Briggs 17.5 OVH Intek stumbling / mis-firing at mid-range RPM

I have a Hustler Sport zero turn lawn tractor with a 17.5 HP Briggs & Stratton OHV Intek engine - model #31G777-3304-G5. The unit is about 5 years old, very well maintained and has seen moderate residential use.

For about 6-8 weeks now, I have been struggling to diagnose what I would call a stumbling or mis-firing only in and around half throttle in terms of engine RPM. At idle it runs just fine, and at near to full throttle it runs fine as well.

See the video/audio at this link to hear exactly what it is doing as the throttle is adjusted. This is without the PTO / blades engaged, but it does the same thing when the blades are engaged, and it results in a noticable vibration in the unit given the engine stumble, which again will go away if I take up to near full or full throttle.

[video]https://photos.app.goo.gl/4DbZuIEudtoNh7dS2 [/video]

I installed a new platinum plug, new air filter and new fuel filter this spring as part of routine maintenance. I was originally thinking it may be a fuel starvation problem, especially since the fuel filter housing is translucent and I can see that it is not full of fuel and has air in the top. To troubleshoot this, I ran it for a few weeks with the fuel filter removed from the system and I ultimately saw the same stumbling. Also, the fact that the problem goes away when I throttle up to full, would seem to suggest it is not a fuel issue -- unless it is possible that the fuel pump is not delivering enough fuel for some reason at that mid-range RPM, but does at high RPM.

There is also a fuel system vent tube on this unit, with a small foam air filter, and I have also cleaned that to ensure there is no blockage -- no improvement.

Any thoughts or advise on this one. Could this be an ignition issue that only appears at this RMP range / throttle position? I am struggling with what to look at next.

Thanks in advance for any help or suggestions.

Well your problem is not really a problem with the engine
Your mower has 2 speeds, WFO for all driving & cutting.
Slow idle for opening gates or removing obstacles from the lawn.
They are not designed to run at any other speed.

Having said that the carb has 2 totally independent circuits, the high speed and the low speed ( idle & start )
At around 1500 to 2000 rpm the air passing through the carb causes it to switch from one circuit to the other and momentarily it will run either slightly rich or slightly lean.
During this time the engine may falter.
The older and more worn the carb the more prevelant this becomes and consistantly running in this zone makes it worse still

Excellent, thanks so much for that explanation. It makes sense as I tend to run the mower at less than WFO sometimes when doing light cutting.

I am just curious if there is any further information on this from Briggs & Stratton or elsewhere. I am also curious about your comment about the "older and more worn" the carb, as I don't really see a carb as wearing per se, unless you would consider some buildup of gum and varnish over time as wearing. Also, just in the last few weeks, I have switched to using premium fuel instead of regular unleaded simply to get ethanol free fuel. While that has not made a difference, I cannot help but wonder if I had been using such all along, I may not be seeing this issue.

Thank you again.

bertsmobile1 said:

Well your problem is not really a problem with the engine
Your mower has 2 speeds, WFO for all driving & cutting.
Slow idle for opening gates or removing obstacles from the lawn.
They are not designed to run at any other speed.

Having said that the carb has 2 totally independent circuits, the high speed and the low speed ( idle & start )
At around 1500 to 2000 rpm the air passing through the carb causes it to switch from one circuit to the other and momentarily it will run either slightly rich or slightly lean.
During this time the engine may falter.
The older and more worn the carb the more prevelant this becomes and consistantly running in this zone makes it worse still

Click to expand...

I would try adding some choke manually in the range you notice problem. This may require adding a temporary wire or string to operate the choke if both throttle and choke are one one cable. IF this helps, I would open the low speed adjustment IF there is one. Running some SeaFoam per instructions on the container may help.

Ethanol gets blamed for lots of things that have nothing to do with it. That is all that has been available in our area for 20 years or more and I have had no problem with it and this includes a 60 plus year old Wisconsin AKN.

Walt Conner

Air being sucked through the carb erodes the air passages.
Throttle shafts running through the body flog them out oval
Butterflys touching the carb body wears grooves.
Fuel flowing through the passageways erodes them and also gum / varnish depostis in places that are very hard to clean.

Depending upon how the carb is designed the air flowing through the venturi at specific speeds creates enough vacuum to sick the fuel through either one or both of the fuel passageways.
In most carbs the idle circuit is hidden behind blanking off plates because it is not adjustable.
If the carb is calibrated to flow fuel through both the high & the low speed jets all the time, and the idle circuit is partially blocked it will be running lean at the point where the main jet kicks in.
AT higher speeds where the carb is 50+ % on the main this effect diminishes.

As for your mowing practice, you always mow with the engine WFO.
The engine speed determines the blade speed and
the blade speed controls the air flow under the deck.
These are designed to work properly over a very narrow speed range.

Hydros are also designed for specific volumes of oil being pumped at specific pressures.
The fastest way to fill a hydro ( apart from contaminated oil ) is to run it at slow engine speeds.

Ok I will add to this....... I have seen it quite a few times before in my shop............

I agree with Bert on the WOT to cut or 3/4 throttle. I rebuilt quite a few carbs that are like your's that sat for a while and didn't set for a while, and I must say some were nice and some were real ugly.... I am betting you have the Nikki carb on your engine. If it does you have the noodle gasket in it. There are 2 types of those carbs.....

I seen the the video 3 times and listened well........ You have a very good idle speed. Your mid range is screwed up like my back is.... Your high range still has a sputter in it tho. It probably goes away when you engage your blades.......

When you engage your blades your engine needs more fuel so the gov kicks in and your engine purrs like a kitten......

Your carb needs a thorough rebuilt WITH the welch plug taken out and soaked in a ultra sonic cleaner or some carb acid then blown out with some good air pressure. Not 150 PSI just some good medium pressure.

I have seen this many times........

Hope this helps Mon Ami and let us know ~!~!~!

SteveHaller said:

Excellent, thanks so much for that explanation. It makes sense as I tend to run the mower at less than WFO sometimes when doing light cutting.

I am just curious if there is any further information on this from Briggs & Stratton or elsewhere. I am also curious about your comment about the "older and more worn" the carb, as I don't really see a carb as wearing per se, unless you would consider some buildup of gum and varnish over time as wearing. Also, just in the last few weeks, I have switched to using premium fuel instead of regular unleaded simply to get ethanol free fuel. While that has not made a difference, I cannot help but wonder if I had been using such all along, I may not be seeing this issue.

Thank you again.

Click to expand...

Briggs claims that running the engine at less than WOT reduces the air flow for cooling, and will cause the engine to run hotter under load.