Hello, I have a 1940s Cat D2 and am having some problems with the donkey engine. It is running really badly and no amount of fiddling on my part makes any improvement.

The engine has good compression and the magneto is good - it will throw a spark when just turning the flywheel by hand. I have pulled the carb apart and given it a good clean, everything seems ok with it.

The engine starts fairly easily but it never fires evenly. It misses really badly, so badly that even with the throttle wide open it never get anywhere much over what I would imagine to be idle speed. All the while there are clouds of black smoke pouring out of the exhaust which makes me think there is something wrong with the carb and it is getting too much fuel.

The carb is a zenith, I couldn't say which model but if you unscrew the bowl from the side of the barrel there is a long brass tube protruding from the bowl through to the other side of the barrel. Hopefully someone can identify it from this description. There are two fuel mixture screws and I have slowly adjusted both of these in various combinations but they have almost no effect on the running of the engine.

I can't imagine much can go wrong with a carb as simple as this one but the black smoke points to this being the problem. I can't think of anything else that might cause these symptoms. Is there anyone else out there who has had experience with these engines and can offer some advice?

Any help greatly appreciated.

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black smoke obviuosly too much fuel.
trash in seat or sunk float.
set all three screws at 1 1/2 turns. factory default.
(well my carb has three).

If that does not help get a kit and go through the carb.
When you take it apart shake the float listen for sloshing.

Have fun
Dudley

Just a few points.

You might check that the choke is opening properly, ( IN is open ) and the air breather is not restricted by gummed up oily dirt in the packing media inside the cannister.

On the D2/D4 pony motor, carburetor mixture screws work backwards from most small engine carburetors'. You turn them in to richen the mixture. So you might need to back yours out from the initial "1 1/2 turns out from fully in" settings.

The float, needle or seat might be bad as was mentioned earlier. I expect if your float is allowing gasoline to overfill the bowl, if you leave the gasoline valve on with the engine stopped, in a little while you will see gasoline coming out the petcocks on the cylinder heads or out the overflow on the older carburetors that have the fitting on the side of the bowl. Do not run the engine with diluted oil. It holds a quart, change it if it has gotten gasoline in it.

If float is good (no sloshing) and the needle and seat are sealing well, then the float adjustment might be a problem. Take the top of the float bowl off (remove gasoline line and take 4 screws out), and check the float for leakage. Even if it hasn't got gasoline inside, you can make sure a metal float's ok by sinking it in a pan of hot water. A leaky float will make bubbles when the air inside heats up.

Then flip the assembled float, needle and bowl cover over upside down and check that the top of the float as it rests on the needle is parallel with the bottom of the float bowl lid. You have to remember as you read this that the assembly is upside down, but "top" is really top as it is installed on the carburetor bowl and bottom is bottom as it is normally installed, not as you are looking at it upside down. Adjust as needed by bending the small tab on the float that presses on the bottom (head) of the needle.

Good luck!

Sounds to me that the load screw adj link is out of position. This carb has a 90 degree link that when you turn the screw in pulls out on the needle and increases the amount of fuel passing . This arangement is located in the float chamber and requires care to assemble correctly. Good luck.
Let us know what you find. Ron Meeder.

Concerning your D2 starting engine running to rich. Perhaps now I can try doing as some that have been so kind in helping me.
My RD4 has a complete closure for the air intake on the starting engine and it ran that way until I found it and opened it. Yours may have the same. Anyway it sure has to be not getting enough air.

As someone mentioned before, check for oil dilution. Pull the dipstick and see if it smells like gas. If any question drain it and refill with 30W. Every time I have had one get gas in the oil the engine would run exactly as you described. There might not be anything wrong with the carb that shutting the gas off wont solve.

Tom

Tom, good advice and I,d change the oil anyway and for all concerned never just shut the starting engine kill switch off but turn the gas shutoff to off and let the gas burn up in the carb till it stalls out on it,s own and then no gas will jiggle out of the carb. and find it,s way down in the lube oil.I have seen this happen so many times of the gas diluting the oil caused by not shutting the gas off. Also Hareti how are the plug wires on it? Never use only wires with wire in them and not automotive types.