14" Hg the jet of air is sonic velocity in the orifice. With very few exceptions, all OE carbs I can think of are made so idle bypass air is introduced near, and aimed at, or directly below, the idle and T-slot orifices. I have seen reduced UHCs (several times) result from relocating the bypass air closer to the idle fuel. Today's pump gas vaporizes so poorly it needs all the help it can get, particularly with unheated "air gap" intake manifolds.

[enigma57]

Thanks, Tuner! That makes perfect sense. Will drill throttle blades as Holley does then.

Best regards,

Harry

[enigma57]

Might add that carbs in question are Weber DCNF. They are designed for IR intake. However, I am adapting them to plenum intake and must drill holes in throttle blades, as they won't idle on plenum intake without uncovering progression holes unless holes are drilled.

I will drill in area marked '0' on underside of throttle blade......



Best regards,

Harry

[statsystems]

Locating the hole near the curb idle and transfer slot discharge encourages vaporization by impinging the high velocity air coming through the bypass holes directly on the fuel coming from the idle discharge and T-slot orifices. If the idle vacuum is above about 14" Hg the jet of air is sonic velocity in the orifice. With very few exceptions, all OE carbs I can think of are made so idle bypass air is introduced near, and aimed at, or directly below, the idle and T-slot orifices. I have seen reduced UHCs (several times) result from relocating the bypass air closer to the idle fuel. Today's pump gas vaporizes so poorly it needs all the help it can get, particularly with unheated "air gap" intake manifolds.

So you are saying to make the hole away from the throttle shaft and make the hole near the transfer slot?

[MadBill]

A word of possible warning: I'm not familiar with the DCNF models, but many Weber carbs, e.g. DCOE, do not feature a 'power' circuit. Instead they rely on the pulsating flow of an IR runner to enrich the mixture from a lean cruise (where the near-closed throttle blade damps the pulsations) to a correct WOT-rich AFR. Fitted to a plenum manifold and jetted correctly for WOT, they might run very rich at cruise.

[Geoff2]

Some QJ carbs had bypass air capability built in. The air entered the air horn ABOVE the boosters & travelled down a passage in the main body, exiting below the t/blades. The reason for this method of bypass air was the extreme sensitivity of QJ primary air flow, where air flow THROUGH the boosters [ holes drilled in t/blades ] could cause nozzle drip.

Webers. I ran dual 48 IDF Webers on an adapter on a Holley Street Dominator intake manifold. I had the carb mounting pad welded up to increase the opening size. This was on a 440 Mopar. It ran very well & mileage was similar to a 4bbl.

Many people here use similar setups [ often 44 IDFs ] on Holden & Ford V8s.

[enigma57]

Thanks to all. Really appreciate your insight in this.

I have set up several of these carbs for plenum intake. The DCNF carb has an idle air bypass for each throttle bore (used when balancing flow on IR intake). Unfortunately, even when both idle air bypasses are opened all the way, they do not supply sufficient quantity of air to allow correct relationship of throttle blades to 1st progression port at idle speed on plenum intake. I have found that 3/32" (2.35mm) holes drilled in both throttle blades seem to work well with both idle bypasses closed, though.

I have always drilled these holes on the opposite side of the throttle blade from the progression holes and that has worked well. Was just wanting to know if drilling them on the same side as progression holes/transfer slot might work better, as Holley positions their holes on that side.

Many thanks for your input,

Harry

P.S. >>> Bill, as the DCNF is designed for IR intake, it has no power valve per se. So yes, the air/fuel mixture throughout the operating range must be set up using the correct emulsion tubes, jetting and air correctors. A fine balancing act. This is further complicated by the fact that today's pump gas is lighter and has different properties than the heavier fuels these DCNF carbs were originally tuned for on IR intake in the late '60s through the early '80s. So setting one up now of days is pretty much like reinventing the wheel. Especially when reconfiguring it for use on plenum intake, as well. For use on plenum intake with what passes for pump gas now of days, float level must be lowered 1 to 2 mm, choke (main venturi) tubes decreased in size, main jets and pump jets increased in size significantly and idle (slow running) jets increased a few steps. Air correctors reduced in size a step or two. Emulsion tubes sorted out by trial and error. Lots of fun.

