BS 4 horse question

rrlund

Well-known Member
I asked this on the garden tractor forum,I'll ask here too,maybe somebody will know.

I've got an old Briggs and Stratton 4 horse upright with the gas tank under the carb on my power corn rake. There are 2 tubes that go down in to the tank. One long brass one that goes to the bottom,one plastic one that sits down in a little pan in the top of the tank. It'll only run as long as there's gas in that little pan. I took the long tube out,it's clean. I took the diaphragm out of the side where that tube feeds in to. Everything looks good in there. Is there supposed to be some check to stop that short tube from sucking air once that little pan is empty or something? That's all I can figure is going on.
 
Had the same issue with that design too. Other than keep gas tank full, I don't have any good advice. Probsbly related to engine being worn as well. My unit has a Honda on it now. It starts and runs well.
 
I wonder what would happen if I just took a punch and knocked the pan out and clamped a long piece of hose to that short tube?
 
Most likely the problem is a pinhole (a pinhole too small to see) in the diaphragm. It's a common problem with this B&S design as the diaphragm ages. A new diaphragm should have it running fine again.

Removing the internal fuel reservoir and clamping a hose over the shorted dip tube will NOT work. Unless the tank is 100% full the engine will be very difficult to start. Then as the engine runs and the tank level drops, the mixture will become increasingly lean until the engine quits.
 
The long brass tube should have a plastic tube with a screen on the end that draws gas from the tank and pumps up to the small cup in the top of the tank. The gas is then drawn threw the jet from the cup. Gas is drawn up to the cup constantly and excess overflows back into the tank. I would check the screen and also if the plastic tube is loose it can suck in air if gas level is below that. HTH
 
Replace the diaphragm. They are like $1.50 or so at the parts store if you use the Prime Line brand. If they get hard, curled or a pin hole, they wont pump fuel. I have several on hand, but they are becoming dinosaurs.
 
Where's the plastic tube with the screen? The brass tube goes directly in to the bottom of the carburetor. There's a hole that comes directly from it that comes out behind a flap on the diaphragm,then another hole behind the other flap and that hole goes in to the carb somewhere. I sprayed carb cleaner in to that second hole and it came out the bottom somewhere.
 
If its the carb I'm thinking of, the plastic tube attaches to the brass tube and extends to the bottom of the tank.
a164100.jpg
 
It looks similar to that,maybe even the same,but there's a plastic suction tube with a screen attached. It sits in that little pan,or bowl,whatever you want to call it,in the top of the tank. There's only one of those short plastic tubes,not two. That's the thing,it quits when that pan is empty so that tube has nothing to suck anymore. I have to rock it and get gas in that pan again,then it'll start and run for a minute or two again until it's empty.

It wouldn't be doing that if gas was being sucked up from the bottom through the brass tube,then being pushed back out that short plastic tube and in to the pan. Unless gas is supposed to just get sucked up and dumped back in to the pan from a hole in the bottom of the carb that doesn't have a tube?
 
Sounds like the fuel-pump is not working and keeping that little cup/bowl full of fuel. Bad diaphragm or maybe a stuck check-ball in the long intake tube that feeds the fuel-pump.
 
I know that pic isn't very good but the plastic tube attaches to the long brass tube and can be removed. If yours doesn't have it on there,it may be in the tank. Does your tank have foam in it,if so that makes it harder to fish out.
 
That must be different. Mine has two tubes. A long brass tube that goes through one hole in the tank and a short plastic one with a screen that sits in that little pan at the top of the tank. That one goes through another hole in the top of the tank. The short one has to be a suction tube too,because it won't run without gas in the pan.
The brass tube has a flange at the bottom and looks like it should probably have a screen in it,but it doesn't.
 
Randy, Tims pic might be a later version. Sounds like you have the early one. The fuel is pumped up the long tube into the cup which keeps over flowing back into the tank. The short tube flows to the adjusting needle and into the air stream in the carb. The idea being to provide a constant fuel level and more even fuel mix without needing constant readjusting. If it runs while the cup is full and dies when it emptied then the diaphram is bad or the long tube or its screen is plugged.
 
Sorry about that. The Vacu-Jet carb has the check-ball at the bottom of the pickup tube. But it sounds like you have the Pulsa-Jet, so no ball. It uses a flap on the fuel-pump diaphragm as a check-valve.
 
I had one do the same thing and found a hairline crack in the brass tube that let it start sucking just enough air at about 3/4 tank to loose "prime". Cleaned it up and silver soldered the crack shut.
 
If this is a pulsa jet carb you have two problems. Have fixed boat loads of these long time ago. First you need the diaphram kit. Very cheap and the instructions usually come with it . Now here is the other part of the problem. When you remove the carb.....is the top of the tank made out of pot metal and the hole and that sump are all one casting? If it is you have a warped tank. It is an easy fix but you must do what I tell you to do. You need a very large very fine flat finish file. Hold the tank gently in a vise and VERY gently surface the top of that tank just enough to get a shinny surface for all contact area. You don't need the whole top, just enough to make the diaphram seal. Put her back together and you will be shocked how nice it works. Like I said. ...if this is a Pulsa-Jet.
 
The older carb had the brass long tube and the newer ones had the brass and plastic combo. There is no check valve in the plastic tube carb. The tube has to be blocked or the diaphragm is not working properly. make sure you have the little concave washer on the diaphragm spring. Poor compression could also cause it not to pump the fuel. Make sure the passages in the carb are clear.
 

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