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I've got a 24 hp twin. It has about 200hrs on it. Bought it new and never been adjusted. On a tractor. Basic problem is it has a backfire through the muffler. It was hard to start too at times too. I opened the valve covers to check them. I did the set up, TDC and 1/4 past it. On both I could not get a 005 in. The 004 goes in but you have to really push it. According to the book, they should be set at 004-006. My motor is 445667. The rocker arms are tight with the 004. I noticed on videos around even with 004 you get a very slight movement. Should I see a very slight movement? Should I go up to 005?

You should feel a small resistant's in the feeler gauge. Better a little on the loose side then the tight side.

So i should go with 005 and still be within correct adjustments? Would valves become too tight on their own in some cases instead of loose?

Valves can become tighter as they wear. Even though the spec is .004-.006 I don't think you would even know if they were .003. Sometimes to they rocker arms will tilt a small amount and make the clearance appear smaller then what it really is. so you may want to wiggle the rockers to make sure they are not tilting, and appearing to give a smaller clearance than what you have in reality.

IMO "screw-type"rocker valve adjustment is an acquired skill. Having to push (force) a feeler gauge thru says it is not at that feeler gauge thickness, but a thinner one. Describing the sensation of a feeler gauge passing thru a clearance so you can accept that value is tough- like describing the first time you balanced on a bicycle, but here goes...Forcing is not acceptable, but neither is looseness. The feedback through the fingers is kinda (IMO) like buttering your thumb/forefinger and sliding the selected gauge thru. If you pinch hard while passing the guage it's not just right, and if you just make contact while passing through -too loose. Also the sensation is different for the small contact on (my) Intek valve tip/rocker tip than say an overhead cam car where the contact patch is as large as the feeler gauge width. Now that this is crystal clear :smile: More blather...

IMO "trapping" the feeler gauge while tightening the rocker screw can result in over -tight clearance because the screw can actually dig in to the feeler gauge. Best tip I can think of is checking the clearance after tightening and being prepared to change the setting until correct. The "ball " type rocker IMO is tricky to tighten because it moves around. I have seen the youtube demos and the professionals have a lot of experience at this, but this is to encourage you not to get discouraged with this type of rocker adjustment. There are a number of hand held valve adjustment tools on the market which promise easy adjustment. I have not tried them. Patience and practice.

Kruzer said:

I've got a 24 hp twin. It has about 200hrs on it. Bought it new and never been adjusted. On a tractor. Basic problem is it has a backfire through the muffler. It was hard to start too at times too. I opened the valve covers to check them. I did the set up, TDC and 1/4 past it. On both I could not get a 005 in. The 004 goes in but you have to really push it. According to the book, they should be set at 004-006. My motor is 445667. The rocker arms are tight with the 004. I noticed on videos around even with 004 you get a very slight movement. Should I see a very slight movement? Should I go up to 005?

Click to expand...

For B&S engines it's 1/4" inches past tdc., not 1/4 turn. Post model xxxxxx, type xxxx, trim xx from engine. When you have valves adjusted correctly, you should feel a light drag with feeler gauge between rocker arm and pushrod. Let us know how it goes, thanks.

Thanks for the replies,

I just went thru the process. I got 004 to where it glides in with minimal drag. Ran it and rechecked. I still get a mild sputter through the muffler which is what the original problem was. Should I go with 005 or go in to a different direction?

Kruzer said:

Thanks for the replies,

I just went thru the process. I got 004 to where it glides in with minimal drag. Ran it and rechecked. I still get a mild sputter through the muffler which is what the original problem was. Should I go with 005 or go in to a different direction?

Click to expand...

Absolutely not, 0.004 is just fine. Remove & clean spark arrestor from muffler & give it a try.

Engine & Carburetor Backfire | Mower & Small Engine Repair | Briggs & Stratton FAQ

FAQ | Lawn Mower Safety Tips | Briggs & Stratton

Kruzer said:

Thanks for the replies,

I just went thru the process. I got 004 to where it glides in with minimal drag. Ran it and rechecked. I still get a mild sputter through the muffler which is what the original problem was. Should I go with 005 or go in to a different direction?

Click to expand...

Post model xxxxxx, type xxxx, trim xx from engine.

mechanic mark said:

Post model xxxxxx, type xxxx, trim xx from engine.

Click to expand...

Not sure if these are the numbers but they are off the valve cover
445677
type:0827
080318yg

mechanic mark said:

Absolutely not, 0.004 is just fine. Remove & clean spark arrestor from muffler & give it a try.

