Predator 9500 Generator backfires like a shotgun on shutdown

[StarTech]

Tiger, That just should how well informed the salespersons are not. It like when you call tech support and they can't the simple should be known things.

I feel like I am dealing with idiots at times when I ask for help of these support personnel. It like the other day when I needed the PTO end size of a new engine's crankshaft, all they keep giving me was the bolt size.

 

[Tiger Small Engine]

Tiger, That just should how well informed the salespersons are not. It like when you call tech support and they can't the simple should be known things.

I feel like I am dealing with idiots at times when I ask for help of these support personnel. It like the other day when I needed the PTO end size of a new engine's crankshaft, all they keep giving me was the bolt size.

Initially, going to this trade show was mainly about gathering, listening, and learning as much as I could from other people about mechanical and technical knowledge on small engines and outdoor power equipment in general. What I quickly found out was that most people do not know much more than the basics. This year when I go to the trade show it is mostly about seeing new products, talking with people, asking questions, but not so much emphasis on learning specifics. It has really improved my confidence by realizing just how far I have come in the 7 years I have running a small engine shop.

 

[kjonxx]

Hello, I have been working on a predator 9500 inverter generator for a family friend. He requested a full service along with a few small repairs. Customer was telling me that generator has been neglected, and oil hadn't been changed in a long time. The customer runs a concession business and likely uses generator all day. Generator had 1,670 hours on it. Customer also stated that the generator has been backfiring on shutdown. He said that it started doing it a couple months ago intermittently and now it backfires every shutdown. Customer also stated that pull cord had been broken by some kids playing with the generator. (not sure of when) I asked the customer how he had been shutting the generator down and he said he had been flipping the switch rather than turning the fuel shut off dial like you are supposed to. ( Manual claims the shutoff switch is for emergency shut off and you should turn the fuel shutoff dial on front of the generator to shut it off instead. I figured this was the issue, since you are shutting an engine down from decent rpms to nothing but my understanding is that the customer has been using the switch the whole time he has owned it and he claims it only started backfiring a few months ago.

I went on with service as usual, oil change, full carb teardown and cleaning, new air filter, spark plug (gapped), and I tore apart the front of the generator to gain access to the crank so I could spin the engine over to TDC for a valve lash adjustment. (Exhaust valve in spec)( Intake was loose) I put everything back together and ran it (I figured it just needed a valve lash adjustment) Only ran it for about 2 minutes, flipped the switch and no backfire.

Tonight, I tested it again after it got hot ( ran for about 5-10 minutes) and I flipped the switch.... it slowed down and just as the engine came to a stop BOOM!! slight flame shot out as well.

I was wondering if anyone can give me suggestions on diagnosing the issue or what might be causing this? Is it possible that the kids that broke the pull cord pulled it while it was running and the engagement of the recoil on the running engine caused a sheared flywheel key? Would a sheared flywheel key cause a backfire after engine has basically came to a stop? What else are possible causes? Bent valve? Engine does have almost 1,700 hours on it. Could it be blow-by that is causing excess air to move past the piston rings and into the exhaust?

Hoping I don't have to dig in too deeply, I probably have almost 5 hours of work into this unit between tearing the front of it apart, sizing out 0-rings and wheel bearings, and replacing everything. I only charged the guy $40 in labor but I don't want to charge him too much more.

If anybody has any ideas or suggestions.. all help is appreciated!

Over time carbon builds up and that is igniting the gas on shut down. Idle it then shut down should eliminate most of the problem.

 

[bentrim]

The afterfire solenoid is used to prevent fuel from entering the muffler on shutdown. The reason it is used is the exact reason you are having. Don't forget as the unit ages deposits can build up in the muffler and as the engine slows is is still pulling fuel thru the engine, without the solenoid functioning properly. As the fuel passes thru the muffler it mixes with oxygen and gets set off by something hot within the muffler --- so BANG ! As you said the operator is supposed to turn the gas off to shut off the generator that way there is no unburned fuel entering the muffler so no bang. Some generators that I have worked with, especially pull start units, have an antifire solenoid that is open except on shut down. It closes until the generator stops then reopens.
The best way to "fix" the problem is for the operator to shut down as recommended, without him doing that you can replace the muffler to possibly eliminate any hot spots inside the muffler -- but it will start banging after a period of time due to buildup within the muffler.

