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Hello this is kind of the sequel to another post I made about a week ago regarding a predator 9500 generator backfiring on shutdown. I have been working on the unit for a family friend. He asked for a full service and a few repairs. One of his complaints was that the generator backfired on shutdown when flipping the shutoff switch. On these generators there are 2 ways to shut them down.. 1:Turn the fuel valve on the front of the generator (proper way according to the manual. 2: Flip the electric shutoff switch (manual states emergency use only.) The customer claimed he had always shut it down using the electrical shutoff switch instead of doing it the proper way. He claimed it started backfiring occasionally on shutdown a couple months ago and now it backfires every time he shuts it down. Generator has 1,690 hours on it and figured a valve adjustment would get back to normal. After the full service (including valve adjustment) the generator would still backfire when it had slowed completely down and was hot.

Because it backfires when the engine has practically come to a stop, and only when it has time to get warm, I assumed it was fuel related. I tore into the carburetor to clean it during the service and the float was not stuck and neither was the solenoid at the bottom of the bowl. I assumed the solenoid had failed internally and was not shutting off the fuel on shutdown like it should. In one of the videos, you can see me testing it with a 12v battery hooked up to my powerprobe. Thanks to some help from ILENGINE on my other forum, I found out that the anti-backfire solenoid runs off power from the generator on shutdown rather than battery voltage like is commonly seen on other small engine applications. In the other video, you can see that it gets plenty of A/C voltage on shutdown but doesn't necessarily continue to provide adequate voltage all the way until the engine stops. Do you guys think it's providing enough voltage until the engine stops? If not, what would be causing this?


What could potentially be the problem here?
Any further diagnostics I could use?
Any thoughts or help is appreciated
Thanks for your time!

Shut it down at half throttle
Oz

The voltage will drop as the engine comes to a stop. Nothing can be done about that. In your first video it appears that the plunger is retracting after a few seconds even with power still applied. Other than that everything looks normal.

They use to recommend removing the head and clean all the carbon off the head and piston each year . When the carbon gets heavy enough it will glow like charcoal and ignite any fuel at anytime , the backfire and shutting off the fuel overcomes the problem. Before the fuel solenoid it was a connecting rod breaker.

Rocky J said:

They use to recommend removing the head and clean all the carbon off the head and piston each year . When the carbon gets heavy enough it will glow like charcoal and ignite any fuel at anytime , the backfire and shutting off the fuel overcomes the problem.

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Rocky J said:

Before the fuel solenoid it was a connecting rod breaker.

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I put an oscilloscope down into the cylinder and the top of the piston was surprisingly clean.

Diesel_Dude08 said:

I put an oscilloscope down into the cylinder and the top of the piston was surprisingly clean.

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No way you did that. An endoscope yes but not an oscilloscope.

StarTech said:

No way you did that. An endoscope yes but not an oscilloscope.

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Whoops! that’s what I meant

Diesel_Dude08 said:

I put an oscilloscope down into the cylinder and the top of the piston was surprisingly clean.

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That is a pretty small O-scope you got there.

slomo said:

That is a pretty small O-scope you got there.

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New model

slomo said:

That is a pretty small O-scope you got there.

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That's what She said!

does the kill wire have a good ground or is it possible rust has built up so the kill wire allows some small amount of voltage to the coil. I've seen a couple videos where the cause of irregular shutdowns was rust and corrosion built up in the kill system, loose spade lug on the coil etc.. Use an ohm meter to see what kind of resistance the kill wire has. It should be around zero ohms, so if your meter zeros at say 2 ohms, then the resistance might be 2, maybe 3 ohms. Wiggle the wires too a tiny bit while checking to see if that changes anything. Also, the glowing carbon that can ignite the fumes after shutdown can be in the exhaust or near the tail pipe too, it does not have to be in the combustion chamber.

Ozcub said:

Shut it down at half throttle
Oz

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If you can manually adjust throttle, simply idle engine down for 5-10 seconds and turn off.

sgkent said:

does the kill wire have a good ground or is it possible rust has built up so the kill wire allows some small amount of voltage to the coil. I've seen a couple videos where the cause of irregular shutdowns was rust and corrosion built up in the kill system, loose spade lug on the coil etc.. Use an ohm meter to see what kind of resistance the kill wire has. It should be around zero ohms, so if your meter zeros at say 2 ohms, then the resistance might be 2, maybe 3 ohms. Wiggle the wires too a tiny bit while checking to see if that changes anything. Also, the glowing carbon that can ignite the fumes after shutdown can be in the exhaust or near the tail pipe too, it does not have to be in the combustion chamber.

