SP740 mountfield mower is not doing anything when i push the primer button. i used to hear and feel liquid but its empty just hear air. i have oil and petrol in it, what am i doing wrong

If you have any length of stud sticking out then after applying Liquid Wrench, grip the stud with vise grips and try to turn out.
If studs are broke flush with base then they must be drilled out and taped. You may need a fix-it shop to do that for you.

no it doesn't. the primer bulb is on the air filter. it will just pop off with a pair of pliers and push the new one in with a socket. fairly easy job. good luck

sound like you need to clean the carb the primer bulb is only used to start not run

There is a tab on either side of the carburetor right where the primer bulb sits. You can depress both of these tabs with a screwdriver and pry out the plastic ring that holds the primer bulb in place. You can purchase a new primer bulb (with a new plastic tab) from a local mower shop or from a small engine repair shop. When you install the new primer bulb, put the rubber bulb in first then put the plastic ring on top and press it in until the tabs click on each side. It should work correctly after the tabs are clicked properly. You can take off the air filter and look into the carburetor to see if the unit is priming correctly. I hope this helps you out.

Yes you can. The only reason small engines have the primer button/bulb is to get gas to the carburator to make it easier to start it. If your mower starts OK without using the primer bulb you will not harm the mower.

check the magnito there may be build up of rust on it.
it could also be a bad sparkplug

You have overfilled the oil tank. White smoke is only a hassle, not a serious problem. You have to empty some of the oil and change the gas. Clean the air filter if it is the spongy type with Dawn Dishwashing Liquid, rinsing and drying it well before reinstalling. The white smoke will stop soon as it is still in the gas that is in the carb. If the case is that you haven't filled it with oil prior to this smoke then you must be mowing on a hillside and the gas tank was on the lower side allowing oil to leak into the gas. Same solution.

The primer bulb might be damaged, or the carburator diaphram could have been damaged if carburater cleaner was used.

either the magneto or the governor is damaged check them

Have you Contacted Mountfield about a Manual? You can Contact them Using the Options at this Site Addy;
http://www.mountfield-online.co.uk/contact?jssCart=13a24ca4de2a2610f7213de8a0539ffb .
For an Engine Manual you can Contact www.outdoordistributors @ 877-707-2787 or Contact Tecumseh @ 800-558-5402.
Please, Do Not Hesitate, If you Hit a Snag or this Does Not Correct the Problem, I am Here if You Require More Assistance.
Hope this Helps. Let me Know What Happens, Please. May the All Mighty Bless You and Yours. Be Safe and Be Happy. Thanks.

Good Luck

Respectfully

jbridger (John)