OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs and Stratton Running Rich

Hi, I just joined after finding a simple answer to a question I was unable to get answers for from anywhere, this site is priceless. I now need to find a way of sorting out what appears to be over rich running. The model is a 281707 12HP 1990 Briggs on a ride on. It is unmodified. When running it tends to puff dark sooty exhaust gas and also hunts a lot under load until it settles down. I removed the existing spark plug and cleaned and adjusted the gap. It is difficult to start but once running tends to cut out or alternatively over revs on idle when hot. Also hard to restart when hot. When I re-examined the plug it was very sooty after just a few minutes running. As far as I can see the choke is not set to cold start although the linkage (part 216) is not fixed. I am about to fix it having found the answer on here but I am certain that this is not the problem as the choke is not in use. I think it is running too rich but any suggestions on what is going on and how to fix would be great. Thanks in advance.

Hi Tank and welcome to the forum.
If you could post the type and code numbers as well i'm sure Grumpy will jump in tomorrow morning with the illustrated parts list and owners manual. From there we can determine the type of carby it has and possible fixes to your problem. A photo of the carby and associated linkages will go a long way in helping identify the issues you have.
Also while you're at it do you have the same symptoms with the air filter removed? Sometimes a blocked/dirty air filter will contribute to the symptoms you have described.

Tank, as bigted said, we need the Type as well as the Model to point you toward the Illustrated Parts List and Operator's Manual. Meanwhile as far as I know, all 281707 engines use the large one-piece flo-jet carburetor. The symptoms you have described sound unusual, but as bigted said, the first step is to remove the air cleaner and see if that makes a difference. If it does, the air cleaner is clogged and needs to be cleaned.

The large one-piece flo-jet is about as reliable a carburetor as can be found in the world: the nearest thing to a characteristic fault is that after about 20 years' running they sometimes have suffered some wear to the needle and seat, which then require replacement. You can read about this in some detail in the following thread:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=15306

You will notice that the symptoms Nathan reported are a bit different from yours, depending on how we interpret your reference to hunting, which normally is caused by lean mixture, whereas most of your symptoms sound like rich mixture. There are simple faults which can cause mixture to be alternately rich and lean, in particular a sticking float lever in the carburetor float bowl under the needle. However, please post the engine's Type, and also report whether the carburetor floods when left for 10 minutes or so with the fuel turned on (see the other thread referenced above, for details of what flooding is, and how to detect it).

This problem should be simple to find and to cure. We just need to all be aware of the detailed symptoms, and background facts like whether the machine has been left parked with the fuel turned on. (Incidentally, never do that. Sooner or later the carburetor will flood, resulting in a large amount of petrol ending up in the engine's lubricating oil, with very unfortunate effects on longevity.)

I am very impressed with the depth of knowledge here. Thanks both Grumpy and bigted. The Type is 0206 and has the same carb as the one I originally read about with photos dealing with the choke linkage part 216 that had been modified by a previous tenant. I have managed to get the B&S owner manual and parts list from their website although the manual only extends to routine servicing. I have ordered air+fuel filters and sparkplug. Will try running without filter later today and take a photo of the carb. Thanks again.

Here is the Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6ntzHVJ1DajI.pdf

Here is the Operator's Manual:
http://bsintek.basco.com/BriggsDocumentDisplay/mssAIQ-K_ny7tH217FzoPu.pdf

Tank, I dreamed up a possible explanation of what is going on. Please test it against what your engine is doing.

Suppose the needle and seat set is leaking, so when the mower is standing idle, the float bowl slowly over-fills with fuel. This means it will start rich, even if you don't use the choke. Suppose the previous tenant was troubled by this but instead of fixing the problem by replacing the needle and seat, he adjusted the main mixture control toward lean. The outcome would be, the engine would still start rich, and would remain rich for perhaps several minutes while the float level slowly lowered itself until the needle and seat began to operate correctly. The problem, however, would be that he'd adjusted the mixture to be lean, so it would hunt after this several minutes had passed.

There is a simple way to test whether this is what has happened. After running for several minutes, so the engine begins to hunt, turn the main mixture control (Item 4 in the illustration below) anticlockwise when viewed from below (that is, screw it out) a fraction of a turn and see if the hunting is eliminated. Turn it back and forth until you have it running as well as possible, and see if it is then running properly.