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what coil are you talking about exactly? I did do a resistance test and think I got around 2 ohms

the ignition coil has a grounding wire (kill wire) attached to it so once it is turned off it has no spark. If that is intermittent due to corrosion at the coil if a spade lug, or at the kill switch then it can ignite the mixture again even though it is turned off. If the exhaust is full of unburned fuel and oxygen, the new burn will ignite it in an exhaust backfire. If the backfire is out the carb then that is a different type problem.

Diesel_Dude08 said:

I put an oscilloscope down into the cylinder and the top of the piston was surprisingly clean.

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That was probably a bore scope, not oscilloscope. Totally different animal. O-scope is for analyzing electrical signals, much like a volt-ohmmeter, but with a screen to display the waveform of the signal. Extremely handy when troubleshooting electronic devices, but about as useful as a screen door on a submarine for looking inside an engine cylinder. A bore-scope is a small camera on a steek, much like Jose Jalapeño. I’m now convinced you lack adequate expertise in electrical testing and measurements and how to use equipment that can get you hurt if not dead.

Here's your diagram, or at least a Chinesium version. It says for 7 KW, but I doubt seriously there's any difference in the wiring of a 9.5 KW. It's a pretty cheesy drawing, but now it makes what you described a little more recognizable. Still makes no sense, but I can see what you say now with this. Ignore the bits to the right of the Charging Module (Starting motor, Starting Relay, and Battery) because you have a pull start system, but the rest of it should match up.

It does indeed look like the battery charging coil under the flywheel is connected to the fuel shut-off valve in the lower left-hand corner when the engine switch is turned to the OFF position. Contacts FS (fuel shutoff) and G (comes from the charging coil [in gooderer Engrish]) are connected when the switch is off. One more thing I just noticed is that the connection to the coil (by way of the on/off switch FS/G contacts) is that it does NOT go through the charge controller for a battery. That would mean that the coil is getting an AC signal, NOT DC. Set your DVM to AC to check the signal to the Solenoid. After seeing this drawing, I would not test that solenoid with DC. Find out what the AC voltage is from that charging coil by checking from the G terminal on the engine switch to the block. Then, you could use a variac to test the solenoid. It's typically going to be something around 24V AC, but that's just a SWAG.

Ignore the bits for the starting circuits if it's a pull start, but all the rest will be identical on the engine. It's usually pretty easy to add a starter to these engines because of them all having the battery charging circuits built in above some magical horsepower limit. The difference is in the external wiring, and now, I'm guessing the way the solenoid operates. That's about as bass-ackward as it can get, but they didn't ask me when they designed it. Normally, a fuel shut-off would be a fail-safe device, but I guess not on small generators. I'll eat my crow pie about that. It was just completely unexpected to find a safety device that had to be powered up to operate. That breaks a LOT of safety standards. But, in short, there should be no connection between that solenoid and the excitation of the generator. The source is the battery charging (sic) "Coil", not the generator output or excitation.

So, my guess of what's happening as the engine/generator spins down and gets to low speed, there isn't enough voltage output (from the "Charging Recoil") to keep the solenoid energized so it's dropping back out before the engine is fully stopped. Typically, there's only one diode in that charging circuit, so you only have half- wave rectification and NO regulation to start with, so it isn't a long reach to understand why it would drop out at low speed. This may be happening even before it's going slow enough for the compression relief to do it's thing under about 500 RPM, which would usually make it back-fire through the carburetor if it was an ignition issue. That would allow raw fuel/air mix to get dumped into the engine, pumped out into a red-hot muffler, and BOOM! Not surprised at all about the back-fire.

This suggests that perhaps the charging circuit isn't functioning properly so that it provides enough power to keep the fuel shut off until the engine's slow enough to prevent the backfire. It isn't a problem with the ignition system, otherwise, the engine would simply try to rev back up or run very poorly on a lean mix. The ignition is shorted out by the contacts at IG and E. IG is for IGnition and E is for Earth (a European/Asian way of saying GROUND). With the ignition shorted out, it cannot provide spark to ignite the fuel/air mix, but high muffler temps could certainly ignite it.

