Briggs and Stratton only runs with full choke, despite carb work

I’ve got a John Deer L108 riding lawn mower. It was working pretty well until one day… BANG! Suddenly I’ve got a half dollar sized hole through the side of the engine and bits of the connecting rod in places bits of connecting rod should not be (for instance, outside the engine.) I am not particularly mechanically inclined, but even I can tell that isn't something that's likely to be fixed with a bit of JB Weld and a few rolls of duct tape.

But, as luck would have it, I managed to get my hands on a used, compatible engine. (My old engine was a Briggs and Stratton 18.5hp 31P777. This one is a 31Q777.) After some trials and tribulations, mostly cause by my lack of mechanical skills, I managed to mount my new (used) engine. I filled it up with fresh oil, a new oil filter, and a new air filter. My battery had apparently gone dead in the interim, so I used some jumper cables to hook up to my car, turned the ignition annndd…

And the engine runs! Albeit a bit roughly. But, here’s the deal breaker, it ONLY runs well with the choke set. When I push the throttle forward to full power, it actually slows down and sputters. It doesn’t, quite, die. But it’s far from healthy sounding. Lowering the throttle any further, or even engaring the mower blades, kills it every time.

I was told that the carburetor was in excellent shape. But I took it apart anyways. It has a solenoid controlled valve-thing (I did say I'm not very knowledgable about engines, right?) at the bottom of the carb bowl. I took the bowl off, cleaned out the tiny bit of rust there, blew out the jet and the whole thing with carb cleaner and reassembled it. Started it again and…

Same situation. No change.

But everything I see, hear, and am told by others tells me that my symptoms sound like a carb problem. So tonight I completely ripped the carburetor off the engine and swapped it out for the carb from my old, dead engine. I know for a fact that this was working just fine when the internals of my old engine made their bid for freedom. So, with a definitely operational carburetor in place I try to start the thing up again anndddd…

Same situation. No change.

So I am now at a loss. I’ve got fresh fuel. I haven’t changed the in line gas filter yet, but I can see fuel flowing through it, it worked fine with the old engine, and the carb bowl is full of gas when I remove it. I’ve cleaned out the carb, and even swapped it for another one, then cleaned that one as well. Still no change. I’d like to think that that rules out the source of the problem being the carburetor, but that leaves me with no ideas as to what the problem COULD be.

So, anyone have any advice for a poor lost (and frustrated!) soul?

 

Does the engine run better at idle? With you having to use the jumper cables to start it and the symptoms you described, I almost wonder if there is a valve problem. Have you checked the clearances?

 

Okay, pardon my ignorance, but what exactly do you mean as idle? My throttle starts with full choke at the top. Right below that is full speed (represented by a racing rabbit for morons such as myself) and at the bottom is a turtle for low speed (which is what I assume you mean by idle.) With the choke on, it runs fairly well if a hair roughly. The moment I switch over to full speed, the engine actually slows down and runs rougher, sputtering. It WILL run like that though. If I try to throttle down at all from full speed, it dies immediately. So moving it to slow/idle shuts it down every time.

Also, when running on full choke, I can start the mower blades. I rattle quite a bit, and I think it’s struggling to spin them, but it will manage. If I even TRY to engage the mower blades while I have the throttle at full speed, it died immediately from the added resistance.

As for the jumper cables, I need those because the battery is dead. I don’t have the drive belt installed at the moment (it’s badly frayed, and I don’t want to bother replacing it until I’m sure I can get the engine running.) So the alternator isn’t charging the battery at the moment. And when I say the battery is dead, I mean it’s completely dead. It doesn’t even have enough juice to light up the headlights without being jumped.

Now, when you say it might be a valve problem, what exactly are you referring to? I’m not sure which clearances you mean, or how to check them. Is that likely to be something someone with mediocre mechanical skills such as myself can get at / check / fix?

I should also mention that I checked the spark plug, and it seemed to be fairly new, clean, and not at all crapped up.

I’ll say this much: For the money I spent on this used engine, even if I don’t get my mower working, I’m certainly getting quite an education on small engines!

 

problem might be the dead battery. Maybe it's drawing so much from the alternator that you are not getting enough voltage/current to hold open the anti-backfire solenoid on the bottom of the carb so only a small amount of gas can get into the jet.

 

38> That's an interesting possibility. But why would the engine draw more power when running at full speed as opposed to when it's running with the choke on? The only thing it needs electricity for is the spark plug, isn't it? Is there a way I could test this without spending a few bucks on a new battery I may not need? Perhaps disconnecting the mower's battery and using the jumper cables to power it fully off my car battery?

