The little brass plug in the base of the bowl, if I open it up to check the channel behind that runs under the bowl how do i reseal it?

Thanks everyone for the tips and advice.....all noted and hope to get back to the machine shortly

There is no gasket on that jet just thread it back in. 5F7639 jet is also still available from Cat.

The brass jets aren’t usually a problem to clean since you have a straight shot through them once the top of the bowl is off. Just don’t enlarge or damage the jet in the process.

The problem area is the jets feed into passages drilled in the pot metal alloy bowl that turn 90 degrees just below the jet and run some distance across the bottom of the bowl before turning up to feed into the throttle body. With that configuration, they’re a great sediment trap and also the last place that varnish remains after residual gasoline evaporates. The one that connects the high speed/main jet and the high speed metering well carries the majority of the gasoline to the engine and seems to be the most susceptible to clogging.

All the passages in the carburetor are drilled when manufactured and then plugged somehow. A few are threaded for example the high speed metering well has a threaded hex head plug because there’s a removable orifice tube fitted in it. Other drilled passages are simply plugged off with “soft plugs” of lead. These are easily drilled out to ream the passages as part of a real once in a generation cleaning. I believe most folks use .177 caliber lead air rifle pellets to tamp in once they’re finished cleaning. Though I’ve heard of using sealant coated BB shot or even threading and installing short machine screws. In a pinch I’ve melted a bit of soft electric wire solder to get a bead to tamp in. I don’t like the idea of the hard BB shot since they would be hard to drill out next time compared to the lead pellets etc.

If both plugs are dry after lots of cranking you aren't getting fuel, I'd go completely through the fuel system (tank, lines, carb) and as others mentioned, pay special attention to the lower passage way in the carb. I bet it will probably start first or second pull after doing that and new plugs. If the factory fuel line is still in-tact i'd by pass that (leave it on the machine in-case you want to restore it back to original later) and run a new rubber line with a new inline shut off valve and inline filter between. Also a fresh oil change if that hasn't already been done, that should be done at LEAST once a season if not more, it's only 1 quart and there is no oil filter, cheap insurance.

Jordan

I had another go at this thing this evening, I had the bowl off and have run brake cleaner through that well and can see it going down and coming up so am assuming its clear.

Now what does this "Non-adjustable Jet" do? Its the long one that connects with the centre port of the bowl and runs through the verticle port.

It is clear through the middle but I don't see how the gas actually gets in to the tube once its installed. Hollow at one end where it locates in the carb body and at the other there are two very small holes around the circumference. Surely there should an opening allowing fuel in to it?

Thats part of the problem, thats the idle jet and is broken off, and it is adjustable, it should come to a point with a very small pin hole on the end. The trouble is some guys will pull the carb off the throttle body and not remove the idle jet first and damage it.

ccjersey wrote: "... the passageway across the bottom of the bowl from the main jet to the removable high speed metering well so clogged with crud that only reaming with a drill bit will restore them. I've had one that I could blow compressed air through that was almost like a new engine AFTER DRILLING THE SOFT PLUG and proper cleaning..."

Where is the "soft plug" that you referred to and what did you use to replace it?

"Delta wrote:

ccjersey wrote: "... the passageway across the bottom of the bowl from the main jet to the removable high speed metering well so clogged with crud that only reaming with a drill bit will restore them. I've had one that I could blow compressed air through that was almost like a new engine AFTER DRILLING THE SOFT PLUG and proper cleaning..."

Where is the "soft plug" that you referred to and what did you use to replace it?

this is a great pic...thanks....the long needle i pulled out was as tight as hell to remove and seems to me to be a bit bent, sounds like i need a new one of them

I’ve been asked to get a finicky pony started a time or two. As much as I hate to suggest this, but fill the tank with good gas, wipe the filler end of the tank, cup your hand and BLOW into the tank. Often a little pressure applied to the system is enough to get things flowing and a hesitant pony going again. I’ve seen guys use an air nozzle, but usually end up with too much pressure and flooding things instead. Often I open a drain while pulling on a rope to allow any excess fuel to drain off, or better yet, listen for subtle pop and easier for a fat guy like me to pull.
Good luck. JM

The soft plug for the main jet passage across the bottom of the bowl is approximately where the shaft of the arrow labeled #7 on the diagram crosses the edge of the bowl as it’s drawn there.

If the bowl was turned 90 degrees so the main jet was toward the viewer, the plug in the passage would be just below the main jet with the high speed adjustment screw if the top is installed on the bowl. On the later model tractors where the main jet adjustment screw is oriented horizontally to clear the gas tank, you would just have to look in with the top off to see the main jet as the screw engages a small bell crank to actually lift and lower the needle into the main jet. That brings up the point of direction of adjustment.

All of the Zenith TU4C downdraft carburetors use an idle mixture adjustment that changes the amount of AIR admitted to mix with a set amount of gasoline. So turn screw IN to richen the mixture and OUT to let in more air to lean the mix

If your carburetor has a high speed/main mixture adjusting screw that’s oriented vertically through the top of the bowl, it is direct acting and you will turn the screw IN to restrict the flow of gas through the main jet to lean the mixture. Back the screw OUT to make mixture richer.

However if the main jet adjustment screw is oriented horizontally into a small “doghouse” attached to the top of the bowl it operates the main adjustment needle through a bell crank and will act opposite from the vertical screw type. Additionally the bell crank in that little doghouse can be bent away from its original 90 degree shape and make the recommended initial setting be way wrong or even make it impossible to properly adjust the mixture at all. This type is illustrated in the diagram referred to earlier.

When looking for soft plugs, remember that all the passages in the carburetor are drilled so there’s no curved ones, only straight sections that intersect with another straight drilling so always there’s a corner and frequently there’s a plug put in to block the access used to drill one or the other or both the intersecting straight sections. Soft plugs are put into a shallow countersunk opening that’s drilled slightly larger than the passageway, so the plug has something to stop it going into the passageway and make it expand as it’s tamped into the opening. They aren’t hard to find and aren’t hard to drill out just the plug allowing one to replace it quite easily once the passage has been cleaned.