# HMSK80 pops/misses



## paulm12 (May 22, 2015)

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


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## cpchriste (Jan 19, 2014)

Plug wire and cap is new?


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## paulm12 (May 22, 2015)

cpchriste said:


> Plug wire and cap is new?


yep, new with the coil

tx


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## paulm12 (May 22, 2015)

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


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## Motor City (Jan 6, 2014)

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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## paulm12 (May 22, 2015)

Valve lash has been set. It certainly seems like that should be the issue, but it isn't.


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## classiccat (Mar 1, 2014)

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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## paulm12 (May 22, 2015)

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


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## Blackfin (Jan 25, 2016)

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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## 2muchsnow (Oct 22, 2019)

I agree with the carbon deposits or maybe sticking valves? Have you tried sea foam? 

Sent from my Pixel 4 XL using Tapatalk


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## classiccat (Mar 1, 2014)

paulm12 said:


> 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.



paulm12 said:


> 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.



paulm12 said:


> 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!


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## paulm12 (May 22, 2015)

2muchsnow: I did a full cleaning of the combustion chamber, including head, pistons and valves. 
Blackfin: the muffler seems fine. 

tx


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## paulm12 (May 22, 2015)

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


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## 2muchsnow (Oct 22, 2019)

paulm12 said:


> 2muchsnow: I did a full cleaning of the combustion chamber, including head, pistons and valves.
> 
> Blackfin: the muffler seems fine.
> 
> ...


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


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## strtch5881 (Oct 6, 2018)

If it is oil on the plug, with all the other symptoms, sounds like a worn valve guide.


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## paulm12 (May 22, 2015)

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.


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## classiccat (Mar 1, 2014)

paulm12 said:


> 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:


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## paulm12 (May 22, 2015)

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


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## JayzAuto1 (Jul 25, 2016)

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


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## paulm12 (May 22, 2015)

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.


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## classiccat (Mar 1, 2014)

Sweet! open heart surgery!

yeah that piston skirt was definitely making some contact. Where you able to hear any rod knock by chance?

I typically measure ring gap and bore

the way I measure ring-gap, remove the rings (a set or ring pliers make this super easy) and note the orientation and order! Carefully insert them (one at a time) into the bore using the piston face and measure how much gap there is between the ring-ends using a feeler gauge (_I don't bother with the oiler/bottom ring_).









For measuring bore, use bore gauges. The way I do it is insert the appropriately sized gauge at 3 depths (top/middle/bottom of ring travel) and measure (0 and 90 degrees)...so 6 measurements in total.









If the ring gap is high and bore is good, then you might just need a new set or rings. if ring gap & bore are bad (large &/or oval-shaped) and this doesn't have a cast iron sleeve, then I'd probably look for a new short block (or repower).


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## paulm12 (May 22, 2015)

CC: yeah, I think I did hear rod knock, but not sure thats what it was, cause again I am new to this part of engine work. There was some piston play at TDC. I measure the ring gaps the same way as you, is straightforward. Though I was surprised that the top ring was tight, but the 2nd ring (wiper ring?) was loose, but that could explain some of the oil. Next I will measure the crank journal and the rod bearing.

and yes, I need to get a bore gauge of some kind, its on my XMas list. 

I still think the valve guides are bad, so this block may not be worth a re-build.

Thanks again.


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## paulm12 (May 22, 2015)

so I started taking some engine component measurements on this HMSK80 that runs poorly, has low compression but good leak-down, and fouls the plug quickly.

intake valve stem = 0.310
intake valve guide = 0.3125 in the middle (tightest spot) with ~ 0.313 at the top of the guide. Wobble is side to side (relative to piston)
exhaust valve stem = 0.308
exhaust valve guide = 0.3125 in the middle (tightest spot), with ~ 0.315 at the top of the guide Wobble is front to back (towards and away from piston)

piston skirt is ~ 1 thou under spec at the bottom of the skirt, and at bottom of spec towards the middle and top of skirt.

top piston ring gap = 17 thou (spec is 10 to 20 thou)
middle piston ring gap = 25 thou (spec is 10 to 20 thou)

a plastigage measurement on the main crank journal to connecting rod bearing was between 1-1/2 and 2 thou. that seems ok. 

I think these valve stem and guide numbers are ok, anyone have any input? The ring gaps are high, I need to get some good cylinder bore measurements now. Also, do these pics of the camshaft lobes appear to show heat damage? also, the wear is to one side of each cam.

Finally, I still wonder if the compression release was hanging up at times. I need to further investigate that. 

thanks


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## classiccat (Mar 1, 2014)

Yeah that ring gap is definitely on the large side...especially deeper into the bore. 

the uneven cam lobe appearance is interesting...however it doesn't appear worn at the top where it actually "lifts" the tappet.

are the tappets wobbly as well?

Do you think you're the 1st person working on this engine's guts? 

Did you inspect for cracks in the cam bearings (sump and block)? it may have thrown a rod once-upon-a-time and the "rebuilder" missed casualties outside of the kill radius.

How were you measuring inside of the valve guide?


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## paulm12 (May 22, 2015)

thanks for the reply CC. I am enjoying trying to diagnose this engine. 

- yes, the cam lobe wear appearance is different, from my limited experience.
- the tappet fit seems fine, but both tappet flats show off-center wear, see picture.
- I wonder if this engine has been worked on. When I checked specs, a few dimensions were not per the manual, as if it has a different crankshaft and connecting rod. But with these engines there were quite a few times where designs changed and were documented by a suffix on the serial number, or date code, so I'm not sure.
- I can't see any main bearing cracks, but I will look some more.
- I bought a set of small hole bore gauges, and they work really well. With a little practice I can get fairly repeatable results. The valve guides are definitely hour-glass shaped, but only in 1 orientation (if that makes sense). And it was interesting (to me at least) that the intake guide was wore side to side and the exhaust guide was wore front to back. 

