Had to pull the fuel shut off solenoid in carb, concerns?

fastline

Member
Well, I finally lost this solenoid in the carb of my 18hp Briggs twin. Had to chop the tip to get going again.

Reading online, it sounds like a bunch of people just bypass them and ignore but I am wondering if I will now get into backfire situations that can hurt the engine? The last thing I want is to do head gaskets on this engine which is a nightmare to get to!

I read that some put a solenoid valve inline with the fuel line but that won't do a thing with fuel already in the float bowl.


The one scenario I can think of is when I hit a bump and I azz comes off the seat while mowing which trips the seat switch and kills the ignition for a moment.

Thoughts? Do I need to go get one of these stupidly priced valves that are not built rugged enough for the environment?
 

Not sure what YT thinks of disabling safety features but the fuel is still controlled by the float and needle, I dont see how it can cause a backfire.
 
Lance, when you shut off a HOT engine by disabling the spark only, it continues to draw fuel as it coasts to a stop, but goes not ignite/burn it so the hot fuel vapor passes through the engine and into the HOT muffler where it often lights off from the heat causing a "backfire".

A properly functioning fuel cutoff solenoid stops the flow of fuel at the float bowl and prevents the unburned fuel/backfire issue.

Oh, yeah, and the feature also keeps the EPA happy by stopping the discharge of unburned hydrocarbons! Which is the primary reason for the solenoid in the first place!

As to whether a "muffler backfire" is harmful to anything else but the muffler and your ears, likely NOT.
 
You can install a fuel valve in the gas line going to the carb, as posted the float and needle valve should stop the flow, if your still concerned, shut the inline valve off and let the engine idle till all the gas is out of the float bowl, the solenoid valve does nothing in the event you bounce in the seat and the engine momentarily stops as it is controlled by the ignition switch.
GB in MN
 
To better explain as I have inspected these systems and understand them, the solenoid acts to restrict the fuel specifically to the main jet circuit. The fuel sitting in the bowl cannot flow into the passage with the main jet unless the solenoid is retracted. The fuel supply at the main is otherwise extremely limited.

This could not be compared to any fuel line shutoff because there is fuel in the bowl! An engine like this will run for several minutes just on the fuel in the bowl alone.

As to why it occurs, The carb and governor are stupid devices. The carb will flow fuel as long as it has it and there is vacuum to pull it. The governor will apply as much throttle as required to maintain a selected RPM. When you kill the engine, the engine speed drops and the governor responds by opening the throttle to chase more engine speed since it does not know you want to stop the engine.

The solenoid blocks fuel nearly instantly to the main. I know this because when I was having issues, the engine would idle but not take more throttle. The pilot circuit stays open. The needle/seat only works to maintain fuel level in the carb.

As to the seat switch, when the PTO is engaged, it is in series with the ignition switch and would kill the engine the same as if I turned the key off. That condition would hold until I either make the switch on the seat or disengage the PTO switch.

So it would be the same condition as shutting the engine down as the governor will be requesting more fuel by going to WOT and fueling the engine while the ignition is cut out. Then when my butt hits the seat again, there will be some extra fuel there. I am sure I could just go test it but rather not! lol
 
The ignition on a B&S is magneto, to stop the engine the magneto coil is grounded the seat switch is wired in series wioth the this grounding circuit so when the seat is up the magneto is ground stopping the engine the fuel shut off solenoid requires 12 VDC to open it completely different operation and circuit from the ignition, the solenoid is powered off the key switch that has +12 VDC as soon as it is turned on no matter if the key is in lights run or start. Keep in mind that fuel shut off solenoids are that came into being in the last 15 or so yrs for 50 before that engine were shut off and that was it no head gasket damage or other.
GB in MN
 
Woops, I had to glance at the schematic again but you are right. Actually, the safety circuit is on a different ground loop than the key switch but ultimately if one of the switches are made, they provide ground reference to the mag coil.

Actually a good catch and glad I asked!

Matter of fact, I always get a little cough when I hit a bump that brings me off the seat for a sec. And as well, I was just last night discovering a new noise that sounds like an exhaust leak.... I cannot tell you the nightmare in working on this Grazer! Great when running but wrenching is NOT fun!

