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Dear all - copying and pasting my efforts of last 4 days of typing at home. I hope it is useful. Here goes.
Conversion of Premier Padmini engine to run on a Mikuni carburetor:
Why this need? – The original carburetor on the Padmini is one of the series of ubiquitous Solex BIC / PBIC. It is becoming almost impossible find a new one now. A very few shops that have it are quoting ridiculous prices (and sitting on it for years). Also, more often than not, it just leaks off from somewhere and @ 56 rupees for a liter of petrol, it’s not worth wasting fuel, that’s why something needed to be done. So I did it. I am providing details. Needless to state, this is an indicative procedure only.
What is the basic difference between the two carburetors from the mounting (packaging) point of view? – 1 - The Solex is a single barrel downdraft carburetor whereas the Mikuni is a twin barrel downdraft carburetor. This is the basic difference. The Solex mounts on two studs whereas the Mikuni mounts on 4 studs. So the base is completely different. 2 - As viewed from the driver’s seat, the throttle plate of the Solex rotates in anticlockwise direction when the throttle is opened whereas the throttle plate of the Mikuni rotates in the clockwise direction. 3 – The Solex does not have a fuel return line (there are numerous advantages of having one however), the Mikuni comes design-equipped with it. 4 – The Solex (except in S1 cars) comes equipped to be operated by a throttle operating rod (push) whereas the Mikuni comes equipped to operate the throttle by means of a cable (pull). So, essentially they are as different as chalk and cheese.
What are the ways? What are the correct way and the incorrect way? - There are 2 ways. One is the "normal" way, mostly adopted by almost all that have done it; the other is the "correct" way. The normal way is the easy route or rather a shortcut, which most garages go for. I do not recommend it.
The normal way - or the "easy" route is to fabricate a so-called "adapter" having the Solex as well as Mikuni mounting faces. I have seen examples of adapters fabricated from a half-inch thick steel flat to sand casting around 50 mm high. So the Solex is removed, this "adapter" is fitted onto the intake manifold (with solex packing) and the Mikuni is fitted on the adapter. The next thing to do is to connect the standard accelerator "rod" to the Mikuni throttle shaft. So the accelerator link comes off the Solex and gets attached to the Mikuni. The Solex has dowel locators, the Mikuni has a slot locator. Fortunately shaft diameter is same. The Solex plate does not have a slot, so chances are that the plate will slip on the Mikuni throttle shaft after sometime so the throttle will not operate properly or fully. For the push rod, some ingenious hand bending has to be resorted to. The rod is supposed to locate on the left side of the throttle now instead of the right side (direction of rotation being different) so its shape has to be different. The intake manifold will physically foul this rod and it may slip off its ball joint. All this ruins the feel of the accelerator pedal completely, so how anybody can realize good performance is beyond my understanding. As the car itself does not have a provision for fuel return (it needed one but was never seriously pursued), in many cases I have seen the return port of the Mikuni just blanked out. A rubber hose is put on it; it is pinched and tied with wire. It is easy to do but it is not correct, as the Mikuni by design needs the return line. Around 95% of the fuel pumped is returned which cools the fuel as well as the mechanical fuel pump. Fuel temperature increases by around 12 degrees C if the fuel return line is blocked. In some cases if a return is indeed provided, more often than not, a badly routed loosely hanging plastic pipe is used to connect the return somewhere to the tank. It would usually go to the tank vent which is easy to do. So the vent then gets blocked so the tank cannot breathe. Also this pipe passes dangerously close to ignition components and can become some sort of a fire hazard (remember, spark plug firing voltage is around 25000 volts). As this return line has to perforce go under the floor, it is just tied with some wire so a stray stone can spark a leak and a fire. Normally a Maruti 800 air cleaner is used. Its PCV port needs to be blocked otherwise unfiltered air would enter the engine.
I would very clearly state here that in absence of any other adapting device, per-se there is nothing wrong in using an adapter but all other parameters need to be taken care of perfectly (means provision of return line and the accelerator control system). However, the adapter by its nature will restrict some airflow so engine performance can get affected to some extent. It is for the person concerned to decide how to go about it. In any case, it is not easy to just make an adapter. It involves cutting, turning, drilling the center hole and building a gradual taper in it. Further for mounting, drilling 2+4 holes, tapping them to M8*1.25, then installing studs in them and then mounting the whole thing on the car. Then the car has to be started on the Mikuni, hoping that the device does not draw any residual air from anywhere, which will prevent the engine from idling properly. I do not endorse this route at all.
