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Carb issues on my 8N

N

norm s

Member
OK, so I got this 8N last summer, finally got it running (but it needed major work) but I did get a kit for
the carb and rebuilt it. Carb worked great. Took the tractor in the shop last fall and tore the engine
down. Fast forward to a month ago when the engine was back together and running. Engine will not idle
without some choke on. Runs fine at speeds above idle. I have had the carb apart at least a half dozen
times, cleaned every passage, etc. and yet it refuses to idle. Doesn't matter what I do with the idle
adjustment screw, nothing changes. Then tonite, for whatever reason, I started to adjust the main jet
needle valve. Found out that it doesn't matter what I do with it, the engine still happily runs fine above
idle. I've verified that there is no vacuum leak from carb to manifold to engine so that's not it. Now I've
been rebuilding carbs for at least 40 years but this one has me baffled. Where do I go with this??? I'm
ready to throw it out and get another carb but that seems ridiculous.

Thanks

Norm
 

It does seem that something is plugged up in the carburetor. These carburetors are a simple design and a person isn't likely to miss anything during an overhaul. One thing I would suggest is to run a steel guitar string through those small passages. There might be something stuck in there that an air hose won't dislodge. Cleaning a carburetor in an ultrasonic cleaner does a good job cleaning out everything. Use a mixture of paint thinner and naphtha as a cleaning agent in the ultrasonic cleaner tank. 75% paint thinner and 25% naphtha. Make sure you keep a fire extinguisher handy though. I've never had a problem using this mixture, but it's better to be safe than sorry.
 
Norm,Are you sure you have the venturi orientated correctly? It is easy to get it turned around backwards. Larger open part goes toward the butterfly.
<image src="http://forums.yesterdaystractors.com/photos/mvphoto5879.jpg"/>
 
When I read your response I had one of those EUREKA moments because I would have sworn I had it in the wrong way. I got up, pulled on a coat and went out to the shop. Took the carb off, took it apart, and.....it was in correctly :( Man, I really thought that might have been it. I wish that would have been it! Thanks for the thought though.

Norm
 
While you have it off and apart, run a wire or banjo string through
all the passageways. It sounds to me like one has a spec of dirt in it.
Pay particular attention to the idle circuit passages.
Also remember that the idle air mix screw adjusts air, not fuel.
So out is lean, in is rich and it is adjusted below 500 rpm.

All of that assumes you have good spark and fuel flow through the
carb. Have you pulled the drain plug out and checked fuel flow?
It should run a pint in two minutes or so in a fairly steady flow.
 
Hey Royse,

I would agree with you. I have run a wire through every passage I can but the idle passages have
some angles in them that prevent a wire. I have done things like spray WD40 in one end and see if it
comes out the other. So far, everything appears open. Good fuel flow, float is set correctly, I've
even taken it apart full of gas and the level is fine. I believe I have another carb from a
different tractor that may fit so I am going to try it and see what happens.

Thanks for your suggestions.

Norm
 
This sentence tells me that this is an area of trouble: "I started to adjust the main jet
needle valve. Found out that it doesn't matter what I do with it, the engine still happily runs fine above
idle. " Seating this needle should cut off ALL fuel to the engine. Yes, including idle. If this does not immediately kill engine, then fuel is bupassing this point &amp; you need to find &amp; stop the bypass. Maybe a hair line crack? Bad needle? Bad seat?
 
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