 

[Alan46]

The afterfire solenoid is used to prevent fuel from entering the muffler on shutdown. The reason it is used is the exact reason you are having. Don't forget as the unit ages deposits can build up in the muffler and as the engine slows is is still pulling fuel thru the engine, without the solenoid functioning properly. As the fuel passes thru the muffler it mixes with oxygen and gets set off by something hot within the muffler --- so BANG ! As you said the operator is supposed to turn the gas off to shut off the generator that way there is no unburned fuel entering the muffler so no bang. Some generators that I have worked with, especially pull start units, have an antifire solenoid that is open except on shut down. It closes until the generator stops then reopens.
The best way to "fix" the problem is for the operator to shut down as recommended, without him doing that you can replace the muffler to possibly eliminate any hot spots inside the muffler -- but it will start banging after a period of time due to buildup within the muffler.

Well said!

 

[Old_Paint]

The solenoid gets is power to block the carb jet with the exciter voltage when the switch is turned off. There is no power supplied by the battery or the charge system to close the valve. So the engine has to be running for it to work, which can't be done with it removed from the carb. Maybe able to apply voltage to the solenoid to see if it works, but I don't know what the input voltage is. Could be 6 volts and a 12 volt battery could burn out the coil. Also would need to test voltage coming to the power wire.

Maybe able to put a meter in series with the power wire for the solenoid and watch the voltage on shutdown.

If you put the meter in series, you better be looking for current, not voltage. A parallel connection(solenoid terminal to ground) is needed for voltage if you also expect the solenoid to operate.

Backfire is usually caused by a rich exhaust mix in a hot exhaust pipe or muffler when fresh air gets sucked in backwards by a decrease in air flow (exhaust) out of the engine. I get a little pop once in a while if I shut down my B&S 22HP with the throttle wide open and don’t cool down a bit after I finish mowing. Normally I drop to idle for a few minutes, then shut it off. All I get then is the “oop, oop, oop” from the compression relief sucking air through the carb. I just like to cool down my air-cooled engines a bit before shutting down. A hard learned lesson from owning an air cooled engine for the better part of 40 years. Cool down at idle solves and prevents a lot of problems.

The easiest and quickest test for the solenoid is to simply unplug it with a hot engine running. If the engine stops and pops, you’ve found the problem. If it doesn’t stop you still found the problem. Likely a stuck or fouled needle valve on the solenoid. Thoroughly clean the carb and the needle/seat of the shutoff solenoid and try again. Honestly, a little pop from the muffler really isn’t a big deal as long as there’s no dry combustible material nearby and the spark arrestor is intact. It may well be the engine is running a little richer from reduced compression caused by normal aging and wear. Just learn to expect it if you shut it down at full throttle. Best bet is to drop to idle for 5-10 minutes before shutting down. See how that goes. I suspect the backfiring has been happening since he started shutting it down at full speed. Maybe not as loud at first, but if the muffler baffling is less effective, then it’s going to make more noise

 

[ILENGINE]

If you put the meter in series, you better be looking for current, not voltage. A parallel connection(solenoid terminal to ground) is needed for voltage if you also expect the solenoid to operate.

I want the positive lead connected between the power side output from the generator and the negative lead connected to the other side of that connector going to the solenoid to check voltage supplied from the windings to the solenoid both for checking for output and that the solenoid isn't open which is series not parellel. generator solenoids have 2 wires that connect to the generator wiring and I suspect is powered the entire time the generator is running but is ground side switched at shutdown.

Could disconnect both leads and put the meter in place of the solenoid and check voltage. I would be worried about trying to check amps without knowing what the possible amps are since most meters are only protected for 10 amp fused.

Backfire is usually caused by a rich exhaust mix in a hot exhaust pipe or muffler when fresh air gets sucked in backwards by a decrease in air flow (exhaust) out of the engine. I get a little pop once in a while if I shut down my B&S 22HP with the throttle wide open and don’t cool down a bit after I finish mowing. Normally I drop to idle for a few minutes, then shut it off. All I get then is the “oop, oop, oop” from the compression relief sucking air through the carb. I just like to cool down my air-cooled engines a bit before shutting down. A hard learned lesson from owning an air cooled engine for the better part of 40 years. Cool down at idle solves and prevents a lot of problems.

