On a John Deere D110 with a B&S engine 33R8 77 0004 G1 it started a couple of weeks ago the engine surges forth and back at all rpms but when I hold the throttle steady the engine runs just fine. The minute I let go the governor starts going forth and back and everything goes crazy. The engine misfires and I can see the throttle linkage going forth and back trying to find its sweet spot but can't for some reason. If I assist the throttle manually the engine runs like a dream. Help!!!

Maybe somebody can answer this question. What is the solenoid for on the bottom of the carburetor?

SRT, where are you????
 
I did some more testing. I disconnected the governor completely from the throttle linkage to test the governor without having to get into the sump. I started the engine and manipulated the throttle manually with one hand and had my other hand on the governor lever. As I went up and down on the rpms the governor responded accordingly, smoothly and the lever went forth and back indicating to me that the internal mechanics are functioning properly. Looks like I don't need to open the crank case. But I found out something else. I was only partially right when I said that the engine was running like a dream. That was only in the higher rpms. Now I discovered that the engine always wanted to stall in the lower rpms. Now I suspect that there might be a problem with the carb after all. It's on my bench right now. Hence the question about the solenoid. Also in normal running operation is the solenoid energized or not. What is its function? My gut feeling tells me that the governor and its related parts, springs etc. are not the problem. I suspect the carb, I ordered a new one for the hell of it. It's only 15 bucks. If that doesn't fix it, back to the think tank. SRT I miss your input!!!

The solenoid on the carb is to block the main jet when you turn engine off. To prevent backfire or afterfire.
Surging is usually caused by dirty carb or restricted fuel system, varying fuel causes governor to see saw. I also have a d110

Thanks, Mike. So with other words the solenoid is always energized when the engine is running. One more question. What is holding the main jet in its place when the solenoid is energized?

That is correct. Main jet could be press fit or screwed in

Sorry, I didn't get any notifications of new posts...
 
As Mike said, the solenoid, when activated with 12 volts (crank/run) opens and allows fuel to flow.
 
Here is your engine's parts diagram:
https://jackssmallengines.com/jacks-parts-lookup/manufacturer/briggs-stratton/briggs-stratton-engine/200000-399999-series/33r800-to-33r899/33r877-0004-g1 
 
As Mike said, the main jet can screw in or be "set" in place.
There's 2 carbs available for you engine.  The Ruixing or the NIKKI (see diagram on above link).
 
The solenoid just screws into the bottom of the float bowl.  If they sit a long time, they can get stuck 
in the closed/extended position.  If any doubt, I remove the solenoid and cut off the pin that extends(and stops fuel) from flowing..   The solenoid now doesn't need power/can't go bad as you 
removed the pin...  As long as the carb float/needle are properly working, it won't hurt anything.
 
The Ruixing is the basic/ built like most carbs.  
 
The NIKKI, IME, is junk.  The large plastic part that holds the float/needle/jet (jet is stuck right down the middle) uses a funky "gasket".  Some NIKKIs have a separate o -ring that goes bad, leaks or just doesn't seal...  
 
Did you re-set the governor with the carb?
 
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Scott

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Yes, the carburetor in question is a Nikki and as I mentioned before is sitting on my bench. My gut feeling just told me that the governor was not the problem, all the springs looked good, so I concentrated on the carburetor. I actually ordered a new one because the are so ridiculously cheap like 14 bucks, but I messed with the old one anyway out of curiosity and when I sprayed carb cleaner thru all the holes etc. my hand was getting wet and i noticed spray coming out of the side of the carb. There are 2 long screws holding the intake manifold to the carburetor and when I removed one of them a welch plug came falling down. I knew right then that this was my problem. I carefully put the welch plug back, slightly hit it with a punch to make sure it won't come out again, put everything back together and sure enough the engine surge is gone. The engine runs like a dream in all RPMs. Problem solved!