HMSK80 runs poor after being worked hard. - Snowblower Forum : Snow Blower Forums
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post #1 of 68 Old 03-26-2015, 09:47 AM Thread Starter
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HMSK80 runs poor after being worked hard.

As others may recall I've been chasing a problem. Assumed it was the carb and possibly fuel line. Now I'm running out of ideas.

I changed the sparkplug in my Tec HMSK-80 on the weekend but have not run the machine yet because there's really no point.

The engine was missing under load and opening the main jet only seemed to make things worse.


What do you think the chances are this plug was causing the issue? My opinion is close to none.



I didn't bother starting the engine after changing the plug because this problem only seems to happen after the engine has been working fairly hard for a while. Without snow, I'll never get it to have the problem anyway. New Tecumseh OEM pre-emissions carb, new fuel line, filter and the problem seems to persist.


At this point, I have no idea what would cause a problem only when the engine is good and hot. Runs great before you actually use the machine for 40 minutes or so.

Any ideas?
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post #2 of 68 Old 03-26-2015, 09:52 AM
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I agree that plug doesn't look too bad, although for $2.00 I'd try replacing it anyway.

Have you checked valve clearances? Those are temperature-sensitive.

Another possibility is bad plug wire or magneto. Temperature can affect those, and they also could be affected by snow (=moisture) drawn into the engine's cooling fan.

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post #3 of 68 Old 03-26-2015, 10:00 AM Thread Starter
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Quote:
Originally Posted by ELaw View Post
I agree that plug doesn't look too bad, although for $2.00 I'd try replacing it anyway.

Have you checked valve clearances? Those are temperature-sensitive.

Another possibility is bad plug wire or magneto. Temperature can affect those, and they also could be affected by snow (=moisture) drawn into the engine's cooling fan.

Haven't checked the valves, though I probably can.
Whatever I do, I'd like to somehow get to a point where I know I can depend on this thing when the next storm comes.

It also seemed like letting it run without a load on it for a bit caused the problem to fade until it started working hard again.

Are the valves on these engines adjustable?

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post #4 of 68 Old 03-26-2015, 10:04 AM
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Are the valves on these engines adjustable?
With some difficulty... you have to grind the stems. If you search here there's a lot of discussion about it, and I think there are some videos on youtube showing the procedure.

But *checking* them is fairly easy... you may have to remove the carb & manifold to get access, but then it's just two screws to remove the breather, rotate the engine to the proper position, and use a feeler gauge.

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post #5 of 68 Old 03-26-2015, 10:08 AM Thread Starter
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Quote:
Originally Posted by ELaw View Post
With some difficulty... you have to grind the stems. If you search here there's a lot of discussion about it, and I think there are some videos on youtube showing the procedure.

But *checking* them is fairly easy... you may have to remove the carb & manifold to get access, but then it's just two screws to remove the breather, rotate the engine to the proper position, and use a feeler gauge.

This may seem like a really dumb question but how would the stems become too long from use?

I would think the cam / stem would wear more than the valve surface, no?

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post #6 of 68 Old 03-26-2015, 10:17 AM
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The clearance decreases as the valve seat and bottom of the valve wear.

https://www.google.com/#q=youtube+ch...ecumseh+valves

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post #7 of 68 Old 03-26-2015, 10:19 AM
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This may seem like a really dumb question but how would the stems become too long from use?

I would think the cam / stem would wear more than the valve surface, no?
No... The exhaust valve burns into the seat over time and the clearance between the lifter and valve stem tip gets smaller. Things expand with heat and the problem gets worse as the machine warms up.

You will need to remove the muffler and intake manifold to get to the valve cover / breather. The gaskets are cheap to replace.

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post #8 of 68 Old 03-26-2015, 10:47 AM Thread Starter
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Something else I didn't realize until just now.
These valveseats are aluminum, aren't they?

Sounds like everyone think's there's a good chance it's the exhaust valve?
Is this common on an engine that doesn't have that much time on it?

Should I buy a valve grinding tool for it while I'm at it, or no?

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post #9 of 68 Old 03-26-2015, 11:48 AM
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The valve seats are in the cast iron block. The aluminum head is just a cover for the combustion chamber. The intake valve is usually ok, it's the exhaust that wears. I doubt anybody bothers to cut new seats, just lapping is good enough.

If you find that the valve lash needs adjustment, you'll want to get a new head gasket too.

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post #10 of 68 Old 03-26-2015, 12:06 PM
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its more than likely the exhaust valve, the stems expand when the engines get older, happened to my hm80 and i threw it on cl for parts and got a honda clone
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