Ready For Copy!

Hi Guy's, I'm sorry it took so long to get this post together. But as you will see, this is very in-depth and detailed.

You can Thank "Mother Nature" for making it rain all day
Labor Day. Which prompted me to take the Carburetor off one more time to make those final adjustments. So I took this opportunity to take some pictures to document what I've done with my Edelbrock #1901 Q-jet.

First off the #1901 is an Emissions legal Carburetor. It is set up on the lean side to be Emissions legal. Yet has a rich enough idle circuit to accommodate many engine combinations. Not that that is a bad thing, just a fact. It works well, But like anything Boss Hoss, There is more performance in there. You just have to go and get it.

The Quadrajet is one of the Best Carburetor's ever designed. It is capable of Outstanding Performance and yet still deliver good fuel economy and all around great drivability. When it was introduced in 1965, It was considered "Very High Tech." It may look a little complicated. But once you understand how it works, It becomes very easy to set it up the way "We" need it to be.

I was having some trouble getting mine to perform right. The #1901 is listed as a pre 75 carburetor, and that was what threw me off. I was using pre 75 float settings and they were too high for this style of Carb. Main Body.

What I found out was, the body is a copy of a 75 and later type Q-jet. But it utilizes the 74 and earlier Jets, Primary Rods, and Ranges of Travel. This is what was throwing me off. But I've ironed that one out. Thanks to Lars Grimsrud, the gentleman who wrote, One of the best How-to articles I've ever read on the Q-jet Carb. He's the one who told me to try the .420 float level setting. And that was just what it needed. And I have also learned through trial and error what works and what doesn't for a Boss Hoss.

First we must think about what we are asking the Carburetor to do. Remember "Our" application is not a car with a 3 or 4 speed Tranny. In my case, I only have one gear to use. And the 2 speed's have the relatively same first gear ratios. So "We" need the idle/cruise circuit to be lean and crisp for good gas mileage and off idle response. A good primary (under load) jetting setup to give us good power until it's into the secondary circuit, and get us there as fast as possible. And a Secondary circuit that doesn't open too fast and cause that dreaded "BOG." Yet fast enough to give instant response on the roll. Now that's a pretty tall order to ask from your Q-Jet Carburetor. ----- But we are in luck! The #1901 is up to the task! So lets get started and see how it's done!

First off, As you look at my Carburetor, you'll notice that I have removed the choke butterfly. Since it is not functional on my Bike why have it there to slow the air down. I know many of you have made this a working part of your Carb. That's OK to. This is just what I opted to do.

Now the second thing that I'm sure you've noticed, Is that I've cut small "Transition" slots into the front edge of my upper secondary butterflies, not the rear as the carb picture shows.The lower pics of the butterflies are correct.These transition slots are to help speed up the fuel flow into the secondary's upon opening. These tips are as follows, From one of many articles I used to tune my Q-jet.

Some "performance" versions of Quadrajets were built with "transition slots" cut into the air valves which made for a smooth transition from primary to secondary operation. If you can find some of these, use them!

From an amendment attached to this article.

