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

Mini Bike & Go-Kart Parts

#1
Hey everyone, I have been restoring this old Briggs engine for a mini bike project. Before I took it apart to paint it, I replaced the magneto coil, and it fire right up. Once I finished the paint and reassembled it, I tried to start it, and it would not go. I sprayed starter fluid into the intake, and tried to start it, all it did was fire for a few seconds, but didn’t run. The spark plug is new, I replaced the magneto coil, and cleaned the carb thoroughly, I don’t know if the gaskets are the cause of the problem, since I reused the old ones. I am stuck. Fyi, this Briggs does not have a float bowl on the carb. It’s the old style from the 70s
 

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#2
Without knowing how much you took apart, it's kind of hard to tell but if you reused the gaskets for the carb, that may be a problem. These carburetors have an internal pulse pump and if they aren't sealed, they won't pump fuel. I'm not sure but I think that the replacement coils for these have a ground wire that would connect to the throttle mechanism like the newer engines have, if not connected, it may be grounding out on something?. Other than that, points may need to be replaced or converted over to solid state.
 
#3
Without knowing how much you took apart, it's kind of hard to tell but if you reused the gaskets for the carb, that may be a problem. These carburetors have an internal pulse pump and if they aren't sealed, they won't pump fuel. I'm not sure but I think that the replacement coils for these have a ground wire that would connect to the throttle mechanism like the newer engines have, if not connected, it may be grounding out on something?. Other than that, points may need to be replaced or converted over to solid state.

I upgraded to a solid state coil, which did not have a ground wire, so I know the points shouldn't be and issue. In terms of the disassembly, All I removed was the carb, gas tank and muffler and valve cover. I didn't touch the valves in anyway. I am wondering if any of the gaskets could be the source of the problem. Thats all I can think of at this point.
 
#4
These carburetors are about as simple as it gets but if you removed the pump gasket/diaphragm, it probably stretched a bit and isn't sealing well. There's two "U" shaped cutouts on the diaphragm, those are your fuel pump, if they don't seal well they don't pump. I don't know if you removed the mixture screw when you cleaned it out, if not, something may have dislodged and found it's way into the jet, heck, it may be as simple as your mixture screw not set correctly after re-assembly?. I just checked one of mine and it's approximately 1-1/2 - 1-3/4 turns out if that helps.
 
#6
It appear this sheared flywheel key may be part of the problem. I have to figure out the torque specs for the flywheel. Hopefully this should fix the issue here.
55 FT Lbs.
Also, make sure that you use an aluminum flywheel key, not steel.
Michael
 
#9
Did you change flywheels if so you may need to get some valve grinding compound and make sure the crank and flywheel match i guess even if you didn't might need to since it sheared the key probly scratched it up a bit
 
#10
Funny, I read [art of the first post and thought, "broken flywheel key".

To tighten the starter clutch without the tool use a big pipe wrench. Be careful not to break the fins on the flywheel. I have never seen one too tight, too loose was common. If left too loose it will break the flywheel key. In the shop I usually would loosen them with the tool and an impact. I would always tighten them with a pipe wrench.
 
#11
Funny, I read [art of the first post and thought, "broken flywheel key".

To tighten the starter clutch without the tool use a big pipe wrench. Be careful not to break the fins on the flywheel. I have never seen one too tight, too loose was common. If left too loose it will break the flywheel key. In the shop I usually would loosen them with the tool and an impact. I would always tighten them with a pipe wrench.
That is a "get -er -done" technique and is not going to work if you want to torque it to specifications PROPERLY.
Michael
 
#13
If it runs on starter fluid "I sprayed starter fluid into the intake, and tried to start it, all it did was fire for a few seconds" it may be the fuel pump diaphragm is shot. Very common problem on the Briggs.
 
#14
If it runs on starter fluid "I sprayed starter fluid into the intake, and tried to start it, all it did was fire for a few seconds" it may be the fuel pump diaphragm is shot. Very common problem on the Briggs.
The engine in the photos provided by the OP, that Briggs engine has a "Vacu-Jet" carb which does not have a diaphragm. The "Pulsa-Jet" carbs are the ones that had the diaphragm.
Michael
 
#15
That is a "get -er -done" technique and is not going to work if you want to torque it to specifications PROPERLY.
Michael
I have to agree. Every single nut and bolt on every engine, every frame, washing machine, dishwasher park bench and etc. has an exact torque spec. Few people do that, and even fewer have their torque wrenches calibrated. It would amaze you to see how far many of them are off, even when new.
 

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