Briggs and Stratton that is hunting and surging

StigaParkPro204wdApril 8, 2014

I have this 20hp Briggs and Stratton that is hunting and surging. Sometimes it stops and not able to start. Black smoke and black dry spark plugs. Getting worse the hotter the engine gets, and worse with heavy load. I did suspect to rich fuel mixture, and I cleaned the carburettor , adjusted the valves, new spark plugs again, and it was running fine for aprox. 0,5-1hr, then back to this rich mixture symptoms. Its now so bad that I can�t use the rider for anything . The symptoms came gradually. The choke is ok (fully open), the symptoms are the same if I remove the air filter, not possible to adjust mixture on this carburetor ( only idle ). Used a lot of sparkplugs, because the mixture is killing them. So please help me. Its a twin engine with 2 coils, so I do not believe both coils could be bad at the same time ?? Both coils give sparks, but I�m not impressed ( compared tho a car engine :)) The flywheel is rusty, the magnet on the flywheel was rusty. It�s a Stiga Park Pro 20 4wd from 2007 with a horizontal Vanguard B&S V twin. 358777-0112-E1 . Do not know the name/nr on the carburettor, but its a "twin in one" ( 102 jet for one side, 104 for the other cylinder.) the other jets are 48 for both cylinders. The engine have aprox. 350hr

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A 358777-0112-E1 is a Vertical shaft engine.

Often, when working on a carb, "manipulating" the fuel line often causes a piece of crud to break off and end up at the carb needle/seat preventing the needle from shutting off fuel flow.
I'd remove the float & needle, flush the fuel line for a couple seconds and reassemble.

    Bookmark   April 8, 2014 at 9:47AM
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Hopefully, tomplum will be able to add something about this particular carb. I seem to recall he posted something once about a nozzle assembly deforming and thereby letting copious amounts of unmetered fuel out of the carb.
But I could be way off base on what I think I recall.
Have you disassembled the carb?

    Bookmark   April 8, 2014 at 11:19AM
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Teikei carburetor. Briggs part number 791230 699709.

Sounds as if the High speed mixture screw is set to rich.

    Bookmark   April 8, 2014 at 11:45AM
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No magic bullet here. I did toss it on parts look up and it shows 2 carb choices depending on date code. Personally I would A) remove the flywheel and check the key. B) verify that the carb vent is not being restricted. C) Verify that the choke isn't wanting to flap shut as it runs up. D) Go as far as removing the kill harness from the coils and do a test run. (Provided you feel you can safely do so and are able to shut it down.) One would think that a vacuum leak at the spacer or intake mounts would create a lean condition, but give them a blast of carb spray and see if the idle kicks up. Does the exhaust sound any different then it did before?

    Bookmark   April 8, 2014 at 11:01PM
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thank you for feedback and suggestions. Yes of course, its a vertical engine, sorry :). The carburetor is not the same as Teikei 791230 699709,but similar, float housing is square, not round. As said: no way to adjust the mixture at high speed, just idling. The carburetor has been disassembled and cleaned two times with carburetor cleaner and compressed air. Tested the float/needle function . I have searched for vacum leaks with "start gas". What will removing the kill harness from the coils and do a test run tell me, is´n this just to turn off the engine? I have spark. Exhaust does not sound different, but that said, this is a problem that has gradually become worse, so it might have change over time? After a drive yesterday it has become worse. The choke does not try to flap shut when running. Now it will not start with choke on, even when cold. Before I had to have full choke to start when cold. It does start without choke. Symptoms of very rich mixture. My compression tester is faulty, but maybe I'll have to to check the engine's mechanical condition, and yes,-the flywheel ( ignition timing ). What is the compression spec on this cold?

    Bookmark   April 9, 2014 at 3:09AM
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carburettor looks like this ( but I have one extra pipe on the top, probably vent ? )

Here is a link that might be useful: carburettor

    Bookmark   April 9, 2014 at 3:22AM
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Others have addressed carburetor issues. My experience is this carburetor requires very careful assembly to eliminate all internal fuel leaks. Here is one additional idea.

If the fuel pump diaphragm has failed, fuel and fumes may be entering the crankcase. The fumes may be discharged through the breather into the carburetor inlet. The training techs at MWE shared this idea and I was able to solve an identical problem with the same engine.

    Bookmark   April 9, 2014 at 10:21AM
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***"What will removing the kill harness from the coils and do a test run tell me?"***
That was brought up due to the fact that the ignition system here (in vee twins) is configured as 2 separate magnetos...........joined together into a single kill wire circuit, for convenience when shutting down the engine.
With magnetos, you must ground one leg of the primary coil circuit in order to "kill" the spark generation.
A problem or obstacle arises when you have an engine configured with multiple cylinders and multiple magnetos.
The focal point in this is that in order to kill both magnetos with a single kill wire circuit, you gotta join the kill wires from each magneto together so that both mags are killed simultaneously when a command to stop the engine is issued. This "command" might come from the ignition key switch when the machine operator turns the key switch to OFF, or.......the command may be issued by one of the safety interlock switches or relays (depending on the make of machine).
The problem or obstacle when simply connecting the kill wires from each magneto together in the form of a "hard wire" splice is that connecting the kill wires together will cause serious and detrimental operating effects on BOTH magnetos when the engine is running.
In theory, the kill wires can be kept separated from each other by providing a separate kill wire circuit for each magneto..........and running the pair of kill wires all the way to a specially designed key switch having 2 individual kill wire terminals (1 for each mag) which remain completely separated from each other until the key switch is turned off.
Now, that in itself is a bit more complex than the single wire system that is industry standard.............but it gets worse.
Because in modern OPE, there is a safety interlock system, with multiple switches wherein each switch can issue the command to kill the engine spark.............each switch would have to have double circuits for the magneto kill function, or a master relay for magneto kill having 2 separate circuits would have to be employed. All this double wire stuff and special switches in various locations could conceivably add hundreds of dollars per unit built to the OEMs cost of manufacture, and that cost would be passed on to the consumer.
To overcome this obstacle, Briggs overcame the problem by connecting the kill wires together with 2 diodes, so that the separate magnetos stay separated electrically from each other during normal operation (and therefore do not interfere with each other's spark making business).
In this manner, the kill wire function can be controlled by the same simple type of switches now in use.

Now, for why this is something to be concerned about if you own one of these Briggs engines.
Occasionally, one of the diodes can become defective.
If becomes defective OPEN, there may not be any noticeable difference in how the engine runs, but when trying to stop the engine, one cylinder may continue to have spark. If this is on an engine that has a fuel solenoid, you might not even notice that you have a cylinder still getting spark, because the fuel solenoid stops main jet fuel when the key is turned off, and the pilot jet fuel is not enough to keep the "live cylinder" running against the dead cylinder load.
But, if one of the diodes becomes SHORTED, or any sort of continuity in both directions develops, there will be some problems in how the 2 cylinders run.
Tomplum's suggestion of isolating the magnetos from their kill wires is a way to determine if there is a problem of 1 cylinder being dead.
But to separate the mags from their kill wires properly, you must disconnect each wire from the magneto itself, and that means taking off the flywheel cover. If you remove the flywheel cover, put it back on before running the engine or you will overheat it because with the cover off, there is no air flow across the cylinders.
You will also have to choke the engine to a stop if running with the kill wires removed.

This post was edited by mownie on Wed, Apr 9, 14 at 13:21

    Bookmark   April 9, 2014 at 11:05AM
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