Briggs Classic 3.5 hp Won't Run

[Hokiefyd]

Hey guys,

I'm working on this mower for somebody at work, and can't figure it out. I've pretty much gave up on it, and will give her one of my "spare" mowers that I have as a permanent replacement (I always seem to have a few "spares" around).

It fires up on the primer gasoline and runs for about a second or so, then quits. It's clear that it's not getting fuel through the carburetor, save for the primer bulb. So I know the primer bulb is good. And there's no choke, so no issue there. This has the carburetor mounted right to the gas tank, and uses a Pulsa-Jet carburetor with the little diaphragm fuel pump. I figured that the diaphragm was bad, not pumping fuel. I put a new diaphragm on it, but no change in how it runs. It will fire up on primer fuel, but won't stay lit.

The carburetor is plastic. I think the material is actually Minlon(R), or so says an old Briggs & Stratton shop manual I have that seems to cover this vintage of engine. There's not much to the carburetor at all, but I feel that there's GOT to be a port that's plugged somewhere. Short of soaking the entire silly thing in carburetor cleaner, I'm not sure what would fix it at this point. A new carburetor won't be worth it, as the mower isn't that valuable at this point (it's a REALLY beat 20" Murray).

But since I'm going to give her one of my spares, I might eventually try to get this to run, just so I have IT as a spare for a later date. What do you all think? Short of replacing the diaphragm, there can't be much more to this thing than a blocked port in the carburetor, right?

 

[SuperDave456]

Have you tried to put in a few ounces of a carb/fuel system cleaner into the tank?
Run it, let it sit, Run it, let it sit. See if it gradually gets better. Sounds like the gas in it has gone bad in the past and something is clogged or partially clogged with that gelled gas.

I had a ancient Toro I played around with as a teenager that had a similar problem.
I eventually put it on the curb and within a 1/2 hour it became another persons problem.

 

[JTK]

I've gotten Pulsa-Jets going again by holding my hand over the carb opening while pulling the recoil. If you changed the diaphragm to no avail, is the gas cap vent plugged or the screen on the p/u tube for the carb?

Joel

 

[Hokiefyd]

I took the carb off and sprayed carb cleaner on every surface. Looking at the bottom of the carb, it looks like there are a number of small passages in it that may or may not be easily cleaned. A new carb was about 20 bucks as I recall online. I doubt I'll spend that to get a junky spare mower running. I may take the engine off the mower, put the deck down in the Bermuda triangle (at the bottom of the driveway), and tear the engine down and see what years of neglect really do. The oil in this poor engine was as thick as (and as black as) tar.

 

[Hokiefyd]

Originally Posted By: JTK
I've gotten Pulsa-Jets going again by holding my hand over the carb opening while pulling the recoil. If you changed the diaphragm to no avail, is the gas cap vent plugged or the screen on the p/u tube for the carb?


I can see through the gas cap vent holes, and the screen and p/u tube are clear. The B&S shop manual I have says that this carburetor does not use check balls in the p/u tubes. The gas cup in the fuel tank DOES fill with fuel, so I know the long p/u tube works. I took the screen off the shorter p/u tube and there was a black plastic cap on the end of the tube. I don't know if there's supposed to be an open orifice in this cap (like in the center of the cap). I'd think there'd have to be somehow. But I couldn't see it that well. There didn't appear to be an open orifice, but I wouldn't know what I was poking at to try and unplug it.

 

[Hokiefyd]

For what it's worth, the crankcase breather tube gets REALLY dirty fast, and after I started it a few times, I took the air cleaner back off (to check the carb again) and the throat of the carb was coated with the thick black oil that was in the crankcase.

Figuring that it may be overfilled from the owner, and fouling the mixture, I drained the oil out and put in fresh (which I was going to do after I got it running anyway), but that didn't change anything.

 

[JTK]

Hmm.. I've never seen a Pulsa-Jet w/ out a check ball (check valve) in the p/up tube, but it's been years since I owned one.

Joel

 

[Hokiefyd]

Is it at the very bottom of the short p/u tube (the one that picks fuel up from the fuel cup, not the main tank)? Maybe there is a stuck ball at the center of this tube and I can just press through it.

 

[boraticus]

Have you confirmed that you do or do not have fuel in the combustion chamber? Take out the spark plug and turn the engine over then check to see if you can smell fuel at the spark plug hole. Put in a dry spark plug and turn the engine over a few times to see if it gets wet. If it doesn't it's obviously a fuel issue.

The reason I'm asking is due to your stating that excessive oil is being forced from the crankcase into the air breather and coating the carb. I find that very unusual, particularly if the oil level is correct. With that much oil being blown out of the crank case, it makes me think of some serious blow-by caused by worn piston rings and/or a valve not completely closing. However, even if oil is getting into the carb, it should burn off. If a valve isn't closing, you won't get proper combustion either.

If I were working on the carb, I'd run a fine wire through any visible orifices, blow WD-40 through the holes then some compressed air. I'd also look for dirt and debris on the bottom of the fuel tank. There might be something in there obstructing the intake for the carb.

 

[Hokiefyd]

Fuel is getting through the carburetor via the primer bulb. I can prime the engine, and it'll run for no longer than a second on the fuel that I primed it with. If I try to dry-start, with no priming, it won't fire at all. It's definitely a fuel issue. It's odd how fuel gets through the carb via the primer (you can see it squirting into the throat of the carb), but it won't pickup normal fuel to run. There's got to be something plugged up inside that carb somewhere. I've soaked the whole thing with carburetor cleaner. A lot of grit gets pushed through the crankcase vent along with oil. It looks nasty. Half me wants to get this thing to run just so I can clean that crankcase out with a few oil changes. But it just doesn't seem worth the time and effort.

 

[RUSHCO]

We tried several times to clean these things. Replace the whole carburetor. Most plastic carbs are around $20.00 to $30.00. We replaced many of these at the shop.

 

[yeehaw1960]

Did you blow compressed air through the hole at the base (near body of carb) of the pickup tube (that hangs all the way into the tank)? Be careful to put your thumb on the primer bulb and not put too much air through there if you don't want to pop it. Are the screens clear on both pickups? Did you take the one (slip on) screen off and poke a wire through the passage way under it? Is the diaphragm and the gasket married up so there isn't any vacuum leaks?

When assembling, did you put the diaphragm down first (against the tank), then the gasket on top of it?

I've had to replace 3 or 4 out of hundreds of this type carb. It's almost always gunk in the reservoir and/or a bad diaphragm.

And, I'm not trying to treat you like you're not intelligent, (I’m the king of not intelligent), but do you always completely empty and blow out the tank? You’d be surprised at how many people will try to save that “good” gas left in the tank.