16.5hp Briggs surges FIXED!
 

I've got a throw away MTD mower with a 16.5 hp Briggs I/C engine that has a surging problem that I can't figure out. :bigthink: It acts like there's something goofy with the governor but I'm not sure if it's internal or what. With the throttle wide open it revs between full throttle and about half throttle and I can watch the governor arm move back and forth opening and closing the throttle. Below about half throttle it just dies. If I engage the deck it smooths out and runs better. Pulling the choke cable out a little helps smooth it out too but it has no power with the choke on.

Before I tear into it, I'd like to know if it's something I can fix without opening up the engine to look at the governor.

I've already looked at the carb but it's a fixed jet with no adjustment. I also checked for vacuum leaks between the carb and engine.

The only reason I want to fix it is so I can sell it this spring. It's getting replaced by a Cub.:biggrin2.gif:

I'd bet a dollar to a donut it is running too lean.
They are set way lean from the factory because of emissions,
Then add a little varnish & crud and it surges, hunting for fuel.
I'd take the carb apart and clean it up using laquer thinner and blow out the ALL the passages.
It can't hurt and will cost little if you can save the gaskets.

funny thing. I have 2 throw aways : both seem to have a different problem, but identical in both engines. Both engines have
GREAT spark. No question this would run any engine. But neither get any fuel. Both have a solenoid cutoff on the carb bowl, but one engine is a bs, and the other is a Kohler. I took out the solenoid on the kohler and got it running with a bolt/ nut and gasket to replace the solenoid. runs great . solenoid works fine outside tractor. why not in tractor when its jumped with a car?

As you probably know, the sol valve closes with no power applied.
Make sure you are supplying power to it to keep it from plugging the main jet when running.
They were installed to do 2 things:
Stop fuel flow from the float bowel to emulsion tube/venturi for emission control,
and to prevent Dieseling (run on)

IHinIN, I have the same problem with my crapsman with a 19.5hp twin B&S. There is a sweet spot on the choke I have to find for it to run smoothly. I have rebuilt the carb, replaced vac. line, and checked for air leaks(none found). Nothing helped. I also talked with a local lawn mower mechanic and he said there is really no fix.

ol'George, I'll check the wire for power.


That's not what I wanted to hear. I may have to sell it as-is. It still runs fine as long as the deck is engaged so I'm wondering if it's because it puts a load on the engine or if the blades spinning has a flywheel affect that helps the engine run smoother. I'm just trying to think of all possible causes. I'm thinking if it was a carb issue it would still run bad or even worse with a load on the engine.

Could it possibly be a bad governor? I've tried messing with the spring on the governor arm with no success. I tried a shorter spring but then the engine would start revving too high so I put the stock spring back on.

IHinIN, you said you checked the vac line from the engine to the carb, did you remove it and inspect it? That line can crack where it slids on the nipple to the engine from all of the heat it gets.

Fixed it! :biggrin2: Turns out it must have been an intake leak. I took off the carb and plastic intake manifold, cleaned the carb, and put it back together with some silicone between the intake and head, and carb and intake. Now it runs just like new. Time to sell it now. The Cubs are starting to give it dirty looks.