Been a headache of a morning...

StarTech

StarTech

Lawn Royalty
2012 Craftsman 247.288842 with a Briggs 31P677-0912-G5 and a Nikki carburetor.

Initial complaint was fuel in the oil. Okay easy enough to fix.

Now comes the problem they didn't tell me about. It was surging through the entire rpm range unloaded as no deck.

Normally if you hold the throttle vane steady the problem usually either goes away or the engine dies. Well it stayed running. So I did a static governor adjustment to no avail.

Next was to add a little choke which did smooth it out until it got running too rich. Adjusting the idle mixture did absolutely nothing.

It took doing a modification on the Nikki to correct the problem. I have once done this several years ago on a Nikki clone so I had an idea where to make the mod.
 
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How did you do the modification?

Sounds like an air mixture bypass jet or port is plugged up. It's fighting to get the proper mixture at all RPM ranges. That or the Yost tube has dirty passages.
 
Well it is not a clogged air bleed, port.or main nozzle passages. It is a design problem is some Nikki single barrel carburetors. They put a too small of a restriction jet in the idle mixture fuel pickup port, probably an EPA thing. You can only go leaner the jet max fuel allowed flow. Air and fuel does mix in the idle mix in the idle circuit.

The solution is to enlarge this jet to around .028". #70 - drill bit or a .0292 #69 drill bit. This should get you to a middle of the road adjust ability. This time I went with a #68 .031 which was just about too much.

I need to get a new hand drill as the current one is worn and would not go below a #68.
 
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Note> Only do this as a last resort as cleaning is always the preferred method but on rarer cases jet resizing is necessary. Also got to be extremely careful when working with these fragile bits. This is because these bits are under a 1/32".
 
You won't believe this but I got a second Nikki, this time a clone, doing the same thing on a JD D105. The customer installed the carburetor and threw away the old carburetor. Darn it at least I know what to do to cure it.

That makes two in a row.
 
Hi Star Tech,

You know the game. LOL

Aftermarket companies make these make these (generic) carburetors at one basic specification to fit a variety of similar engines, but they are not tuned/ jetted exactly right to operate on the different engines.
 
liveliness of the fuel can change how an engine behaves. The deader the fuel, the leaner it acts. Jet perfectly with dead fuel, add new fuel, and too rich. The whole thing about mixture is how far apart the burnable molecules and oxygen are during combustion. Not enough fuel and they are too far apart to pass the flame front to one another. Too many are close but the O2 is too far away from them. Same effect. Dead fuel - the highly burnable molecules are too far apart because lower flammable molecules have taken place of the flammable ones. The manufacturers assume the fuel is to their standard when they jet it to make EPA happy.
 
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