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Hi. I am new to the forum and have made a post earlier today to introduce myself and my interests in the appropriate section of the forum.

I am working to improve the idle quality of my 1970 Cutlass which has its original Q-Jet P/N 7040250. In the Knowledge Base I found an excellent article by Damon Nickles titled the same as the subject line of this message.

Damon's article is clear on what to do and I shall follow the steps he outlined including drilling the opening for the idle mixture screw larger and then, if needed, to drill out the Idle Channel Restriction a few thousandths.

My question has to do with a feature Damon did not address. Starting in 1970, the Cutlass Q jet has an added idle air bypass circuit. It is a direct air bleed passage from the area where the top cover attaches to the main body through channels in the main body and down into the throttle body where it dumps the air in holes (one on each primary hole) beneath the throttle plate.

So my question is this: Should I be blocking this idle air bypass off? It seems a no-brainer to me to do so, but wanted to see what guys on this forum think before I do it.

Does anybody know how I could shoot the question directly to Damon Nickles who wrote the Knowledge Base article?

Thanks for any help! :)
 

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Crush.I also have been dealing with these carbs lately.On a 74 Camaro,we installed a 73 Olds Qjet.Vacuum signal would not go above 15"hg no matter how I adjusted the mixture screws.After doing many trials I decided to plug the leak holes you mentioned.Idle quality immediately picked up.Now I can get 18"hg and a smoother running engine.Also I noticed it has no vacuum advance port for off idle advance.I carefully drilled and made one.Drivability improved considerably.
 

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Thanks for the info, Paulo I. I will block off those air bypass holes. I think I will also go ahead and drill out the idle discharge ports (where the idle screws go in) as recommended by Damon Nickles paper on the Forum Knowledge Base. If you haven't read those yet, they are super.
 

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OK.See,what I did here and got good result was to enlarge slightly the tiny brass holes at the top of the main body where the gasket is.It has to do with idle and transition circuits.Fuel is sucked up from one channel reaches the cover turns down and goes towards the discharge holes you mentioned.I did not enlarge the discharge holes themselves but I have not read Damon's paper yet.Besides the gasoline available in Brasil is totally different for it contains 25% of ethanol.What happens is that every carburetor becomes lean down here.After I did so the lean sensation off idle disappeared and instead of 3,5 turns at the mixture screws now it takes only 1,5 turns.What is your complain?
 

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Discussion Starter #5
Paulo 1 My complaint is rough idle and rough idle transition, same as you. Your description of circuit flow is exactly right and also matches Damon's description. He explained you only need to drill out the hole on the outer side (which is the idle restrictor). The other brass hole is really a long "straw" that has the real idle orifice at the bottom, too deep to reach with a drill. So drilling the outer ones is all you need to do. Drilling the inner ones (the top of the "straw") will not hurt but will not change flow. The results you report match what Damon says. It sound like you do not need to drill out the main discharge where the idle screws go. But this option is something you can keep in mind if your gasoline composition changes again. I am leaving for 10 day vacation but when I get back I will drill my idle discharge holes and see how it runs and then if I need to drill the brass idle restriction. I will post my results here when I get done. Will probably be at least two weeks.
 

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OK Crush1.Indeed one of the holes had a weird forever drilling feeling.See,I did not have to many choices of bits.The one I used was 0,055".Slightly larger than stock.Since I turned by hand,I quit one the inner ones afraid it was going to brake deep inside.You know the feeling when it gets deep and starts grabbing such a weak drill.by the way on those "vacuum leak holes" only one of them actually leaked.The other died at the gasket.I used small machine screws.Good luck and trip.
 

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Idle and transition problems on a Q-jet can sometimes be the the fault of worn throttle shaft bores. This is cured by installing bushings.
 

