**HELP** Briggs and Stratton 3.5hp, fires but won’t start

#23
I'm curious, how does fuel get from the bottom of the tank, to the carb, without some kind of pump?
Suction/vacuum
You are correct, if that diaphragm is being sold for the carb listed in the original posting. I do not believe anyone has said that no Briggs engines use diaphragms, just that the one in question doesn't.
 
#24
Suction/vacuum

You are correct, if that diaphragm is being sold for the carb listed in the original posting. I do not believe anyone has said that no Briggs engines use diaphragms, just that the one in question doesn't.
Admittedly, I have never taken a vacu jet apart, but, there are lots of ads for a diaphram, for them. Are you trying to say the engine pulse is what draws the fuel?
 
#25
Admittedly, I have never taken a vacu jet apart, but, there are lots of ads for a diaphram, for them. Are you trying to say the engine pulse is what draws the fuel?
When the piston drops and the air rushes into the engine the Vacuum created sucks, pulling fuel through the jet, drawing in fuel and air to the engine. It works just like putting a siphon hose into a bucket of water and blowing air sideways across the end of the hose, starting the siphon.
 
#26
the shallow tank briggs do not need a diaphragm,the depth is about a 1 1/2 same as the fuel bowl on the deeper tanks which need a diaphragm to pull up the fuel 4 1/2 inch,I guess you could call the shallow tank a large fuel bowl,not much different then a tecumseh carb the bowl depth is about 1 1/2 works the same
 
#27
When the piston drops and the air rushes into the engine the Vacuum created sucks, pulling fuel through the jet, drawing in fuel and air to the engine. It works just like putting a siphon hose into a bucket of water and blowing air sideways across the end of the hose, starting the siphon.

Yes, I do understand the basics of an internal combustion engine. That was my question. It would explain why the vacu jet was not a well liked carb.
 
#29
When the piston drops and the air rushes into the engine the Vacuum created sucks, pulling fuel through the jet, drawing in fuel and air to the engine. It works just like putting a siphon hose into a bucket of water and blowing air sideways across the end of the hose, starting the siphon.
yes this is correct with that carb initially mentioned by the original poster. pulsa jet carbs have diaphragms,vacujet carbs do not. the carb in the picture as posted does not have a diaphragm.
 
#31
the shallow tank briggs do not need a diaphragm,the depth is about a 1 1/2 same as the fuel bowl on the deeper tanks which need a diaphragm to pull up the fuel 4 1/2 inch,I guess you could call the shallow tank a large fuel bowl,not much different then a tecumseh carb the bowl depth is about 1 1/2 works the same
again correct.
 
#33
How exactly does a sun set? How exactly does a posi-trac rear-end on a Plymouth work? It just does.

Every Briggs "Pulsa-Jet" carburetor has a diaphragm fuel pump no matter the size of tank, the only difference is the length of the pickup tube. In the picture below referencing the "Pulsa-Jet" carburetor for a 3.5 Briggs model 091212 with 1qt. SHALLOW tank, you'll see three parts circled in red. There's the diaphragm, spring and the little metal ring that keeps the sharp part of the spring from tearing the diaphragm. As the piston travels down for the intake stroke and the intake valve opens, air flows through the carburetor. This air creates a suction or vacuum, one that works against the spring in the diaphragm PUMP. Once the intake valve is closed, there's no longer any suction/vacuum and the spring returns the diaphragm to it's "rest" position". This back and forth motion of the diaphragm IS THE PUMP!. The two little "U" shapes (yellow arrow) in the diaphragm are the pumps "valves", one coming from the tank, the other to the internals of the carburetor. If the diaphragm or spring is damaged, it won't pump fuel. I have also included pictures of the parts, part numbers and their descriptions.


091212.png
Screen Shot 2021-05-17 at 11.43.17 AM.png
Screen Shot 2021-05-17 at 11.44.52 AM.png
Screen Shot 2021-05-17 at 11.44.29 AM.png
 
Last edited:
#34
How exactly does a sun set? How exactly does a posi-trac rear-end on a Plymouth work? It just does.

Every Briggs "Pulsa-Jet" carburetor has a diaphragm fuel pump no matter the size of tank, the only difference is the length of the pickup tube. In the picture below referencing the "Pulsa-Jet" carburetor for a 3.5 Briggs model 091212 with 1qt. SHALLOW tank, you'll see three parts circled in red. There's the diaphragm, spring and the little metal ring that keeps the sharp part of the spring from tearing the diaphragm. As the piston travels down for the intake stroke and the intake valve opens, air flows through the carburetor. This air creates a suction or vacuum, one that works against the spring in the diaphragm PUMP. Once the intake valve is closed, there's no longer any suction/vacuum and the spring returns the diaphragm to it's "rest" position". This back and forth motion of the diaphragm IS THE PUMP!. The two little "U" shapes (yellow arrow) in the diaphragm are the pumps "valves", one coming from the tank, the other to the internals of the carburetor. If the diaphragm or spring is damaged, it won't pump fuel. I have also included pictures of the parts, part numbers and their descriptions.

/QUOTE]
I am puzzled why you are talking about the pulsa-jet and not the carburetor that the original poster has. Your statement that every single pulsa-jet has a diaphragm is correct, but has nothing to do with the original engine/carb in question.

So no one is confused, THE CARB IN THE ORIGIONAL POSTER HAS ON HIS ENGINE DOES NOT USE A FUEL PUMP!
 
#36
I've stated that very clearly from the very beginning of this thread, however, one member in particular just never seemed to get it.
Michael
I used to think that the Pulse-Jet carb was the simplest thing on earth to me until THAT guy chimed in, now his method of thinking/reasoning is considered to be the simplest thing on earth to me.
 
#37
The carb is not a Pulse-jet its a Vacu-jet which is used on all briggs shallow tanks.if you look a the picture of the Vacu-jet No diaphragm. They are only used on shallow tanks which are 2 1/2 deep and and pickup directly from the bottom of the tank,All pulse-jets have a diaphragm and are used on the deeper tanks .I have not seen a 3 1/2 with a small tank and vacu-jet and think it might have been swapped
 

Attachments

#38
I've stated that very clearly from the very beginning of this thread, however, one member in particular just never seemed to get it.
Michael
Well, you have attempted to correct me in the past and been dead wrong. Sorry if I more readily accept that of a published auther, over someone I know to be just as falable as myself.
 
#39
The carb is not a Pulse-jet its a Vacu-jet which is used on all briggs shallow tanks.if you look a the picture of the Vacu-jet No diaphragm. They are only used on shallow tanks which are 2 1/2 deep and and pickup directly from the bottom of the tank,All pulse-jets have a diaphragm and are used on the deeper tanks .I have not seen a 3 1/2 with a small tank and vacu-jet and think it might have been swapped
Yes, the easiest way to spot the difference between “Vacu-Jet” and “Pulsa-Jet” is “Vacu-Jet” is a single pickup tube whereas “Pulsa-Jet” had two pickup tubes. I was under the impression that this was a “Pulsa-Jet” carb because all the “Vacu-Jet” carbureted engines I’ve come across have had the “B&S” stamped in the blower housing, did they even make a “Vacu-Jet” with the non-embossed housing?.
 
Top