Engine & Carburetor Backfire | Mower & Small Engine Repair | Briggs & Stratton FAQ

FAQ | Lawn Mower Safety Tips | Briggs & Stratton

Click to expand...

When it does it, it's more when it's running not when I shut it off. Not ether. Actually sounds like one of my older horses. Walks and farts at the same time. I'll try what you suggest.

I'm still getting the backfire or more a sputter from the muffler. On adjusting the valves, I know you get it to TDC and then 1/4" past it. When you check the exhaust valve do you keep the motor where it is or do you spin it again to another position? I tried all the other suggestions on fixing the sputter with no results. It does have a hard time starting even with a fresh battery.

Do as was advised by others....find TDC and 1/4" turn of fly wheel.... if both valves are closed (method) at this point you should not have to turn/re-adjust your flywheel... I do repeat the process until happy tho...if adjustment is good then its good... other issues at hand...albeit small ones...:forgot to add... have a look at your push rods and guides for any wear...if you pull rods out make sure to put them back as you found them 2cents

I checked them and double checked them. All seem fine. I remember when this problem started. It was after I ran out of gas and the motor died. From that point on it started to have a mild backfire through the muffler. At times it would run fine but most times it would do it. I'm not sure if any of these were possible solutions but I tried running with out the cap, change the filter, ran seafoam through it. I didn't take the carb apart yet mostly because I'm not too sure what I'm looking for. The reason for thinking it's the valves is from different threads I read. Probably too much info I guess. The motor has about 250 hrs on it.

Just a shot in the dark here....not sure on muffler set up over there as it can vary... had a honda mower in some time back with a removable spark arrestor screen that was clogged...just removed and cleaned.... other than that with no fuel line, spark, issues etc....id replace the gasket and diaphragm in your carb and a general clean....if your diaphragm is dead it will feel a little dried up/ hard....cheap and easy to replace...did you hit anything with it lately?...tree stump etc... does engine seem hard to turn over as if battery is weak?

Thanks for the reply,

I'll try the spark arrester. It was mentioned before. Not sure where it is or if its removable. I didn't hit anything, just ran out of gas. It started do thing shortly after. I do mulch and maybe something worked it's way into the carb? Nothing was hit too. Not sure as the battery as it has one season on it. It has plenty of power as I put a snow plow on it and it doesn't struggle at all. I'll try looking at those suggestions. We're getting that blizzard soon here in Mass. This might be put on the back burner for a few days.

It would be in the muffler itself...if its a sealed unit then remove it and check if its towards exhaust port end (where it bolts to cylinder) only guessing as it might not have one but worth a look...might be worth checking the flywheel key while your at it...all I can say after that is to check air and fuel manifold just to illuminate any extra air intake etc :confused2: Happy Hunting :biggrin:

In order to back fire through the muffler you need to be putting unburned fuel in there which gets ignited either by something that is red hot or by the following exhaust that is still burning.

Only 3 reasons why fuel does not burn in the cylinder.
Wrong air: fuel ratio
Compression too low
no spark ( includes wrong time spark )

Now the first two would cause consistant back fires
So the last is the prime suspect.
First try a new plug

Second remove the ignition cut off wire at the engine .
It is a small tubular connector with a funny cap.
This isolates the coil/s from the entire mower so if you have a bad safety switch it is not in circuit and neither is the IGNITION SWITCH . ( shouting intended to get your undivided attention )
Note THERE IS NO WAY TO TURN OFF THE ENGINE UNTILL YOU RECONNECT THIS WIRE so some caution is necessary .
If the problem goes away then tough luck as you now have to work out which switch is giving you grief.

Next trick is to run the mower without its hood & the engine without its cowl, in a dark place for a short time.
Carefully inspect all the wires coming out of the coil and you are looking for a spark sneaking out of the system some where escaping onto the engine and this goes all the way back to connector you dissconnected in the previous test.
This is best done after dark as some sparks can be really hard to see.
Also note I said a short time as running for more than 10 minues or so without the cowl can cause local overheating and distort the engine turning it into land fill.

Gone thru just about all suggested. As of now if I run it with the choke very slightly pulled out, the random backfire stops. This thing has been working its *** off as we have had over 40" of snow in the last couple of weeks ( and lot more on the way) and my snow blower crapped out. I have craftsman snow blade on it. I just put a new 340 CCA battery on it and starts easily so I'm guessing I got the valves right or really close. Where should I be looking? I didn't try Bert's last suggestion yet. I wanted to mention the choke first. If I have to pull the carb, I might wait and run it like it is if it don't hurt it. I'm not really confident in taking that apart.

