Posted by ChadS on April 03, 2009 at 07:55:14 from (4.224.78.65):
In Reply to: 362173r91 carb posted by karl f on April 02, 2009 at 22:48:18:
You need to inspect this carb to properly ID a "Triple Port" carb. They are different from your typical 300-450 carbs, they have an extra fuel passage to help fuel delivery and keep a high quality idle at the same time. Best of my knowledge, they were only made from January 1955, thru July of 1955. There is a stamped date code, in the gasket surface of the carb flange, but have not seen theis on carbs that were older than 1949 on the M-450 carbs that Ive been into.
Only reason I belive this style of carb existed back then,, was for a gasoline conversion,, for when someone wanted to take an LP tractor and convert it to gasoline and still use the LP engine componets. The carb requirements to deliver fuel, kind of exceeded the stock design to do that kind of conversion without suffering idle quality.
To ID a triple port, your looking for the location of the third passage in the 300-450 style throttle body. When you look at the throttle body upside down, (float and needle removed) your looking at the holes that are right behind the float,, close up by the venturi bore,, not inside the venturi bore, on the outside, where they have acces to the fuel bowl,,,, On the plain jane 300-450 carbs wit the R92 part number,,, you will only see 4 holes for passages. If you see 5, you have a triple port carb.
Follow this passage up into the throttle body,, you will find, it exits right below the throttle butterfly, and only gives fuel when the throttle plate is more than half open. It operates by throttle position,,, when vacuum hits that passage, it draws fuel right out of the bowl, up, into the passage and exits above the venturi, but right below the throttle plate.
I have seen R92 carbs with the hole below the throttle plate, and the passage from the bowl plugged by a brass plug, or the passage not drilled, or completed, but the top exit hole was installed, for if any reason someone wanted to convert the R92 carb into a R91 style carb.
Info on the typical 300-450 carbs, they are twin port carbs. Here is what they did,,,, this goes back to the M, which is a single port. This means, the carb has only one passage (besides the idle circuits) to deliver fuel, in the M, it was thru the main jet and metering stem, and during a certain postion of the throttle plate,, the idle passage has a circuit that does pull fuel right out the bowl too,,,,, but that circuit exists in most of these carbs, and is not tunable unless you really get into the carb, and know what your doing. When the 300 and 400 was built,,, they changed the carbs internal passages, and created a second port,,, or they redesigned the passage mentioned abou the M idle passage,,, either way, they gave the idle circuit its own exit passage way up high above the venturi,, but they kept the location of the secondary passage, pretty much the same location as the M carbs idle passage. Still operated off the throttle position, but routed entirely different,, either way, fuel delivery now had 2 ways of fuel delivery,,,,, other than the regular main jet/metering stem single delivery. Hope this helps,,, Id go cleaning on that carb you have,,,,, a tiny set of drill bits help clean,,,, a passage Ive found thats blocked when the idle is acting up,, is located when you remove the tiny brass idle tube in the bowl,,, once that stem is removed, run a tiny drill bit down the hole,, it sould go deep to the main jet,,, dont drill, just see if anything comes out,,, you might find it plugged in tat location,,, and it will make for bad and erratic idle condtions. Need help, call me. 574-893-7437, or email ChadS3@hotmail.com ChadS The Tractor Shop.com
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