Balancing Throttle Butterflies

Since getting the car registered i have had to do some sorting that was always inevitable.

So here are the areas that have needed attention.

• Engine Intake
• Oil Pressure Failsafe
• Panel Fit
• Exhaust Fit
• Balancing Throttle Butterflies
• And more........

First some photos of my fabricated "throttle balancers" and a description of the problem I had that I bet I am not the Lone Ranger in having.

On the last dyno session, while trying to tune for suburban traffic situations (very light load, low revs) it became quickly apparent that my throttle butterflies were all over the place. At idle, a flow meter showed 4 cylinders had zero flow and the 4 others had all different readings. I knew it was always going to be very tricky to try and get them the same (when going by eye via a feeler gauge and trying to be careful and consistent when tightening the throttle arms to throttle shaft), but I was very surprised to see I was that far out!

The trouble is that as you tighten up the arms on the shaft, the arm may or may not pull around the shaft ever so slightly. And with my big 55mm butterflies, even a tiny variance makes a big difference. At least this explains why I would fowl some plugs up and not others. So what to do?

Over a couple of nights thinking, and finding ZERO on the internet, I decided to build an "adjuster cage" for each throttle arm - and run 4 of them per bank. This means the common throttle rail that pulls open 4 throttles on each bank, opens the cage, and the cage then pulls open throttle arms.

The trick is that the arms have some adjustable movement inside the cage so you can adjust each throttle arm individually. Here is what I have fabricated as a set of prototypes until I can swap over to some nice CNC machined units. The first issue is to make 8 of them, and to have them be as identical as possible with just home handyman tools. Here is what I did:

1.    Cut 16 pieces of 1.5mm stainless sheet into a suitable "cage shape" (after creating a cardboard template using the throttle arms, shafts, adjuster screws, etc to get the overall shape correctly done). I then sandwiched them in the vice and ran a bead of stainless weld across all plates at each end to keep them all aligned and stable for drilling and shaping. This gets them all pretty close to identical.

2.    Create a jig to drill 3mm holes in a 10mm diameter brass rod in prep for cutting threads in these holes. The thread needs to be pretty fine to give yourself good adjustment fidelity. 

3.    Mine are threaded for a 4mm stainless hex head screw. Once the brass rod has all these holes threaded, slice the rod up into 7.5mm sections ensuring the threaded hole is in the centre and these will be each actual throttle butterfly "adjustment mechanism".

4.    The width of the cages needs to be a hair bigger than the width of the 8mm throttle arms, so I cut some 8mm stainless strips, then drilled a 4mm (for the adjuster bolt to pass thru) and 12 mm further along a slightly bigger 5mm hole (for the 3mm Allen key to pass thru). This strip will "capture" the hex head of the 4mm bolt. But when bending this strip, slide the bolt into its 4mm hole then bend the strip into a "U" shape back over the head of the bolt. Use the Allen key located in the head of the bolt as you are bending this strip to ensure its adjustment hole lines up as you bend it. Weld the "U" to one side of the cage such that the threaded brass adjuster can run up/down the bolt thread along the lower edge of the cage. In the photo below, imagine the lower edge of the cage has its "side" in position. This stops the brass adjuster dropping below the bottom edge of the cage. I trust you can see that (for the bottom cage in the photo below), as the bolt is turned clockwise, the brass adjuster draws up the thread and the throttle arm can rotate downwards.

5.    Next clamp both sides of the cage together to weld the cage into a single unit. Note that the throttle arm is put into position and I also have thin slice from a flexible paint scraper in there to give me some clearance once only the throttle arm remains. 

6.    It is important not to overtighten as you weld and make sure everything moves freely at each step. 

7.    With all 8 units fabricated, a good test was to sandwitch them all together and run an 8mm brass shaft thru the lot to mimic them being connected to a throttle shaft. This way I could see how much rotation the throttle arms had and how uniform they were to each other. In the photo below, this shows the throttle arms on the right in a more or less "equal" position with the brass adjuster sitting in the same position on its "adjuster bolt", while the left two arms have been "adjusted" to show the angle of throttle arm rotation available within each cage.

8.    The "underside" shot below gives another view and shows how I have run some of the adjusters up/down the threads and how it allows the throttle arms to rotate a few degrees within each cage. Again, the two adjusters on the very left show the movement angle possible with the end adjuster at full thread and the next adjuster at half thread.

9.    Now seeing this throttle arm movement within the cage means the stainless cages rotate slightly on the throttle shafts, I thought it best to run the cages on bearings so they did not "dig into" the softer brass over time. Here I have my new jig in place to line up the bearing cups. To create these cups I purchased some 8mm I.D. and 14mm O.D sealed stainless bearings which determines my cage bearing cups need to have a 14mm I.D. The simplest solution was to purchase some 18mm O.D. 2mm wall stainless stainless tube that i then sliced up to size (the size of the bearing thickness). I lightly hammered a small dent in the outer edges of the cups after inserting the bearings to keep them in place.

10.    To install, i just put the adjusters at approximately "half thread", connected all 4 up to the common throttle rail and simply held each butterfly shut as I thightened the throttle arms to their respective shaft. Something I thought that would be important is to have a stiff spring inside the cage that pushes the throttle arm back against the brass adjuster. This ensures the arm does not "flap around" inside the cage and creates a slight resistance to the adjuster bolt so it stays in place and does not "self adjust". Note that I have designed these adjusters such that as the throttle is opened, the common throttle rail pulls on all 4 cages, and each cage pushes against the brass adjuster, which pushes the throttle arms around (rotates them). It is a solid connection of one component pushing on the other. I did not want to get this backwards and have the cage push on the spring, which then pushes the throttle arm around, as this would introduce variance across the butterflies. FYI, these springs are 7.5mm diameter and 15mm in length, similar to an idle adjustment screw spring on a Holley carb, but a bit stiffer.

11.    So finally, to adjust the position of each butterfly, I now have very fine tune adjustment capability for each of them simply by turning the adjustment screw in each cage. The stiff spring within each cage pushes each throttle arm back against its brass adjuster. A keen eye will see the adjuster hole for a 3mm allen key on the nearest cage in this photo (No. 1 cylinder in this case). I can now adjust all 8 butterflies to equal flow in approximately 1 minute using a CFM flow meter as the measuring tool. And i can do this repeatedly in 1 minute each time i pull things apart for inspection or otherwise fool around with throttle linkages, or to get into the engine valley, or whatever! Well worth the effort I say.

As for panel fit, I got some time to get things back to where they were before it all went to the paint booth, and I also got the bumpers re-chromed. What a difference the new bumpers make! I cut and shut them, so they needed a lot of work, but they came up beautifully. The next few photos shows things as they are as of November 2020 and it seems to be a real head-turner.

Notice the Frenched in reverse lights in the rear bumper. They are nice and bright when in use and clean up the rear of the car in my opinion.