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Al

Addressing After-fire.

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Al
Hi, I notice there are many comments referring to “backfire” on shut down. The problem is after fire and is caused when the engine is turned off after running for a while. The interior of the muffler is red hot. The engine is running and the ignition is turned off. No spark, and the engine winds down. Each time it intakes fuel, the mix is exhausted without being ignited. This fuel goes into the muffler and after a couple of seconds explodes. It causes no harm to the engine, but an older muffler may rupture. A common term for it is “BSS”, (Brown Seat Syndrome) because it usually occurs as you are throwing your leg over the steering wheel getting off the tractor. Idling the engine for 45 to 60 seconds and lowering the idle speed before shutting it off may help some, but with older engines without carb. solenoids this about all that can be done. Next generation. Newer engines have an anti-after-fire solenoid added to the carburetor that shuts off the main jet when the key switch is shut off. If you shut off the engine at idle, there is no benefit of the solenoid and after-fire will continue to occur. New engine new thinking, shut the engine off with the engine running at ¾ to full throttle. (Read the manuals with the engine) Sequence: engine wide open, key off no ignition, no solenoid voltage, no gas through main jet. Engine has a long wind down. No fuel, air only to the muffler cooling the muffler down and the muffler is filled with air. Now the wind down gets in the idle jet range and the idle circuit starts to put a small amount of fuel in the cylinders and to the muffler. The muffler has cooled AND the small amount of fuel mixing with all of the air in the muffler is too lean to fire. NO AFTERFIRE. If you shut the new generation engines down by idling them you will still have after-fire. Shut them down like the mfrs recommend, wide open and the problem will be gone. Thought I would comment on this to explain what is happening. Al Eden

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Ketchamized
Al, Thanks for sharing that. It always makes me cringe when I ever have an engine backfire. What about the head gasket, though? Would it help damage the gasket? It's not much for a new one, but a pain to order another one and put it on and all that.

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BigSix
Al, Thank you for this very helpful explanation. I don't have modern stuff, but I help my neighbors, occasionally, when theirs doesn't run. Very interesting. But it raises a question: doesn't the idle circuit bleed fuel continually, to blend in with what is coming from the highspeed jet, when the engine is at speed? I mean, I could still see how your explanation of "afterfire" would be unchanged, practically speaking, if the idle speed circuit bled a small amount of fuel continually, on the "downwind." IOW, it could just be too small an amount to cause an afterfire even if it bled it's normal amount of idle fuel continuously, from 3,600 to 0, after the ignition was chopped. I want to know the answer generally, because when I am adjusting carburetors, be it on an older tractor, chainsaw, etc..., I just always assumed that if the engine would suck the idle circuit's flow in at idle rpm, it certainly would do the same at 3/4 or full throttle, you know? I mean, vacuum is vacuum, right? Or is there something I'm overlooking, that deactivates the idle flow? I just can't imagine that "too much" vacuum (when at speed) could somehow stop the idle fuel flow, when a much smaller amount of vacuum was enough to pull it into the venturi. And if there is a way that the idle fuel cuts off above idle, is that only on modern, afterfire-solenoid-equipped engines, or on all these tractors, chainsaws, etc...? Thank you--it's a very interesting topic to me. Peter

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Al
Peter, You are basically correct. It is just that at high rpms, this is an insignificant amount. The fuel is bled in at the edge of the throttle plate when the plate is nearly closed. When the engine is shut off at high speed, the governor pulls the plate wide open If shut off at low speed this doesn't happen and dump the air in. This reduces the vacuum. In the idle circuit the air speed is increased at the side of the venturi, where the throttle plate is nearly closed, leaving the space between the edge of the disk and the side of the carb body at the point the idle passages enter the carb the only high velocity area. Once the throttle plate opens, the idle circuit becomes less significant. This is why one should always adjust the main jet, then the idle, then the main, then the idle and then finish with the main jet when adjusting the carb. Changing the idle has a very small effect on the total mix when it is adjusted. Sorry I didn't make that very clear, I appologize. Al Eden

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tillerman
This is what I do to stop backfires and it may be the wrong thing to do but I usually shut it down by choking if I am ready to quit for the day or take a long break. I read this somewhere several years back. The only thing that bothers me is I sometimes choke it down and then it runs the battery down because I forget to turn the key off or turn off the switch. Thanks Al for this post. Richard

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DMedal
Al- Thanks for a really nice tech tip. Question? Am I understanding right that afterfire is a single pop/BANG after shut off vs what I'd call dieseling or the attempt of the engine to attempt run briefly without spark? Two very different symptoms. Now I have to remember which tractor I'm running at the time of shut down, as you describe my newer engine (Intek) will do a real good BANG if shut down at idle once in a while. Wow. When I worked for Caddy I had a customer call about 1/2 mile from the dealership. She'd attempted to drive in with the ignition off (to prove it did it, I guess) and it finally died. Or maybe she just wasn't able to shut it off. Those big block V8s which never ran much above idle could really diesel ugly if not kept in tune. -Don

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Mike
I always let my 16hp kohler idle for at least 5 minutes, before I shut it down, and have no problems. It will buck and snort if I shut it right down after use without letting it cool down. Mike