Of course after it has been standing with the fuel turned on, it will start, and initially run, richer than ever, and this will have to be dealt with by servicing the carburetor.

The full details on overhauling all of the B&S side valve single cylinder engines made after 1984 can be found in B&S Part Number 270962, Single Cylinder L Head Tech Manual. If you can't find a free download anywhere on the internet, that and all of the other Briggs tech manuals can be downloaded from Outdoorking, but an annual subscription is required. Alternatively you can just follow the process in the thread I referred you to previously. You may notice that 168,000 people have already referred to that thread, which may give you some idea how common worn needle-and seat is in very old 28 cubic inch Briggs engines.

Many thanks grumpy. I will be doing all of this over the next few days between other farm work and will surely get back here to let you know how it goes. Fantastic service!

Thanks Tank, coming back and telling us the outcome would be appreciated. A thread without a validation of the diagnosis isn't very useful to others who have the same problem in the future. And of course if there are still issues with the engine, we are here to help: just continue the thread.

Hi Grumpy. Followed all your advice and also stripped down the fuel bowl to the carb and checked the needle per the earlier thread. I found through vibration (I guess) that the needle had ground a very shallow indentation in the soft alloy seating so I tweaked the seating very slightly by lifting it with a small screwdriver and after putting it back together made sure it operated correctly before fixing the fuel bowl. You were correct in saying the bowl would be full - it was when I removed the carb. Replaced air filter, fuel filter and spark plug and hey presto it fired up and runs pretty well. Also found how the choke linkage works (per your site) and fixed that too. I have replaced all the belts and again thanks for the advice on how the belt comes off of the gearbox pulley (another thread) bloody fiddly but it is a good feeling to have got it all done.

There is a but however. Starting and tickover is fine, increasing revs generally ok but the engine sometimes labours as if the choke was pulled on and slows before recovering. I noticed this was happening when I engaged the cutter blades before doing all this work. After a short few revolutions the engine picks up again and runs sweet. Is this a problem? or is it they way the thing works? I haven't touched the carb tuning so wonder if it needs tweeking? I looked for a tip on tuning but can't find it if it is there as it would mean reading through many pages of non related advice because some headings are like mine starting out with one problem but ending up with a heap of good and relevant help from you guys.

Thanks once again and please let me know if there is anything I can do about the engine revs. Cheers Tank

Tank, you have committed a misdeed by setting the float bowl seat slightly lower, rather than replacing it. By doing that you lowered the float height, and you also did not properly fix the flooding problem, because the seat is still worn out. You also do not seem to have replaced the needle, which you said has a groove in it. Most likely the issues you now have, stem from that, combined with the fact that you have not yet tuned the main mixture adjustment.

You cannot expect the engine to run perfectly when it now has a low float height, an unadjusted main mixture control, and a slight tendency to flood because the needle and seat still leak. What you have sounds like the same faults you had in the first place, mildly alleviated because moving the seat may have slowed the leakage slightly.

I suggest you replace the needle and seat, then check the float height. When the flooding has stopped and the float height is correct, it is the work of a moment to adjust the mixture.

Grumpy, knuckles wrapped I have ordered the parts and will look forward to getting it right this time.

Sorry if I was a bit heavy-handed there Tank, I was confident you could handle it and I did want to make the point before you lost confidence in the repair process. Note there are two types of seat used over time on that carburetor: a soft one that you pull out with a small hook, and a hard brass one that you have to tap a thread in and then pull out with a screw. Since you moved the one you have easily, it sounds like it is the soft one.

Hi Grumpy

Long time!

I just got a birthday card from you guys which prompted me to thank you for the help. I did all the repairs you recommended and hey presto the mower has worked all season without a hitch - the engine now runs sweet as a nut.

Many thanks

Hi Tank,

I'm very glad to hear that you've gone through the process, and fixed the faults on your own. Most likely you will find a whole range of carburetor repairs relatively easy to make in future.

My own recollection is that the more things you learn how to fix, the more confidently, and enjoyably, you approach your whole relationship with the machine. It is never fun to operate something without believing that you understand it, that you receive its distress signals, and that you are pretty sure you can respond to them effectively without causing additional harm.

Nobody goes from beginner to pro in an instant, and we will be here to help if you ever call on us. Hopefully knowing you can get assistance is one of the factors that will make you comfortable to use that machine effectively, and enjoy doing it.