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Discussion Starter #8
Paulo 1.... I have completed mods with good results. By the way, I find the book on QJets by Cliff Ruggles to be outstanding. You can buy it a number of places on internet including his website: [email protected] He rebuilds and also sells QJet parts, including idle tubes, idle restrictors etc. He does not list everything on his website that he sells; you need to contact him. He explains in his book how to remove idle tubes and has lots of info on what to drill out and how much. Of course you would need to modify further because of your 25% ethanol fuel but I think you will find his info a great help. I think you would need to move in the direction he talks about for highly modified engines.

For my stock 1970 Cutlass (7040250) I did the following: (1) blocked idle bypass air, (2) removed idle tubes, cleaned area good, and drilled them from original 0.033 " diameter to 0.035" ...would have gone to 0.036 but my bit that size was broken, (3) drilled out the idle down channel restrictor orifice from a stock 0.048" to 0.055", (4) drilled out idle mixture screw orifice opening from a stock 0.086" to 0.096", (5) sink wells did not appear to be leaking but I applied gasoline resistant epoxy over them anyway, and (6) installed new primary throttle shaft bushings. Car now idles smoothly, and there are no flat spots in off-idle transition. I am able to get the car to idle best at about 3.5 or 4 turns out on idle screw which most experts say is about what you want, that if it takes more than 4 turns out that you need to enrichen further. I want to get a brass float too, since today's gas can cause problems with original type. I would be interested in seeing list of all changes you have made to your carb to get it to work on 25% ethanol. What p/n QJet do you have?
 

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OK Crush1. Back to Quadrajets. See,after I decided to go ahead with the drilling mods and picked up some good result,the car was sent to the paint shop to be buffed.I have not done any additional modification. But when I drove it to the shop it felt better.I can only attribute it to the idle/transition channel restrictions I enlarged to 0.055".

Now on your mod #2,what exactly is this idle tube?

And what are these sink wells on #5?

I don't have the carb number here.I will get it to you.I found out that the little cam on the secondary air valve,you know the one raising the secondary needles,was missing on our cover! So we got another carb,this time a donor carb and swapped the cam onto ours.It is made of plastic! Now the secondary air valve raises the needles and works.

But a new problem is bothering us.Most times we use the secondary it does not fully return.It is even dangerous to drive it this way in traffic.I have checked everything and can not find out why...
 

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Paulo 1 .... Ok, the idle tubes are pressed into the main body right beside the idle restrictor orifices you drilled out to 0.055" in your carb. They are next to each other in that little kidney bean shaped place. The idle tubes are brass, about 1/16 inch OD and have a reduced ID at the bottom. The passage it sticks down into is full of fuel at the same level as main float bowl. The fuel for idle and idle transition flows up through the idle tube, then across in the kidney bean shaped area to go down the idle restrictor you drilled out to .055" and from there down a passageway that leads to the idle screw discharge port in the throttle body. In the little opening above the kidney bean shape idle air is mixed in with the fuel before it goes down through the idel discharge orifice you drilled out to .055" and it also gets more air mixed in as it goes down to the idle screw discharge ports. So the idea is if drilling out the idle channel restriction (like you did) is not enough, you can also (1) drill out the idle screw discharge hole in throttle body which lets more fuel/air mixture get sucked in beneath the butterfly, and (2) remove the idle tubes and drill out the opening in the restriction at the bottom of them (but you have to go easy because this is where the idle mixture really gets established - mine was 0.033" stock and I opened mine up to 0.035" but 0.036" would be ok if you have the bit handy). You can do all three mods (drill out idle screw discharge hole, drill out end of idle tubes, and drill out idle channel restriction) which is what I did. You need Cliff Ruggles book. It explains it all and shows how to remove the idle tubes. You can get his book from his website in link I put in earlier post or other places on-line. I got a good price from here: http://www.themotorbookstore.com/howtoreandmo1.html

OK, on to sink wells. If you take throttle plate off main carb body, you will see two sink well plugs where the factory origianlly drilled out the secondary jet holes and another smaller set for primary. These four locations have pressed in plugs but sometimes they start to leak and can cause too rich a mixture when cruising which will kill gas mileage. Also will make the car flood when turned off and can make you crank with starter for a while with hot engine before it will start. Some people just put a gasoline compatible epoxy over these four locations. A better job results by drilling out the factory plugs, threading, and installing a flat head screw and putting the epoxy glue on the threads for a good seal.

