OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines SB45 - B&S 3hp

Hi my sb45 has problems idling it hunts and eventually dies the governed moves back and fourth the model number is 91212

Hi Michael. The symptoms you describe are common, and nearly always result from lean mixture. With Briggs engines the usual cause of lean mixture is that some gum has built up between the carburetor's needle and jet. I suggest you try turning the mixture adjustment screw anti-clockwise slightly and see if that clears up the problem. You may only need to adjust it about an eighth of a turn.

Note that yours is of course a horizontal crankshaft engine, and the horizontal crankshaft Briggs engines have flange-mount carburetors rather than O-ring seals as the vertical crankshaft ones do. It is not unknown for the carburetor's two attaching bolts to come loose, allowing some air leakage across the flange. You should check that the mounting bolts are done up firmly before you begin the tuning process.

Where is the adjustment screw?

Grumpy, Don't forget some these carburetors uses the fixed idle jet instead of variable mixture screw that need to be removed and cleaned. Also a failing fuel pump diaphragm gasket can cause problem especially at idle speeds.

Thanks AVB. I note this one is a relatively late engine for a Scott Bonnar 45 mower - earlier they used the 60102, 60202, and the equivalent 8 cubic inch engines, and from memory all of those had the adjustable main jet. However this one is a 91912, and may well have the fixed jet. We need the engine's type number, so we can have a look at the Illustrated Parts List.

All sorted but shall I take that screw out and spray throttle body cleaner in there? The screw was all the way in tight. And it did die on me half way through my lawn but restarted with choke on

If the carburetor is the later model shown in AVB's illustration, the screw is supposed to be all the way in there and tightened. You need to remove that screw, clean the holes in it, and spray cleaner into the recess behind it, under pressure, to clean the drilling in the carburetor body. However I'm not clear on just what you have and haven't done. Did you remove the carburetor, clean the two filters, clean the main jet, and inspect and most likely replace the diaphragm? The carburetor is almost certainly still in need of attention, since it stopped running and needed choke to restart.

Ok I've done everything you said apart from replacing the diaphram and it seems to be fine now and it was that exact carb avb posted. Thank you both for your great knowledge! I now have a excrllent running sb45

Michael, it sounds as if it starved for fuel when you had run it for a while. By far the most likely thing to cause that would be a deteriorated diaphragm. The diaphragm is the key component of the fuel pump.

Yeah I will change it just to be sure

Good thinking, Michael. It is probably about 25 years old, which is a very long time for a synthetic rubber flexible diaphragm - especially if the previous tenant ever used fuel with ethanol in it.