One problem leads to...

Hi all,
Many here have helped me with my David Brown 25 petrol problems, but today I put a Zenith carb off a TEA onto the 25, and it works well.
Only...how do you connect a Zenith carb to a DB? It only runs idle or screams uncontrollably then I had to shut it off fast with the switch as the governor threw on so hard I couldn't force it back to an idle...
I would appreciate any thoughts and help!
Thanks!
Joshua

I assume that you put the control rods on in what appears to be the same way that they were on the old carb. This would make it appear that the crank arm for the throttle butterfly works in the opposite direction on the replacement compared to the original.

I think showcrop is right, the throttle linkage is opposite of the original carb, so the governor is opening the throttle instead of closing it when up to speed.

What you can try is have an assistant sit on the tractor and operate the dash lever.

With the engine off, disconnect the governor to carb linkage, hold the link in an unrestricted position. Have the assistant move the dash lever to idle position then to fast position while you watch what happens with the linkage. With the dash lever in the idle position, the carb linkage should be pushed against the idle stop position, in fast position it should be wide open.

If necessary modify the carb, not the governor, to make this happen. Use the original carb as a reference to get the length, angle, and position as close as possible.

Once the modification has been made, cautiously start it up, being ready to kill it if it starts a runaway. If still not right, you can disconnect the linkage, have the assistant start the engine while you manually operate the throttle at the carb. With the dash lever set for fast, manually vary the engine speed from idle to a reasonable upward speed around 2000 RPM. You should be able to hold the governor link and feel the governor work as the RPM changes.

Theory of operation, the throttle spring wants to pull the the carb throttle plate open, the spinning weights in the governor want to push it closed. It's a balancing act between the spring and governor, determined by RPM's.

With the engine revved up to around 2000 RPM, and the dash lever in the fast position, the governor linkage should line up with the carb linkage when it's in the idle stop position.