Briggs & Stratten Mower Engine Surging/Hunting. Suggestion For A Fix Please

Engine Runs Rough | Engine RPM Fluctuates | Briggs & Stratton

You tube videos are a good source too:

Briggs and Stratton lawnmower carburetor repair for surging engines

From another blog: " I partially disassembled the "non serviceable" carb and sprayed it very well with carb cleaner, did that a couple of times. Totally drained the tank and refilled with new gas which had Stabil added, also some carb and fuel injector cleaner. Couple of tanks later the problem had essentially disappeared."

Good Luck.

PS: to my OP. All basic 'good practice' methods have been tried. The fact that the engine starts and runs suggests fuel and ignition is OK when running in neutral - hunting is regular and rhythmic - not coughing or spluttering

That points to a governor type problem, but as far as I can tell, there is no adjustment.

Reading thro' Lyn's links - thanks - gives some ideas to check tomorrow.

tcmtech is a bit 'one-way' - ie, I drill the hole too big I'm knackered if it's wrong. I will see what size the jet is first.

There used to be sprays, starter sprays, that you could spray on all the various intake parts, when you hit the air leak the engine note would improve.....

Haven't tried it in many years though!! I am sure that many here will have better and similar ideas to try....as it does sound like a mixture problem to me....

Is the petrol in the tank old? Try a refill of the best/highest Octane available....

If you are using the old carb and air cleaner, try running without the air cleaner first..... Then clean the carb!!

Have you used the original carb and other bits, or is the motor completely new. If completely new, make a video than take it back to the seller......show him the video.

Here's a shot in the dark. My mower kind of does that just before running out of gas (petrol). Is it possible that it isn't getting enough fuel, i.e., a clogged or crimped fuel line or a bit of debris somewhere restricting fuel flow?

Well assuming you let it warm up first, my first guess would be an obstruction in the fuel flow , which could be any number of things....so I would blow out the fuel line, check the screen, look for kinks, check for any water in tank...add some sta-bil...

http://www.briggsandstratton.com/eu/en/support/faqs/fuel-recommendations

http://www.jackssmallengines.com/diy/small-engine-troubleshooting/

Usually an unstable fuel system, take a cloth wrap it around a small stick or old tooth brush soak it with either ( starting fluid) move it around all the hoses from fuel tank to the carb it engine changes tune indicates a vacuum leak. Make sure the fuel cap vent is working and allowing air to equalize the tank. There may be a filter partially blocked and the comments about a good fuel cleaner/conditioner is good inexpensive advice.

Acronymfinder: "MILF" = Mowers I'd Like to Fix.

This is going to sound dumb but I swear it is true.

Because of all the sand in my area my lawn mower blades take a huge amount of wear. As my blade becomes out of balance my B&S mower engine surges and slows to a near stall over and over. I never timed it but I probably go through about 15 surge/slow cycles per minute. After I replace the blade all is well for about a year.

I would assume that I get an oscillation between the spinning blade and all the automatic safety crap that prevents me from having any throttle control. I long for the good old days when a mower had a throttle level and you could push in or pull back to control the speed.

I would drain the gas tank and clean the entire fuel system, especially the carb and bowl. It sounds like it is a fuel flow problem. I had a brand new mower that ran fantastically, one pull starts and no bowl priming required, for about 4 weeks. Then, it started running exactly as you describe and it turned into a multi pull fiasco , trying to get it started. I drained and flushed the fuel tank as well as disassembled and cleaned the carburetor. Good as new. Then, a month or so later, the same behavior. I started looking at the mower closely and made a startling discovery; in their infinite cheapness and greed, the manufacturer deleted the inline fuel filter! :-O Remember, the gas one gets at the pump is far from clean and not to mention, whenever you fill the tank, there is always dust from grass clippings floating around, that would just love to settle in the tank. I have found it pays to get a pack of cone shaped fuel filters and, when filling the tank, pour the gas through them. I was pretty amazed what got caught in the filter. An inline fuel filter would be the ultimate fix.