Note the Idle Speed Switch near the center of the drawing. This suggests (as well as does the stepping motor) that there IS a throttle control on the machine. Most modern generators have an "Economy Mode" that lets them idle down when power is not needed. That's a PITA feature with LED lighting because they simply don't draw enough current (only a few milliamps) for the control module to detect the load and keep the engine at full throttle, so turning the Idle Control Switch OFF makes the generator run at full output capacity at all time. If you're using it out in the boonies somewhere, that economy mode might make a big difference in how long you have power available. The benefit of an inverter output generator is that it's typically taking a DC input from the generator and synthesizing AC power at proper frequency and voltage. That makes economy circuits a little more friendly to LED lighting and lets them use a lot less fuel by idling down under light or no load conditions. Make sure nothing is plugged into the sockets, turn that switch to the ON position, and after a preset time, the engine should slow to idle. Let it sit in idle for a few minutes to cool down a bit, then turn it off and see what happens. If it still backfires, and you've tested the solenoid (with a 12V battery as suggested, but a 6V lantern battery may do the trick), odds are the charging circuit has probably spat the dummy. You can check for voltage at the fuse (straight below the Oil Level indicator in the diagram). Just put the common lead anywhere on the engine block and the positive lead on the 'hot' side of the fuse to see if the charging circuit is actually working at all. If it is, it may still be weak.

Here's a proposed test. Disconnect the solenoid, which apparently isn't really used except for shut-down. Start the generator, get it hot (plug some stuff in), Then stop it. It will likely go BANG when it stops from full speed. Start it again, get it hot, then, unplug all the loads with the Idle Switch set to ON and wait for it to slow down. It REALLY should slow down, but I have no idea how long the timer takes. If it slows to idle, let it do so for 5 minutes or so to allow much cooler exhaust to cool down the muffler. Then shut it off and see if it goes bang. I kinda think it won't because the muffler will be cooler and the compression relief should open pretty quick which SHOULD dilute the air/fuel mix some to prevent the backfire. It isn't getting a chance to do that when stopping from full speed. On occasion, I don't give my B&S 21HP Intek time to cool down and shut it off with it throttled up, and yeppers, it'll go bang, too. BUT, if I slow the engine speed and let it idle for 30 seconds or so, it NEVER goes bang. OOOP OOOP OOOP (compression relief) when it drops below about 200 RPM, but no bang. The only thing that can ignite the fuel air mix is a spark or something that's still hot. Let it cool a bit at idle before shutting it off SHOULD help.

@Old_Paint If you had seen some of the things they do to portable generators as far as wiring you would be going crazy. How about the electric start generators that don't have a battery charge coil to recharge the battery. The generator comes with a separate battery maintainer that connects to a plug on the side of the generator panel and either plugs into a wall outlet to recharge/maintain the start battery, or you can use up one of the generator outlets to recharge the battery.

ILENGINE said:

@Old_Paint If you had seen some of the things they do to portable generators as far as wiring you would be going crazy. How about the electric start generators that don't have a battery charge coil to recharge the battery. The generator comes with a separate battery maintainer that connects to a plug on the side of the generator panel and either plugs into a wall outlet to recharge/maintain the start battery, or you can use up one of the generator outlets to recharge the battery.

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Yeah, that would probably drive me insane. What's the point of electric start if it can't recharge itself without additional electronics?
I think I would be very likely to get the parts and fix that issue. I'd also be very likely to replace that solenoid valve with one that operates fail-safe. Nothing like fuel pouring into the cylinder with the engine stopped for a nice quick hydraulic lockup, and then having to replace the oil because the fuel wasn't shut off properly. I won't say I'd be surprised at all. Frustrated, probably, but surprised, no. We live in the day when saving one penny per widget can make or break a major corporation. It's all about what can be sold for the least amount of cost, regardless of whether or not it will be safe to use, dependable, or for that matter, if it will work at all.

I see you're a certified B&S engine Master Service Technician. Good to have on the forum. What I know about small I've learned the hard way; other than the training I had in high school (about 50 years ago when rural schools were still funded for agricultural/craft education). I also owned an air-cooled VW for 35 years, which is basically a very large 4-cylinder lawn mower engine. Once you understand how air-cooled engines work, it really isn't hard to keep them running well. Somehow, the importance of being able to tune up an air-cooled engine, or sharpen a chain saw, or set the kerf on a handsaw all took a back seat to pronoun awareness and diversity sensitivity. Meanwhile, we have about 4 generations now that the majority of can't tie shoelaces or look up from their phones long enough to have an intelligent debate about anything.

Old_Paint said:

Yeah, that would probably drive me insane. What's the point of electric start if it can't recharge itself without additional electronics?
I think I would be very likely to get the parts and fix that issue. I'd also be very likely to replace that solenoid valve with one that operates fail-safe. Nothing like fuel pouring into the cylinder with the engine stopped for a nice quick hydraulic lockup, and then having to replace the oil because the fuel wasn't shut off properly. I won't say I'd be surprised at all. Frustrated, probably, but surprised, no. We live in the day when saving one penny per widget can make or break a major corporation. It's all about what can be sold for the least amount of cost, regardless of whether or not it will be safe to use, dependable, or for that matter, if it will work at all.