I thew in a picture of the cylinder bore wear at the bottom, compared to the original cross-hatch. I have no idea how good or bad this is. I'm planning to have a beer or 2 tonight and ponder all of this.

tx for following along


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## paulm12 (May 22, 2015)

well CC, I had a few beers Saturday, and I still have no idea what is wrong with this engine. I did get a good bore gage, and all measurements (top mid bot at 0 and 90 each) are within the +1 thou spec. I was amazed. I then measured a different engine, the HSK70 that I replaced with a Predator because it just got tired, and burned some oil, and its bore measurements are all within 1-1/2 thou over, which is within 1/2 thou of the spec. I am starting to wonder if just the loss of cylinder cross-hatching is really a bigger issue than I thought on these engines. And maybe a good cylinder honing would bring back some compression and also prevent the oil burn? 

as Aristotle wrote, “The more you know, the more you realize you don’t know.”

tx


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## tadawson (Jan 3, 2018)

Check the rings - they wear far faster than the cylinder, and also make sure that the end gaps are staggered. Granted, not a Tec, but I rebuilt a GX Honda that has at least 1000 hours and 30 years on it, and the cyl was within about 1/1000 of new. A light hone and rings, and back to zero oil consumption, and I would expect most cast iron sleeved engines to be similar.


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## KennyW in CT (Feb 24, 2015)

Does it run better with a little choke? Those aftermarket carbs tend to run a bit lean resulting in misfiring. Also try a new plug even if you changed it sometimes they are intermittent.


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## paulm12 (May 22, 2015)

tadawson: thanks for reply. As mentioned above, the top ring gap was around 17 thou and the middle ring 25 thou. I have not yet tried any cylinder honing, and was wondering if it really makes a difference on the oil blow-by. What honing tool do you use?

thanks


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## tadawson (Jan 3, 2018)

The honing is to get the cylinder walls prepared for new rings to seat to them. If you leave the hard, smooth surface, the new rings may not seat. For a hone, I just got one from the loaner pool at Autozone - they will go small enough, and it took very little time to just knocknoff the glaze (the goal is *NOT* to remove much material). 

(You can get away with a low end hone like this since you are not overboring, etc., so alignment issues don't come into play . . .)


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## classiccat (Mar 1, 2014)

That's good investigative work there!

Oh man...that cast iron sleeve changes things a bit! I have one as well on my old HM80.

I've used a flex ball hone in the past just to rehatch and seat new rings…I only needed a few passes on a cool-bore engine. I’ve never used it on cast iron.

The downside of that flex ball is that it’s only good for a certain bore diameter range...where other hones are adjustable.

You are going to sacrifice a little of the bore and starting ring gap will then be slightly higher however you should see a much larger improvement over what you have.

Make sure you clean the heck out of that block when you’re done honing. I scrub with dishwashing soap over and over until a white paper towel comes out clean. 

So the issues so far,
- wonky cam; I’m still suspicious of the ends…either the block/sump bearings or the cam itself.
- rod knock (play in the rod bearing); did you mic the diameter of the cam journal as well as the rod “bearing”?
- popping/missing/oily combustion area; a lot likely getting up past the worn rings but I think you’re also sucking up from the breather cavity through the wallowed-out valve guide. I can imagine that it will also have some impact on crankcase pressure (should be under slight vacuum).


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## paulm12 (May 22, 2015)

CC; the camshaft looks alot like others of that sized that I've looked at on Google, EBay, etc. so maybe that isn't so out of ordinary. The camshaft journals are in spec, I don't have means to measure the bushings that they ride in. But they look ok. I may get a flex-hone for this bore, even if just to practice on. I am wondering if the honing goes well (gets rid of most scratches and imparts a good cross-hatch) if it would be better to get 10-over rings, even if they need to be filed down a bit. My thinking is that if the bore is 1-1/2 to 2 thou over nominal diameter, that translates to 5 to 6 mils over on the rings. And for nominal rings that amount uses up alot of the 10 mil to 20 mil ring gap tolerance. I had noticed that when I re-ringed by HSK70, even the new rings ended up at 20 mil gaps to start. 

We'll see. For now I have the block and parts set aside. And I will look for another engine in newer shape in the meantime. 

tx


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## Doc (Feb 22, 2021)

This is for the initiator of this post. There are different times when people use the term "backfire". A backfire through the exhaust, which is what I believe you are saying you have, is generally cause by excess fuel after combustion has occurred or a valve issue. Most of the time the engine need not be dismantled and miked up for deficiencies in crucial measurements. The rule of thumb if it runs, it's not in need of an overhaul.

Your particular problem sound like too much fuel because it is burning on the exhaust stroke. Start the engine and wait for the backfires then either pinch the fuel line or shut the fuel off completely and let it run out of gas and then see how it runs as the fuel is being depleted. If the backfire smooths, you have too much fuel. If the backfire persists, there is a valve issue.

Most of the time it is that simple.


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## RIT333 (Feb 6, 2014)

Great explanation


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## paulm12 (May 22, 2015)

thanks for replies. Since then, I parted out the machine due to too many other issues. 

I don't think it was too much fuel though, I had tried an adjustable jet carb, and cutting back the jets did not remove the "pops". As I worked on the engine more I started to think that the plug was fouled with oil, not gas. So that led me down the piston rings and/or valve guide path as others had suggested. Also, the reason I broke the engine down was mainly for learning, and while I did learn alot, I never truly diagnosed and fixed the popping issue. At the end I just felt that the valve guide slop was too much to spend any more $$ on parts.

tx


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