New you say. Did the instructions say you need to oil the air filter? IF it is the foam type they used to oil them for dusty environments and that restricted airflow which made the mixture richer. If it is the folded paper type then it's not that. Does the regulator arm have a spring and various holes to put the end of the spring into?

Other than that, earlier posts are good.

Jim

Lots of ideas from everybody to explore, thank. Raining at the moment so will have to wait before trying them out.

But the solution will be linked to something that has gone wrong since first fitting the engine because it ran OK the first time - and had no trouble cutting the overgrown grass.

The previous engine was always giving trouble and losing power that I put down to blocked petrol filter. And on a couple of occasions found water in the fuel bowl. I have fitted a drain cock in the tank to dump water - where does it come from. The mower is not used when it is raining and is kept under cover.

The filter was not easy to get at or open to clean it - so I fitted new one - this time one with a clear glass body for visual inspection. I notice now that it always shows signs of loose crud, so is working as a filter - but there is an air bubble in it - but not stopping the fuel because it pours out when the hose to the carb body is disconnected - and the air bubble stays there. But I will check out pipe runs to align the filter at the lowest point not to act as an air trap and to show if water has been collected.

Further inspection of the parts manual show a plastic cap.

The bit marked 1091 is a plastic cap that only turns 90^o and has no effect.

I will try to remove the cap and turn the screw to see what happens. From what has been said I guess this violates emission regs. How will I know.

I might also fiddle with the governor linkage. There is only one 'hole' for the spring and wire control rod. I will drill a couple of extra holes in the arm to change the ratio of leverage.

I will report back.

I've tried messing with those different setting on the governor linkage many times andI have never been able to get them to prevent engine surging at light loads. All they are good for is setting the throttle response curves and what setting works best a light load is way off at a heavy one.

As other have said a good clean out and service helps to some degree but from personal experience anything that has a surging problem at light load that only goes away with a load or by running with the choke part way on needs to have the main jet opened up.

I have B & S, Honda, Kohler, Tecumseh, Wisconsin/Teledyne, Kawasaki, MTD and a number of their foreign copies of small engines of every size from string trimmer and chainsaws to 24 HP two cylinders that all have the modern non-adjustable carbs on them and so far everyone of them that does not have an adjustable main jet screw has required a jet drill out at some point to make them run properly.

It's just the nature of the design to meet emission regulations. They build them and set them up to work at the factory on idea fuel and working conditions to pass the tests and pretty much say screw their performance out in the real world if you run anything off from their test spec fuels and whatnot.

To be honest drilling a jet is not that big of a deal. I have a number of micro drill bit sets (1/8" shank style) that cover every size from a thousandths up to several 10 of thousdandths and the are easy to work with.

Just find the bit that will just fit the stock jet hole and pick the one the next size larger and ream the hole out with the bit carefully turning it with your fingers then put the carb back together and give it a try. If it got better but not great it may need to go one size larger still.

At worst you over drill the jet and have to spend a dollar or two and buy a new one and start over. Surprisingly I have only ever done that once and that was simply from not paying attention to what I was doing and drilled out a jet that had already been drilled twice.

I'd like to see that drill that is .001" in diameter.

Sorry, I don't have any .001" dia drill bits.

.0039" is as small as mine go presently.

If you want a set here you go!

http://www.ebay.com/itm/10PCS-PCB-CNC-Print-Circuit-Board-Carbide-Micro-Drill-Bits-Set-Tool-0-1mm-1-0mm-/121819963886?hash=item1c5d0921ee:g:Ci4AAOSweuxWTzh7

No thanks. I could't see anything that small anyway.

Still raining so haven't looked at carb yet. But I have ordered a set of drills - just in case - but I will fiddle with the bleed screw first.

A supplementary question arises - the drills are all described in eBay as PCB drills - which is fine if that's what you want them for - but as pcb's are usually not made of metal does it mean the drills are no good for metal, do you know?