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Actually the fuel solenoid doesn't shutoff the fuel coming into the carb. It only is to shutoff the fuel that is being drawn through the carb jets when shutoff. So if the float valve fails it will still fill the crankcase with gas, fuel solenoid or not. And that includes any small engine with a float style carb no matter what is it mounted on.

But in my case I never turn off the kill switch on a generator. I always turn off the external fuel valve and let it run the carb dry as possible. Helps prevent the afterfire effect especially on generator that don't have the bang solenoid and also drains the carb for storage, since most people think about starting their generator about every 5 years after setting in the garage with stale gas in it.

ILENGINE said:

Actually the fuel solenoid doesn't shutoff the fuel coming into the carb. It only is to shutoff the fuel that is being drawn through the carb jets when shutoff. So if the float valve fails it will still fill the crankcase with gas, fuel solenoid or not. And that includes any small engine with a float style carb no matter what is it mounted on.

But in my case I never turn off the kill switch on a generator. I always turn off the external fuel valve and let it run the carb dry as possible. Helps prevent the afterfire effect especially on generator that don't have the bang solenoid and also drains the carb for storage, since most people think about starting their generator about every 5 years after setting in the garage with stale gas in it.

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Yup, I know exactly where it shuts off the fuel. And I meant exactly what you pointed out, that the float valve often swells and fails and hydrolocks small engines that are stored for long periods. The solenoid valve on my Intek 22HP is actually the main jet and solenoid valve all rolled up into one. Replaced it a few years ago when I replaced the carb. I took the original apart and cleaned it just to have a clean one on the shelf when the varnish and crud stops up the replacement. The solenoid just pulls the plunger down below the bottom holes of the jet to let it pull fuel from the bowl.

I added external valves to three of my small engines for the same purpose (to run 'em dry). All three are run intermittently, so rather than let ethanol blend gasoline screw them up, I just shut off the fuel valve and when they start sputtering, flip the choke on to pull as much as possible out of the carb. That's also a good time to hit 'em a little with either some Seafoam or carb cleaner to help dry the bowl even more. I've still had to clean the main jet (which is often what holds the bowl on), but I've never had a stuck float again since I started doing that. I run the 22HP Intek often enough that I didn't bother with an inline cutoff. I run it almost as much picking up leaves in winter as I do cutting grass in summer. Sitting in storage is any engine's worst enemy, especially with fuel in the carb.

I know you might throw rocks at me when I say this, but the little Predator engines seem to be pretty good quality. They're very close knock-offs of B&S engines. Dunno if they'll have the longevity of a B&S, but for the price, I can buy 3 of them for the price of 1 B&S of similar ratings. I repowered an old Troy-Bilt Horse tiller that my mom bought in 1976 with one of the Predator engines. It originally had a Tecumseh 6HP with a cast iron block. I got it from her back in 2004, and it was using more oil than gasoline, i.e. worn out. I tried to find parts to rebuild the Tecumseh, but it just wasn't meant to be. I would have loved to restore that old beast to original condition. I still need to overhaul the entire tiller and replace the tines. I don't have a big garden, so it hasn't been a priority.

Old_Paint said:

I see you're a certified B&S engine Master Service Technician. Good to have on the forum. What I know about small I've learned the hard way; other than the training I had in high school (about 50 years ago when rural schools were still funded for agricultural/craft education). I also owned an air-cooled VW for 35 years, which is basically a very large 4-cylinder lawn mower engine. Once you understand how air-cooled engines work, it really isn't hard to keep them running well. Somehow, the importance of being able to tune up an air-cooled engine, or sharpen a chain saw, or set the kerf on a handsaw all took a back seat to pronoun awareness and diversity sensitivity. Meanwhile, we have about 4 generations now that the majority of can't tie shoelaces or look up from their phones long enough to have an intelligent debate about anything.

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There is a story behind my MST. Back in 2001 when I first became a Briggs dealer my area rep for Briggs was an A--. So his remarks to me was I don't know when I will be able to get you set up for the 4 day Briggs factory school, but if you take the MST test and pass it you will be exempt from the factory school. At that time the test was a 4.5 hour hands on timed test with a 80% first time failure rate. The day I took my test there was another person that had been a tech for close to 30 years that was taking it for the 5th time.

Anyway I took the test and received an 86% which made me one of only 2 in the state of IL to ever pass the test without going to the school first. And I later becomes friends with the other person which got an 88% on his test So after that the Briggs field rep never give me a hard time.

But like you, I learned to work on small engines by trial and error, mostly error. No formal training as far the basics. Had to do the Kohler and Tecumseh factory schools to become a dealer but no other prior training.