Snowblower Forum banner

1 - 20 of 32 Posts

·
Registered
Joined
·
812 Posts
Discussion Starter #1 (Edited)
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
 

·
Registered
Joined
·
812 Posts
Discussion Starter #4
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
 

·
Premium Member
Joined
·
719 Posts
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.
 

·
Registered
Joined
·
812 Posts
Discussion Starter #6
Valve lash has been set. It certainly seems like that should be the issue, but it isn't.
 

·
Registered
Joined
·
1,740 Posts
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)
 

·
Registered
Joined
·
812 Posts
Discussion Starter #8
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
 

·
Registered
Joined
·
101 Posts
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.
 

·
Registered
Joined
·
106 Posts
I agree with the carbon deposits or maybe sticking valves? Have you tried sea foam?

Sent from my Pixel 4 XL using Tapatalk
 

·
Registered
Joined
·
1,740 Posts
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.
np paulm12! That Smartach sounds neat! I like the optical tester (Thexton) for seeing the color, strength and might even catch a gross ignition miss if you're turning the flywheel with the starter or drill. If armature gap was too large &/or the flywheel magnet was weak, then I'm guessing you'd see it in the potential being generated by the coil secondary.

The carb on there now is a new aftermarket unit, so no play in the throttle shaft.
thanks
Yeah if the problem didn't modulate when you swapped the carbs, then it's probably not a loose (leaky) throttle shaft causing a lean condition.

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
I'm referring to the intake manifold/pipe gaskets (carb/pipe & pipe/block). If you carefully apply starter fluid around those interfaces/gaskets and RPMs move, they're leaning out the fuel mixture; that test is harder than it sounds b/c some inevitably gets sucked into the carb intake with the pressure that those cans expel the stuff.

Does the problem go away under load? Have you tried richening-up that idle mixture (turn-out)?

Like motor city said earlier, these engines often have occasion pop when running high-speed / no load...hopefully you're not chasing your tail on this one!
 

·
Registered
Joined
·
812 Posts
Discussion Starter #12
2muchsnow: I did a full cleaning of the combustion chamber, including head, pistons and valves.
Blackfin: the muffler seems fine.

tx
 

·
Registered
Joined
·
812 Posts
Discussion Starter #13
CC: changing the carb main and idle jet settings have little change on the performance. Also, I could not find any leak around the intake areas. And the missing/pooping is at all speeds, and is different than the usual and occasional no load type of pop. After looking at the spark plug after 10 minutes of running, I'm starting to think maybe it is oil and not gas on the plug. Is it sucking up some oil through the intake valve?

The engine is off of the machine, so I cannot test under load (unless I try to grab the output shaft and hold on !!)

tx
 

·
Registered
Joined
·
106 Posts
2muchsnow: I did a full cleaning of the combustion chamber, including head, pistons and valves.

Blackfin: the muffler seems fine.



tx
Just my 2 cents. Had a guy give me an mtd with a tech engine because it wouldn't run. Cleaned the carb and got it running ok, but had similar popping issues. I drained the gas, added new gas with sea foam and ran it through for about 20 minutes while moving the throttle up and down. After that it ran great.

It's worth a shot, you aren't going to hurt anything.

Sent from my Pixel 4 XL using Tapatalk
 

·
Registered
Joined
·
812 Posts
Discussion Starter #16
I am back looking at the HMSK80. Today I removed the head and valves to take another look. After 3 or 4 runs of only 10 minutes each, I noticed alot of oil on the plug, and on the piston. Both valves have play in their guides, more with the exhaust, but I'm not sure how much would result in oil issues. (note that the exhaust valve is new, so the stem is good) I also tried to measure the cylinder bore with a Central Tools bore gauge, to see if maybe it is wore, but I can't get repeatable results.

I think I need more experience to determine if the valve slop is too much. For now I will set this engine aside and look for a possible replacement.

Thanks all for your inputs.
 

·
Registered
Joined
·
1,740 Posts
I am back looking at the HMSK80. Today I removed the head and valves to take another look. After 3 or 4 runs of only 10 minutes each, I noticed alot of oil on the plug, and on the piston. Both valves have play in their guides, more with the exhaust, but I'm not sure how much would result in oil issues. (note that the exhaust valve is new, so the stem is good) I also tried to measure the cylinder bore with a Central Tools bore gauge, to see if maybe it is wore, but I can't get repeatable results.

I think I need more experience to determine if the valve slop is too much. For now I will set this engine aside and look for a possible replacement.

Thanks all for your inputs.
yeah you shouldn't have alot of play/wiggle in the valve guides. I'm guessing that this engine doesn't have replaceable sleeves (or oversized valves) like a Briggs. I suppose you can get some oil sucked-in during the intake stroke. Briggs has a 1/4" and 5/16" valve...maybe one of those guides are interchangeable...undersized enough where you can ream it to the tecumseh valve dimension?

I wonder if most of this however is autocorrelation....if the valve guides are worn down on a snow engine, imagine how badly the cylinder and rings are worn; blow-by would my 1st guess for the source of the excessive oil in the combustion area. The counter to that theory is that you said leakdown was good...maybe it's lyin' to ya :grin:
 

·
Registered
Joined
·
812 Posts
Discussion Starter #18
thanks again for following along CC. I don't thing the guides are replaceable on these engines, and not worth it to me (I don't have machinist capabilities) to try to repair. And yes, the good leak-down reading (taken several times) has me confused. That's why I tried to measure the cylinder bore.

Every time I think I'm starting to know what I am doing, I am taught otherwise.

tx
 

·
Premium Member
Joined
·
463 Posts
Keep in mind, that Cylinder Leak Down tests Should be performed with piston at TDC.... with both valves closed. That will test the engines ability to seal. So a good reading, (under 10%) will indicate the rings are tight and valve/seats are tight. It will not indicate worn valve guides, unless they're so worn, that the valve is no longer seated....in which case, it may not even run. Also, the Leak Down test will not take into account Cylinder Taper.... The Taper is generally near the bottom of the cylinder. Trying to use a Cylinder Bore Gauge with the Piston installed is not an accurate test. The fact there is that much oil in the combustion chamber so soon after cleaning, indicates some major work may be needed to clean things up. In the end, a replacement or full overhaul may be the answer.


GLuck, Jay
 

·
Registered
Joined
·
812 Posts
Discussion Starter #20 (Edited)
so a few observations and measurements on the HMSK80 that runs sloppy and has a lot of oil in the combustion chamber. First a few pics showing the wear on the piston skirt, and the wear near the top of piston. I don't have experience reading pistons, so any observations from the engine experts? Also, is the wear at the top of the pistons, by the rings, normal? The gap in the last pic is around 5 thou. 2nd, I took some measurements: the piston skirt is around 1 thou under spec at the bottom, the top compression ring gap is around 17 thou, the 2nd ring is over the 20 thou spec. I want to get a better bore gauge and get some repeatable measurements on the cylinder. We'll see. And I still want to somehow measure the valve guide slop.

Thanks for any inputs. And yes, I think and talk in "thou"s, picked that up from my dad. I do write the dimensions down properly.
 

Attachments

1 - 20 of 32 Posts
Top