Drilling the Slide Many who have done this on non Dyna-jet equipped carburetors report improved performance and better throttle performance. If you are using a Dyna-jet kit, chances are the Dyna-jet instructions already recommended that you do this. Some Dyna-jet kits include a drill bit, however not all Dyna-jet kits do. There are some who have done this on Dyna-jet kitted carburetors when Dyna-jet did not supply the drill bit for drilling the slide also had good success. Keep in mind, if you deviate from what Dyna-jet recommends, then their jetting recommendations will also probably be invalid. If you pull the slide out and look at the bottom, you'll see 2 holes. One dead center, that is where the needle is installed, the other is offset from dead center. This is the hole you enlarge to 1/8" Do not drill the center hole. Drilling the slide allows the slide to open quicker since intake vacuum has a path of lower resistance to top of the diaphragm to lift the slide. Raising or lowering the needle with shim washers after drilling the slide may be required to achieve best results.
Plugs chops in the middle throttle positions should be performed to insure the mid range is not too lean and adjust needle height accordingly. The engine must be under load but not lugging. A CV carburetor will self lean out when the engine is under light loads. This is normal and desirable for best mileage but it does make plug reading a little more difficult when jetting jets for best performance.
Clipping the spring
This is a modification for stock slide springs only. This mod or idea originated from copying the replacement springs that are contained in some Dyna-jet kits. Some of these kits include a shorter spring then a typical stock spring. The length of the Dyna-jet replacement spring is approximately 4 coils turns shorter then a stock spring. If you are using a Dyna-jet kit, and the kit did include a shorter spring, I would not cut a short Dyna-jet spring shorter.
So take it as it currently stands. The two modifications together seem to work for the most part for most people on a stock carburetor. If your carburetors mid range is already close to giving good A/F ratios in the middle throttle positions for your altitude & atmospheric conditions, the modifications may not deliver as much performance increase as it does for others. Dyna-jet kits are good. They seem to have their act together and put together quality jetting kits. The needle alone sometimes greatly improves performance. Do I recommend you to drill and clip? Maybe, and less so recommended with a Dyna-jet kit installed already. If you do, proceed under your own risk and make sure you do plug chops to make sure you don't end up too rich or too lean at mid range throttle openings. Worse case, you'll have to purchase a replacement spring and or slide to get back to stock conditions.
How to do the slide Mod
Remove or rotate your carburetor sideways to obtain access to the 4 screws holding the diaphragm cover in place.
(Note on my 06 H1 I can get access to the carb slide just by loosening the clamps on the intake and outlet boots and carefully rotating the carb 90 degrees sideways towards the right. Be careful with throttle cable, electric choke wire and the vacuum impulse line to the fuel pump if you do it this way. Belt air air outlet will be taken loose at the bottom and pushed up and forward out of the way. Yours may be different. )
Remove diaphragm cover. Underneath is a rubber diaphragm, spring & slide. (Careful, don't' loose parts since spring loaded.)
Remove diaphragm and slide. Carefully noting how it all comes apart. The needle is held into place by the spring and spring retainer only, there may be washers and other tiny parts involved. Note the order everything comes apart.
Cut 4 coils from the length of the spring. Discard the short piece.
On the slide look at the bottom there are two holes a little less then 1/8" diameter, One dead center (Where the needle pokes through) the other offset forwards about 1/4-3/8 inch maybe.
Drill the offset hole larger with a 1/8 drill bit clean any burrs and drill shavings. DO NOT DRILL THE CENTER HOLE
Re-assemble everything in the exact order it came apart. Getting the diaphragm stuffed back under the cap may be a real pain in the ass, if it leaks it will not run right. Be persistent. Sometimes it helps to put the diaphragm in the freezer for a while this stiffens up the rubber making it easier to control.
Note: Depending upon how rich your carburetor was on the needle before this mod you may be able to get better performance by raising your needle height. This will enrich your mid range. If so use #4 stainless steel machine screw washers to shim the needle so it rides higher in the slide. Each washer is about 20 thousands of an inch thick. One or two may do it, believe you could use maybe up to 4 washers if you had too. Plug chops or O2 testing equipment with the throttle 1/2 open will determine if your needle is rich or lean, Jet a tad rich on the needle for best results. No hard fast recommendations on the number of washers to use, you just have to try it at different heights to see what works best.
(Chances are; [depending upon your current jetting and altitude] one or two washers raising the needle will be better than stock. On Dynajet adjustable needles, the 3rd, 4th or or even 5th notch from the top seems to be the sweet spot for many. )
The screws used on the slide cap are cheap and screw slots will strip out easily, good time to replace with 4 stainless steel allen head screws from hardware store. Just match thread and length, I've replaced the 4 on the slide cap and the 4 on the carb bowl myself.