Mikuni RS Carburetor Conversion

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Clifford Skinner
6 years ago
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1 Mikuni RS Carburetor Conversion After putting your carbies on the bench or the kitchen table if the wife is out, you will see that the linkages may be in different positions depending on which brand of bike they have come off. Photo below shows two different positions, linkage is on the outer left on the upper set and in between 2 and 3 in lower set. Does not matter, goes back together different anyway. In this exercise we will call the carbies No s. 1, 2, 3 and 4, going from left to right. When I do the conversion I select carbies No.1, 2 and 4 which means that I have to modify carby No. 4 to accept a needle roller bearing to support the Throttle Shaft. You will notice that only two carbs actually have the needle rollers in them. Understanding that you would not generally have access to a milling machine we have supplied a Fuel Rail Bung so No. 3 can be used in lieu of No. 4. This will give you 2 bearings for throttle shaft support. Following conversion is for Carbies 1, 2 and 3.

Note one has no adjusting nut, this is the main carby so to speak, rest are

2 Remove the caps off the top of the carbies and undo the four adjusting bolts that attach the slide to the throttle shaft. Note one has no adjusting nut, this is the main carby so to speak, rest are adjusted to match slide height. Knock out the 2 x 3mm roll pins that attach the linkages to the throttle shaft. There are two 3 mm pins.

3 You can knock out the 2.5mm pin that attaches the accelerator linkage to the throttle shaft but be careful, you need to separate the Nylon section from the Alloy Activator bar to allow the pin to punch through. Or, you can leave this to last as everything comes off before it anyway, lot easier that way. Carefully remove the throttle shaft, undo all the retaining bolts on the spacer plates and remove the spacer plates from the carbies. The spacer bar screws can be real tight sometimes, try an impact driver with some reserve when hitting, do this before you damage the Phillips Heads too badly. Note how the three springs are mounted before pulling everything apart, including the tensioning spring on the Accelerator linkage arm. Note how it clips over the white tab and also hooks onto the carby body in below. You should have the pile below by now.

You can just cut the Pump transfer hoses as you can not use them anyway as they are to

4 The only thing left holding the carbies together now are the fuel rails and accelerator pump transfer hoses. You can just cut the Pump transfer hoses as you can not use them anyway as they are to short. Should now look like this. Is best to remove the slides at this point so they do not get damaged but try and keep them matched with their respective bodies.

Remove Carb 4 from the pile but first take off the Fuel Bowl which needs to be changed with Carb 3. No 4 has a blanked off Accelerator Pump circuit being the outer bowl.

5 Remove Carb 4 from the pile but first take off the Fuel Bowl which needs to be changed with Carb 3. No 4 has a blanked off Accelerator Pump circuit being the outer bowl. At this stage inspect all the slides for wear and select best three, remove the needles and have a look at the needles you have. Needles that we recommend are 9DZH5-50 with the circlip set on the middle of 5 grooves. Make certain that the nylon washer is not lost and is put back onto the needle. Remove the bowls and check the jetting we recommend for 1,000 s:- Pilot Jets - 15 Main Jets ( this is generally just on the rich side but is where we prefer to start) Needle Jets P4 Air Jet.-0.8 Note that the bowl and gaskets for the Carby with the Accelerator Pump is different to the rest so don t get them mixed up. Spring goes down and when looking at the Piston, raised piece goes up, Spring fits into the recess section on the bottom of the piston. Clean the carbies as required make sure all the air passages are clear and reassemble. Note if you have to remove your floats support the post with a small socket as shown before knocking the pin out.

6 Failure to support the post will result in the post breaking and then the Carby is stuffed. Also at this stage remove the idle mixture screw and make certain the fine point has not been broken off the end. Best way of checking is to shine a torch in through the carby throat and look down the hole making sure you see a pin point light. If no light, shit, big trouble. See separate attachment for guidance. Set the screws at one turn out initially, is just a starting point to fire the beast up. Make certain when you seat the screws you just seat them lightly as the tips are tapered and they will lock into the seat and break off when you back the screw out. This is a very common problem with carbs that we buy 2 nd hand and is not easy to repair. BE VERY CAREFUL. Remember you need to swap float bowl No. 3 for float bowl No. 4 so the Accelerator Pump transfer port is now blanked off. At this stage also tap in the Fuel Rail Bung into the right hand Carby, No 3 now. See Black Nylon bung in below photo.

Now slide over the shaft, the Nylon Arm, then the Spring followed by the Metal Ring with minor diameter

7 Reassembly. Look at the photo below, install the Accelerator Pump Alloy piece as shown and knock in the 2.5 mm Pin. Now slide over the shaft, the Nylon Arm, then the Spring followed by the Metal Ring with minor diameter facing towards the Nylon arm. Spring clips over the top of the Nylon Arm. Make certain that the Metal Ring goes over the Nylon spigot, inside the Spring, it tries to avoid the spring thus wearing out the Nylon Arm.