They sound OK too me but it does not say in the spec - I am more concerned they might be weak and break off in the jet.

They are carbide so they will drill through pretty much anything.

And yes the smallest ones are rather fragile so that's why I recommend doing the drilling with fingers rather than a Dremel or powered drill.

My father in law uses a large darning needle. He drops it in the jet lets it sit on the tapper of the needle and then just pushes it a little to expand the bore. If more needs to come out he uses a needle out of a pneumatic rust de-scaler. They are about 2mm dia, (0.080" for the imperialist dogs) x 150 -200mm (6" - 8") and made of spring steel. He has ground a fine triangular tapper on it and uses it the same as the needle accept with this one you drop it in and give it a light twist.

BAB

Thanks for the tips. I'm trying the choke stop first. The drills are on order, but if necessary I can borrow a needle from my wife -although she might not take kindly to grinding triangular flats on it.

Hello Horace.

I am in the states and not familiar with your countries fuel supplies.

Most of our gasoline, i guess you say petrol, is mixed with ethanol.

Ethanol attracts and absorbs water.

A blend of 10% ethanol and gasoline can absorb 7000 ppm water.

Octane decreases as the fuel becomes contaminated.

Conventional gasoline was usually good for a couple of years. An ethanol blend is only good for 100 days and should be discarded.

I pour my blended 10% ethanol / gasoline mix from my plastic gas cans into a clear glass gallon jug before pouring it into my mower. If there is water contamination two layers of fuel will be seen. Upper layer will be water contaminated gasoline and the lower layer water contaminated ethanol.

I have watched the tankers waiting to fill up at the terminal with their tank lids open in the rain. There are lots of ways for water to find it's way into fuel before we pump it.

We had a mentally challenged kid in our little town for a while who thought it was great fun to pee in people's gas tanks.

Crud in the filter could be coming from your supplier or it could be the result of the contaminated fuel tearing up your fuel system. There can be up to 1000 different ingredients in these new fuel blends.

Myself, i wouldn't mess with the linkages till the fuel is right.

Hope this helps.

GA.

Ethanol was my immediate thought re. the water problem.

We had an instance here not too long ago where a whole heap of premixed two stroke was found to have been made with an Ethanol blend.

The company had to withdraw it, but I'm sure there is still some out there, and it is possibly happening elsewhere as well.

There have been numerous instances of premixes not being as expected, with either too much or too little oil in them, or even incorrect oils especially in outboard mixes.

Always best to mix your own to be sure you have what you want.

I thought B & S only made 4 stroke engines for lawn mowers.....

Here I have only seen 4 stroke at least.....

That could be Horace's problems!

Some feed-back:

It has stopped raining and it is a lovely day.

I can reduce the hunting by a very slight adjustment of the choke by poking my fingers in the carb linkage to close it slightly - and if too much I get black smoke (sure signs of running rich I know) but there is no mechanism to hold the choke at that position so the engine hunts as soon as I let go of the link.

Another thing to note is the occasional hiccups in running at high revs when I hold the governor off. Misfiring in a sense, but not the sort due to ignition problems, because there is no back-firing in the exhaust. It all sounds like fuel problems.

Fiddling with the choke suggests a weak fuel mixture and tcmtech is right about increasing the size of the hole in the jet to cure the problem.

But first, rather than dismantle the carb to drill out the jet (I have not got the drills yet anyway) I will make a 'temporary' stop to hold the choke open slightly to prevent hunting, and try to cut the grass with the engine under load.

Watch this space for a further announcement!

You are on to it, well done!

Welcome to the dark side of defeating emissions systems to make life work the way it's used to and is supposed to work!

Lyn will be along any moment to inform you that drilling that jet out will lead to earthquakes and pollution on his land.

Bwahaha!

BTW if you have a baseline measurement of fuel use and engine power at load drilling that jet out correctly usually and counter-intuitively picks up both just a bit.