Very odd since I do not believe this noise was there before but I started mowing with my brand new fuel pump, knowing that it was going out but I could let it catch up to get me done for a few weeks. Then it just nose dived last night, 15min in. I chopped throttle to idle immediately and got an idle. I knew it was like hitting the kill switch but I chased around on the carb and pump for 30min before wiggling the dam solenoid to realize it was an issue.

I never did get any backfires or anything while wrenching so still puzzled.... Getting that muffler off will probably take pulling the engine! No kidding, they suck!
 
Ha, and with all my luck, sure enough, a head gasket is already blown out. Why it chose yesterday to rear its head, I have no idea...

Guess the only good news is this is a flathead so if I can gain access, it should go pretty quick as intake and exhaust can stay put.

I have not found the exact point of the leak yet but rotating by hand, it is obvious and comp test shows 90 psi on that cyl and 115 on the other...
 
I don't see how a backfire could possibly damage a head gasket.

Cylinder pressure is highest during the combustion process, and the head gasket survives.

A backfire is nothing more than a combustion event occurring at the wrong time.
 

The solenoid doesnt stop the fuel from leaving the the float bowl towards the engine, it stops the fuel from the tank to the bowl, IF, the solenoid remains open and the spark is cut off at full throttle then fuel could be drawn thru both the solenoid and the needle for a few seconds, but under normal circumstances with the ign switch cutting the spark at low throttle it wont draw enough fuel from the bowl to create a backfire... the seat switch could cause it I suppose if its not working properly for some reason but that would be another issue.

Backfire blowing a headgasket.... possible, if the engine rotates in reverse during the backfire it could close the exhaust valve and trap the compressed air inside the chamber for an extended time... not very likely, but possible. Sounds more like a timing issue imo
 
"it stops the fuel from the tank to the bowl,"

REALLY?

That's good to know! Thanks for correcting my ignorance. (Or NOT?)
 
No no, I do NOT have any current backfire issues and I do not suspect backfire to be the cause of the head gasket issue. I am still quite puzzled why the gasket is toast. I know some probably experience this as "life" but I suspect something caused it. Pooling fuel degrading it, loss of head bolt tension, just not sure yet.

If I have time, I will open it today.

Quick question though - Will I need to drain the oil in this opposed twin to pull the gasket? IIRC, it is not required but I can't remember.
 
Bob is correct on this issue. The solenoid does not stop fuel from entering the bowl. It does stop fuel from leaving the bowl to the jets.
 
Grrrrr..... It seems my head bolts lost tension.... I will need to gather the correct torque spec for these but a couple were easily removed with one hand with a 1/4" ratchet. Im gunna guess these are 30-40ft/lb?
 
You answered your own question. The 'combustion event' is at the wrong time. It is also uncontrolled.

What most people refer to as 'backfire' is actually an explosion in the muffler. If the exhaust valve is open the UNCONTROLLED explosion can blow right through the head gasket gasket as it is the weakest point in the combustion chamber. It can also get the exhaust manifold gasket too. I have also seen mufflers swell up too.

Every year I see several engines that I am certain the head gasket failure was caused by a backfire, most times during shutdown.

I don't modify fuel solenoids nor do I recommend anyone else do so either.
 
"If the exhaust valve is open the UNCONTROLLED explosion can blow right through the head gasket gasket as it is the weakest point in the combustion chamber."

- - - - - - - - - - - - - - - - - - - - - - - -- -- - - - - - - - - - - - - - - -

Please explain how the cylinder pressure of an "UNCONTROLLED" explosion, especially if the exhaust valve is open, can be higher than the pressure inside the sealed cylinder during a normal combustion event. ...The head gasket routinely survives this event.
 
what was your engine actually doing,cause i dont see how backfire will damage head gaskets, seen some muffler damage
once while but thats about it. are you sure that was your issue i see very little of those be issue, new units out of
crates yes because they have not ran for long time sometime and will be dry stuck. turn your motor off wide open and
bet it stops backfire,thats what the solenoid is suppose to do but only if you leave the motor 3/4 throttle or more. i
run my units like no throttle control on them put it in wide open and forget its there. in fact several motors now days
have no throttle control they are full throttle or nothing. solenoid on carb is NOT what shuts the engine off. you can
take it clear out replace with carb bowl nut/gasket go. shut her down full throttle be done with it. far as bouncing
off seat switch (you must be awfull small)slow do or do like i have one small lady do she puts small back back on back
of her seat with some drinks for weight works fine.
 

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