Briggs & Sratton 12.5hp intermitten rough running (backfire through exhaust)

[Semicrazy]

Hello,

I thought I would pick the great minds of TDR first with this issue.



I went to use my 60in trail mower, with B&S 12. 5hp vertical shaft model 289707, a few weeks ago with it in the 90s out and after I warmed up the engine for a couple minutes I ran into this problem.



It will run decent/smooth with the choke on and if it is on full throttle it will be rough (the governor keeps moving back and forth) and it pops/backfires out the exhaust. Anything less than full throttle or if I try to engage the electric pto it just slowly dies out.



I thought it was fuel related so I tried a new fuel filter and I thought the problem had been fixed because I useed the unit for 4 hours after that without issue.



I went to mow today and used the unit for 2 hours and then ran into the same problem. I tried running without the fuel filter. I tried taking the carb apart and cleaning everything inside, but everything looks good inside.



So I just started playing around with the throttle and choke and it seemed to clear up for a few minutes so I tried to continue mowing but the problem kept coming back. I continued to keep trying to play with the choke and throttle and that lasted for a little while but then I just got too frustrated.





So now I am stumped as to whether this I have an intermitten fuel problem or if this might be some kind of electrical/ignition problem?:{



Anyone have any ideas or know if a good LAWN MOWER FORUM that I may be able to try? I would appreicate all the help I can get.



Thanks,

Nate

 

[scootergmc]

Did you check the fuel tank for unwelcome goo and such?

 

[Semicrazy]

Thanks scootergmc,



I will try cleaning the tank. The new fuel filter that I put on is clear with a metal mesh in the center.



The filter is always full of fuel and I can not see any debris that is why I thought the issue was down stream in the carb.



But I am willing to try anything at this point.



I will let you know if I find anything. Thanks,

Nate

 

[Wayne M.]

It definetly sounds like you have a fuel problem, weather it's the carb/ delivery or quality, I cant be sure, but your comment about the choke is the giveaway. You must check the carb out thoroughly. totally disassemble it and make sure the jets are clear. I use a jumbo can of carb cleaner and sometimes compressed air to confirm flow through all passages. If you're not experienced in carburetors or tuning them, bring it to a pro.

 

[PToombs]

Try turning the hi speed screw out a little. It's the one that goes either straight up thru the float bowl, or straight down thru the top. If it's in the bottom and this seems better, pull the float bowl and check the main jet. It's inside the center tube. Sometimes they come loose, and unscrew.

Hope this helps.

 

[Semicrazy]

Thanks for the help.



I am pretty sure it is not an issue with fuel from the tank. I pull the line going to the fuel filter and fuel flows from the tank very quickly and I filled a 2 gallon gas can in a matter of a minute. Plus it is not the vent cap because the problem will occur without the cap on as well.



I had to run it for one hour yesterday with the choke partially on so I am pretty sure that I can rule out the ignition if I can make the problem go away with the choke. I think I also ruled out a governor problem based on a diagnositc on a mower website that stated that if you hold the governor constant and you still have a varying RPM then you have a fuel issue.





PToombs, I saw what you were talking about the first time I took the carb apart and sprayed carb cleaner all through it, it is a brass item right in the center? I will try turning that out a little, what do you think like a half a turn?



I plan on ordered a carb rebuild kit and going through it. From the Briggs website I printed off the exploded parts view, it is a Walboro carb if anybody is wondering.



Thanks for tall the help!!! I appreciate it. I will report back when I have more info.

 

[PToombs]

Semicrazy, that's the jet, inside the tube. Make sure it's screwed IN. It should be snug in there. If it's loose, that's the problem.

If you want to adjust the fuel, turn the SCREW out. Sounds like you're on the right track.

 

[Bill Stockard]

I had to run it for one hour yesterday with the choke partially on so I am pretty sure that I can rule out the ignition if I can make the problem go away with the choke... . PToombs, I saw what you were talking about the first time I took the carb apart and sprayed carb cleaner all through it, it is a brass item right in the center? I will try turning that out a little, what do you think like a half a turn?

Running with the choke partially on leads me to think that the gasoline to air fuel mixture is too lean. I had a similar problem with a B&S engine. After a careful inspection of the carb parts, I found the main jet partially clogged with fiber looking stuff. I don't know where it came from since the carb fuel bowl and the fuel filter were clean. After cleaning out the main jet, it cured the problem.



The main jet clogged up a couple of times after I found it the first time. I don't know if it had anything to do with it, but I started using a gasoline fuel stabilizer all year in my gasoline powered air cooled engines and haven't had the problem re-occur.



Bill

 

[tgordon]

Is this a new mower or one you have used in the past? If new it could be a bent needle. If you have used it prevoius seasons and have not had the carb apart I doubt that, but about everything else I can come up with has been listed.



Troy

 

[daveshoe]

Valve problems could also cause problem

If your valves get build-up on the stem area,and do not seal properly, this can result in symptoms you describe. When the piston fires,both intake and exhaust valves are supposed to be sealed. If the intake valve does not quite seal,hot fuel-air mixture can backfire in to the intake and rapidly put varnish on the valve stem. Eventually the valve will stick open.

This happened to me, and the fix was to pull the head,clean the carbon from it,and clean and adjust the valve clearance and clean the valve stem where it goes in to the guides. Get a good Briggs book from the library,and it will show you how to do it. Some manufacturers do not want you to remove the valve completely from the engine as it could damage the guide.