Briggs tankmount pulsajet diaphragm question

[Vandoog]

Hi y'all. I have a tank mount 3hp briggs with a pulsajet carb. I am trying to get it to run and am waiting on a new diaphragm to come in the mail.

Is the sole purpose of the diaphragm to pump fuel from the large tank to the smaller well within the fuel tank?

I ask because I can't get my motor to start. Under the assumption that my diaphragm is bad figured I could bypass it by filling the smaller well within the tank thus allowing me to start my engine and see if the diaphragm is in fact the problem. Now I am wondering if the diaphragm also has a second function of somehow creating vacuum for the shorter pickup tub that brings fuel to the needle valve? Anyways thanks for the help, any info is much appreciated.

 

[bikesisbikes]

The diaphragm is one of the things that fail, on this carb. The other major item is the pickup tube will crack. In very simple terms, everything operates because of the engine vacuum. The diaphragm sucks fuel out of the tank, based upon a pulse from the engine. That fuel is deposited into the secondary tank, (jet tank), where another pulse draws fuel past the metering jet and into the air flow. Like any other carb, that same engine vacuum draws air in, through the intake port, where it is mixed with the metered fuel and sucked to the combustion chamber.

 

[Vandoog]

The diaphragm is one of the things that fail, on this carb. The other major item is the pickup tube will crack. In very simple terms, everything operates because of the engine vacuum. The diaphragm sucks fuel out of the tank, based upon a pulse from the engine. That fuel is deposited into the secondary tank, (jet tank), where another pulse draws fuel past the metering jet and into the air flow. Like any other carb, that same engine vacuum draws air in, through the intake port, where it is mixed with the metered fuel and sucked to the combustion chamber.

Thanks for the response , so if I am reading this right the diaphragm also provides the vacuum that pulls fuel to the jet thus making it not possible to bypass the diaphragm by filling the smaller fuel well to test my engine?

 

[bikesisbikes]

Yes. The diaphragm pulls the fuel from the main tank and pushes it to the secondary tank. I will not say it is impossible to fill the secondary tank and run for a test, but I will say, I tried and could not get it to function. But, I got frustrated with putting the carb back on the tank before I gave it a try. In my opinion and only my opinion, as I have not read anything either for or against, if you do not have a complete vacuum function, inside the system, you cannot have a vacuum function in an individual part of the system. The carb functions off of the vacuum pulse of the engine. If you have no fuel in the main tank, or there is a crack in the main tank pickup, or the diaphragm is faulty, you lose vacuum and therefore function. That said, someone may have figured out how to do it.

 

[Vandoog]

Yes. The diaphragm pulls the fuel from the main tank and pushes it to the secondary tank. I will not say it is impossible to fill the secondary tank and run for a test, but I will say, I tried and could not get it to function. But, I got frustrated with putting the carb back on the tank before I gave it a try. In my opinion and only my opinion, as I have not read anything either for or against, if you do not have a complete vacuum function, inside the system, you cannot have a vacuum function in an individual part of the system. The carb functions off of the vacuum pulse of the engine. If you have no fuel in the main tank, or there is a crack in the main tank pickup, or the diaphragm is faulty, you lose vacuum and therefore function. That said, someone may have figured out how to do it.

Thanks! actually got a diaphragm today from a local mower repair guy but I still can't get the thing t start! I double checked that the carb is clean and I have spark. I don't have a compression tester and am pretty stumped as to why it wont start.

 

[bikesisbikes]

Start looking for air leaks. For example, many people use two gaskets at the fuel tank, to ensure a good seal. Did you check the pickup tube for cracks? A suggestion made to me, that seems to work: use a tiny bit of fuel in the carb throat, instead of starter fluid. If it tries to start, you know your ignition is good. If it still you may have problems there. Invest in an $8 spark tester. It will show if you have enough spark.

 

[electrathon]

Prime it and see if it starts for a few seconds. If so, it is a fuel issue. If it does not start, pull out the plug and connect it to the plug wire. Hold it to the top of the engine, is there spark?

 

[Doc1976]

check your mixture screw setting if equipped, too lean will be a bitch to start.

Did you check to be sure the small well does not have any rust holes? if there is a hole in it it won't hold fuel.

for checking spark, I like to use a spring clamp to hold the plug against ground to verify spark. I don't like getting shocked.

report back with findings. please.