# tecumseh hmsk 8 hp backfiring



## Julien Morrissette (Oct 7, 2017)

Hi,
My ariens st824 was giving me trouble, the needle was sometimes getting stuck in the open position so it was overflowing and then after i fixed it, it wouldn't idle anymore so i decided to change the carb. Now the problem is at high rpm i can't seem to adjust it properly the engine is either surging and/or backfiring at high rpm. it is running fine at idle. any help would be apreciated maybe the new carb is bad? the governor always worked properly on that unit and there doesn't seem to be any air leak


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## crazzywolfie (Jun 2, 2014)

have you checked the valve clearance on the exhaust valve? those Tecumseh seem bad for needing that adjusted eventually. if you run the engine at night outside you will generally notice the muffler starting to glo. what carb did you buy for it?


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## dman2 (Sep 22, 2019)

Was it running fine at high rpm after you fixed the old carb? If so, you know where the problem is. New carb can be bad out the box, or you just bought the wrong one. The carb is new, but the rubber rings and tube inside it could be all cracked and not seating properly. Your engine is old, so it could be anything.

It is not likely to be the spark plug, but when that happens, I would want to replace the spark plug and its wire. They are cheap to replace and might fix your problem. The part on the flywheel that generates sparks for you, it maybe rusted. I don't remember what it is called.

Like you, I would look more into fuel and air leak.

It sucks if you have to adjust the valves. Might want to do a compression test.

Not a mechanic. Just want to chime in.


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## dman2 (Sep 22, 2019)

crazzywolfie said:


> have you checked the valve clearance on the exhaust valve? those Tecumseh seem bad for needing that adjusted eventually. if you run the engine at night outside you will generally notice the muffler starting to glo. what carb did you buy for it?


They all split out fires and pop if you removed their mufflers, so I wouldn't be surprised if they glow at night.


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## Julien Morrissette (Oct 7, 2017)

i don't know the muffler is loose too so it could be an exhaust leak i will definetly look into that i put in a cheap carb that i had bought multiple times before for other machines. when i came to start it with the old carb it was hard to crank like if it had flooded. i don't know how this could happen though. So yeah might be valves


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## Julien Morrissette (Oct 7, 2017)

dman2 said:


> Was it running fine at high rpm after you fixed the old carb? If so, you know where the problem is. New carb can be bad out the box, or you just bought the wrong one. The carb is new, but the rubber rings and tube inside it could be all cracked and not seating properly. Your engine is old, so it could be anything.
> 
> It is not likely to be the spark plug, but when that happens, I would want to replace the spark plug and its wire. They are cheap to replace and might fix your problem. The part on the flywheel that generates sparks for you, it maybe rusted. I don't remember what it is called.
> 
> ...


it was running better but not perfect i might have underlying issues somewhere, i have two spare engine so i must have the parts i need


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## crazzywolfie (Jun 2, 2014)

if the muffler is loose that will definitely make you hear the popping of the engine. you definitely probably want to try tightening up the muffler or replacing it to see about getting the engine tuned. 


dman2 said:


> They all split out fires and pop if you removed their mufflers, so I wouldn't be surprised if they glow at night.


but it is sometimes harder to see the metal of the muffler glowing during the day unless unless the exhaust valve is leaking bad at which point ya you see the muffler glowing red hot. on top of that the Tecumseh's are known for exhaust valve last getting out of spec with age.


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## rod330 (Oct 9, 2015)

crazzywolfie said:


> have you checked the valve clearance on the exhaust valve?


I agree with crazzywolfie. The first thing I would do when the engine is cold is check the valve clearance. It takes just a few minutes with these OHV engines. Intake and exhaust is 0.004 at TDC. At least you can eliminate that and it probably needed to be done anyhow.
Afterwards, I'd go back to the original carb and give it thorough cleaning (clean orifices with carb spray, torch tips, compressed air and ultrasonic cleaner).


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## crazzywolfie (Jun 2, 2014)

on the Tecumseh's the exhaust valve clearance should be 0.012.


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## rod330 (Oct 9, 2015)

crazzywolfie said:


> on the Tecumseh's the exhaust valve clearance should be 0.012.


Hmmm...isn't the ST824 an OHV engine? Maybe I misread but looked at this spec in the Tech handbook


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## crazzywolfie (Jun 2, 2014)

its a hmsk 8hp. the info is only in the title and almost overlooked. if it was a OHV engine i don't know if exhaust valve clearance would be as much of an issue. i just know it is a common issue with the flat head engines.


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## rod330 (Oct 9, 2015)

crazzywolfie said:


> its a hmsk 8hp. the info is only in the title and almost overlooked. if it was a OHV engine i don't know if exhaust valve clearance would be as much of an issue. i just know it is a common issue with the flat head engines.


Ahhhh....great catch....my mistake!


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## Jackmels (Feb 18, 2013)

Tecumseh OHV engine model numbers begin with OH such as OHSK80


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## CO Snow (Dec 8, 2011)

I’d start by replacing the muffler. I had similar issue with a Tecumseh - backfiring, glowing muffler - and all resolved with a new muffler.


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## dman2 (Sep 22, 2019)

CO Snow said:


> I’d start by replacing the muffler. I had similar issue with a Tecumseh - backfiring, glowing muffler - and all resolved with a new muffler.


I don't think it is the problem unless it is all rusted out and has big holes in it. That would allow much oxygen to get in, and with excessive fuel, you would get those little popping noises and backfires. That would cause the muffler to glow even more.

Little popping noises without the muffler installed is normal, but he said backfiring, or too much excessive fuel. His engine probably ran like crap and wanted to die.


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## dman2 (Sep 22, 2019)

I like backfires, but that means my engine is not running right.


