Strange carb problem

[Dave Downs]

Backstory - neighbor calls me, do I want a Cub? It was given to him, has a Woods 59 mower deck and IH snow plow. He can’t get it running. He wants it gone for the cost of the parts he’s got in it - new front tire, new battery, new carb, fuel line and sediment bowl. Has one leaking rear tire, will need a new rim.

I go look, 1949, straight, mower deck looks almost new, plow has surface rust. Turn on the fuel, leaks at carb connection and steady drip out of the carb drain. It does start but won’t stay running (no surprise!).

Trailer it home, fabricate new fuel line but can’t get carb to stop dripping. He gave me old carb, I put it on, tractor runs great, can even start it with crank (magneto ignition). Run it around, seems good.

Put mower deck on, tractor has no power under load, barely pulls tractor and mower. Half choke helps but I’m sure I have a fuel problem. I make all the fuel system checks, everything is OK.

When I cleaned the main jet I thought it looked a little small, so I pull the jet out of an old carb in the parts bin and it is way bigger diameter then the one in original carb so I put it in original carb and tractor runs fine under load!!!

I can’t figure out how this tractor ran for years with that small jet in it and why/how it got there.

Hope the picture turns out, comparing with jet from new (leaky) carb.

[PFord]

What does the side of it looks like?

[Dave Downs]

Identical except for inside diameter

[PFord]

Identical except for inside diameter

Interesting. My cub (1975) has the smaller diameter end hole but runs just fine. I don't have my carb here in front of me to take a photo, but when that main jet screws into the body of the carb, that end goes into a blind hole if I remember correctly, but the hole on the side is open to the fuel bowl. Maybe your setup is different than mine.

[staninlowerAL]

Count your blessings Sounds like you done alright on this one.

[Bob McCarty]

I can't tell from the photo, but it looks like there is a smaller hole at the bottom of the bigger hole. A lot of jets are that way. That being said jet sizes varied from .034-.040. The old jet may have a bigger orifice even though it looks opposite of that.

[Dave Downs]

I’m curious, went and looked closer, yep, new jet has counter-bore but main orifice is significantly bigger. The only thing I have to check diameters with is a set of pin drills that I don’t have sizes for, but the drill that fits through the new one is way too big for original. Unfortunately I have no way to determine if one is .034 and the other is .040.

[Don McCombs]

If you have a wire type spark plug gap gauge, you may be able to get approximate measurements.