Doesn't work like that for generator since there is no throttle control and most don't even have idle down setups. So they are fixed speed and shutdown at full throttle.

The easiest and quickest test for the solenoid is to simply unplug it with a hot engine running. If the engine stops and pops, you’ve found the problem. If it doesn’t stop you still found the problem. Likely a stuck or fouled needle valve on the solenoid. Thoroughly clean the carb and the needle/seat of the shutoff solenoid and try again. Honestly, a little pop from the muffler really isn’t a big deal as long as there’s no dry combustible material nearby and the spark arrestor is intact. It may well be the engine is running a little richer from reduced compression caused by normal aging and wear. Just learn to expect it if you shut it down at full throttle. Best bet is to drop to idle for 5-10 minutes before shutting down. See how that goes. I suspect the backfiring has been happening since he started shutting it down at full speed. Maybe not as loud at first, but if the muffler baffling is less effective, then it’s going to make more noise

Can't do that with generator solenoids because they are normally open and are closed on shutdown, whereas rider fuel solenoids are notmally closed and use battery voltage to hold them open while operating.

 

[S.A.E.]

Hello, I have been working on a predator 9500 inverter generator for a family friend. He requested a full service along with a few small repairs. Customer was telling me that generator has been neglected, and oil hadn't been changed in a long time. The customer runs a concession business and likely uses generator all day. Generator had 1,670 hours on it. Customer also stated that the generator has been backfiring on shutdown. He said that it started doing it a couple months ago intermittently and now it backfires every shutdown. Customer also stated that pull cord had been broken by some kids playing with the generator. (not sure of when) I asked the customer how he had been shutting the generator down and he said he had been flipping the switch rather than turning the fuel shut off dial like you are supposed to. ( Manual claims the shutoff switch is for emergency shut off and you should turn the fuel shutoff dial on front of the generator to shut it off instead. I figured this was the issue, since you are shutting an engine down from decent rpms to nothing but my understanding is that the customer has been using the switch the whole time he has owned it and he claims it only started backfiring a few months ago.

I went on with service as usual, oil change, full carb teardown and cleaning, new air filter, spark plug (gapped), and I tore apart the front of the generator to gain access to the crank so I could spin the engine over to TDC for a valve lash adjustment. (Exhaust valve in spec)( Intake was loose) I put everything back together and ran it (I figured it just needed a valve lash adjustment) Only ran it for about 2 minutes, flipped the switch and no backfire.

Tonight, I tested it again after it got hot ( ran for about 5-10 minutes) and I flipped the switch.... it slowed down and just as the engine came to a stop BOOM!! slight flame shot out as well.

I was wondering if anyone can give me suggestions on diagnosing the issue or what might be causing this? Is it possible that the kids that broke the pull cord pulled it while it was running and the engagement of the recoil on the running engine caused a sheared flywheel key? Would a sheared flywheel key cause a backfire after engine has basically came to a stop? What else are possible causes? Bent valve? Engine does have almost 1,700 hours on it. Could it be blow-by that is causing excess air to move past the piston rings and into the exhaust?

Hoping I don't have to dig in too deeply, I probably have almost 5 hours of work into this unit between tearing the front of it apart, sizing out 0-rings and wheel bearings, and replacing everything. I only charged the guy $40 in labor but I don't want to charge him too much more.

If anybody has any ideas or suggestions.. all help is appreciated!

Almost ALL. new gasoline small engine product's carbs have a fuel shutoff solenoid. You should be completely familiar, but if not

.

 

[outdoorpowermike]

Turn gas valve off. Remove solenoid, jump solenoid wires with 12 volt battery. Battery can be from a riding mower, or any 12 volt system, just ground 1 wire, does not matter witch wire, then tap other jumped wire to other solenoid wire. should move about 3/8 to 1/2 inch. If no movement replace solenoid with correct one. You can look up correct part with the eng/model numbers because there are different sizes and threads. Some solenoids hav e small plastic tips on them and some don't. Also when you pull start it and engine pulls rope out of your hand check to see if cylinder has gas in it. This is another sign of a bad solenoid or flywheel key starting to brake.

 

[sgkent]

duplicate thread https://www.lawnmowerforum.com/threads/predator-9500-generator-backfiring-on-shutdown.78808/