Vintage 3.5hp Briggs and Stratton 148cc Engine won't go past idle

[VegetiveSteam]

I just worked on my personal 1980 B&S 3.5 and used an aftermarket diaphram. It ran worse that it did before I worked on it. I got a genuine B&S diaphram and it now runs great. Yours may use a different diaphram as my carb is metal not plastic. Got the spring and cup on the carb side of the diaphram?

 

[volvofan94]

Thank you everyone for the suggestions!

The replacement OEM B&S breather arrived yesterday; will install then try starting the engine again. If that doesn’t work then it’s time for an OEM diaphragm as well as another round of carb cleaning.

The carb is metal as well - only plastic items are the fuel pickup tubes.

Got the spring and cup on the carb side of the diaphram?

Yes, all springs are in the correct spots. The spring below the choke is there, however I didn’t fully attach it to the rod controlled by the auto-choke below the diaphragm. It’s resting on top. Should it be connected?

Then the spring below the throttle plate is in the recess of the carb above the diaphragm.

 

[VegetiveSteam]

If I am understanding correctly then, yes. That spring needs to be attached to the diaphram as shown in the diagram below in for lack of a better term the U shaped area on two sides of the rod.

 

[Rivets]

Yes, pages 20-22 of section 3 in the manual I posted explains installation and preloading of the diaphragm. Might want to read it before going to far.

 

[volvofan94]

We may be onto something with the choke and how it pumps fuel into the carburetor. I adjusted the spring on the auto-choke plate to keep the choke plate fully open then the engine was able to maintain a running speed just above idle. It didn’t rev to full speed, but it was enough to adjust the mixture screw.

I‘ll study the manual Rivets provided further then try again. This time, perform the preload procedure to the diaphragm.

 

[kjonxx]

set 3/8 bolt in carb to hold choke in correct position then tighten screws to set. Check gas pickup tube by blowing and sucking on it. (some have check valve in) or replace the tube.

 

[VegetiveSteam]

We may be onto something with the choke and how it pumps fuel into the carburetor. I adjusted the spring on the auto-choke plate to keep the choke plate fully open then the engine was able to maintain a running speed just above idle. It didn’t rev to full speed, but it was enough to adjust the mixture screw.

I‘ll study the manual Rivets provided further then try again. This time, perform the preload procedure to the diaphragm.

Something I forgot to ask. Are you running the engine with the air filter off? If so, put the air filter hold down screw back in. Many of those older Briggs would run rich if you tried running them with that screw removed as it would let unwanted fuel come up through that open screw hole.

 

[kjonxx]

Check the butterfly for throttle to be sure its not binding or loose. I have seen the screws come out of these. Just a thought.

 

[grumpyunk]

Have you checked for warpage of the top surface of the fuel tank? The bottom of the carb where it seals to the tank? As I understand it can be necessary to add a second gasket to get them to seal up if there is warpage. Without the seal, it won't pump the fuel properly.
tom

 

[SmallEngineHead160]

Have you checked for warpage of the top surface of the fuel tank? The bottom of the carb where it seals to the tank? As I understand it can be necessary to add a second gasket to get them to seal up if there is warpage. Without the seal, it won't pump the fuel properly.
tom

warpage is a problem but a second gasket wont work on these!