View Full Version : Stall and Backfires - Needs Expert Help

08-08-2007, 08:54 AM
Hi guys, I have a problem with my Exmark Lazer Z with 27 hp. Hoping someone on here can help me out. The problem is the mower backfires after slowing down at or getting ready to go reverse. It does it more on a small grade lots, going down and getting ready to turn, almost want to stall then backfires. This backfire is unpredictable. Sometime it doesn't't backfire but want to almost stall out. It have stall completely a few time. I've replace sparks plug. Check the fuel pump. Check Air filter. Replace fuel filter. Taken carbuator apart to clean out any dirt or water that may be in there. Even put in higher grade gas (89) after shop recommended. I know most of us use regular. Everything looks good but still can't figure it out why it still doing it. I am running out of option and hoping you guys can throw me some ideas, thought, comment or anything. I'm starting to lean toward a bad carbuator because that my only thought left, that's why i'm here, i don't want to have to buy a new carbuator if it's something else. The engine have almost 450 hours on it. I must say, Taken the spark plugs out , It looks too lean (not getting enough gas). The gap is at 30 which is recommended. Again, please let me know what you think.

08-08-2007, 04:58 PM
Please help

Bill Kapaun
08-08-2007, 05:21 PM
I think it points to 2 possible things-
1 You have water in the gas (bottom of carb bowl) and the engine gets a "sip" of it when it gets tilted or direction gets changed quickly. Try a few oz of Gas Dri per tank.
2. "Shifting" of the frame/seat is causing a marginally adjusted safety switch to "do it's thing".

The Captain
08-08-2007, 05:22 PM
Does the carb have adjusting screws for the mixture" and what about the idle speed? I Think you're on track with it being a carb/fuel problem. Just a guess as to the solution.

Good luck and let us know how you correct it.

08-08-2007, 05:40 PM
Bill and Captain, the carb be taken apart 4 times and been spray down, so there's no water. Water was the very first thing came to mind, but problem not solve. Carb is Clean Clean Clean. The second thing you mention is the frame/Seat......This was also checked out still nothing found. Alot of the lots are smooth, so it does alot of it backfireing or stalling as you getting ready to slow down or reverse. I'm not a rough driver so i take an easy with it, i'm not falling out of seat. Captain, the carb does not have screws for the mixture. Bill, i did stop by the shop and have them look at it, the mower was running, they can't see the problem since the problem is not happening right there. But at some point at the time, I think i overheard that they mention about the Accerlater Pump may be sticking, but then nothing was said anymore about it. I'm not familiar with Accerlater pump, but if i think about it, if you going full speed one way to the end of lot then pull back to slow down and revrese, can the accerlater pump be sticking and making it backfire or stalling? Please, let me know if there anything else that i can check, I'm up for anything. I pretty much done everything, but missing something simple such as the accerlater pump sticking or the whole carb (which i hope i don't plan on buying unless i have to). Hope to hear from you or anybody again.

08-08-2007, 05:42 PM
Bill, I've also done the gas-dri to clean out the system last week, still nothing.

08-08-2007, 06:22 PM
This is a Kohler 27 hp? ( No engine #s posted). If this is a Kohler, and it does have an accelerator pump then I would check it, but most that I Have seen fail make the unit run very rich not lean. I would check for intake leaks were the pipes connect to the heads. Spray (carb spray) around this area while the unit is running and see if there is a difference in how the engine runs.

John Stiles
08-08-2007, 07:25 PM
Seems to me the governor could be adjusted up a tiny bit, and it would take care of the problem....~john

08-08-2007, 08:36 PM
In some cases backfire is caused from inter-minting spark (a spark plug skips a fire process).

Given the fact that this only happens when on a incline/moving forward to reverse brings this to mind;

Some OEM's install a mercury switch on the engine to kill the ignition should too steep of a incline is tackled or a roll-over occurs.

It looks like a over-sized black pill capsule and has two black wires coming out of one end. On Kohlers it is mounted to the flywheel cover screw just above the starter.

Check your unit for one of these, Make sure the wires are facing STRAIGHT up. I have seen these installed spun more sideways from tightening the screw and the slightest incline or jerk cause the mercury to kill spark.

We all know how easy mercury moves around.

Just a thought.....

John Stiles
08-08-2007, 10:31 PM
Ah, yes...the evil bank angle sensor!:dancing: :) ~john

08-09-2007, 09:35 AM
Thanks for all the respond, Still working on it. I did disconnect that mercery switch yesterday but have not ran it yet, this was the last thing i did so far , i will post an update. As for the accelator pump, again, i'm not familiar with it, but as it post eariler by Mowermedic - if it wasn't working right, it will run rich, correct? if so, then that wouldn't be the problem. Guys, tell me if this correct....... Maybe it just me.....but for sure the spark plugs looks lean (white and dry, for those who understand this) is this normal on lazer Z?, if not, is the backfire and stalling problem cause this problem on the spark plugs? and the mercury switch could correct the problem as for as for backfire and stalling, plus the lean of spark plugs? Guys, thanks for helping, hopefully we'll find something. I'll post an update later today.

08-10-2007, 04:30 PM
Well, with the mercery switch being disconnect, no more stalling out or backfires. That little piece can ruin your days. Seem to be ok now after using it all day, but now i'm getting the engine rev up and down every second or so, i'm getting this without the blade going, with the blade going, everything seems fine. Is this something i should be concerd about. If so, please let me know. Thanks for your help.

08-10-2007, 06:36 PM
Hmmm, Yep those little switches can kill a days work :cool:

The reeving up/down is referred to as hunting, Is it doing this at all speeds or only at full throttle with the blades off ?

08-13-2007, 08:23 PM
the "hunting", I believe it's doing this without the blade running, usually at full speed, not 100% sure....may do this at slower speed but for sure at full speed. Let me know Thanks

08-13-2007, 09:18 PM
Well, Most engine OEM's say 3600 rpm is where the high speed should be set. Some equipment OEM's tweak them up to 3750 which when not under load (deck engaged) will hunt at wide open speed. Check you rpm and see where it's at, As long as you don't run FULL bore with the blades off you shouldn't have any problems.

Dixie Chopper for one does this and I just tell the customer's not to run wide open unless mowing.