Carb problem

Second, the throttle adjustment screw (at govenor linkage) dosen't seem to have a definate hard stop. After reducing the RPM with the throttle leaver, I can still move the shaft forward some by hand which causes the engine to cut off.

I've removed and cleaned the shaft and carb body and all appears ok.

All comments and suggestions appreciated...John A

Let me add, I'm not having any problems with the operation, runs like a top, just won't idle down to 400 - 500 RPM.

Come to see us....John A

see you soon.

Roy

The issues are tied together when it comes to tuning the carb. The idle circuit does not come into play except at very, very low RPMs. This is different from cars, chainsaws, lawnmowers, and other engines you many have adjusted. Unless you get the engine down to less than 600 RPM or so, the idle mixture screw will have no effect, since the carb runs off the main metering circuit. I suspect your carb has no effective idle circuit, which is why it "cuts off" when you get the throttle fully closed.

The idle mixture screw works in reverse. It opens and closes an air bleed that adds additional air to the idle fuel mixture. If the idle fuel mixture is already too lean, with the screw all the way in, you can unscrew the adjustment (and add more air) until the cows come home, and it will not help.

If you want to try to make the idle work, you will have to get the throttle blade where it needs to be to allow the engine to run slow enough. This involves fiddling with the throttle linkage, and sometimes opening the hand throttle and shoving it closed quickly, to overcome friction and slop. Turn the stop screw in until it becomes effective, so that you have a repeatable point to do your tuning from. If this is above 600 RPM, fix the linkage. If it won't run this slowly (because it "cuts off"), try Zane's "polish the case halves" of the carburetor fix, and otherwise try to figure out why the idle circuit gets too little fuel.

The Chinese/Indian/Turkish clone carbs seem to have this problem new out of the box, by the way. I had to make several modifications to mine to get it to idle at 400 RPM.

John A

The best way to research the problem is to disconnect the governor arm and attach a spring to pull the throttle towards the closed position. Adjust the idle stop screw to a level that will allow the tractor to start and continue to run. (Your 600 RPM).

Now slowly unscrew the idle stop screw. As the throttle blade is closed, the engine will slow down. If the idle system does not work, the engine will also start to falter. Try lightly seating the idle mixture screw. This takes the idle mixture as rich as it will get.

Still no help? Do a thorough vacuum leak patrol. Any source of extra air (carb gasket, manifold gasket, intake valve guides, hole in manifold) will lean out the mixture and take the idle system outside of its tuning range.

If you think you've eliminated any chance at a vacuum leak, try the following test. Remove the air cleaner pipe. Get the engine to the "begin to falter" point. Obtain an unlit propane torch. Crack the valve on the torch, and induce some propane into the air flow flowing into the choke horn. If the idle steadies out, you have verified that the mixture is too lean, because the carb isn't flowing enough fuel. If the idle does not improve, something else is wrong, like the ignition system.

If propane helps, and you are SURE there are no vacuum leaks, you will have to open up the carb. Inspect for blockages in the idle passages. Perform the Zane polish-the-case-halves ritual.
Check the float level.

If nothing helps, you can consider carb mods. Buy yourself some idle jets and a set of numbered drill bits. Drill out the idle jet a few drill sizes and see if you can lower the RPM the tractor will operate at. This should allow more fuel to flow at low engine speeds, and bring the idle air bleed system back into its adjustment range.

If you want to venture into really advanced carb theory, it is my opinion that the idle transfer passage in the Marvel Schebler is poorly located. It is possible to get a solid 400 RPM idle, and still have lean missfire in the 600 RPM range. The idle transfer passage is too far down in the throttle body, and doesn't kick in soon enough. You need at least a drillpress to consider this mod, however.

Before investing all this time and effort, consider that the main reason for the 400 RPM idle spec is to allow the tractor to be started with a hand crank, and to cater to weak electrical systems in cold weather. It's kinda cool to hear the engine ticking over so slowly, but it really isn't necessary for day to day use of the tractor, assuming it starts easily.

John A