Belligerent Butterflies

By Spud Miller
Contributed to Vintage Fuel Magazine Vol.2, Issue 1

One of the most common problems folks who run stack injection seem to encounter is getting things setup initially for a good idle and an even temperature throughout the motor. After having reinstalled the injector, many people wonder why things don't behave as before even though they didn't touch any of the adjustments while the casting was off the motor. The idle may be all wrong, with some cylinders running cold and others hot, and the oil may end up very milky.

Even though that old injector casting may be "one piece" (and not one of those newfangled fancy three-piece setups), a significant amount of adjusting is needed every time it's put back on the motor. The aluminum and magnesium castings move around an incredible amount when the bolts are torqued! A procedure must be followed to restore things to optimal after final tightening of the casting. On alcohol or nitro, the following procedure must be carried out with the engine COLD. A gasoline application should be final-adjusted as below with the engine HOT. (Be sure to read through these steps first, as the sequence of steps for your set-up might vary.)

  1. Back off any idle stop screws completely so they are not effecting throttle placement.

  2. Disconnect the crosslink that ties the two banks together.

  3. Remove the stacks if necessary and loosen the screws holding each butterfly to the throttle shaft until they can float freely in the shaft.

  4. Gently force one throttle shaft into the closed position by hand while lightly tapping individually on the butterfly blades with something soft (like plastic). It may take opening and closing it a few times to get each butterfly positioned optimally and seated in its bore. The shaft should want to "stick" closed when all are seated correctly. As you tap, you'll notice the sound will change when the butterfly seats in its bore.

  5. Gently tap one end of the throttle shaft all the way one direction until it stops. Then tap it the other way paying attention to the distance it moves. Finally, tap it back the other way and split the difference - try to achieve a place in between the two extremes. Close is good enough.

  6. Tighten the butterfly screws and repeat steps 4 & 5 on the other bank.

  7. While both banks are in a fully closed "stuck" position, replace the crosslink adjusting as needed to assemble without disturbing the butterfly positions.

  8. Perform the following procedure on the bank that has the idle stop first: One at a time, close the butterflies on a feeler gauge and draw it out noting the resistance. Adjust the idle-stop until a uniform, light drag is achieved. It should be very similar on all butterflies on the bank. A .005"-.007" feeler gauge is a good place to start. If one seems very different on the feeler gauge, go back to step 3.

  9. Now adjust the crosslink so that the other bank of butterflies has the same drag with the feeler gauge as the first one. NOTE: If you utilize an idle stop on both throttle shafts, perform step 8 for each bank before completing step 7.

  10. Cap off the secondary bypass port (if you use one) in the barrel valve and adjust the barrel valve to the desired percent leakage with a leak-down tester. Note which direction leans and which richens the mixture. It is important to know this, so draw yourself a picture or whatever you need to do to remember which direction is which.

A good number to start with on a normally aspirated motor on alcohol is 25% leakage. This means a leak-down tester set to 100 psi on the left gauge will read 75 psi on the right gauge. The higher the number on the right gauge, the leaner the motor will be at idle.

Fire it up! As soon as it starts, turn the barrel valve adjustment link in the lean direction until the RPM starts to climb a few hundred RPM. Now back it up (richen it) until the RPM quits dropping. You are looking for that spot where the RPM just barely starts increasing. Is the idle RPM now higher than you want? If so, back off the idle stop screw to the desired RPM. Now, readjust the barrel valve as described. Changing either the idle stop screw or the barrel valve separately will change the idle air-fuel mixture. If you are just trying to change the idle RPM, you must adjust both - first the stop, then the barrel valve.

Once the motor is running, a Uni-Syn can be used to balance the banks perfectly to one another. A Uni-Syn is a small device that inserts into an injector stack. A small ball is visible in a clear tube, and it floats to give you a relative reading on the amount of vacuum beneath it. Some stacks have a flat on one side, and a Uni-Syn won't seal on the top of the stack without rigging up some kind of adapter. If you don�t have access to a Uni-Syn, another option is to check the temperature of the exhaust tubes side to side. A small handheld temperature gun can be had for less than $100. Check the first tube on each bank and fiddle with the crosslink until you get them as close to the same temperature as you can. As the engine warms up, check the temperature of the front of each cylinder head. If you can get them within 25 degrees of each other from a cold startup, you are doing pretty well. Now check each exhaust tube to see where you are. Where there are Siamese exhaust ports (like the middle two on a Chevy) expect about an extra 50 degrees on the pair.

While checking the exhaust tubes about 2" out from the head, a motor on alcohol should be between 350-450 degrees. The more nitro you run, the hotter this will be (60% nitro will generally make 525-550 degrees at a good idle). If it's cooler than you want, lean the barrel valve a flat or two. If it's much hotter, richen it a few flats. Depending on your application and preference, you may wish to keep it cool or let it build heat. A cold motor usually won't respond as well at the launch, but if your starting line procedure takes a while, you might want to opt for the cool side of things to compensate for that extra time the motor will be running.

Try to achieve a setup that has the throttle actuating linkage or cable, idle stop screw, and return spring all together on the same section of the same throttle shaft. With the pull at one end, the spring at the other, and the stop screw somewhere else, your shafts will become very twisted! You'll experience uneven temperature, and it will be impossible to achieve an even pull between blades in the adjustment procedure with the feeler gauge.

With alcohol/nitro, expect very milky oil during the initial adjustment. If you've gone through the adjustment procedure and you're still getting very milky oil at the track, consider leaning your barrel valve a flat or two. Most milky oil happens at idle, not going down the track! Drag machines spend a lot more time idling than at full-tilt, and at higher RPM most unburned fuel escapes out the exhaust, not past the rings.

After you are satisfied with the settings, check your barrel valve with a leak-down tester and the butterflies with feeler gauges to see where things ended up. Write it all down! This will save you a bunch of messing around in the future and help to get you going quickly after a teardown.

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