He also suggested using the air valves with "transition" slots. Most Pontiac Q-Jets have small orifices (ports) located directly in front of and at the center of the air valves, called the accelerating ports. These ports function as accelerator pumps for the secondary's. When the secondary throttle plates begin to open, a vacuum is reflected up into the Carb, and this causes a small amount of air to begin to flow around and through the air valves before they begin to open. The transition slots are intended to concentrate that initial air flow past the accelerating ports. As you will all remember from your physics classes, air flow across an opening will lower the pressure at the opening. The fuel inside the accelerator wells, tubes, and orifices immediately begin to travel towards the low pressure area and is sprayed into the air stream at the transition slot. This initial shot of fuel is intended to prevent bog until the secondary metering system begins to meter fuel through the secondary discharge nozzles. This is a great system, and normally works well on stock carbs. One of the changes Randy mentions is to open the air valve quicker, because this provides quicker secondary throttle response. At some point in the increased air valve opening rate, there may not be enough fuel from the accelerating ports (or it may not begin quickly enough) , and the dreaded bog reappears. To cure this problem, we have made the transition slots smaller. We obtained non slotted Chevy air valve plates, and cut much smaller openings in them (about 3/8" wide and 1/4" deep centered on the accelerating ports). Although the actual size is not terribly important, both plates should be identical. The smaller openings concentrate airflow and increases the velocity at the accelerating ports, and results in much quicker and stronger fuel flow from the ports. Again, while this change may not result in measurable acceleration improvements, it does sharpen the throttle response and allows the secondary to begin to add power almost simultaneously with the primaries. Your reaction times, and 60' times will definitely be more consistent when there is sharp and instantaneous throttle response! Before trying to remove the air valve shaft/plates retaining screws, file or grind the ends of the screws where they have been deformed. Using a large blade screwdriver, carefully remove them using a technique of twisting CCW, CW, and again CCW, while using WD-40 for lubrication. These screws may break, but with care, they can be removed intact. When all check-out is finished, a medium strength of thread lock (in case you desire to remove the screws later) should be used to secure the screws in the new plates.

Another trick worth trying is to rough up the bottom of the secondary air valve plates with a rough file. This can aid in fuel atomization starting early as the fuel discharged from the secondary fuel tubes strikes the bottom of the plates first.

OK, Now you can see how I modified mine.


It dawned on me that I may have made a mistake. Accelerator port's and discharge tubes are two different things.

I made a correspondence with Lars, (The writer of that great article.) I asked him about the slotted Air Valves and if he had one I could copy from.

I found out just how Human I am. As you can see in the picture I've slotted the Air Valve at the wrong end.

This wont cause a problem. It just wont do what the slot's and intended for.





Since we are on the secondary's at the moment, lets stay with them until the they are finished. The next thing I'd like to address is the wind-up spring tension. There are many schools of thought on this one. 1/4 turn to 1 1/2 turn. Which is right?

My thinking is we are in a high gear to start with. So timing is everything. We want them to open fast but not too fast. My engine likes 7/8 turn past zero tension. I know many have had good luck with 1 full turn to. The 7/8 is just a tad faster.

As you can see, there is a #10 torx screw on the bottom side of the upper secondary shaft. This is the set screw for the wind-up spring screw. Back it out until the wind-up screw is free. It's best to do this while the Carb. is apart. This way you can actually see the spring come into contact with the anchor pin. This is your zero tension. Note and mark the Carb where it is. Turn clock wise until you reach 7/8 to 1 full turn. Tighten the torx set screw.

Next is the Vacuum Pull Off or Vacuum Pod. I drilled out the orifice down inside the vacuum hose tube to a larger size. This allows more air to pass threw it and it will open faster. In stock form mine took over 2.25 seconds to open. But the drill bit I used, was too big. The Vacuum Pod opened in about 1/3 second afterwards. So I had to lay a bead of solder at the end of the vacuum tube and drilled it out to a smaller size. I then tried .017". But it was also too big. I then heated up the solder and let it cool. This made the orifice smaller each time I did it. I repeated it again and again until I got the timing just right. I set it up so the vacuum pod opens close to 1 second in the range of travel that opens the upper butterflies. But you'll need to get the full travel in the area of 1.25 seconds for it to be 1 for the range of travel. My best guess would be to drill it out to .011" to .012". Then go from there. You'll want the 1.25 seconds timing to be as installed on the Carburetor, with all the rods and springs mounted.