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olcowhand
I don't think it's as good for the valves for engine to be shut down full hot. The idling helps valves cool a bit before cooler air can get to them after engine is shut down. Probably insignificant, but doesn't matter in my case anyway as all my tractors are not equipped with the solenoids. I always let them idle a tad less than a minute & almost never have a backfire. The backfire won't hurt a head gasket as even if the exhaust valve was open, with the open ended muffler there wouldn't be enough pressure to blow the gasket or harm anything except the muffler...& the cat lying in the shed! :D

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BigSix
Al, Don't apologize, please! You opened my eyes to this whole after-fire issue. And just now, I think you've taught me something else--I hadn't considered that the governor would pull open the throttle plate, when the ignition is chopped at high RPM, as you indicated. But I guess it would, because as the rpm dropped below what the throttle setting is calling for, the governor would attempt to compensate, right? Hadn't considered that.... So...just so I understand, once the engine is at or above idle rpm, the idle circuit's flow is constant, and becomes a (small) part of the total fuel flow at hi speed. In other words, that circuit doesn't "shut down," it just becomes insiginificant at higher speeds (given the larger amount of air ingested), and is (generally) insufficient to cause the "after-fire" ignition that is being complained of. Which is why the WOT shutdown procedure you explained will eliminate the after-fire condition. I appreciate knowing the proper sequence to use, when adjusting carbs. I never gave much thought to it, but I always adjusted the idle at idle speed and the hi speed at WOT. I can't remember if I've ever tweaked the idle mix after setting the hi speed, but I may have...but only at an idle speed. :I (In addition to idle and main adjustments, I find an additional variable to be dealt with is that changing the idle mixture often requires readjusting the idle speed setting.) I'll follow your procedure next time and perhaps it will speed up my tuning. :) As I said, I appreciate learning the proper sequence you described, for carb tuning:
quote:
Once the throttle plate opens, the idle circuit becomes less significant. This is why one should always adjust the main jet, then the idle, then the main, then the idle and then finish with the main jet when adjusting the carb. Changing the idle has a very small effect on the total mix when it is adjusted.
I read the above with the understanding that one is adjusting the main jet at WOT rpm, and adjusting the idle at an idle rpm, obviously. This makes sense to me because, if I understand you, your sequence eliminates the idle circuit as a variable when setting the hi speed mixture, right? Under your procedure, one sets the idle mix, and it "is what it is," i.e., it becomes a constant, and the last variable is the hi speed mixture, right? If one did it in the reverse, one would be, essentially, "fine tuning" the hi speed mix with the idle speed mix, a sort of "tail wagging the dog" scenario? I can see that this would not be good because the idle speed circuit does not have the ability to flow the larger amount of fuel necessary at higher engine speeds. (Plus, it would likely cause an incorrect idle mixture, because it wasn't being set under "idle" conditions.) So you want to set it first so it becomes the "baseline" or a "known quantity," as you determine the hi speed setting--is that the reason for the adjustment sequence you laid out? I always learn so much from you! Thanks again, Peter

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skunkhome
At our school we have a couple of 13hp Honda powered pressure washers. We found that if the ignition is turned off at any speed other than a idle it will backfire every time. I have instructed all My operators to bring the machines to an idle before cutting them off. The Hondas also start better at an idle than with an advanced throttle.

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firefoxz1
It was always my understanding and from looking at diagrams (cut aways) that in most carbs the idle circuit fuel flow actually feeds through the main jet first (even in cars). All fuel flows through the main jet to exit the fuel bowl for use so shutting the fuel off at the main jet with a solenoid also shut fuel off to the idle circuit. This is the reason to adjust the WOT, main, mixture first and then the idle mixture. One exception I can think of off the top of my head is the Briggs vertical opposed twin carb which actually has a small feed passage for the idle circuit straight from the bowl.

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andrewk
quote:
Originally posted by skunkhome
At our school we have a couple of 13hp Honda powered pressure washers. We found that if the ignition is turned off at any speed other than a idle it will backfire every time. I have instructed all My operators to bring the machines to an idle before cutting them off. The Hondas also start better at an idle than with an advanced throttle.
I don't think that that pressure washer has an after-fire solenoid unless it has electric start, in which case the "old technology" would apply. The idle startup is probably more of a pressure washer thing than a Honda thing- If the engine starts at mid throttle, it will start producing pump pressure before the engine is in the powerband, leading to chugging and stalling, or at least thats the only thing I can think of to explain it- I have seen briggs powered pressure washers do the same thing-

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Al
Hi, I believe the idle circuit comes out of the float bowl and is routed to a tiny passage just ahead of the throttle plate where it is closed. When the throttle plate all but shuts the incoming air off. Since there is for all practically no flow through the venturi and the main nozzle or emulsion tube, the only high velocity air where there is a pressure differential is at the edge of the bore. The engine side is the low pressure side and causes the fuel to be drawn in through the little passage right at the point the throttle plate is almost touching the body. In our full throttle shut down, the governor is still holding the plate open and the amount of fuel from the idle circuit is nil. If the engine is shut off with the engine idling the throttle remains closed. Shutting the main jet off has very little effect because the idle circuit is still functional. Wide open for all practical purposes disables the idle circuit. Al Eden

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