Advice please, re 25-40

We have just finished rebuilding the engine on my 25-40 Thresherman's Special. The new valves seal really well and the engine now runs fantastically, except that it will not run without the choke fully on, even when hot and on petrol. I cleaned out the carb with toluene when we had the engine in bits, so I know that the problem is not dirt in the carb. We have checked and found no air leaks at the manifold, carb flange etc, so I am guessing that the problem is that the fuel level is too low (Zenith C6V carb). Any other ideas on how to solve the problem would be greatly appreciated (Fred, can you help?)

The fuel flow through a C-6 is not very hard to figure out. When you say you cleaned the carb did you take it apart and clean it or just run some cleaner through it?  Tolulene wont remove rust from passages.  I used number drills and small wires to clean ours. There is a valve in the C-6 that allows the fuel to flow through the main jet, commonly called a "powervalve" here in the colony.  A spring closes it and vacuam opens it, located  in the bottom of the main body, hard to miss with the bowl off.  You check the fuel level in a C-6 by screwing a hose barb in the drain valve hole and attaching a couple feet of clear tubing. Hold the end of the tubing above the carb and turn the fuel on. The fuel should be about 9/16" ( I think) below the top of the fuel bowl. Do not think for a minute that if ithe fuel level is just a little low that you have found your problem. We routinely shut our As off by turning off the fuel and they run just fine right up the time they run completely out. The carburetor shop has a kit for the C-6 that contains needed gaskets and a needle and seat. www.thecarburetorshop.com Larry Lietzow has brass floats, if you dont have one now you soon will need one as modern fuel will destroy the cork float in short order, at least USA fuel will. Past the carb I assume you have checked for leaks in the manifolds?

There are no leaks at the manifold etc and I did clean the carb out pretty well. The float in mine is brass, so no problem with unleaded fuel. I.m wondering whether the main jet may have been changed for a smaller one when the hot box was smashed off the original maniflod to run on gasoline when the tractor was in the States.

I have now found one of the problems - when I cleaned out the carb, I cut a new gasket to go between the bowl and body and I missed cutting out a hole for one of the jets. I've put this right and now the tractor runs really well on gasoline, but I can't get the mixture rich enough for kerosene to use with the new hotbox manifold which Will Woodward made. I guess that the carb must have been changed or fitted with smaller jets when they smashed the hotbox off the original manifold for gasoline operation during its earlier life in the USA, so now I either need to drill out the main jet a little and rely on the adjusting needle for mixture control, or get a fresh carb which is correctly jetted for kerosene from somewhere - can anyone help me with that one?

Edited by Tricky Dickie - 29 May 2010 at 12:07pm