Now that we have the secondary's opening timing set. Let's get the hanger and Rods dialed in. GM makes hangers from B to V. "B" being the richest and as you work down to V, moves the rods down into their seats by .005" with each letter size. "V" being the leanest. You could go out and buy a new hanger of any letter or you can just bend the one you have now. The rich setting of a B hanger are as follows. 41/64" from the top of the Air Horn to the top of the Secondary Metering Rod Hook with the upper butterflies held wide open. But there is a twist! The Edelbrock #1901 uses a .050" thick gasket. It is .023" thicker than the stock
Rochester gasket. So you'll need to compensate for that. The measurement should be set at 21.236 /32" or .663625". I found this out first hand as the rods would get hung up once in a while and would flood the engine with fuel when you'd get out of Wide Open Throttle. It was just a stubble with a lot of Black smoke. So after you have made this modification. Do this little trick to make sure yours are not sticking. Turn the Carb. upside down. Open the top butterfly fast with your finger. If it sticks you have it too high and the Secondary Rods are getting jammed. Keep lowering or aligning the hanger until they close properly. This was what I needed to fix on Labor Day.   

The #1901 uses DR Secondary Rods. These are lean rods. But transition to full rich very fast. So I feel there is no need to change them.

That's it for the Secondary's.

Next I'll give you the right float setting and show you the float bowl modifications I've made. Like I said in the beginning this is a 75 and later style body. The correct float setting from Edelbrock is .470" with the gasket in place.  .470" minus the .050 gasket is .420. Or 13.44/32".  Just like a 75 and later Carb. Any higher and you'll have the gurgling over problem like I was having at the beginning.

Now heres a little trick from an article.

The worst inherent problem to overcome with the Q-Jet is the small bowl size. In stock form, there is barely enough gas stored in the bowl to take a very strong running Musclecar from one end of the quarter mile to another. Raising the fuel pressure available to the carburetor is not the answer as Quadrajets can work fine with as little as 3 PSI inlet pressure, but the volume of gas must be there! If you are sure that your fuel pump is capable of keeping the bowl full under several seconds of WOT conditions, and your gas tank has sufficient fuel to keep the pickup tube covered under acceleration, proceed to the following.

Remove the plastic "baffle" that is located in the area around the power piston. Do not leave the baffle out, but we are going to modify it in order to increase fuel bowl capacity. I use a coping saw to cut about one quarter of an inch from the bottom of the baffle, then use a sanding block to smooth the surface. Some of these baffles are plastic and others are phenolic, so be careful; and do not break them as they are brittle. The purpose of this modification is to remove some material that takes up space that would be better occupied by fuel.

OK lets see here I covered the secondary's and the float bowl area,------- Oh yeah ! ! The Primary's!
Here is the heart and soul of how the engine run's. Your engine run's on the primary's circuit 95% of the time. So getting this right is the key for a well tuned carburetor.

My stock jetting was as follows 69 primary jets and 36 Rod's. All #1901's have these jets and rods.

I have found that my engine loves the 71 primary jets. The 72's are just a tad too rich and make's the engine run a little sluggish. So I would think the 71's would help out just about everyone who has modified their engine in some way. I've also changed the metering rod to a 41 at first. But I found them to be wore and smaller then the stamped size. I happened across a set of good 40's and they are doing very well. Now the next and most important thing is to precisely measured the primary rod to the jets to be sure they are right on with the metering rods tapers. This is the meticulous part. Every Jet and metering rod are slightly different. So getting the measurements down first is crucial. My 71 Jets measure .120 from the top of the Jet to the top edge of the true jet size.

Most of the rods vary some to. So what you'll need to do is measure these two thing. The distance from the top of the jet to the upper edge of the true jet size. Then the distance from the top of the rod to the bottom edge of the true rod size. There is an area of taper that is about .075" long. (from the .026 tip to the actually rod size.)  And the range of travel up and down that the power piston moves is about .095" to .100". This leaves you with .010" to .0125" above and below the tapers. 

I have a rod that is 2.168" from the top to the bottom edge of the true rod size. And .120" from the top of the jet to the true jet size. Subtract the .120 from the 2.168  2.168 - .120 = 2.048 Then I gave it another .007&" to sink it into the true rod size a little.

Now with the power piston pushed into the down position I need to have 2.041" from the top of the rod to the top of the Jet to have it set correctly. Just bend the T hanger which every way you need it to go. Make sure your measurements are right on and you'll have a crisp running Carburetor. Even with the head work and cam the power piston spring is soft enough to allow it to do it's job correctly. I would think this orange spring will be fine for everyone.

