Many discussions about carburetor cleaning mention that a rattle should be heard when shaking a carburetor. It’s said that if you don’t hear the rattle the carb will not perform correctly and should be scrapped.
The rattle has been a confusion as I have only heard the rattle in a few carbs. I figured that carbs are not all alike and the 4-13 hp Tecumseh engines I usually work on were not all outfitted with carbs of the rattle type.
Some discussions mention that the rattle is related to metering fuel but it’s never explained so not so understandable as to what the rattle is and how it figures into metering.
The rattle does exist in low power carburetors for Tecumseh engines. The carb has to be shaken in a certain way and it’s very faint in most carbs.
Here’s the deal – The shoulder on the main jet leg is drilled to allow a passage for fuel to get up into the Welch area to feed the small holes drilled into the carb throat. This drilled passageway is vertically, parallel to the main jet leg.
A second hole is drilled through the outside of the shoulder up about ½ an inch from the base of the main jet leg. That gets gas from the main jet area over to the passageway.
A thin dowel pin is put into the passageway and sealed in with a ball. The ball is fixed in place but the pin is loose. The pin is roughly the same diameter as the pin that holds on the float but shorter. Purpose of the pin is to restrict the flow in the passageway which is the metering part. Part of the purpose may also be to dislodge particles or scrape away evaporation buidup.
When there is no rattle it may be the pin is stuck from varnish and the passageway plugged. This is supposed to cause the engine to hunt or run rough and why some say junk the carb. This is a confusing part. The fuel goes to the Welch area so in the low idle circuit. Some carbs respond to a low idle setting change and others not so much but both can idle fine and run fine at high speed under load. It could be that the pin is playing a role here. The pin could be stuck and blocking the passageway or partly stuck allowing a restricted amount of fuel but fuel enough to get by with. ??
Another thing I’m told is part of the purpose of the idle jet is to keep the pin from falling down into the hole too far and blocking the fuel flow. The end of the jet supports the pin and allows fuel to get into the throat.
Most cleaned carbs work out but some have a stubborn minor hunt or run rough and a few impossible to cure and scrapped. It may have been due to this pin being stuck. Now that I understand the pin, how to shake it to get the faint rattle I’ll have to look for this in stubborn carbs.
If anyone has some comments about this please post them as I’m not sure if I have everything right about this.
This message was modified Oct 28, 2010 by trouts2