Pretty sure that's not a lawn and those aren't lawnmowers there.

LOL!!!

So now we know it needs to be richer, as suspected. Again i suggest looking at the air cleaner. Is it properly seated? Does it need oiling (foam type )? Is there air getting in between the filter and the engine? If it is all good, drill away! You may already have a set of Oxy Acet tip cleaners. They are more robust than the carbide pcb drills. Tip drills come with a plastic knob on the end to enable finger drilling and the size is metric i.e. a No 8 drill is 0.8mm etc. But they may not be small enough, however, the file type cleaners go down very small.

Pic may not go through. It is from eBay search Oxy Acet tip cleaner

Jim

Re the air cleaner - it's dry pleated paper - I took it off to gain access to the carb - and ran the engine to see what effect fiddling with the choke had on hunting - and although this seemed to 'cure' the problem - I was running the engine without the filter.

I will see what happens when I put the filter back on.

I note what has been said (by others) about 'drills' and jets - but I must admit that it is not something I would have done even if I had thought of it - this harks back to the early days of National Service in the RAF when it was drummed into us that in no circumstances must foreign objects (pieces of wire) ever be used to clean carb jets (they had to gently and passively 'washed' out and blown out) - memories linger - ".. it could alter the diameter..."

Now we are talking of using violent external force to deliberately drill out the diameter - how times change.

I somethimes wonder what else I have been taught to accept without question - that could act as a brake on progress.

I note what has been said, even so, I still think my problem is solvable by governor control. Hunting (I learned in my early days) is due to incorrect damping - which to me is to do with the ratio of the lever movement compared to the throttle movement.

For the moment the best I can do is a bodge job on the choke opening. The grass has grown a lot due to rain in the last few days.

Damn the torpedoes, full speed ahead!

I love the picture, the spray is flying everywhere!!!

Hello Horace. Good to hear you are making progress.

In your post # 14 you said the problem will be linked to something that has gone wrong since first fitting the engine because it ran ok the first time.

You did not give the model and serial number on the engine so i do not know if you have the pneumatic or mechanical governor. Still, settings on either governor should not have changed since the first run.

The pneumatic governor could change if something (grass) is preventing it from moving freely or air pressure from the flywheel is blocked.

The mechanical governor would only change if the flyweights inside have possibly loosened or the external linkages have been bent.

Barring any changes in the governor assemblies it is sounding like a lean fuel mixture.

Old gasoline or contaminated gasoline will do this. Another possibility is some crud may have gotten past the filter and reducing the flow in the carb. Removing the plastic covers on the mixture screws and taking out the screws and blowing out the passages with compressed air may dislodge the crud.There are also aerosol cans of carb cleaners which may clean the passages if the air alone does not work.

Hope this helps.

In a recent post (#35) from Horace40, he posted:-

Re the air cleaner - it's dry pleated paper - I took it off to gain access to the carb - and ran the engine to see what effect fiddling with the choke had on hunting - and although this seemed to 'cure' the problem - I was running the engine without the filter.

Which sort of demonstrates that the motor is more than likely running rich, that is when the air filter is in place....as in my experience, engines tend to "lean out" when the filter is not present (the "suck" is reduced on the fuel supply, so less fuel is burnt!)......that is assuming a simple, no electronics, no injector engine as I am sure this one is!

Modern car engines, adjust themselves far better with or without the air filter generally!! A blocked air filter simply reducing engine power generally....or even posting a FSC to be read out from the OBD codes. Is there a lawn mower with an FSC display for small gardens???

Hello Andy.

In Horace's first post he said the engine will not stop hunting/surging. I have to assume he meant the problem was happening with the air filter installed. Having adjusted a lot of B&S engine carburetors, turning the mixture screw in too far leans the mixture and the engine hunts/surges as described.

In post 27 Horace said he was able to reduce the hunting by closing the choke slightly. This implies the fuel / air mixture is too lean .

