B&S 3.5 hp engine help !! plz.

[bigredhead]

Ok.. so my friend has an old Briggs Stratton 3.5 Hp Classic..

No throttle, just safety pull bar and primer.. carb is part of the gas tank...

Darn thing will fire if you prime it but sputter to a stop in no time.

Air filter is clean as a whistle.

Put in a new plug tonight Champion J?19LM, same as what was in it.

Gas is new, tank is clean, put stabilizer in it for the heck of it.. no difference.

Removed the carb. cleaned it out throroughly and still same problem.


I had it here about a month ago and it would not even start.. i cleaned it up, the carb was filthy, has a bunch of debris stuck to the mesh screen in the tank preventing it from getting fuel. I cleaned it spotless and flushed out the gas tank, no rust btw.. and it fire right up first try.

Now. the only thing i can make sense of is the " Bladder " .. the black rubber membrane that seperates the carb and gas tank assembly.. it's plit in 2 layers and one part of it .. there is a spring that pushes aginst it.. is all caved in sorta deal. Canadian Tire did not have the replacment for it.

I told him to get one tomorrow and i'd put it in. it's the only thing i can think of.

The plug sparks fine... good strong spark. Fresh oil , i don't know what else it could be !

[Tri-ZNate]

when you say carb is part of the gas tank do you mean it is mounted on top of the tank?

The only thing I can figure is when you use the primer it sucks gas into the carb so it can start but the carb cant suck without you primering it. I think the way your gas gets transported to the carb is messed up

[bigredhead]

yes.. the bowl section is molded/ milled into the tank body.. and the top of the carb bolts right on.

[Tri-ZNate]

ok how does gas hget to the carb? a pump or vacume or what?

[bigredhead]

Well... vacume from the engine sucks gas thru a plastic tube that is mounted to the carb section going down into the gas tank.

From there it fills the bowl below the bladder, into the jets and mixed with air into the cylinder.

There is a spring from the choke assembly to a metal arm, this part makes no sense tho.. the airflow from the engine's top ( fan blades on top of the motor ) push against a plastic arm that turns the choke off once it revs fast enough to generate an air current.

[Tri-ZNate]

I would say you might be on the right track with your bladder theory. Try it out and let us know.

[MTS]

YEP its that diahphram between the tank and the carb, i just rebuilt one of them exact motors....you can get the diaphram from any small enginshop....heck i think sears carrys parts for them too,,,OH and NAPA dose...prolly the best bet :)

[Tri-ZNate]

NAPA carries anything Briggs. I walked in when I needed points and they had a wall just for points on Briggs and stratton engines.

[corndawg]

Well... vacume from the engine sucks gas thru a plastic tube that is mounted to the carb section going down into the gas tank.

From there it fills the bowl below the bladder, into the jets and mixed with air into the cylinder.

There is a spring from the choke assembly to a metal arm, this part makes no sense tho.. the airflow from the engine's top ( fan blades on top of the motor ) push against a plastic arm that turns the choke off once it revs fast enough to generate an air current.
that plastic thing is the governer, very simple how it works, once too much air is generated by the flywheel it blows it over and slows the eng down and if there isnt enough air to blow it over it pulls the throttle open

[bigredhead]

hey corndawg... yes, that's right...

However one of the problems here is that the spring that would normally keep it " Open and full throttle " is either stretched or got re-located along the years cuz it don't put no tension on the thing.. it just sags there doing nothing.

I'm looking for a diagram or schematic for the carb assembly but those suckers are hard to find online or i'm not looking at the right place.... You'd think they would be all over the place with the popularity of those engines.

Will look some more today... i'll get that thing running at some point !

[MTS]

if ya wait till i get home tonight i can get a pic of that spring off the one i got sittin in thr garage that i know works....

[bigredhead]

That would be awsome.. you would need to remove the bolt that hold the air filter and pop off the air filter to see how it mounts !

if you can do that, i'd much apreciate it !!! thanks !

[diggerzmound]

Yup the rubber gasket is a fuel pump..

And the governer spring hooks drictly to the pivot assembly that the throttle cable hooks to. The spring will not have tension on it until the pivot piece is moved to full throttle. The spring tension acts against the governor vane for just the right engine RPM's..

[Pistonhead]

I'm actually rebuilding that exact motor right now, I bet that gasket in mine is toast too, except I snapped the pullstart cord so I rebuilt a new one but I been away from home for like 2 weeks though so I need to get on it right away soon.