The correct way - In 1997, around 200 S1 cars were introduced with factory equipped Mikuni carburetors.
The inlet manifold - the complete inlet manifold was redesigned. This design was derived from the original NISSAN design of the 118NE-inlet manifold. The first thing to do is to get hold of such a manifold, essentially a Mikuni mount at the carburetor end, with the Padmini cylinder head boundary dimensions. It is not easy to find but once obtained, remove the PCV non-return valve and block it by local welding as the Padmini engine does not have provision for PCV (except for some very late S1 cars). Refit the valve to block the open hole in the manifold. This manifold also had a charge heating pot with connections to the cooling system. For all practical purposes, this can be discounted. However the bottom plate will have to be removed to provide adequate clearance with the exhaust manifold. Minor grinding will be required to provide working clearance with the bell crank lever of the column shift gear mechanism to operate in reverse gear. The exhaust manifold may need to be slightly ground to provide working clearance.
The Mikuni carburetor - You need a Mikuni carburetor. First preference is 118NE, second preference is Maruti Zen and third preference is Maruti 800. There are changes to be made in the Zen and 800 settings, which I will explain separately. If you use a 118NE carburetor, use a Maruti derived accelerator outer cable-mounting bracket on it. In this procedure, the 118NE bracket will not work, as it does not have a face locator, the cable is located by friction. If you use a Zen carburettor, please interchange the throttle cable operating lever from an 800 carburettor because for some odd reason, the zen uses a larger diameter end connection which will not match the Maruti Omni cable. Remove the inlet and exhaust manifolds, replace the inlet manifold with one suitable for Mikuni and refit both manifolds. Remember to use a new sealing gasket. Fit the Mikuni carburetor on it with suitable packing (Maruti 800 or Esteem packing is good). Use M8*1.25 flanged nuts only to mount the carburetor.
The cable operated accelerator control system - The accelerator will need to be converted to cable operation for which you will need the operating lever which came in S1 cars / 137D cars alongwith it’s fulcrum bracket. This fulcrum bracket is welded on the bodyshell. Any broken down 118NE or Padmini S1 bodyshell has one welded to it. Just cut it and take it off. For the accelerator ratio, the radius of the operating lever of the Solex on its fulcrum is 19 mm whereas in Mikuni it is 28 mm. Both throttle shafts rotate by 82 degrees (8 degree taper on the throttle plate gives perfect sealing against the bore). Increase the length of the top lever by around 40 mm or use the top end from another 118NE lever (make one lever out of two). This lever has a 6.5-mm diameter hole in it, in which a rubber bump stop is mounted. Remove the bump stop, weld a M6*1.0 nut in its place and use an M6*1.0 bolt to set the lever height and pedal angle to your liking. How to position this lever correctly in the car? Remove the utility recess panel and remove the lever holding the ball joint to the accelerator pedal. By design, the accelerator pedal is tilted towards the right side of the car by 6 degrees and is positioned at 19 degrees to the vertical in idling position. Check whether your car’s accelerator pedal resembles a similar layout. If no, replace the pedal. You can use "Ema" make pedal with a new fulcrum assembled with an M6*1.0 bolt. Get the pedal in the right location. A good approximation is sufficient. Just keep sufficient angular movement to obtain full throttle. Now locate the fulcrum rod by its ball joint fixed to the pedal and weld the fulcrum bracket by tack welding on the bodyshell. For RHD, you will need to cut some portion of its top end, as it will dig into the RH dash panel reinforcement. For a good spring action for lever return, use the spring of the S1 or you can fabricate one by purchasing a regular accelerator return spring and cutting it as required. Weld a piece of wire on the dash panel to hold the spring. Once the lever is installed, you need to drill the dash panel to locate the center of the accelerator cable. Drill a small hole through the center of the forked top end of the lever and then enlarge the hole to around 14-mm diameter. For RHD, this will come out next to the steering ear if you use 137D lever and it will come out on the right side of the steering gear mounting pillar if you use the S1 lever. This point is where the accelerator cable will locate on the dash panel. Take an M8*1.25 flanged nut (Maruti 800 carburetor mounting nut) and working from the engine compartment, locate it in the 14 mm diameter hole so that the flange is towards the front of the car. Now gas-weld this nut in place after placing a bolt through it for temporary location and so as not to spoil its internal threads. After this is done, remove the bolt. So now you have an M8*1.25 welded nut in place. Now take the Bajaj Pulsar accelerator cable adapter (it has an M8*1.25 thread and has a locator which