briggs and straton 3.75 horsepower engine,starts with a primer bulb. My oldest son (14 yrs old) put the wrong gas in it,he put gas mixed with oil for the weedeater,he has done this the last 3 times he cut the grass. Now when the mower is running,the engine speed is only about half what it should be,and the engine surges high then almost dies,it does this the whole time it is running. Smells like it is running rich,and it is putting out a little black smoke. I put in a new spark plug,the old one looked like it was fouled,and dumped the old gas out and refilled with fresh gas. no change,still runs the same. I even squirted some carb cleaner in the carb while it was running,no luck. Did putting the wrong gas in it have something to do with the way it is running now,or is it just coincidence? I just need some other ideas what to check/clean replace. Not good with small engines,never had to do much to them besides change oil,air filters,and change plugs. thanks for any ideas!!

 

I'm no expert but I would take the exhaust apart to be sure it is not plugged with half burned oil.

 

I would yank the carb off and give it a good cleaning.

 

Yep - pull the bowl, take out the main jet and blow it out. Careful not to lose the needle!

 

Yup....going to have to clean out the fuel system. That oil residue will continue to cause it to run like garbage until it's completely burned out. You'll probably have to change that plug three times before that happens, so it's probably just best to clean out the carb completely, and get that mixed fuel out of the bowl.

 

One tankful of pre-mix for use in a weed eater accidentally put into a 4-cycle lawnmower engine will make it smoke a bit, but shouldn't give you a major problem. It may foul the plug, but it shouldn't plug the carb - unless you've severely over-mixed the pre-mix to begin with (would also give you a problem with your weed eater, too!)..

Sounds to me like you've plugged up the air filter with dust from the mowing operation, and the engine can't get enough air. This'll drive the fuel-to-air ratio much too high, and engine speed surging you've described is the typical result. Pull the air filter off and clean it. If it's a sponge-rubber type, wash it in soapy water, squeeze it dry in a paper towel, put about 1 teaspoon of oil on it, re-install, and you'll be good to go. If it's a paper-type filter, just replace it (or blow it out from the inside with an air hose).

Please post back as to what you find with the air filter. Thanks.

PS: DON'T yell at or lay a guilt trip on your son! He's not to blame for anything. You're air filter is just plugged up from the dust of mowing! It's no big deal!

 

Take the top fan shroud off, and check to make sure nothing is blocking the air intake on the fan, and that the large air paddle moves freely. Also clean all of the throttle linkages and springs with WD40 or carb cleaner. Also check the throttle valve by hand to make sure it moves freely and easily (accounting for the resistance of the spring that is )

A little bit of two stroke mix gas will never hurt a 4 stroke. Helps lubricate the cylinder and valves if anything.

 

Forgot to mention in my 1st post that I did clean the air filter also. I even tried to run it without the air filter for a minute,same result. I did not yell ,or even say much about it to my son,just told him he was using the wrong gas to fill it up,and he told me he had used the same gas the last 3 times. I showed him again which can has the regular gas in it,guess it is time to get the sharpie out and mark the cans Thanks for all the ideas so far,guess I will have to tear into it a little farther than I hoped

 

lead head,I am not sure what you are referring to as the top fan shroud. The only shroud the on the mower is the one that cools the head of the engine and also has the pull string wrapped up in it. is this what you are referring to? Air comes in through the air filter,correct? not the shroud? I am confused about what you mean by the air intake on the fan,shows what I know about small engines,is the mower getting air from somewhere besides the air filter?

 

The engine is air cooled. There is a little blower located underneath the fan shroud (the black rectangular cover over the top of the engine with the pull start).

These engines have a governor which tries to keep the engine between 3400-3600 RPM. The governor on the briggs is a little plastic flap near the cooling blower, and based on how much air is flowing over it, it adjusted the throttle position. Sometimes that can get gummed up. If I thinking of the right enigne, there should be 3 bolts total holding the shroud on.

Take the air filter off, and clean all the throttle linkages you can see with carb cleaner.

 

No,what he is referring to is cooling air,,not intake air. I agree,that restricted cooling air will cause it to run poorly if it is running too hot but,from what you have described,this happens all the time,not just after the mower has been run for awhile. I also agree that 2 stroke mix put into a lawnmower occasionally will not harm it. Heck,I have done it myself,,intentionally! Got the yard almost done and ran out of regular,,so tossed some mix in it and finished the yard. A trick you might want to try before tearing anything apart. Drain the tank,,buy a can of injector cleaner,,,remove the spark plug,,pour the injector cleaner in the tank and pull the rope a few times to get it into the carb. Let it soak overnight,then in the morning,,,pull the rope a bunch of times to pulsate fresh cleaner from the tank through the system. Then drain what is left out,,remembering to drain the float bowl,,fill it with fresh fuel and a new plug. If that doesn't show a noticeable improvement,then your problem is not a little oil in your fuel system.Also,,if indeed he has run mix through it multiple times,,then it may very well be a restricted muffler,,,easy enough to check,,take it off and fire up the lawn mower,,if it runs ok,,buy a new muffler. Sure you can try and clean it,,but I would replace it,they are not expensive.

 

> Did putting the wrong gas in it have something to do with the way it is running now

No, I run my chainsaw mix 40:1 in my 4 cycle engines all the time.

From fixing a bunch of B&S engines I have fond two things usually lead to this up and down cycling. Bad coils and the float. The float bowl gets clogged and the needle sticks. Some have springs, some do not. If you get the float kit it should come with a foam bushing the needle pushes into. Replace that along with the plastic float.

Do not blow out the housing when you take the carb off, you can damage the little (reed?) valves. I did that once...

I look up the B&S kit numbers and then order them from Tulsa on E-Bay.

 

Thanks for the ideas! I THINK(yeah,scary I know) it is fixed. we(my son and I) took everything apart that involved the fuel system and cleaned everything out. seems to run like normal now. Only question I have left is,how do I set the "coil"(magneto?)I put a piece of paper between the "coil" and the "flywheel" thinking i remembered this from high school way back when,but not sure if that is the right space,and i don't want to burn something up. Funny thing when we were working on it,my son kept asking me if I knew what I was doing and had I ever done this before. I had to be honest and say,no we are learning this together!! then he started to get really worried about the lawn mower never running right again!!

 

Coil gap is .010 thou, but a business card will work. The coil will fire at 3 times that gap, FYI.

First thing to check is the air filter. Pull it off, run the engine. If there is no change, you know the problem is not restricted air flow.

Second thing to check, given the conditions, is the spark plug. If that is ok, the next step is to look at where the governor spring hooks up to it's support bracket. Bending this bracket forward will increase speed, and bending it back will decrease speed.

Third is to go into the fuel system. The carburetor in question has no float or needle assembly. It is a diaphragm carb. While your at it, you may as well replace the diaphragm, as they go bad often in these engines.

The exhaust on this particular engine is nothing more than a pepper-pot muffler. It would take a very, very rich 2 cycle mix to clog it.

I realize this is a pretty redundant post, but maybe it will help-

Regards,
Andrew