The possibility of an electrical problem did cross my mind. I have just learned that the alternator is part of the engine itself, and not related to the drive belt (once again, I'm an idiot) so my battery should be getting recharged even if I'm just sitting there. And yet, even after five or ten minutes of running, my battery doesn't have enough juice to light the headlights. I'm not sure if this is because the battery went from fine to completely dead over the week or two I wasn't using the mower... Or maybe it's a sign of an electrical issue of some kind?

One of my coworkers also mentioned that it sounds like I might be loosing vacuum somewhere. Once the weather clears up a bit in a few days I plan to check all the seals / spray some WD40 around and see if that helps. That may be a few days though, since playing with jumper cables in the rain sounds like a rather poor idea...

 

Well, the weather decided to cooperate for a bit and hold off on the rain, giving me a chance to perform a few more experiments.

First off, I don't think it's a problem with the fuel filter. I detached the fuel line and gasoline poured out of it QUITE readily when I held it below the level of the tank. So no problems there. On the other hand when I removed the screw (or in this case, solenoid) from the bottom of the carburetor cup, only a little gasoline poured out. Someone mentioned that gas should continously pour out? I'm not sure if that is correct however.

Now, another interesting observation: I hauled out my mutlimeter and checked my battery's voltage. It was hovering around 2V, i.e. dead. My car battery reads 12V (as it damned well should!) When I jump the lawn mower and leave the cables attached, it will run more or less indefinitely. REMOVE the jumper cables however, and it will (eventually) sputter to a halt.

In one test, I left the lawn mower running with the jumper cables conncted for a good 10 minutes or so. When I disconncted the jumper cables, the battery was reading 10V and the engine was running as well as it ever does on full choke. HOWEVER, over time I could literally watch as the voltage dropped down. After another five or ten minutes it dwindled back to 2V or so, and the engine died. Obviously trying to turn on the headlights hastened this demise greatly. Oddly enough, it doesn't seem to matter much if I leave the engine in choke (and running somewhat well) or put it into full speed (and have it running roughly and slowly)

So, this tells me... I have no idea what. Is this normal for a dead batterly? Or does it signify a short or electrical issue somewhere? Shouldn't the battery just be needed to START the engine, with the alternator providing enough power to keep the engine running? Might this mean the alternator is bad? Could that be causing the behavior I'm seeing with regards to the choke / no choke issue? Should I buy a new battery? Should I swap out the alternator? Where IS the alternator? Should I just bang my head against a wall until I get amnesia and forget this damned thing exists??

HELP!

 

Firstly, you need a new battery. The engine doesn't need a battery for spark, but it does need it for the fuel solenoid. If the battery is internally shorted like yours appears to be (you shouldn't see the voltage dropping like you did even if the alternator wasn't working), it will not take a charge from the alternator anyway. A shorted battery consumes the current from your alternator. Your car may have a 150 amp alternator with it's power windows, computer and other current sucking devices, but this little mower only has a 15 amp alternator and only that much at full speed, which you are not attaining.

 

Cheese> Ah, I see now! Thank you for the explanation. One way or another, I guess I'll need a new battery. At least that's a problem I can actually fix!

Of course, that still leaves another question: What are the odds that a new battery will fix the general problem of the engine not running properly except with the choke enabled? Could a shorted battery be causing that? Messing up the timing of the spark plug or something, even when connected to a properly functioning battery?

I just don't want to throw more money down a rathole if I'm not going to be able to get this thing running right.... On the other hand, I suppose I can always find a use for an extra battery...

 

If it ran ok with auxiliary battery without choke then likely it's as I suggested the lack of power to the carb solenoid. It's restricting fuel but letting a bit through. By closing the choke you are restricting air which now matches the restricted fuel and achieves a gas/air ratio close to normal. Opening the fuel supply will allow yopu to open the choke as I assume you saw with the auxiliary battery

 

The problem is your anti-backfire solenoid. It is mounted on the bowl on the bottom of the carburetor. If you cannot afford to replace it, use a pair of dykes to cut the end off of it that goes into the bowl. It will run then, I guarantee it. However, if you shut it off without idling down first, you may cause a backfire that could result in a fire.

 

Joey this thread is 3 years old, they have either fixed it or blown themselves up. Have a good one. Geo

 

Joey,

It's now May 24th 2016 and I am soooo glad that I saw your response. THANK-YOU sooooo friggin' much. I re-built my carb and knew that I did it right, but it didn't help. After reading your thread, I finally found a solenoid for almost $100 ! Screw that.....I always idle down before shutting it off. I cut it off & problem solved, it runs like a dream ! Thanks again !

Larry