Sunday, November 2, 2014

Tecumseh 1099 Carburetor -- Repair Kit 632347

I have a Noma Gran Prix snow blower with a Tecumseh HM80 engine that's misbehaving. The engine runs fine at governed speed, so the machine is still useable, but it will not idle. If I try to idle it down, the engine surges (hunts). If I close down the idle mixture screw completely while it's surging, there's no effect whatsoever. It appears that the carburetor's idle circuit is inoperative.

I've had the carburetor apart, and cleaned it more than once, to no avail. So, I've obtained the repair kit that has replacement welch plugs in it. Now I can open up the carburetor completely, and hopefully resolve the problem. If this doesn't work, I'll be left somewhere on the distant side of mystification.

Here's a view of the carburetor and the repair kit.

Here's the kit spread out so you can see better what's in it.

There's also a note detailing the installation procedure for the inlet valve seat.


Teardown of the carburetor is straightforward. You readily get to here.

To dislodge and extract the float valve seat, blow compressed air into the fuel inlet. Keep a finger over where the valve seat will emerge. The valve seat is a light, tiny thing that's liable to fly off for parts unknown if it's not restrained. Here's the float valve seat shown by the carburetor.

Aside from removing the two welch plugs, I've nothing to gain by dismantling the carburetor any further, so I'll leave the throttle and choke butterflies alone.

The Welch Plugs

The large welch plug in the 'roof' of the float chamber can be pried out fairly easily. All it covers is the passageway from the primer tube's nipple to the float chamber. Here's a view of that opened up.

There's really nothing that could ever go wrong in there.

The smaller welch plug on the side of the carburetor is the cover for the idle mixture well. That's the area that's of particular interest to me here, given the symptom that I'm looking to cure.

I punctured and pried out the plug per the advice that's invariably given by the small engines 'literature'. That method worked for me, after a fashion, but I found it clumsy and difficult.[1]

Here's a view of the side of the carburetor with the idle mixture well exposed. (Down below is the demolished welch plug.)

The exposed orifices are all clear.

What remains unexposed and inaccessible is the fuel passageway from the main well to the idle mixture well. All you can do to check that is to force solvent up into the main well, and observe that it emerges into the mixture well. I tried that using the spray nozzle of my parts washer's pump, and the passageway appears to be passing fluid.

All of that leaves one more item that may be at fault -- the main jet nut. And lo and behold, look at this.

Note the very tiny upper orifice in the new nut on the right. In the old nut on the left, that orifice is completely blocked. That tiny orifice is what passes fuel from the main jet's well to the idle mixture well's supply passageway. That blockage may explain a lot; we'll see.

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Reassembly -- SUNDAY, NOVEMBER 2, 2014

Welch Plugs

For a flat-ended punch to install the welch plugs with, I squared off an end of a suitable length of 10mm diameter steel rod on the lathe. That worked well for the large plug underneath.

And it worked equally well for the smaller plug on the side. The key to getting that one installed was to solidly support the carburetor while I hammered on the welch plug. A socket wrench served nicely, like so.

Tecumseh advises to seal the edges of welch plugs with nail polish after installation. I'm not sure that should be necessary if the plug is truly seated, but I took their advice anyway to play it safe. I didn't have any nail polish, so I used CA adhesive.

The Float Valve Seat

The little instruction sheet that comes with the kit makes it clear how the float valve seat is to be oriented (grooved side up -- away from the needle). The seat goes in more easily if it's lubricated with WD-40. Press the seat into place with a suitable punch, or the shank end of a 5/32" twist drill.

Float Height Adjustment

I'm not convinced that this adjustment is as critical as it's made out to be. If the float is roughly parallel to the seating rim for the bowl when the carburetor is held upside down, you're probably close enough. For what it's worth, Tecumseh calls for 11/64" between the bowl's seating rim, and the top of the float, with the carburetor held upside down. Use the shank end of a twist drill for a gauge.

Final Assembly

The rest of it is straightforward. Everything goes together better and more easily if it's lubricated with WD-40.

And The Outcome

The engine now idles correctly; the surging is gone. That plugged orifice in the main jet nut was the source of the idle problem.

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[1] If I ever have to remove a welch plug again, I'll try a different method. That punch/puncture/pry method looks to me like orthodox hogwash that keeps getting recycled for no reason other than that it's become orthodoxy by virtue of having been recycled so often.

On the drill press, I'll carefully drill just barely through the centre of the plug, with a drill that's about 1/2 to 2/3 of the plug's diameter. That should leave me a 'ring' that can be easily pried out, and avoid the hammering/punching/puncturing nonsense. We'll see.

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  1. Very helpful piece. Thanks for putting it up!

    1. You're very welcome. 'Glad to hear that it was helpful.