Murray - Briggs and Stratton 4.5HP - Surging

[gibbywmu]

Hi All,

Trying to restore a mower for my mother. I purchased a new carb, air filter, and sparkplug about a month ago. I'm pretty sure I fired it up and let it run for a while at her house (didn't mow), and it was fine. She recently told me she thought she put some old gas in it, and it started running funny. She did empty the gas out, and put new in. I just tore the carb off again and inspected to see if anything was abnormal. Seemed fine. I also sprayed with carb cleaner just to make sure no old gas was in there.

When you hold the governor steady, the engine run normally..........it just hunts and surges and won't stay a steady RPM. What could this possibly be? Every bit of research points to a dirty or problematic carburetor. How could that be when this one is brand new?

Any direction on this would be helpful.

Thanks!

 

[Russ2251]

Surging is due to a lean condition.
The fact that it is (was) a new carb has been negated by the use of old fuel.
There is a restriction somewhere.

 

[gibbywmu]

I've ran it so much the past 24hrs there can't be any old fuel left. I went back and retightened all of the 6 screws that hold the carb on the tank, in case there was a leak. The air intake look fine, no cracks. I can't figure out where this thing is sucking in air. I also made sure the vent holes were clear in the gas tank cap.

Should I just buy another carburetor and test it out, even though this one is brand new? I figured since they are only $10 or so on Amazon its worth the shot. I'm completely stumped.

 

[Rivets]

When you changed the carb did you change the carb to tank gasket and the carb to manifold oring? Spray your carb cleaner around these areas and see if the surging disappears.

 

[bertsmobile1]

It might be a local thing but a lot of those Briggs I see have a crack in the manifold tube.
Always underneath where you can not see it.
Be careful not to overtighten the two 1/16" bolts that hold it in they are easily stripped.
Also fit a new gasket it is very thin and wears then leaks.

 

[gibbywmu]

Thanks everyone for the help! Finally, after changing out carbs, gaskets, and a gas tank from my older B&G Pressure washer, I finally got it working. I actually fully removed the intake manifold. It appeared normal with no cracks or wear marks. After I reinstalled, and assembled everything back together, it didn't have a surge! Maybe I didn't have that tube from the carb fully seated onto the manifold? Not really sure.........lol.......one of those things where you just start swapping stuff and it works! Not on to my pressure washer rebuild!