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TomF

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TomF
I had to chuckle re. the earlier thread on Onans...I myself was cursing orange paint just yesterday (what got into me?) during a difficult reassembly. Or rather I should say, I was cursing (figuratively speaking, of course) our TOTALLY IRRATIONAL affection for anything painted Persian or Corporate Orange. Back to Onans...I have two brothers-in-law, one of which runs a portable sawmill with an Onan powering it. He can't get it to stop backfiring upon sudden throttle-down. The other bro-in-law (who owns a nursery) said, "We had an Onan as well on a piece of equipment, and NOBODY could get it to NOT backfire on throttle-down. Being an engineer, I find that difficult to accept. Does anyone have any suggestions on how to remedy this apparent flaw?? Thanks! Tom Feldpausch

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HubbardRA
I have a B/S Vanguard and a 12 Hp Kohler that also do that on sudden throttle cutback. Let me know if you have an answer. I know in an automobile it is usually a combination of fuel mixture and ignition timing. I had a Ford that shot a 3 foot flame from the exhaust if you let off suddenly. It was a worn out pivot on the plate controlled by the vacuum advance. Fixed that, no problem, didn't change anything else. Rod H.

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JeffG
I don't usually get the backfire when throttling down. However, when I shut the ignition off immediately after throttling down - then it will backfire everytime. I found the secret is to slowly throttle down to idle, allow it to idle for 15-20 seconds and then turn it off. Jeff

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Rossco
I follow the same procedure on my K301. I think if you throttle it down too quickly and shut it off with a hot engine, there must be too much raw fuel left in the combustion chamber. Just a hunch.

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Dutch
Start by reading the operator's manual for the specific engine application. My Kohler CH18 was backfiring until Jeff Nemes straightened me out. I was idling down just like I do with the K-series and B&S flatheads. Never bothered to read CH18 manual because I "knew" how to do basic stuff. Well, I was wrong. When all else fails, read the manual.......... Thanks Jeff.

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Al
Hi, Afterfire also known as BSS, Brown Seat Syndrom, results when the muffler is very hot inside and the ignition is cut. The engine turns over several times and the unburned gas from no spark goes in the muffler, vaporizes and BANG. Used to be, slow idle to cool the muffler down and key off would be OK. Now mufflers are quieter and more baffles inside, more hot metal. Many new engines now have AAF solenoids [Anti After Fire] These shut off the main jet when the ignition is shut off. Problem: Idle circuit still lets fuel into the engine even with the main jet shut off. Same problem. Kohler solution open the engine to 3/4 throttle or more. Carb is on the High Speed curcuit and the solenoid shuts it off. No fuel to the cylinders and the engine has a long wind down. This sends a lot of cool air [throttle open] into the cylinders and through the muffler, cooling it down. With the throttle plate open the idle circuit never gets into play. By the time engine winds down the muffler is cooled internally. No afterfire. Without the solenoid, low slow cool down idle is the answer. Good luck, Al Eden

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Woodydel
A lean mixture could be creating hot spots. I think most people would suspect too much fuel as the cause but maybe it's the opposite of that. I will not believe that Onan would run their engines with backfire built in. When I use generators with automatic throttle up upon demand there are many fast throttle down situations created all day without backfire. I also remember in the early days of emissions controls on automotive engines that they had a solenoid which slowed down the closing of the throttle. This was pre-computer control days. Chevy had such a throttle control and may be adaptable to the Onan. I'm out working but I'm sure I have shop manuals with that throttle control shown somewhere.....If I remember right it worked off the engine vacuum. Under full throttle the spring loaded plunger would extend and when you backed off suddenly on the throttle the higher vacuum would slowly retract the plunger allowing the throttle to close less abruptly...Woody

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Woodydel
Roy, you are right about the dashpot. I remember them on many cars and trucks. Whether one can be adapted or not is another story but I'm sure that would work. An idle stop solenoid could also be used to fully close the throttle when the ignition is turned off. The solenoid was/is used to prevent dieseling and run on. Tom, could try out the solenoid idea by backing out the throttle idle screw all the way so the throttle could close completely. Slap the throttle shut and turn off the ignition at the same time. If it doesn't backfire the solenoid would work. Turning on the ignition switch activates the solenoid. You set the idle with the solenoid activated (extended). When the ignition switch is off the solenoid retracts fully closing the throttle starving the engine of air flow...I still think a lean engine will backfire. Anytime my Briggs engines had a gummed up carb the engine had a tendency to backfire..Woody

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