Outdoor Power Equipment and Small Engines - Why does my Briggs mower tend to surge

As a woman, I've overcome the female fear of trying to fix things. Not having the money to give away to other folks has largely been the reason for the dawning of my courageous attitude. Being relatively mechanically inclined and having had a few successes in repairing things, it has only amplified my belief of my abilities. I've received wonderful advice from the people that frequent this website. For that, I am very grateful. Here's my latest problem.
My mower has a tendency to surge when at lower RPMs. I have spent the large part of the day trying to set the carburetor settings of my 18.5 HP Briggs Twinn II engine. The carb is a diaphram type with one adjustment screw (needle valve) located below the rectangular diaphram on the front side of the carb. The engine seems to run better when this is set at one turn out. To the left of that screw is another adjustment screw that acts as a stop for the governor. If out too far, the mower dies. If set just a little bit in, the idle is too high. When the mower is running, I can watch the governor linkage move back and forth a teeny bit, causing it to surge. The adustment to this screw is very sensitive. There's not a frogs hair of difference between too high of RPMs and engine dying. I'm wondering...if the needle valve is set correctly and the governor is set within limits, what in the heck am I supposed to do to lower the RPMs?
I cannot tell what screw is for high speed adjustments and which is for low speed. What effect will in and out adjustments to the throttle screw do? Help, fellas...

The Carburetor adjustment steps are listed in the owners manual. I posted the page at http://www.geocities.com/strim01/carbadjust1.jpg Hope it helps.

Thanks. The page helped me further understand a couple of things. I really appreciate the time it took to locate that information and post it. You are the BSTheyAre. :p
Nancy

First let me say I've never heard of a frogs hair...but I like the sounds of that. Seriously though, you didn't provide any engine numbers but we'll tackle the problem based on what you have stated and what I believe you have based on my knowledge. First, the carburetor is not a diaphragm type...it is a bowl type. The diaphragm you referred to is the fuel pump (which is an integral part of the carburetor). The first needle you mentioned is the idle circuit mixture screw. This controls fuel flow to the venturi only at idle speed. The second screw you mentioned is the idle speed screw. This dictates the RPM at which the engine runs at idle. I believe you have a fixed high speed (mixture) jet in this carb whereas there is no adjustment for RPM's above idle. And, I believe, this is where your trouble lies. The "hunting" is likely related to this. Just to the right of the idle mixture screw (facing 90 degrees to such) is a plug. Behind this plug (you will need a 5/8" wrench) is the fixed jet. A special tool typically is used to remove this so as not to ruin it but you can use a small flat-blade screwdriver to remove it as well. The small hole machined in this likely is plugged partially. If I were to do the job, I'd want to pull the carburetor off the manifold and service it fully with a complete carb rebuild kit (which includes fuel pump rebuild parts as well). The average cost of the kit is around 20 bucks. Have some spray carb cleaner as well as compressed air available when you take apart the carb and fuel pump. A Briggs service manual (part #271172) available at most Briggs shops will help you out greatly when servicing such. Good luck and let us know how you make out. If you don't have the tools or willingness to do this, you could still bring the carburetor, alone, in to a Briggs shop and have them do the work for you. I can't speak for other shops, but we charge $12-20 to service a carb, depending on the carb.

With your help, I just may know what I'm doing when this is all over. I have a couple of non-carb parts on order, so work on this mower will resume when they come in. The carb is not running Ed Zachary like I'd like it to, but it is better. The RPMs are a little high and there's a little stutter when I go from low to high RPMs, but when work resumes on this baby, a copy of the information you so generously provided will be right there with me. Thank you soooo much. I will definitely let you know how it turns out.

Nancy

P.S. A frog's hair is verrrry fine. I guess that's why most have never heard of ....or ever seen one. :p