OK, Before we put the Carburetor back together, There is one more modification to be made. This has to do with the accelerator pump curcuit. You just need to make sure the hole on the Gasket that passes the Accelerator pump shot is large enough. I used a 1/8" drill bit.

Once you have the drill through the hole in the Gasket. Take a razor and cut the bulgeing gasket material off. This will enlarge the hole, and allow more of the accelerator shot to pass through.

You can also use a shorter Accelerator pump to get a bigger shot of fuel. But be careful. Too much fuel will drowned the engine and cause a too rich bog.

And lastly, The most important things to get right, Is the Idle mixture screws.

This is what I have found to work the best in getting them set right. And this goes for everyone's carburetors, Even the bone stock ones. 750's and 850's.

Set the bike up on jack stands at LEVEL. Without raising the front end up too much. With the engine at operating temperature. In Neutral set the engine idle to 900 Rpm's. Now set the idle mixture screws using a vacuum gauge to find the " Least amount of turns out to hold the highest stable vacuum." The key words here are "Least amount of turns out" and "STABLE" = Without having the vacuum gauge needle bouncing around too much.

With the engine idling. This is done by backing the idle mixture screws (one at a time) out {ccw while watching the vacuum gauge and Tach. You will see the vacuum and Rpm's go up as you back out the screw. It will keep going up as you turn it out. Until it reaches a point that turning it out more will no longer have an effect. Keep resetting the idle to 900Rpm's if it happens to go above 1,100Rpm's. Once you have the screw to the point that turning it out "MORE" no longer has an effect. Turn the screw back in until the Vacuum needle and Rpm's just start to drop. Now do the same procedure on the other side. Reset the idle speed to 900 Rpm's. This is very important. Keep resetting the idle to 900Rpm's if it happens to vary over 200 Rpm's UP from there. Once you have both screws at the rich end of the spectrum. You'll want to find that sweet spot at the lean end. Your getting closer at this point.

Now turn one of the screws slowly {cw} back in and watch the gauge. The vacuum and Rpm's will start to come back down. Turn the screw in and get the Vacuum as low as you can with the needle staying relatively "STABLE." You'll know when you have turned it in too far. This is when the engine starts to run rough, the Vacuum needle starts bouncing around, and the rpm's start dropping off even more. It's OK if the engine's rpm's go down to 700Rpm's while doing this part. As long as it is idling smoothly with a stable Vac. needle. Turn the idle mixture screw in or out until to find the position that keeps the needle as "STABLE " as possible. You will need to Rev the engine from time to time to clear is out. And get it idling smoothly again. Then quickly double check the screw setting.

You will have to then do the other side the same way. Then repeat the final leaning process a couple more times to make sure the screws are on the mark with a lean but stable running engine. Then turn both idle mixture screws "clock wise"or in 1/16 turn from there. Reset the idle to your original idle speed. Your done !

Make sure the screws are somewhat even with the same number of turns out. There may be a difference of up to 1 full turn or so. But don't have one out 4 turns and the other at 1-1/2. 

Test ride the bike. Now turn the throttle very slowly without activating the Accelerator pump. Just bring it off idle slowly. The engine should idle up into the transition ports smoothly. Then try it a little faster. Again without getting into the Accelerator pump. There should be a millisecond flat spot on the faster short twist ONLY. You'll have to play with it until you find this sweet spot. For the most part you'll never find this to be annoying when you are riding the bike. You'll only hear it once in a while if it's set right. It's very seldom that you'll turn the throttle in this manor anyway. The engine doesn't falter. It's just as I said "a millisecond flat spot." It shouldn't stumble the engine. If it's too noticeable back the screws out a little until you find this spot.

This will give the engine the sharpest " THROTTLE RESPONSE ! ! ! "

I think that should about do it. Thanks for listening
Michael Levesque
" Carburetor Mike "
BHRA President