will accept the Maruti Omni’s outer cable OD) and assemble it on the welded nut with a locknut in place for fine adjustment. Now you will get a hollow tapering locator for the cable. Take a Maruti Omni accelerator control cable and observe both its ends. They are similar. They have M6*1.0 nut type sleeves on both ends but one end has an anti-kink cover on it. Do not disturb this end. By gas-cutting, cutoff the end connector at the end of the inner cable. Let the inner cable cool and then slowly remove it from the outer cable. Cut the end connector of the outer cable on the side opposite to the anti-kink cover and throw it out. Now assemble the outer cable on the Mikuni carburetor’s throttle control plate, and then pass the inner cable through it. The inner cable will come out of the other end. Cut the outer cable to suitable length so that it forms a nice long loop from the carburetor to the dash panel. For RHD with battery on the LH side, you may loop it behind the fender support. For RHD with battery mounted on the RH side, you may decide best routing which will not kink the cable. Pass the inner cable through the Bajaj Pulsar locator and locate the end of the outer cable in it. Now the inner cable will be located exactly inside the forked end of the lever. Locate the outer cable on the carburetor and the Pulsar locator on the dash panel at almost center positions (to provide for fine adjustment at both points to adjust final free play). Working from inside the car, put an M5*.08 nut over the open end of the inner cable till it touches the fork and carefully braze it to the inner cable. Take care not to braze it to the fork. Alternately, you may use a screw-on connector taken from the gearshift cable adjuster of a Bajaj scooter but this may slip off so I do not prefer it. Now operate the accelerator pedal. If required, adjust free play from the carburetor / dashpanel ends. Cut the utility recess panel to provide working clearance with the lever and fulcrum. Then refit the utility recess panel to the car. This system if fitted properly will work perfectly. Believe me, I have gone from Mumbai to Simla and back with this system adapted to a solex with differential angular operation. This system worked flawlessly.
The fuel return line – Remove the fuel tank from the car. Remove the 3 small nuts holding the fuel outlet pipe in place and remove the main pipe from the tank. Obtain a metallic fuel pipe discarded from an S1 engine and cut it at its curved end at around 50 mm. Then heat the triangular mounting face. It is soldered to the pipe so solder will melt away. Enlarge the center hole slightly and locate the 50-mm long pipe alongwith the original pipe in this hole. Braze both the pipes in this hole. Take care to maintain original height of the pipe in the triangular plate. Refit the assembly in the fuel tank. Now you have the tank, which will accept fuel return from the Mikuni carburetor. Now one long main fuel line from a discarded Padmini car and locate it underfloor next to the main fuel line. Alternately, you may use a brass 5/16" pipe. In any case, it must be routed and fitted perfectly to the underfloor of the car. You may route it inside the handbrake cable-mounting bracket for additional support. Now your car has a fuel return line into the engine compartment. Obtain fuel inlet and return adapters of the same size as in your carburetor, also obtain 2 nos. Solex fuel inlet adapters and braze the two together after passing a welding wire through them. The flared ends must face opposite sides. This is required, as the Padmini fuel lines are BLG3 (8 mm), Mikuni fuel lines are BLG2 and BLG1 respectively. Use Maruti fuel line upto these adapters and then use Padmini lines throughout the car. Locate these adapters on the choke-mounting bracket of the Mikuni. If you want to use a choke cable, you will have to fabricate one but more often than not, the choke is not required so I have not used it. Use good quality clips on all fuel line connections. This is imperative for safety and must be done carefully and correctly. Refit the fuel tank in the car. You will not be able to use the green plastic cover as the return line will foul it, but it is OK.
Engine starting on Mikuni – Crank the engine and start it. Connect the vacuum advance, adjust parameters as in Solex (already explained by me and compiled by Adheesh). Use a Maruti 800 / Zen / Esteem / 118NE air cleaner, it’s your choice. The center stud in a 118NE car is longer than in all Maruti cars. These are easily available. Use a new Maruti gasket at the top cover of the carburetor if using Maruti air cleaner. Locate the air cleaner angularly on the rocker arm cover by relocating its mounting bracket. Use new air cleaner element. It must be of Purolator make only.
I will give all the fine nuances of tuning all 3 carburetors on the Padmini engine separately.
Please let me have your comments.
Best regards,
Behram Dhabhar
Last edited by Rehaan : 27th June 2008 at 08:58.
Reason: Formatting tags removed.
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24th June 2008, 16:27
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Thanks
