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

[SmallEngineHead160]

It would be a very long reply for me to post everything I know about these carburetors but will try to post something helpful.
I'm not a professional, but I have worked on these engines with these carburetors for many years now as a hobby.

First off, the model number you posted for your engine indicates that this engine originally had a vacu-jet carburetor, not a pulsa-jet.
Both are interchangeable on these engines so there's a good chance you might have a pulsa-jet on your engine if someone has swapped it.
Side note info:
A model 92502 indicates a vacu-jet carb.
A model 92902 indicates a pulsa-jet carb.
5 = vacu-jet (Also indicates that the engine has a vertical crankshaft)
9 = pulsa-jet (Also indicates that the engine has a vertical crankshaft)

So this being said, if you are unsure what carburetor you have you will need to verify for sure what carb. you have.
The easy way of telling the difference if you don't know what your looking at is:
(With the carb. removed from the fuel tank) A vacu-jet will only have ONE fuel pickup tube.
A pulsa-jet will have TWO pickup tubes and a second short spring and a dished washer thing between the diaphragm and carburetor.
You will want to verify what carb. you have before ordering parts, specifically the pump diaphragm, as the vacu-jet and pulsa-jet diaphragms are totaly different.

Number one problem on these is the pump diaphragm going bad, but these carbs also have a problem of warping and not sealing to the tank properly (also the tank surface will warp too)
So sometimes you may have to replace the carb. or the tank or both.
I will post another reply with a list of part numbers that may be helpful to you or others reading.

Something I would like to add is that years ago My brother and I had a 3.5hp briggs that would not throttle up from idle without dying.
It ended up being that the ignition coil had come loose and was scraping the flywheel and wobbling around, probably messing with timing and what not. Just something to check.

 

[Ocean909]

Do you wan to to restore it, or just get it running? If the latter, you can also swap that pulsa-jet with a pulsa-prime carb & tank. I did this when my pulsa-jet tank rotted. Its basically a direct swap except for 2 possible issues on old equipment. You may need the longer screw for the air cleaner and the breather tube may need to be shortened.

Amazon sells the chinesium version of pulsa-prime with the steel tank and plastic carb for around $25. I have one and it works decent, but there is a strong chance you may get a turd. Luck of the draw with that stuff. If you go the knockoff route you'll also need a metric bolt where the tank attaches to the bracket. 6mm I believe.

 

[SmallEngineHead160]

This is a list of part numbers for the carburetors used on most 3 to 3.5 (and some 4) HP briggs and stratton vertical crankshaft engines from the mid 1960's to the early mid 1990's
So first off I will list part numbers for the pulsa-jet carbs
Carburetor pump diaphragm:
391681
Carburetors:
394063
390065
390126
391642 These 4 all require the longer breather tube/pipe
395957 This one is a later variation and requires the shorter breather tube/pipe.
Breather tubes/pipes:
230802 Long
691245 Short
231272 Short
231774 Short
Be carful as ebay sellers sometimes don't know the difference between the two sizes and will list them with the wrong part numbers.
Fuel Tank:
391694

The following is for Vacu-Jet carburetors:
Carburetor pump diaphragm:
299637
I seem to have lost or messed up my info for the part numbers for the vacu-jet carburetor.
I will edit and post when I get that sorted out
Fuel Tank:
396778




.

 

[slomo]

If the tank top is warped, prep it and use Bondo, abrasive paper and a sheet of plate glass. Use one gasket sprayed with high tack sealant. Little skim coat is all you need. And don't over torque the carb when you reinstall. Use your weaker off hand with open fingers to lessen torque. Don't need lug nut torque at all.

 

[SmallEngineHead160]

If the tank top is warped, prep it and use Bondo, abrasive paper and a sheet of plate glass. Use one gasket sprayed with high tack sealant. Little skim coat is all you need. And don't over torque the carb when you reinstall. Use your weaker off hand with open fingers to lessen torque. Don't need lug nut torque at all.
View attachment 66115

Sounds like a good idea except for the high tack sealant, might work on a vacu-jet but not on a pulsa-jet. Seems like it would glue the little diaphragm flaps closed and cause a whole world of trouble.

 

[Rivets]

As was just posted, never use any type of sealer around the diaphragms used on pulsa jet carburetors. You’ll be replacing it very fast, as most sealants will also eat the thin rubber. Experienced techs have learned this the hard way.

 

[volvofan94]

Quick update…no change.

I tried preloading the diaphragm per the shop manual link Rivets’ provided as well as replaced the choke shaft spring. The engine still won’t run properly. I have a genuine B&S diaphragm on order. If it still won’t rev after replacing the diaphragm, I‘m considering throwing in the towel then move onto other projects. Plus I‘d like the garage space back too! lol.

I should add I removed, re-cleaned, and inspected the two fuel pickup tubes. Those are good and clean as are the passageways they support in the carburetor.

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.

The air filter has been on and off while running the engine, but I’ve always had the air filter hold down screw installed.

 

[volvofan94]

Sorry I’m catching up on replies the from last week. I appreciate everyone’s suggestions!!

Thank you SmallEngineHead160 for your detailed description between the vacu-jet and pulsa-jet carbs! The carb I’m working with is 100% a pulsa-jet. It has the two pickup tubes. I’m not sure if the carb (and possibly tank) was replaced sometime in the engine‘s past, but it’s looking more likely now. Also, when I started looking for parts, that could explain why my engine didn’t exactly match the diagram on B&S’ website. I verified I ordered the correct B&S diaphragm. Whew!

Ocean909, my plan is to get the mower running then sell it on.

 

[volvofan94]

Update #2: got the new diaphragm installed, the engine started, revved up, then died. Can’t seem to win with this engine! lol.

Well, suppose next step is to check the tank for flatness. I may be getting fuel into the tank‘s vacuum chamber. The manual Rivet’s provided has the steps I’ll perform on page 19.

 

[SmallEngineHead160]

Im assuming you followed the proper mixture screw adjustment procedure?
If not you're gonna want to do that before anything else.