Small Engines (Lawn Mowers, etc.): Another surging question, mikuni carb, air mixture

Question
QUESTION: Hi John, you provide great info - thank you for your efforts on this website!!  I have a 1990 toro 26624 with a 4hp suzuki 4cycle engine and mikuni carb.  New for this mowing season, the thing surges all of the time without the BBC engaged and sometimes while the BBC is engaged.  Yesterday, after mowing for about a half hour, it died all together (with the BBC engaged) and wouldn't refire.  Leaving the choke on got it running again, but it quickly died again.  I've read your responses to other people with similar problems and I have tried all that you have suggested with cleaning the carb thoroughly (done it several times now).  I have actually replaced the carb with a backup one that I had from an identical mower (same exact carb) - I've thoroughly cleaned it as well.  The mower is running again now, but still surging, though not quite as bad.  Being that I changed carbs, I'm obviously thinking that the problem likely lies on the engine/mower somewhere else...or I have a bad gasket(s) around the air cleaner/carb.  I went and ordered a new governor spring, but I don't have it to install yet.  Do you have any thoughts on other things to check?  Should I be concerned about valve problems?  Is it likely that I have two dirty carbs (despite my efforts to clean them)?  This mower has been great and is otherwise in top condition for it's age - it literally manicures my yard.  I hate to just toss it to the curb, but I'm getting frustrated!  Thanks in advance!  Mike

ANSWER: Hi Mike:

Thanks for the Good Words. I am Glad I can Assist Others and Make Their Yard Work Easier and Less Costly. Don't Get Frustrated. It Sounds to me like you have Found your Perfect Mower for your Yard Work. This is Rare. 99.9% of the Time, People Complain to me that they Can Not seem to Find a Mower that Makes their Yard Look the Way they want it to Look. Did you Try Setting the Idle Air Mixture Screw to Eliminate the Surging? The Following File is for a Carburetor that Uses Both Air Mixture Screws. Use the File as if this carburetor had Both Screws. Just Ignore the Parts for the High/Full Throttle Air Mixture Screw.  Now Setting the Air Mixture Screws:  ****Turn the Idle Air and Main Air Mixture Screws In Until SNUG **Do Not Jam** Then Reverse Both Screws 1 1/2 Turns. Holding the Throttle Full Open, Start the Engine. Turn the Main Air Mixture Screw Clockwise Until Proper Revs are Obtained. Occasionally you May have to Turn this Counter Clockwise to Achieve the Proper Revs. Now Allow the Engine to Idle. Set the Engine Idle Screw (Not Idle Air) so the Engine will Stay Running if Required. Now Set the Idle Air Mixture Screw so there is No Hesitation when Throttling from Idle to Full Throttle. Reset the Engine Idle if Required.**** You can View a Breakdown/IPL of the Engine and Mower at this Site Addy, http://jdpc.deere.com/jdpc/servlet/com.deere.u90490.partscatalog.view.servlets.H and Enter the Model Numbers and Click Find. Then Select your Model from the Search Results Area. Then Select the Sectional File. Then Select the Section of the Mower you Wish to View Online. 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. Thanks.

Good Luck

Respectfully

John

PS: I am Allowed to Answer 10 Questions a Day. If you See I am Maxxed Out, then Try Submitting your Question at or Just After 8pm EST (US). My New Day Starts then. Thanks.

Respectfully

John

---------- FOLLOW-UP ----------

QUESTION: John - thanks!  I have played around with and set the one screw that is on the carb - no luck.  I just came back in from the garage after cleaning the carb thoroughly for the 4th time.  Still, no change in the surge.  I even went as far as to remove the head to see how the head gasket looked - it too looked fine.  I changed the governor spring - no change in the surging.  Do you have any other ideas for me?  Any chance I've got a cracked head or something along those lines?  Thanks again!!

Mike

Answer
Hi Again Mike:

If you Close the Choke Slightly, Does the Surging Stop? It Really Sounds like the Carburetor Jet is Still Restricted. Try Soaking the Carburetor Overnight 1 More Time. I will be Doing Research in the Shop Today, but I will Check with a Contact at Suzuki and See if they can Shed Any Further Light on a Possible Cause. Thanks.

Respectfully

John

PS: I am Allowed to Answer 10 Questions a Day. If you See I am Maxxed Out, then Try Submitting your Question at or Just After 8pm EST (US). My New Day Starts then. Thanks.

Respectfully

John

Hi Mike:

I Spoke with Suzuki and they Feel the Carburetor or Air Leaking Into the Intake is Causing the Problem. The Only Other cause they Could Suggest was the Valve Clearances May Require Resetting. Stanley will be Back in Touch with me Again Today and I will Follow Up if he has Anything to Add. Thanks Again.

Respectfully

John