There is a mechanical enrichening system that comes into play when the throttle blades are opened. This consists of a pump diaphragm on the DCNF which is actuated by a pivot arm that rides on a throttle shaft mounted pump cam and pumps through a pump jet placed high in the flow path. Pump stroke (depth and duration) are controlled by pump cam profile (there are several profiles), pump jet size and a pump jet bypass valve. I have never had to change the bypass valve on a DCNF. Once correct emulsion tubes, jetting and air correctors are sorted out, pump cam and pump jet size are all I have had to work with to get correct pump shot. Not sure if the pump bypass valves for these carburettors are even available now of days, but I've never had to change one out to adjust pump shot. They are a size 40 in all the DCNF carbs I have worked with.

[Tuner]

Locating the hole near the curb idle and transfer slot discharge encourages vaporization by impinging the high velocity air coming through the bypass holes directly on the fuel coming from the idle discharge and T-slot orifices. If the idle vacuum is above about 14" Hg the jet of air is sonic velocity in the orifice. With very few exceptions, all OE carbs I can think of are made so idle bypass air is introduced near, and aimed at, or directly below, the idle and T-slot orifices. I have seen reduced UHCs (several times) result from relocating the bypass air closer to the idle fuel. Today's pump gas vaporizes so poorly it needs all the help it can get, particularly with unheated "air gap" intake manifolds.

So you are saying to make the hole away from the throttle shaft and make the hole near the transfer slot?

Yes.

[statsystems]

Locating the hole near the curb idle and transfer slot discharge encourages vaporization by impinging the high velocity air coming through the bypass holes directly on the fuel coming from the idle discharge and T-slot orifices. If the idle vacuum is above about 14" Hg the jet of air is sonic velocity in the orifice. With very few exceptions, all OE carbs I can think of are made so idle bypass air is introduced near, and aimed at, or directly below, the idle and T-slot orifices. I have seen reduced UHCs (several times) result from relocating the bypass air closer to the idle fuel. Today's pump gas vaporizes so poorly it needs all the help it can get, particularly with unheated "air gap" intake manifolds.

So you are saying to make the hole away from the throttle shaft and make the hole near the transfer slot?

Well then I did do it wrong. Time to order some new throttle blades.

Thanks Tuner
Yes.

[naukkis79]

So you are pig rich at idle? As idle circuits are designed to IR they overfuel when used with common plenum with constant high vacuum. Opening throttle blades more will richen even more as transfer slots open.

Have you tried much smaller idle jets and leaner idle emulsions instead of cutting holes to blades? Tickover probably will still be pig rich is only idle is corrected with additional air bleeded through blade holes.

[naukkis79]

And why do you insist to use IR carbs with common plenum? Setting them up is both expensive and difficult compared to progressive linked dual-stage carbs, and at best you still can't tune them as good as progressive carbs. For common plenum i rather use worn out junkyard Quadrajet than brand new IR carbs.

[CharlieB53]

Two stroke outboard carbs all have a hole drilled into each throttle plate so as to ensure each cyl receives exactly the same idle air. All throttle plates are fully closed/seated at an idle.

This picture shows a small 'dimple' in the throttle plates located new perfectly for this idle air hole. Start small, it is much easier to re-drill to the next larger size.

Might add that carbs in question are Weber DCNF. They are designed for IR intake. However, I am adapting them to plenum intake and must drill holes in throttle blades, as they won't idle on plenum intake without uncovering progression holes unless holes are drilled.

I will drill in area marked '0' on underside of throttle blade......



Best regards,

Harry

[enigma57]

Will do, Charlie. I've always drilled them on the other side of the throttle blades from progression holes/transfer slot, but will give this a try.

Best regards,

Harry

[pamotorman]

when i was doing 7448 350 CFM holleys for NASCAR short track spec engines i had to drill the butterflies to .250 to get them to work correctly and they were drilled on the front side .