I do agree that the fuel mixture may be "slightly" leaner with a clean air filter removed, but usually adjustments on carburetors are not so precise as to have the engine hunt/surge when a clean air filter is installed.

Also my experience with B&S engines is that when the fuel mixture is too rich, they begin to misfire as spark plugs foul, unburned hydrocarbons / black smoke from the exhaust, rpm's/power decrease, but they do not hunt/surge as Horace describes.

Modern car engines i am familiar with have electronic ( Computer controlled ) fuel injection. Plugged air filters still affect performance and do not always set a code. MAP and MAF systems will be affected differently. I am not familiar with fuel injection systems on lawn mowers.

PS: to Horace; I do have a trick i use on lean carburetors. I raise the rpm's high and then hold my hand over the air inlet untill the engine stalls. Vacuum from the engine will sometimes pull the crud out of the carburetor passages.

Hope this helps.

I liked your post.

I also find the infos (nobody is too blame by the way!) confusing....I went through the same thoughts as you did....

Over the years, I have "rescued" several mowers that were set up too rich and just setting the mixture better improved starting. reduced wear and tear on plugs and generally made the mower nicer to use.

I have to admit that this modern mower with its setup, is outside of my limited experience.....but a really interesting problem.

I got given a brand new (2 years old, but has never run!) motor mower a few weeks ago, because it won't run, but I am waiting for better weather before starting on it!

It appears to have missing hardware controlling choke and throttle at a quick glance!! Not the problems we have here....

Some limited success. I tried a 'stronger' spring on the governor. It did not stop hunting - it just speeded up the frequency the hunting cycle. That idea bit the dust.

The drills I ordered on ebay turned up today but they were all too small. So I resorted the the tapered darning needle suggestion to 'measure' the jet - and found I had a drill already.

So sod the torpedoes, I drilled out the jet. It seemed to cure the hunting at a certain throttle openings on no-load at various revs - that then led to misfiring/lumpy running on full-load (long grass) That to me sounded more like fuel blockage due to dirty fuel - so I will sort that out tomorrow.

And just to make life interesting, the 'bleed' screw with a temper-proof cap (if that is what it is) - broke off as I tried to undo it - but it does not seem to have any effect. More tomorrow.

On balance I seemed to have made little progress. One step forward and two steps back you might say.

Which reminds me of a story from schooldays.

Tommy was late for school. "Why are you late Tommy?" asks Miss. "It was the snow, Miss, all slipping and sliding, it was one step forward and two steps back, Miss".

"Well if that was the case and if strictly true Tommy, then how did you get here Tommy?"

"I gave up Miss. I turned round and went home Miss!"

Maybe a high altitude jet...

http://www.ebay.com/itm/like/401110213541?lpid=82&chn=ps&ul_noapp=true

Had same surging problem with brand new Briggs Stratton 125cc mower and using brand new gas. Cranked engine, then removed the top that houses pull cord, so I could see the carb controls. Sure enough, the control that I think opens and closes the butterfly in the carb, was wobbling open, then shut, then open, etc. I think the very cheap governor they use on these things to hold the rpm down tends to choke off the fuel supply as engine revs. When engine falls back to idle, valve opens and engine revs again. Just an ill-tuned piece of equipment or poor design. Anyway, I rigged a piece of coat hanger to the control rod going to bell crank (it's easily accessed from rear of engine), so now I have a manual throttle. Solved the surge problem completely.

Your solution is basically the one I may need to use for my "new" mower! (see my previous post #44)

Great idea.

I subsequently replaced my coat hanger jerry-rig with an actual throttle cable. Perhaps not surprisingly, the little hole that the spring fit in on the engine throttle connection seemed to be the exact size for the bent end of a standard throttle cable. In other words, the original design of the engine seems to have been meant to accept a manual throttle. Anyway, it runs great now...just like the old Briggs & Stratton engines did fifty years ago!