Adding a Carb Gantry to Your Norton Commando
All the new parts that came in Donald Pender’s Norton carburetor gantry kit, laid out and ready for installation.
A few issues ago we installed a pair of Donald Pender’s version of the Lansdowne dampers on a 1974 Norton Commando. While looking through his list of available parts, we noticed another cool upgrade we were interested in trying out. Don manufactures a gantry setup for the Norton Commando. The kit, once installed, does away with the 1-into-2 cable throttle setup and greatly simplifies carb synchronization.
You’ll search in vain for it at tritonmotorcycleparts.com, so instead your best bet is to email Don directly at firstname.lastname@example.org. That’s what we did, and for just $134, shipping included, exactly one week later (from the Phillipines!) we had the kit shown here.
In this How-To we’ll show you the steps necessary to change over from the 1-into-2 cable system to the single-cable gantry. One might ask whether this is really a necessary upgrade, and the answer, of course, is no. Norton Commandos with dual carburetors have been running their stock cable setups for decades and are functioning just fine.
A close-up of the gantry, which simplifies carb synchronization.
This is a modification that would definitely be labeled a “farkle,” but it is a practical one, as the setup delivers a smoother, easier throttle pull. It’s just one of those things you do to your machine to make it your own.
— Keith Fellenstein
1.) Start by removing the seat then the tank for easy access to the throttle cable and carburetors. Next remove the balance tube between the carburetors.
2.) Remove the carburetor top screws and extract the slides and needles.
3.) Pull the return springs back and carefully remove the metering needles. Note the position of the dip on the needle in case it gets displaced and you have to reset it. Free the cable ends from the slides and remove the cables from the carburetor tops.
4.) Once the carburetor ends of the throttle cable are free, disconnect the existing cable form the twist grip.
5.) Unscrew the cable adjusters from the carburetor tops; they won’t be needed.
6.) Take the supplied length of silicone tubing and screw it into the carburetor top as far as possible (usually about 1/4 inch).
7.) Trim the excess off with a razor blade, flush with the top of the carburetor top.
8.) With a pair of needle nose pliers, grasp the new slide cable close to the slide ferrule and push it through the silicone tubing. Repeat for the second carburetor.
9.) Inspect your work. Both tops should look like this.
10.) using your preferred method, replace the slide return springs, using the new ones supplied with the kit. I like to snap the spring coils over the cable and essentially screw the spring onto the cable. IT seems much easier than trying to compress the spring. Hold the cable and slide all at once.
11.) Now we go back to the bike and loosen the manifold to head screws about one turn each. I’ve got a 7/32-inch allen key that’s cut down on the short end to fit between the carburetor and manifold. I’ve also crudely cut a ball end on the long end since I didn’t have a spare ball end key to butcher.
12.) Once the carburetor mounting screws are loose, attach the gantry plate with the carburetor top screws and carefully tighten them. This aligns the carburetors to the plate.
13.) With the carburetors linked together with the gantry plate, re-tighten the mounting screws with the cut down allen key.
14.) Going back to the gantry plate, remove the top screws from one side and loosen the other so that the plate is loose on the carburetors.
15.) Take one new slide assembly and carefully place it into the side of the gantry plate where you have removed the carburetor top screws. I say carefully because it is all too easy to snag the needle on the needle jet when inserting the slide. Take your time and it will drop into place.
16.) Replace the top screws but don’t tighten them yet, then remove the screws on the opposite side and replace the slide assembly there. Once you have both slide assemblies in place, tighten the top screws evenly and snugly, but remember they have farther to go then they did originally, so not as much thread engagement with the bodies.
17.) Place the rotating trunnion in the actuating arm of the gantry, pull the slide cable up through the slot, making sure the locknut rests squarely between the shoulders of the trunnion. Repeat on the other side.
18.) Pull the cable up only far enough to slip it into the trunnion; it is possible to pull the slide up far enough to snag on the gantry top (ask me how I know that). That’s outside the operating range of the slide so not likely to happen in use.
19.) Now that everything’s assembled you can take up the slack in the gantry with the new idle adjustment screw.
20.) When you have the slack out of the new setup, back the old idle speed screws out about one turn each. They now have no purpose in speed regulation.
21.) Fit the throttle grip end of the new throttle cable to your grip. Now is a good time to renew the grease in the assembly and check for good function of the assembly.
22.) Run the gantry end of the new throttle cable through the frame and attach the cable end barrel to the rightmost slot in the gantry arm.
23.) Pull the cable back, attach it to the gantry and adjust it until there is minimal slack. Test for smooth operation and full return.
24.) Refit the balance tube, passing the left carburetor end through the hole in the gantry. Clamp it to the stub. A small outside diameter hose of sufficient strength fits better than the one I originally fitted.
25.) If possible, before you refit the tank, check your carburetor synchronization. I used a TwinMax, but there are many options.
26.) In this case, the carburetor on the left was slightly more open than the right, resulting in a lumpy idle. Use a 9/32-inch or 7mm wrench to adjust the cable.
27.) with the tank and seat back on the bike you can’t even notice the change. What you will notice when riding is a quicker throttle response, so you’ll have to retrain your wrist to the new reality. MC
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