Now insert the throttle shaft into the center carby, but also be aware that you need to tension up the Spring, you will see the fine pin that has been cast into this Carby body, back side of spring

8 Now insert the throttle shaft into the center carby, but also be aware that you need to tension up the Spring, you will see the fine pin that has been cast into this Carby body, back side of spring slips over this pin. Once the spring is seated properly, line up the 2.5mm hole and knock the roll pin into place. Use a pin punch for best results. Now install the non adjustable screw that attaches the Slide Arm to the throttle Shaft. By now you would have worked out that the Throttle Shaft only goes one way. Next step, install secondary Spring mechanism as shown below. Once again you need to take up Spring Tension before installing the Retaining Pin. Amount of tension is up to you, we generally look for around 3/4 of a turn with the spring attached, same as with the Main Linkage. Line up the 3mm hole and knock the pin into place. You should have the above. Next install Carby 1, it is easier then the other Carby. No major problems, just slide it over the shaft and attach with the Adjustable Screw type, has the concentric nut. Do not forget to install the appropriate Fuel Tee first.

You also need to make certain that the hole that had the original Throttle Linkage attached is now filled in with the proper screw, taken from the Carby No 3.

9 You also need to make certain that the hole that had the original Throttle Linkage attached is now filled in with the proper screw, taken from the Carby No 3. Depending on what bike your carbs came from, this will vary between carby bodies. It must be closed off no matter which body had the original linkage. No. 3 carby needs to be set up with the throttle linkage as shown. Also note that you may need to install one or two of the white Nylon Washers supplied behind the Linkage Plate, in between the plate and the Carby body to space the Plate out. This is to line up the Wheel with the Linkage Arm. Have supplied enough Washers for two below each Screw in extreme cases. It has been noted that there is some variance in position, due to manufacturing tolerances and also previous owners of the carbs bending the arms. You need to have the cable entry hole inline with the center of the Wheel to avoid the cable binding. Time taken here getting it all lined up will result in a very light throttle. Just slide the main linkage over the shaft and leave it unpinned for now. Hook the Spring in below the Throttle Linkage Plate and hook it over the Carby Body as shown, see arrow.

Don t forget to install the correct Fuel T before positioning the carby. Install the Adjustable Nut and Retaining Screw into the Throttle Slide and attach to the Throttle Shaft.

10 Don t forget to install the correct Fuel T before positioning the carby. Install the Adjustable Nut and Retaining Screw into the Throttle Slide and attach to the Throttle Shaft. Note that you need to install two opposing Nylon Washers onto the Throttle Shaft as well. These would have been in place when you first pulled the bank of 4 apart. They limit the amount of sideways movement available to the Shaft itself. There are two ways of doing it, you can have both in one carby on opposing sides or have them at the far Carbies, on opposite ends, really depends on how your lower Throttle Slide linkages are lining up. Now slide the Main Linkage wheel back towards the Carby, once again you need to pretension the Spring, Slide it into position with the Tab hooked under the Adjusting Screw and while holding tension with the hole lined up, knock in the 3 mm pin until it seats home. If all has been done well then all will be free, we hope. Now install both sets of Spacer Plates. These have been pre fitted hence the filed out holes so they should go on okay. Do not tighten them until all 12 bolts are started, install a spring washer on each bolt. Sometimes you need to twist the bodies slightly to start the thread and also be very careful to line the Screws up properly, they can cross thread very easily.

11 Once all the bolts are installed, tighten everything up and then install the Fuel Transfer hoses for the Accelerator Pumps. Grease those little Brass suckers first, is a bitch to get the supplied hose on. Feel free to find something else, needs to be fuel line though. Now slide the old Choke Shaft into the Carbs, position the Nylon end as shown above, with a small amount of clearance. Mark the other end with a Marker Pen. You can see the Blue mark in the photo above, to the extreme left. This is where we cut the original shaft off. Now the original shaft has a set of holes shallow drilled that provides a detent system for the Choke System. This basically means that the Choke can be left on without holding it. In Aussie we do not really need a choke so I have never bothered with it. I just start my bike by hitting the throttle twice and punching the button. But with most of you being in the colder climates then you may wish to use the original system which would mean drilling or filing some extra holes. These are for the very small ball bearings and springs that fell out when you removed the Choke Shaft at the start. They slide down inside the small spigot that the choke shaft goes through.

It may be better to just mark the position of the required holes and to just use a Triangular File to file the grooves into the shaft, does the same thing and easier then trying to drill onto the

12 It may be better to just mark the position of the required holes and to just use a Triangular File to file the grooves into the shaft, does the same thing and easier then trying to drill onto the centre of such a small shaft. To install the Airbox Adaptors you need to gently file the casting ridge off the outside of the Air Inlet throats, both on the top and bottom side. Just gently, does not need a great deal taken off, just smooth it out. The adaptors are quite thin and do have some flex in them. By now you should have something looking like the following.

13 Now before putting the top Caps on for the final time, look at the Center Carby Throttle Slide from the Inlet side, looking down the throat. Lower the slide by adjusting the idle screw until the slide is just open. Now compare the outer two adjustable Carbies and see if the gap is the same. You adjust the slides by backing off the screw slightly then rotate the adjustable nut. It just goes around 360 degrees so don t go winding it at will, also, keep the screw just nearly pinched up to see the proper effect. Note the small pin punch dot on the outer of the nut, just keep moving it around within 360 degrees to get the full adjustment. Just keep trying until all slides are even, lock down and then install top caps. Final setting will need to be done on a set of Vacuum Gauges for best results. Best to install some 1/8 Fuel Line on the Bowl Vents, very lowest ones. Make them long enough to join together and go down to the section between the bottom of the motor and the swingarm. Recommend looking on Fleabay for the Fuel Line Guy, sells grea Fuel Line like the Blue Vent lines on the Monty and Triple sets shown below, does look proper flash.

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