MCS Mixture Control Solenoid Availability VS Usability.
Anyone that owns and maintains an 85-88 MK1 Sprint knows, the carburetors on these cars, mainly the Hitachi DFB306 feedback unit, are not the most popular where carburetors are concerned and near impossible to find parts for. However, if properly maintained and rebuilt, they offer great performance and some of the best economy any car has ever offered even to this day.
One of the key elements that makes its impressive performance possible is the Mixture Control Solenoid, aka MCS. This tiny electro mechanical device works wonders where those impressive mpg numbers come in to play. However, in recent years trying to obtain a new one that works and fits seems to be getting near impossible. I’ve heard all sorts of stories about how many among the 3 cyl crowd have purchased what they are lead to believe is an exact replacement, only to find out that it doesn’t work. Many to abandon reviving the car due to the fact they can’t find the proper part. Of course many never go down the path anyway due to the fact they can’t stomach to spend the cost of the part, on a car they only paid pennies more for. Sadly, it is no doubt why so many of these impressive little cars ended up in junk yards. Me personally, I’ve never heard exactly what it was that didn’t work until I myself recently went on that quest of obtaining a new one. What I am about to share is my research, purchase, and eventual modification to make a readily obtainable one work.
If one does an internet search to no end, one will commonly find what the carburetor parts industry refers to as the MX40. According to all sources this is the MCS for any 3 cyl Suzuki engine from 85-88. Prices range anywhere from $108-$279. All of these claim to be brand new units. All sources depict a photo of the same exact unit. I was told by a few here at teamswift, those that know the carbs well, that the unit probably would not work and good luck with the purchase. Most of the suppliers of these devices tell you the parts are non-refundable, (Many electrical devices in the parts industry are this way) such a purchase situation sways many. Of course, non-refundable aside, if you desire to get the carburetor back going what option does one have but to order and see, and then make the thing work or tear something up in the process. I chose to not give up, buy the thing, find out if it would work or if it wouldn’t. If it would not work, then do whatever it took to make it work. If my approach worked, share it here with the rest of you that periodically go on the search.
At first glance they look 100% identical. Overall specs of the metal bore are identical. Internally they are both identical. Coil resistance is identical with both in the 40 ohm range. The first noticeable differences are the Jets that occupy both ends. They are clearly not the same as the old units. My old unit had a #75 on the bottom and a #160 at the top, obvious by stamped numbers. The new one, with no stampings had both at quite a bit larger. I never obtained the exact numbers, but using drill bits as reference could clearly see a difference. Obviously such a difference brought up concerns. If an orifice for flow is set at a certain value in the form of a jet, then obviously it is set at that for a reason. Or so common sense would lead most to think. I did successfully disassemble the old unit to see if those jets could be removed. Also, to learn more about how the internals of this device worked, and could the DIY rebuild one if they wanted to.
Here is what I found on the disassembly. The jets are installed using red Loctite. Red Loctite can only be loosened in one way that is to apply heat. Obviously as heat is not exactly a friend of electronic coils there was concern. Looking at the metal bore one see’s two areas, one at the top and one at the bottom that looks to be a beefier ring type area.
Using this tougher area, I placed the MCS in a vice so as to try and not crush the thing. Also to hope that the vice would work as a heat sink to avoid any coil damage. Using a heat gun vs. a torch I was finally able to remove the jets. Extreme patience is key here, these jets being brass strip very easily. That and no one ever seems to have just the exact screwdriver by which to get them out. But, with heat and patience it does happen. If I ever were to reinstall a set I would use blue Loctite vs. the red. Here is a picture of what the insides of an MCS look like.
Really not much one can DIY at all. There are two small rubber pads on each end that act as seals mounted to metal pads, otherwise it’s a piston rod, an a spring. I guess maybe the rubber pads could be replaced. In the case of mine, the piston rod itself was quite galled, leading me to believe that possibly it was binding in the bore. As for the electronic coil itself, I see no way possible the DIY would ever remove one and rewind it, if necessary, without total destruction of the metal casing. OF course, this is all for education purposes and to aid in finding out how to make this NEW, supposed won’t work/fit unit actually work. My goal was to see if these mismatched jets could be swapped out, and this much I did prove. However, I chose to just for the heck of it install the new MCS as it came with its oversized jets, worst case if I can’t tune the carb, or economy is affected, I’ll remove it and swap jets. But, different jets, is the least of this MX40’s compatibility problems. As stated first glances can be deceiving, as I tried to physically put it in place the real difference popped up, that being its ability to identically mount
The Original MCS has a small rubber boot that slide over the wires and provides some sort of dirt sealing at the top. This boot mounts all the way against the hard black plastic mounting tab found in the the middle of the MCS. The New MX40 has a similar boot, but larger in diameter and the plastic mounting tab has a raised area that is raised approximately 1/8” higher than the old MCS. This raise makes it so that this unit will not fit the top lid of the carb. Though the wires can be unsoldered, this raise cannot be ground down without damaging the wire housing. However, what can be altered is the carb housing itself. There is plenty of metal stock in the carbs lid to do what I planned to do. The old rubber boot is still used, that way the base factory hole stays the same. In the event someone were ever to actually find an exact replacement, leaving the base hole the same allows the use of the old style MCS, However, an o-ring would be needed to make up the depth of the new hole.
The tool I used to accomplish this was a standard drill press, set on high speed, and a Dremel 9900 series tungsten carbide cutter.
I used a 4x4 with a hole drilled into it to hold the carb lid level. The stem that normally holds down the air cleaner is placed in the hole in the 4x4 and acts as a decent holding platform. Set the proper depth of the bit to accommodate the rise in the MX40’s mounting tab. Take your time and grind away.
I also found it necessary to elongate the mounting holes slightly in the body of the MX40. Place a small amount if dielectric grease or Vaseline on the o-ring’s for the MCS and mount. Now all else goes together just like it should. My carb housing can accommodate both the old extinct MCS and the new mx40.
Now came the real test, that is will it run properly. Aside from some other problems with a possible vacuum leak, etc that I won’t go into, The new MX40 performed great. Setting dwell was just the same as with the old style MCS. Even with the larger jets in the MX40 all seemed to do great. Time and MPG will tell just how compatible it is. Worst case I’ll remove it, and use the jets out of the old MCS.
The main point behind all of this was to prove that this NEW MCS, MX40 could be made to work in the Hitachi DFB306. Not a lot of effort to get it done. Does not alter the carb against using original stuff if ever found. But, this is not a task for the squeamish or totally lost in a work shop person. Ultimately you have two choices, do this or sell the car or car or carb. The best of us that still believe in carbs just can’t find a source for the OEM. DO you think you know where one is, please share your findings. It would be interesting to hear if anyone else out there ever bought one of these MX40’s and it fit perfect. I cannot vouch for the fact that all Carb lids are the same.