Home › Forum › Ask A Member › 1976/77 OMC 9.9 & 15 carb needle valve bearing › Re: 1976/77 OMC 9.9 & 15 carb needle valve bearing
you may be right . I am guessing your talking about the cup seal being not used in 79? I do know that it seems like the carb was changed 3 times in a serious way for basically the same power head that was used on 9.9s from like 75 to 91ish to 95ish other than the ignition systems going from points in the 75 then 85 and 91 were also different from each oher I am guessing both were a type of cdi type set-up for 85 and 91. I couldn’t tell much of a difference when setting up the motors regarding ignition systems other than getting the timing of the carb butterfly opening at the correct time with relation to the magneto plate rotation. The big difference to me on slow engine idle was how well the 95 carb performed much better than the 75 85 and 91 carbs.
Again, the whole carb was redesigned in 1987, so the plastic cup seal question discussion only applies to the 74-86 models with the aluminum body/tin float bowl. The question is how engineers were able to eliminate this seal without making other changes to the carb body that would help prevent outside air from entering the mixing chamber through the needle valve/carb body threads. The idle circuit plumbing was changed on these style carbs sometime around 1980, the fuel was drawn up the to idle circuit through a brass tube that ran right up through the venture. The older carbs had a cast in passage in the carb body instead of the brass tube. So, I am guessing this change was simply a cost savings, but the change would not seem to be a substitute for the low speed needle cup/seal.
The "modular"/CPS carbs were introduced in 1987, and that is when all the trouble/reliability issues began. What once was a simple, reliable carb, was now troublesome, and more complicated. The low speed needles on the newer carbs threaded directly into the plastic tops, the plastic threads posed much more interference/tightness when threading the needles in place, which also created more plastic debris floating around. I’m "guessing" this tight interference fit was how the engineers justified not adding some sort of sealing system to the low speed needle.
To be fair, the modular carbs got many improvement over the years, but still not as simple and reliable as the original style.
The modular carbs were changed again when the engines were redesigned in 1993. The 1993 carbs had a design flaw which created an air leak in the plastic top, the only fix was replacing the carb assembly. Needless to say, all these carbs should have been replaced free of charge, but many of the defective carbs that were replaced are floating around on dealer’s shelves now, I see them for sale every once in awhile. It was with this new engine/carb design in 1993 that the needle valves were moved to the side of the carb under the engine cover. I suppose one could argue both sides of the debate regarding this being an improvement. I suppose it does keep the owner from easily messing with the mixture, but also makes it more difficult to make minute adjustments that might actually be necessary depending on temp/weather conditions. These 93 and newer carbs will not fit on the older models. The choke linkage is different, and I don’t think the older air box will fit onto the newer carbs. I really can’t think of any reason why one would want to replace the simple reliable carb design with one of the newer style carbs, I don’t think these new carbs offer any idling advantages when compared to the original design.