Lanse said:
Ive been trying to get my mower working today, and it will run just fine on choke, but nothing elce. It also runs WAY faster then normal, and i cant seem to get the combination right on these screws on the carb, the idle and needle screw. What aint i doing???
Consider the following:
(1) The way faster than normal speed typically is the result of a vacuum leak which acts like an open throttle. Instead of the throttle being open (letting more air in) in the normal fashion, it is getting it's air elsewhere. Check carburetor mounting screws to ensure they are not loose or gasket damaged or damaged/disconnected vacuum hoses, cracked manifold, etc.
(2) The engine not wanting to run on anything but full choke suggests clogged jets or fuel starvation EXCEPT that the full choke rectifies the situation meaning that the carburetor DOES have enough fuel to operate properly and is NOT STARVING. A full choke will not make a clogged fuel filter or other inlet issue pass more fuel. Rust in float bowl more often than not causes float needle to sit in open position causing a flooding situation instead. Meaning... a much more likely condition being a possible vacuum leak such as from the carburetor mounting bolts being loose or damaged/disconnected vacuum hoses, cracked manifold, etc. In order to understand this, you need to know the principle behind how a carburetor works. Air flowing through the carburetor venturis (a narrowing of the carburetor) causes air to flow faster through that narrowing passage which results in sort of vacuum wake that draws fuel into the air flow and mixes to form a combustible mixture. When the air is going into the engine from another source (vacuum leak) rather than through the normal channel, a lean mixture results which you have compensated for by applying full choke.
(3) The idle and needle screws not having any effect is the result of (1) above. Remember that carburetors have many different circuits: power/tip-in/acceleration circuit, etc. The idle circuit is one of those circuits and function only at IDLE SPEEDS. The engine running at "way higher" than normal speeds means that the idle circuit is no longer active and hence, the mixture screws will have no effect. If the high idle is the result of a vacuum leak, then the idle speed adjustment screw will also have no effect because, well, the air entering the engine by means of a leak won't be controlled by the throttle blade position which is precisely what the idle speed adjustment screw attempts to adjust.
My recommendations is to first look for any obvious leaks through a visual inspection. If none, try tightening the carb mounting screws. If not that, some folks recommend spraying some oil into some seams and seeing if that temporarily plugs the leak. You'll know because the idle speed will change. I don't like doing that because I don't like dirty oily engines and it only makes a mess of things. You can also run a propane torch (not lit, of course) around the engine/carb area and see if the idle situation changes. The engine will suddenly run rich if the torch is supplying propane to the area of the vacuum leak. Once you've rectified your vacuum leak, corrected the high idle situation, you can then go back and readjust your idle mixture screws and idle speed screw.
BTW, oftentimes when this occurs, many folks will remove the carburetor, rebuild and the problem goes away. The thinking is that the carburetor must have been dirty which caused the problem not knowing that they fixed a vacuum leak during the removal and installation process by installing new gaskets, replacing old vacuum hoses, proper tightening of the re-installed carb.
Hope this helps and the explanation understandable.
