engine will not max

young99 · Mar 3, 2012

I have a strange issue with my push mower and I was hoping to get your take on what the problem might be.

I have a small little Briggs & Stratton 500 series push mower. It cranks up just fine but the engine will not max out and run at full power in a flat position. I noticed if I lean the mower back a little bit the engine will pick up and run just fine. if i lower it back to the ground the engine will die out and will take a while before it will crank back up.

Originally i was thinking it was a dirty fuel lines or filter. I removed the tank and looked the the filter and it was very clean.

Today I drained the oil out let it set for a while and put fresh oil in. Did not seem to help. I was wondering if anyone may have an idea of why the mower would act this way.

pugaltitude · Mar 3, 2012

Sounds like carb diaphragm/gasket set is needed.
Purchase part no 795083 from briggs supplier.
Remove fuel tank and carb and replace.
Give the carb a good clean while its off.
Check and reset engine rpm and job should be a good one.

reynoldston · Mar 3, 2012

pugaltitude said:
Sounds like carb diaphragm/gasket set is needed.
Purchase part no 795083 from briggs supplier.
Remove fuel tank and carb and replace.
Give the carb a good clean while its off.
Check and reset engine rpm and job should be a good one.

Why would the RPM's rise when he tilts his mower back? Maybe the carburetor is loose and changes linkage adjustment? draws vacuum?

young99 · Mar 3, 2012

pugaltitude said:
Sounds like carb diaphragm/gasket set is needed.
Purchase part no 795083 from briggs supplier.
Remove fuel tank and carb and replace.
Give the carb a good clean while its off.
Check and reset engine rpm and job should be a good one.

I am going to give the carburetor diaphragm a shot. I stopped by my local lowes and seen they had a briggs and stratton part number 495770. The box says fits most 3 to 5 hp push mowers. This looked similar but could not remember the exact pattern. Think this would be a similar pattern or should I order the one you mentioned?

pugaltitude · Mar 4, 2012

young99 said:
I am going to give the carburetor diaphragm a shot. I stopped by my local lowes and seen they had a briggs and stratton part number 495770. The box says fits most 3 to 5 hp push mowers. This looked similar but could not remember the exact pattern. Think this would be a similar pattern or should I order the one you mentioned?

That was the original part no and super seded onto the part no I quoted.
When you fit the new set, tighten the screws for the carb to the tank diagonally and not tighten in a
circle as this causes the carb to distort.
Could also be a cracked inlet manifold or seal but try diaphragm set 1st.

young99 · Mar 4, 2012

pugaltitude said:
That was the original part no and super seded onto the part no I quoted.
When you fit the new set, tighten the screws for the carb to the tank diagonally and not tighten in a
circle as this causes the carb to distort.
Could also be a cracked inlet manifold or seal but try diaphragm set 1st.

I replaced the carb diaphragm and the mower ran correctly. I did notice one new issue and it is with gas leaking from the new diaphragm. I tried my best to screw this back together diagonally. Back right, front left, back left, front right, then the side screw.

It is leaking from the back of the diaphragm/carb area and i notice a little lip of the rubber hangs out of the back. Would this be normal or should I trim it back some so that none shows?

The screws that came with this appears to strip out easy. I will need to replace them as well.

pugaltitude · Mar 4, 2012

young99 said:
I replaced the carb diaphragm and the mower ran correctly. I did notice one new issue and it is with gas leaking from the new diaphragm. I tried my best to screw this back together diagonally. Back right, front left, back left, front right, then the side screw.

It is leaking from the back of the diaphragm/carb area and i notice a little lip of the rubber hangs out of the back. Would this be normal or should I trim it back some so that none shows?

The screws that came with this appears to strip out easy. I will need to replace them as well.

Did you fit the set the right way.
Diaphragm on tank then gasket on top of diaphragm.
The gasket should seal against the carb.

young99 · Mar 4, 2012

I am pretty sure that I put this on correctly. the holes and cut-outs lined up correctly. The thinner flimsy rubbery piece on bottom against the gas tank. the thicker not as flimsy gasket on top against the carb.

I pretty sure I put it on the same way as the one that came off.

pugaltitude · Mar 4, 2012

[/QUOTE] It is leaking from the back of the diaphragm/carb area and i notice a little lip of the rubber hangs out of the back. Would this be normal or should I trim it back some so that none shows?[/QUOTE]

Dont trim it as this is how it should be.
Thats the area that causes the bad running as the diaphragm gets sucked in and they kept more gasket and diaphragm on to compensate it.

9 out of 10 they dont leak but that 1time it could be a faulty set.
All you can do is check everything again and make sure everything is tight.
At least the running problem is fixed and you know the leak is something that has happened since you changed things so it should be easily fixed.
Is there a bit of dirt not allowing the set to seal?

young99 · Mar 4, 2012

That is what i am guessing. I am going to buy new screws for the carburetor. When I replace them I will check re-clean the holes to make sure there is not debris.

Thank you for the help.

engine will not max

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