# HS720 Backfiring



## MANU01 (Oct 3, 2016)

Yesterday, the SB started backfiring like I never seen it before, flames and all shooting out of the exhaust!

Starting procedure like every other time. Choke on, pull the cord (took more pulls than normal). Couple backfires, didn't think anything of it. 30 or so seconds go by, push the choke back in, machine runs slightly rough. 
Engage the augers and start to clear snow and as soon as load is put on the engine, it backfires,
pull back, engage augers, then again backfire.
This happened repeatedly 5-6 times in a row. One of the times, the engine actually died.
Ran with the choke on (halfway) for 2-3 minutes, then pushed it back in and it ran normal.

After 5 minutes go by, then it operated normally.

What is causing the backfiring? It never backfired like this before.

Btw, spark plug was never replaced yet, and use only premium fuel.


----------



## farmer52 (Dec 27, 2020)

My guess is a sticking valve.


----------



## Caper63 (Jun 15, 2021)

I had a similar issue in December with my HS720. Turns out it was low on fuel. I filled the tank. It stopped and has not repeated itself.

If choking it addresses your issue, this suggests it wasn't getting enough fuel.

Comparing your experience and mine, the common denominator seems to be when it is running lean and struggling to get enough fuel. 

Were you getting low on fuel? Seems too simple, but that solved my issue. Otherwise, try cleaning your carb.

My 2 cents.


----------



## orangputeh (Nov 24, 2016)

MANU01 said:


> Yesterday, the SB started backfiring like I never seen it before, flames and all shooting out of the exhaust!
> 
> Starting procedure like every other time. Choke on, pull the cord (took more pulls than normal). Couple backfires, didn't think anything of it. 30 or so seconds go by, push the choke back in, machine runs slightly rough.
> Engage the augers and start to clear snow and as soon as load is put on the engine, it backfires,
> ...


Can you pull plug and post picture ?


----------



## aldfam4 (Dec 25, 2016)

farmer52 said:


> My guess is a sticking valve.


...or valve adjustment.


----------



## MANU01 (Oct 3, 2016)

Caper63 said:


> I had a similar issue in December with my HS720. Turns out it was low on fuel. I filled the tank. It stopped and has not repeated itself.
> 
> If choking it addresses your issue, this suggests it wasn't getting enough fuel.
> 
> ...


Low on fuel, not really but I refuelled just in case before starting the blower today. 

Still backfires when pulling the auger handle. Once it warms up, it kinda goes away but not 100%. 

RPM’s are at 3440/3450. 

Blower never backfired like that until yesterday, a day after clearing some heavy/compacted snow at the end of driveway (12” higher than top of blower) courtesy of the city. The engine did struggle at time, but whatever, that’s what it is meant to do. 

What is involved in order to find out if indeed it needs a valve adjustment or carb cleaning?


----------



## orangputeh (Nov 24, 2016)

MANU01 said:


> Low on fuel, not really but I refuelled just in case before starting the blower today.
> 
> Still backfires when pulling the auger handle. Once it warms up, it kinda goes away but not 100%.
> 
> ...


how's the oil level? doesnt hurt to check valve lash. check at TDC ,. did you remove plug and check? whitish is lean, black is rich.


----------



## MANU01 (Oct 3, 2016)

orangputeh said:


> how's the oil level? doesnt hurt to check valve lash. check at TDC ,. did you remove plug and check? whitish is lean, black is rich.


Will check oil level and post a picture of spark plug tomorrow


----------



## MANU01 (Oct 3, 2016)

orangputeh said:


> how's the oil level? doesnt hurt to check valve lash. check at TDC ,. did you remove plug and check? whitish is lean, black is rich.


Oil level is fine. 
Spark plug on the other hand is full of soot. 

I bit more info leading up to the backfires: 
Wife tried to run the blower while I was away the day before, and just told me the she ran it chocked, until it died and wouldn’t start again, about 5 to 10 minutes. Then she put the blower away. 

Spark plug


----------



## orangputeh (Nov 24, 2016)

okay. good clue about running it on choke. 
clean the plug thoroughly or install new plug
and try. it will probably run fine. 

probably a bunch of soot in exhaust also so it may take a while to clean out.


----------



## Caper63 (Jun 15, 2021)

Ok - the opposite of what I suggested.

Does anyone have a little ketchup? It helps when eating crow.


----------



## orangputeh (Nov 24, 2016)

Caper63 said:


> Ok - the opposite of what I suggested.
> 
> Does anyone have a little ketchup? It helps when eating crow.


well the owner did come back with more pertinent info.....that is machine was run on full choke for 30 minutes??? probably caused this issue. will know after he cleans/replaces plug.


----------



## MANU01 (Oct 3, 2016)

Caper63 said:


> Ok - the opposite of what I suggested.
> 
> Does anyone have a little ketchup? It helps when eating crow.


Any suggestions are good suggestions, I am very thankful for the help received so far. 

I will replace the spark plug with a new one, and will report back once I do.


----------



## MANU01 (Oct 3, 2016)

Finally had some time to go out and buy a new spark plug today. Gapped it correctly (0.7-0.8 mm) and installed. 

Started first pull and no more backfiring!!


----------



## Oneacer (Jan 3, 2011)

I had a neighbor bring me her lawnmower that would not start .... I asked if she hit anything, she said no.

long story short, .... Blade was bent, pulled the flywheel, key was sheared ... Replaced the key, bought new blade .... Mower runs fine.

sometimes people do the saddest things ....  .


----------

