A YARD MACHINES by MTD 4.5HP/21" Snow Thrower

My son picked this up in working condition for $20.00 CDN. He's asked me to look it over before he tries flipping it for a profit.

It's an MTD MODEL 31A-150-500. MFG. CODE 21-074-4411

It's a single stage machine with a two-stroke engine. Fuel mix ratio is 50:1. There's a 120 VAC electric starter feature.

Engine is a Tecumseh:

• ENGINE MODEL HSK845 8204D (H)
• ENGINE FAMILY VTP139UB25RA
• DISPLACEMENT 139
• D.O.M. 7254G

Let's have a look-see and discover what sort of condition the machine is in.

A Motley Collection Of Auger Drive Cover Screws

The heads of the screws fastening the auger drive cover are all different. I'm almost afraid to discover what the screws' threads are. There's a cross-slot, a slot, a PoziDriv and a slotted hex head. there's one screw missing down below.

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The screws are a mixed bag -- mostly threading (sheet metal) screws. I think they should all be 1/4"-20, so I chased the holes with a tap, and now I've got uniform 1/4"-20 screw locations at all five places. I'll install 1/2" long hex socket head screws, split lockwashers and flat washers at final reassembly.

The Auger Drive

The V-belt looks to be more-or-less ok. The tension idler feels lubricant-starved. I'll take that off and oil it as best I can.

Belt Tension Adjustment

The clutch control cable terminates in an extension spring that hooks onto the idler pulley bracket. There are three holes in the idler pulley bracket for the spring to hook into. The lowest hole affords the least belt tension. The two higher up holes afford progressively greater belt tension. Here's a view of that.

Note that the spring in the photo above is hooked into the lowest of the three holes, for least belt tension. That belt tension seemed marginal to me, so I hooked the spring into the middle hole.

Note also the second extension spring higher up. That's the idler pulley bracket return spring. The spring seen in the photo is not factory -- it's someone's kludged replacement. The return spring is not supposed to be hooked into a tension spring hole; it's supposed to hook onto that notch in the bracket that's just below the tensioning spring holes. I don't have a better replacement for the return spring, so I'll just have to leave that be.

The Top Cover

Remove nine threading screws to unfasten the top cover. Remove the fuel tank cap and the discharge chute's wingnut. The cover will be free to come away, though some forceful coaxing will be needed to get the cover past the exhaust pipe. Here's a view of the machine with the top cover off.

Note the electric starter motor directly below the muffler.

The Carburetor

The carburetor is a float bowl type, with no adjustments aside from float level. The float bowl is readily accessible from below, as can be seen in the following photograph.

I took the float bowl off for inspection. (A 1/2" wrench is needed.) There was some sediment in the bowl that I rinsed out. Float level looked satisfactory.

On reassembly, I ended up with a leak at the float bowl screw's gasket. The gasket is an ill-fitting thing. Reassembly and re-tightening solved the leak.

Carburetor Removal

Carburetor removal on this machine looks like a nasty bit of business -- not to be undertaken without a pressing reason for it.

It appears that in order to remove the carburetor, one must unfasten the dashboard to get it out of the way. Then, a ball-ended hex key might be needed to loosen off the carburetor's two fasteners, and I don't have any of those. Since this machine starts and runs satisfactorily, I'm going to leave well enough alone.

The Spark Plug

The spark plug is accessible via a hole in the dashboard. The spark plug in this machine's engine is an NGK BPMR7A, with a 3/4" hex and a 0.025" gap. The plug shows some evidence of oil fouling, but is in good condition. (The oil fouling may be from having run with too oily a fuel mix.)

And That's It

The machine starts and runs, and auger drive looks ok. It can go on Kijiji.

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Update -- SATURDAY, MARCH 8, 2020

I never did put the machine up for sale. Instead, I let it sit unused all winter under the carport. I tried starting it today and it was no go -- the machine absolutely refused to start.

I dragged it into the workshop and checked for spark; that was ok. I uncovered the carburetor's intake and checked for priming. There was no evidence of priming spurts whatsoever. I upended the machine on the front of its auger housing and tried the primer bulb. Fuel emerged from around the float bowl's ring gasket. Bingo!

That explained a lot. With the float bowl's ring gasket leaking, pressing the primer bulb only blew air past the ring gasket; the float bowl wasn't being pressurized so that fuel would be forced up the main jet into the venturi. The priming function was rendered inoperative by the leaky float bowl ring gasket.

A replacement ring gasket solved the problem. A reminder that a small, normally invisible detail can disable an engine.

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