Don't know what to say about your problem with secondary sticking open. It must be something that happened when you worked on carb to install new secondary cam. Look close at external secondary linkage on drivers side of carb to make sure it all looks ok and be sure nothing there is hanging up and also to be sure that the spring there that shuts the secondaries is not broken.
 

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Discussion Starter #11
Ok Paulo 1.... earlier today I made a reply but somehow it seems to not have been posted. So this is try #2. The idle tubes are the brass tubes that are pressed into the down channel right next to the idle restrictor orifice you drilled out to 0.055". The tubes are about 1/16" OD, and about 1.5 to 1.75" inch long. The bottom of the tubes, which sticks into gas at the same level as the main float bowl, has a necked down end that is the primary orifice to control flow of gasoline into the idle and off idle circuits. There is a thin brass sleeve, about 1/4 inch long that is pressed onto the top of the tube and then the whole works is pressed into the carb body. Cliff Ruggles' book (you can get a good price and quick delivery here:
http://www.themotorbookstore.com/howtoreandmo1.html ) explains how to remove them. It is easy. The gas flows up the tube and crosses over in the kidney bean shaped area (where it is mixed with idle bleed air) and then flows down through the idle restriction orifice (the one you drilled out) and on down a channel where it gets more idle bleed air mixed in and finally it reaches the idle screw discharge hole beneath the butterfiles in the throttle body. It appears, after reading both Ruggle's book and the paper in the Knowledge base by Damon Nickles) the best way to fix idle problems, after putting new primary shaft bushings in if needed, is to drill out the idle tube tip orifice a little.... maybe .002 or .003" larger than stock, then drill out the idle restriction orifice like you did, and then drill out the idle screw orifice in the throttle plate. Really high performance engines need the openings drilled out even more. With 25% ethanol you may need to drill larger holes. Ruggles book goes over how to do all this in good detail.

The sink wells are at the bottom of the main body. You have to remove the throttle plate to see them. It is where the factory inserted plugs to block off holes they used to drill the places for the primary and secondary jets. There are two larger ones right under the secondary jets and two smaller ones under the primary jets. Sometimes these plugs leak which makes the car start hard when hot and it can also hurt gas milage when cruising. You can stop it by applying gasoline resistant epoxy over the plugs. A better way is to drill out factory plugs, tap threads and screw in a short flat head screw, using epoxy on the threads and over the top of the head.

I cannot say what to do about your secondaries not closing all the way. It seems you are saying it started happening after you installed the little cam that lifts the secondary metering rods. Be sure you check over what you did carefully. Also put an eagle eye on the exterior secondary linkage on the driver's side of the carb. Be sure something isn't hanging up and be sure the little torsional spring that closes the secondaries is installed ok and not broken. Also, and this is a long shot, the secondary butterflies may be hanging up somehow. I don't know how this could happen unless the screws holding them to the shaft came loose (if you ever removed them).
 

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Hi guys,

I have a long running quaddy idle problem on a Holden 5L V8 with an open plane inlet manifold and a high overlap cam (although only 220 degrees duration).
I have drilled out the idle pickup tube to .036, the down channel restriction to .060 and the idle screw ports to .100.

I have control of the idle and can make it too lean and too rich but I cannot for the life of me get a decent idle and it sounds miserable.

It runs a heap better with a vacuum hose pulled off but the butterflies are in the right position with most of the transition slot above the butterfly edge so I figure if I increase bypass air from factory size (I didn't measure them but they are tiny untouched holes) then I may not be able to reduce idle speed enough anymore.
 
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