HMSK80 pops/misses - Snowblower Forum : Snow Blower Forums
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post #1 of 32 Old 01-22-2020, 05:40 PM Thread Starter
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HMSK80 pops/misses

I've got an HMSK80 engine from a Toro 824XL (1999 model) that won't run smooth. I got it recently, the PO said he never really used it, bought a plow instead. I believe him, was a nice guy, and didn't ask much for it. Anyways, when I got it home, I started it and noticed that it had a bit of a miss or pop, at all speeds, soon after starting. Choke did not change the behavior, so it seemed to be a rich condition, maybe carb or maybe timing. The spark plug is wet after 5 to 10 minutes of use, and I could also see some sparks in the exhaust. So far I have checked the flywheel key, flushed the fuel system, removed the head and cleaned the combustion area, replaced head gasket. I checked valve lash, both were a bit under spec, so I ground the stems a bit, lapped and remeasured. The intake was now good, but I overground the exhaust valve and had to replace. Got a new one, lapped it and installed. Problem persisted. I removed, cleaned and rebuilt the carb. No improvement. I replaced the coil with new one, also new spark plug, and still the same. I replaced the carb with known good one, with adjustable main jet, and still could not get rid of the popping.

I really thought it was a timing or coil issue, some sort of intermittent spark. But now I am at a loss for what it could be. Any ideas what else to check?

also, cylinder leak-down is good (actually very good for a 20 year old flat head).

tx

Paul
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1995 Toro 724 with Predator 212cc, 1988 Toro 521
1980 Toro 421, 1995 Craftsman 524
1997 Toro CCR 2500 & 1994 Toro CCR Powerlite

Last edited by paulm12; 01-25-2020 at 10:48 AM.
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post #2 of 32 Old 01-22-2020, 05:53 PM
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Plug wire and cap is new?
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post #3 of 32 Old 01-22-2020, 06:47 PM Thread Starter
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Quote:
Originally Posted by cpchriste View Post
Plug wire and cap is new?
yep, new with the coil

tx

Paul
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1995 Toro 724 with Predator 212cc, 1988 Toro 521
1980 Toro 421, 1995 Craftsman 524
1997 Toro CCR 2500 & 1994 Toro CCR Powerlite
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post #4 of 32 Old 01-24-2020, 09:03 PM Thread Starter
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question for the Tecumseh L-head experts - can the compression release fail in such a way that it intermittently engages (opens the valve) at speed? I am still trouble-shooting a HMSK80 that pops and misses at all rpms, fouls the plug and also shoots a few sparks into the muffler. I double checked everything above (from my first post) and can't find anything wrong yet. I can't see the ACR failing in this way, but I am running out of ideas on what could be wrong. Any other ideas?

tx

Paul
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1995 Toro 724 with Predator 212cc, 1988 Toro 521
1980 Toro 421, 1995 Craftsman 524
1997 Toro CCR 2500 & 1994 Toro CCR Powerlite
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post #5 of 32 Old 01-24-2020, 09:31 PM
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Those Tecumseh's tend to pop/miss some. But they are notorious for the valve lash tightening up. The seats hammer into the head over time, and the lash is to tight. Probably a good idea to check it.




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post #6 of 32 Old 01-24-2020, 10:13 PM Thread Starter
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Valve lash has been set. It certainly seems like that should be the issue, but it isn't.

Paul
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1995 Toro 724 with Predator 212cc, 1988 Toro 521
1980 Toro 421, 1995 Craftsman 524
1997 Toro CCR 2500 & 1994 Toro CCR Powerlite
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post #7 of 32 Old 01-24-2020, 10:32 PM
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do you have an adjustable-gap spark tester to check spark strength?

do you have any play in the throttle shaft?

are you sure you have good intake seals? (reusing old gaskets, surfaces weren't completely clean, warped intake elbow)


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post #8 of 32 Old 01-24-2020, 10:58 PM Thread Starter
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Hey CC thanks for reply. I do not have an optical spark tester, but I have measured peak spark with my Matco Smartach, with readings up to 11kv, which is similar to what I get on other flatheads. And the coil is new. The carb on there now is a new aftermarket unit, so no play in the throttle shaft. I was wondering about the intake valve seal when I set the lash, but I just re-used the existing. I never really thought through what a failure of this seal would do. How would that cause the problems I am seeing?

I replaced intake and head gaskets when cleaning the combustion chamber.

thanks

Paul
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1995 Toro 724 with Predator 212cc, 1988 Toro 521
1980 Toro 421, 1995 Craftsman 524
1997 Toro CCR 2500 & 1994 Toro CCR Powerlite
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post #9 of 32 Old 01-24-2020, 11:40 PM
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Kind of a long-shot but have you tried removing and checking the muffler? Perhaps it's clogged; some of the "sparks" you see are probably incandescent bits of carbon from deposits inside the muffler or exhaust port.
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post #10 of 32 Old 01-25-2020, 12:04 AM
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I agree with the carbon deposits or maybe sticking valves? Have you tried sea foam?

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