850 with a new zenith carb

Hi,

I've been working for about a week off and on trying to get my 850 ready for the spring. I had a lot of trouble last summer with the carb on it so I bought a new (not rebuilt) Zenith carb and installed it. I've never run a zenith carb before and I was surprised when it came and only had one adjustment screw on it.

I got the carb installed and got it running but I'm having serious problems with carb ice, even to the point that the tractor will die. I did replace the manifold gaskets on it when I discovered an intake leak, which I'm sure was my problem last summer.

The tractor will run fairly smooth for 10 or 15 minutes but then it starts idling rough and eventually chokes down and dies. I can see no sign of carb ice on the outside but when I touch the manifold just above the carb it is wet and cold but not frozen.

Can anyone give me some pointers on how to adjust the zenith carb? It came with an instruction sheet that said to start with the screw at 3/4 of a turn off of full closed.

I'm assuming that the one adjustment screw is the air screw so to lean the mixture I would need to screw it further out, correct? I have tried adjusting the screw 1/4 of a turn at a time starting at full closed all the way to 4 full turns out and I can't seem to get it to stop choking down after awhile. It isn't flooding, I'm doing the adjustments with the intake hose loose so that when it dies I can pull it off and check for flooding in the intake opening of the carb.

I'm not certain that carb ice is the problem but I can't think of anything else it could be.

All advice will be welcome!
Mark

Did you reuse your old short, large diameter 850 sediment bowl assy.? If your carb. now has the 851 style tall, small diameter sediment bowl assy. they may not have changed the hollow bolt to hold it on to the end of the carb., the drilled hole is at a different place and it starves for gas if they are not matched to each other.

Carb icing up is in fact normal and the way a carb works. As the gas atomizes it is super cooled so that in turn causes the icing. As for the carb problem the replacement carbs are junk. Me I always rebuild my old carb or replace it with a factory rebuilt M/S carb.
That all said does choking it help or hurt. If it hurts when was the last time you serviced the air cleaner. If it helps then you got your self a bad carb or one that came new with a clogged main jet circuit