Predator 9500 Generator backfires like a shotgun on shutdown

[Diesel_Dude08]

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!

 

[Its Me]

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!

I would say that the engine valves (exhaust) are very hot that is where the gas explodes, make you a short wire or even string that you can slow it down just enough speed to keep it running but much lower RPM's that it was running, disconnect the power cords, this will get it time to cool down fast, usually 30 seconds to a minute and you won't get the back fire which is spent gas.

 

[ILENGINE]

I suspect that part of the issue is the afterfire solenoid on the bottom of the carb isn't working correctly. Either a solenoid issue or the control circuit that actuates it isn't working correctly. On generators, the afterfire solenoid is normally open and then is closed by the generator output on shutdown. Could also be something as simple or complicated as a muffler issue.

 

[Diesel_Dude08]

I suspect that part of the issue is the afterfire solenoid on the bottom of the carb isn't working correctly. Either a solenoid issue or the control circuit that actuates it isn't working correctly. On generators, the afterfire solenoid is normally open and then is closed by the generator output on shutdown. Could also be something as simple or complicated as a muffler issue.

interesting... I forgot that carburetor had that on it. I know it wasn't seized at least because I remember moving the plunger up and down with my finger when I rebuilt that carburetor. However, is testing it as simple as pulling out from the carb and turning the power switch on/off and see if it moves up and down? What could be wrong with the muffler to cause that backfiring to happen? (the 9500 does not have a spark arrestor) Thank you for the suggestion!

 

[Diesel_Dude08]

I would say that the engine valves (exhaust) are very hot that is where the gas explodes, make you a short wire or even string that you can slow it down just enough speed to keep it running but much lower RPM's that it was running, disconnect the power cords, this will get it time to cool down fast, usually 30 seconds to a minute and you won't get the back fire which is spent gas.

I would agree with you about the immediate shutdown without backing down the revs to cause the backfiring... However, the customer stated that he had always shut it down that way and it only started backfiring a couple months ago... which suggests there is something else going on. Of course... I have already recommended shutting it down the proper way with the slow fuel shut-of switch/valve/dial on the front of the generator.

 

[ILENGINE]

interesting... I forgot that carburetor had that on it. I know it wasn't seized at least because I remember moving the plunger up and down with my finger when I rebuilt that carburetor. However, is testing it as simple as pulling out from the carb and turning the power switch on/off and see if it moves up and down? What could be wrong with the muffler to cause that backfiring to happen? (the 9500 does not have a spark arrestor) Thank you for the suggestion!

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.

 

[Diesel_Dude08]

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.

Oh I gotcha.. I was thinking it was ran off battery power like a normal system. Unfortunately its a pain to put multimeter leads on it because the connection is sealed all the way up into the front of the generator which is sealed off with glued foam... fun times lol. I will try to peel back that foam and see if i cant test the voltage going to it when I shut it down. Seems like it could be the culprit. Thank you for your help!

 

[Diesel_Dude08]

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.

ok so I traced the wire wrong and the cable going to the solenoid actually goes over by the starter area up against the generator end. I hooked up my power probe even though I didn't know if it was supposed to be a 12v connection or not. and as you can see in the video both of the lower connectors are reading ground with the switch off... that didn't make any sense to me. When i would flip the switch on the generator to the ON position... the left lower cable read ground and the right lower cable didn't read anything. I then applied voltage to the top right cable leading to the solenoid and I didn't hear it click open or closed. Did I test this properly and the solenoid is shot or am I not testing this correctly? I cant really test what voltage is going to the solenoid because of the sealed connectors. Im 16 and still learning electrical... not sure if it was tested properly?

 

[StarTech]

It is best to remove the solenoid for testing, this way you see it operate.

As IL these operate right the opposite of the other solenoids. As you shut down the generator it applies 12v which extends the pin. It is unlike a mower engine version which 12V is applied while the engine is running to retract the pin.

As equipment sits used for long periods these pins gets gummed with old fuel deposits and can't move as intended. Many can be cleaned to restore operate via ultrasonics. But those that have rusted due to water in the fuel has to be replaced. They also fail electrically by opening up or shorting.

Now if someone in the past has replaced the solenoid they may have simply installed the wrong version not knowing how these operate.

 

[Tiger Small Engine]

It is best to remove the solenoid for testing, this way you see it operate.

As IL these operate right the opposite of the other solenoids. As you shut down the generator it applies 12v which extends the pin. It is unlike a mower engine version which 12V is applied while the engine is running to retract the pin.

As equipment sits used for long periods these pins gets gummed with old fuel deposits and can't move as intended. Many can be cleaned to restore operate via ultrasonics. But those that have rusted due to water in the fuel has to be replaced. They also fail electrically by opening up or shorting.

Now if someone in the past has replaced the solenoid they may have simply installed the wrong version not knowing how these operate.

I remember being at the Green Industry Equipment Exposition in Louisville, KY about 4 years ago and asking a guy selling pull start generators with no battery about the solenoid on the carburetor. He didn’t have an answer and I didn’t understand it either. Because these solenoids are the complete opposite of normal ones on how they operate, it makes sense. Pintle on solenoid is normally open, and then goes in to shut off fuel to main jet upon shut down. It is often amazing to me how simple some things are once you understand how they work. Clean the solenoid with carburetor cleaner and hit it with air compressor air gun and it should unstick and start working again.