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## tabora (Mar 1, 2017)

dman2 said:


> I like backfires, but that means my engine is not running right.


For what it's worth, on my Toro Zero Turn mower with a 14 HP Briggs & Stratton, it MUST be turned off at full throttle, or I get an afterfire that sounds like a 30-06 firing. (Backfire is through the carb, afterfire is through the muffler.) There's actually lingo in the Owners Manual: 
_To stop the machine, move the traction control levers to neutral and separate to the brake position, disengage the PTO, *ensure the throttle is in the fast position*, and turn the ignition key to off._
*B&S - Possible Fixes for Afterfire:* 

Change to a different non-alcohol or alcohol brand fuel. Gasoline that contains alcohol has a tendency to ignite easier, which can cause afterfire.
Ensure proper carburetor adjustment for optimal engine performance.
Check the anti-afterfire solenoid for proper operation. If equipped with an anti-afterfire solenoid, shut engine off at full throttle.
Check valve lash.


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## rod330 (Oct 9, 2015)

Jackmels said:


> Tecumseh OHV engine model numbers begin with OH such as OHSK80


Yep, I mistakenly thought the ST824 had an OHSK80.,,,next time, I'll read the actual thread title


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## Julien Morrissette (Oct 7, 2017)

thanks guys i might have time to check the valve clearance this weekend. The engine is a flathead hmsk 8hp. It's not the original engine but it was the same engine model originally ( around 1986) i have a spare engine i could use the head which doesn't have stripped muffler bolts holes.


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## paulm12 (May 22, 2015)

and from the Tecumseh L-head engine technicians handbook, the valve clearance for HMSK80 is 0.008" to 0.012" for both intake and exhaust..

tx


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## 140278 (Aug 27, 2020)

Julien Morrissette said:


> thanks guys i might have time to check the valve clearance this weekend. The engine is a flathead hmsk 8hp. It's not the original engine but it was the same engine model originally ( around 1986) i have a spare engine i could use the head which doesn't have stripped muffler bolts holes.


muffler bolts in the head? i hope you miss typed , on a flathead those go into the engine block, you can fix those by installing Heli coils into the block rather than junking


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## Julien Morrissette (Oct 7, 2017)

I meant the top bolt of the muffler








Right now i have the engine appart, The clearances are 13/1000th intake and 10/1000th on the exhaust valve i heard the specs are 0.008 intake and 0.012 exhaust but i could be wrong. It's weird because on the compression stroke the exhaust valve opens a bit is this normal? should i mess around with them with those clearances?
Thanks for your help


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## Julien Morrissette (Oct 7, 2017)

this position the piston go up after the intake valve closed i can rotate the exhaust valve by hand


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## crazzywolfie (Jun 2, 2014)

Julien Morrissette said:


> i heard the specs are 0.008 intake and 0.012 exhaust but i could be wrong. It's weird because on the compression stroke the exhaust valve opens a bit is this normal?


those specs are correct. the exhaust valve being open a tiny bit is the compression release holding the valve open to make turning the engine over easier. i think i have heard you got to spin the engine over backwards to have it not effect the valve lash. you definitely want to make sure that exhaust valve is seated correctly on the compression stroke.


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## paulm12 (May 22, 2015)

crazzy: I have mentioned this before on the forum, but aren't the specs 0.008" to 0.012" for both intake and exhaust? There are documents that conflict on this, but the 0.008 to 0.012" spec is what most Tecumseh documents call for. That is what the L-head handbook has listed. 

Also, that is what my dealer techs said too.

tx


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## Julien Morrissette (Oct 7, 2017)

my valves are seating properly so i guess I'll check the carb since it's the only logical thing that could cause these problems


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## crazzywolfie (Jun 2, 2014)

paulm12 said:


> crazzy: I have mentioned this before on the forum, but aren't the specs 0.008" to 0.012" for both intake and exhaust? There are documents that conflict on this, but the 0.008 to 0.012" spec is what most Tecumseh documents call for. That is what the L-head handbook has listed.
> 
> Also, that is what my dealer techs said too.
> 
> tx


but what valve do you usually have issues with? i always usually set the exhaust valve to 0.12" since it will likely change on its own with time. that is usually not as much of an issue with intake valve so as long as it is withing spec i usually leave it alone.


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## paulm12 (May 22, 2015)

crazzy: I understand your approach, but I just want to be sure that when we mention specs to others, it is what the manufacturers specify. We all can alter them as we want, and, again, I understand the reasoning for opening up exhaust a bit more, but that is different than "spec".

I appreciate your experience and comments, and based on comments here I usually leave the exhaust side more towards the upper end of the spec.

anyways, hopefully Juliene can get his machine running well soon 

tx


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## Julien Morrissette (Oct 7, 2017)

After putting the engine back together, i searched everywhere what could be causing my issues now it was just flooding intensely every time i throttled up. So i still searched and checked everything, even put the old carb back on, with the old carb the engine wasn't flooding but i had to keep hitting the primer bulb (this carb is all clogged up and pretty much wore down that's why i changed it). so i took appart the new carb and a part is missing which leads me to believe it's the issue.

What do you think ?
















the piece missing is the small round thing where the primer bulb line goes to next to the main jet (diaphragm??) that thing must serve a purpose imo.


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## Julien Morrissette (Oct 7, 2017)

#48


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## Julien Morrissette (Oct 7, 2017)

it's a welch plug


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## Rooskie (Feb 12, 2015)

Julien Morrissette said:


> it's a welch plug


They welched you out of a welch plug???
What a world, what a world!
(Apologies to any Welch members!)


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## Julien Morrissette (Oct 7, 2017)

I received the new carb, still no welch plug, will try it anyway before sending it back, this might not be my problem


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