OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs & Stratton surging

Hi all. I'm wondering if someone can help with a problem?. I have a Rover cylinder mower with a Briggs & Stratton that surges at idol and at full throttle.
Model 91212.
Type 1025EI.
Code 0006093A.

I suggest you begin by establishing whether this is a case of hunting, or something else. For reference, and you may already know this, surging is a technical term that refers to a specific effect of lean mixture. Hunting is a term applicable only to governed engines, and can either be due to lean mixture or a malfunction in the governor - usually, friction between two parts rubbing together.

Your engine was made on 9 June, 2000. Here is the Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/18jsBLXBnfBhU7y.pdf

Here is the Operator's Manual:
http://bsintek.basco.com/BriggsDocumentDisplay/flmvtLX-nfBhU7y.pdf

From its date of manufacture, and from the presence of Item 211 on Page 5 of the IPL, it seems your engine has governed idle: there is no idle speed adjustment screw, and the engine is under governor control even at idle.

I suggest you begin by checking whether the problem is in fact lean mixture. You can do this, both at idle and at higher speeds, by letting the engine run until it warms up, then at each speed, try moving the choke lever slightly. If you find that you can make the engine run steadily at any speed by finding the right choke setting, but its speed cycles continuously when the choke is wide open, the problem is lean mixture.

Your carburetor is an emissions type, with no provision for mixture adjustment. Lean mixture could be caused by a blocked vent in the fuel filler cap, an air leak at the carburetor mounting flange (where it is bolted to the intake port of the engine) or by a carburetor malfunction. When you get to the point where you know whether the problem is lean mixture, and whether the filler cap vent is blocked or there is a gasket leak at the mounting flange, please report back and we can talk about next steps.

Hi grumpy. Thanks very much for your reply. I'm a little time poor so its taken me a while to try the things you've suggested. I tried a different fuel cap and fitted a new gasket to the carburetor mounting flange and I'm satisfied the governor is moving freely however there is no change to the way the engine runs. It does however run smoother at idle and revs with the choke half closed. Looking forward to your next suggestion.
Thanks. Bergy.

It sounds like lean mixture then, and you will need to look at the carburetor. Please post pictures of the carburetor. It appears to be a version of the Pulsa-Jet with governed idle and a rotary choke, but let's be sure we are on the same page about that.

Once we are both sure which carburetor we are talking about, you will need to remove the carburetor and attached fuel tank from the engine, then remove the carburetor from the fuel tank. Turn the carburetor upside down and check that the fuel screen is clear (circled in red in this illustration from the manual):



If the screen is clear, we can look at the main jet, but the screen is the usual cause of your kind of problem. If there is dirt or, more likely, grass on the screen, you will need to remove the fouling. I do not recommend following the instruction on the diagram to routinely cut the nylon tube, since you'd then need a new screen: usually you can avoid this disassembly process and clean the screen from the outside with a soft brush, being careful not to push the rubbish through the screen.

Hi grumpy. I removed the fuel tank from the carby and discovered a little fouling of the fuel pick up tube screen, however the other screen which dips into the secondary tank within the main tank was fouled with rusty gunk. I cleaned the screens as well as the fuel tank, put it back together, but to my disappointment made little improvement to the way it runs . See photos below.

It looks as if the fuel tank has had water in it, from the brown sludge in the well that the main jet screen dips into. The main jet is right behind that screen, either in the bottom of the tower or in the side of it near the bottom. Clean that jet and the passages behind it with carburetor cleaner, and inspect it afterward to be sure it is clear. Was the well below the main jet screen full when you removed the carburetor from the tank? The purpose of the fuel pump is to keep that well steadily overflowing back into the tank. The pattern of the crud on the filter suggests it may have been barely reaching down to the fuel in the well, which would imply a fuel pump malfunction. Are you sure the tube that leads to the fuel pump intake screen (the longer tube, with the screen that was only slightly fouled) is clear? I suggest you revisit the fuel pump overhaul you did earlier, and make sure all is well there.

Hi grumpy. I removed the fuel tank again bit could not work out how to remove the main jet. I didn't have any carby cleaner so I put some compressed air through it, put it back together, kicked her in the guts and low and behold she seems to be running just fine now . Thanks very much grumpy for sharing your knowledge, you helped me out big time.

The main jet is not detachable, Bergy, you squirt carburetor cleaner through it in situ. I don't approve of what you did, because it would have made the situation worse rather than better if there had been gum in the jet along with the rust-dirt. At best, you didn't remove any gum that was there, and it will continue to build up slowly. However it appears there wasn't much gum, or you were lucky, and the dirt blew through the jet successfully. I don't think you've done any harm, and you got it working properly, so all is good. I'll close this thread.