Kruzer said:

Gone thru just about all suggested. As of now if I run it with the choke very slightly pulled out, the random backfire stops. This thing has been working its *** off as we have had over 40" of snow in the last couple of weeks ( and lot more on the way) and my snow blower crapped out. I have craftsman snow blade on it. I just put a new 340 CCA battery on it and starts easily so I'm guessing I got the valves right or really close. Where should I be looking? I didn't try Bert's last suggestion yet. I wanted to mention the choke first. If I have to pull the carb, I might wait and run it like it is if it don't hurt it. I'm not really confident in taking that apart.

Click to expand...

Just a carb cleaning issue me thinks if not drawing any unwanted air....is there a fuel pump on her?... not sure of model specs but some will have a positive feed pipe that leads from OHV cover to fuel pump....check for any cracks in pipe etc....:confused2:

Trivial observation: It would be interesting to survey how many first failures are reported at 250 hours... Seems like some kind of un named lawn mower "law" to me. Some one can guarantee his (her)immortality by naming this law. Be sure to tell us what it is. :laughing:


Please note I hope you solve your problem soon. You are in good hands.

motoman said:

Trivial observation: It would be interesting to survey how many first failures are reported at 250 hours... Seems like some kind of un named lawn mower "law" to me. Some one can guarantee his (her)immortality by naming this law. Be sure to tell us what it is. :laughing:


Please note I hope you solve your problem soon. You are in good hands.

Click to expand...

Real simple. It's called the Warranty Law.

SeniorCitizen said:

Real simple. It's called the Warranty Law.

Click to expand...

:laughing: perfect

Kruzer said:

Not sure if these are the numbers but they are off the valve cover
445677
type:0827
080318yg

Click to expand...

Results engine manual

Briggs and Stratton 273521 Repair Manual Intk V2 OHV New | eBay engine repair manual

Michael72 said:

Just a carb cleaning issue me thinks if not drawing any unwanted air....is there a fuel pump on her?... not sure of model specs but some will have a positive feed pipe that leads from OHV cover to fuel pump....check for any cracks in pipe etc....:confused2:

Click to expand...

There is a fuel pump. When this problem started back in the fall it went from running great to where it is now in an afternoon. Right after I ran out of gas. I do a lot of mulching and it gets pretty dusty. I remember tearing the cowl off and checking for anything obvious. Blew it clean with an air gun much like I do after every use. When we have a break in the daily snow fall I'll get into the carb. I'm a little apprehensive about tearing it apart right now as I'm relying on it to plow the driveway and paths. I screw up the carb somehow and I'm shoveling. Thanks to all for the help.

Kruzer said:

There is a fuel pump. When this problem started back in the fall it went from running great to where it is now in an afternoon. Right after I ran out of gas. I do a lot of mulching and it gets pretty dusty. I remember tearing the cowl off and checking for anything obvious. Blew it clean with an air gun much like I do after every use. When we have a break in the daily snow fall I'll get into the carb. I'm a little apprehensive about tearing it apart right now as I'm relying on it to plow the driveway and paths. I screw up the carb somehow and I'm shoveling. Thanks to all for the help.

Click to expand...

Mummmm, very dusty mulching you say, puts a little different slant on things.
Two more things come to mind.
1) crud in the bottom og the fuel tank restricting the supply of fuel
2) crud stuck in the fuel pump air filter reducing its pumping capacity.

Once again you are looking at bypassing the items in question .

To do this you will need some sort of temporary fuel tank and some fuel line if you do it relatively quickly then you can use simple PVC tube just make sure it does not contact anything hot enough to melt it.
There will be enough fuel in a walk behing mower fuel tank to do both of these two test and by the sounds of it you won't be needing the small mower for quite a bit.

First test is to hook up empoary tank to fuel pump and run for about 10 minutes, problem goes away then you ar looking at crud blocking the fuel tank outlet or fuel line between tank & pump.

Second test will require you to hang the temporary fuel tank above the engine so it will gravity feed.
Plum it directly into the carby and try again.

If neither make any difference then you have at least eliminated two lines of enquiry.


Usually a low fuel situation causes the engine to hunt so the revs rise & fall and this will even out a lot by engaging the choke slightly and the engine will run fine with the choke fully off & the throttle stopped down to idle.
Usually an air leak will cause the engine to run faster than it should due to lean burning at large throttle openings and at idle it won't run at all.