OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines B&S 3.5 91202

hi guys, been a while since postsed. Getting warmer so decided to clean up my cylinder mower & get my lawn back to the condition it was in my pic top left. Anyway didnt use it last year so the mower has been sitting for a while without being started. Cleaned spark plug, new fuel & started second pull. But...3 issues hope you can help
1. Engine wont idle. Have tried the idle screw but there is so much vibration in the linkage, see pic circled, that it is very difficult to adjust when running. Should this linkage be loose at all?
2. Hunting/ surging. At say half throttle when I engage the clutch and load is placed on the engine, it will surge. Seems not to like the extra load of spinning the reel etc. Seems generally to be running rough as well (replaced rubber diaphragm couple of yrs ago)
3. Clutch engage lever flicks over and engages by itself when the engine is running. Perhaps due to vibration. Obviously this is dangerous so need to fix thit issue.
Engine ID 91202 0016 04 9408103
thanks for your help- Ross

Here is the Operator's Manual for your engine:
http://bsintek.basco.com/BriggsDocumentDisplay/msvtJR-K_ny7tH217FzoPu.pdf

Here is the Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/18jsBLXBnfBhU7y.pdf

The first point is, I have not heard your engine so I can't tell whether the problem is intermittent ignition, or lean mixture: it is probably one or the other, though a sticking or leaking valve is also possible. To clarify the ignition situation, how easy is the engine to cold-start with choke, and to warm-restart without choke? Does it cut in and out, or does it misbehave completely consistently? When you say it surges, do you mean it continues to fire with a constant speed setting, but sometimes the throttle opens and closes by itself? That would be surging, as distinct from intermittent ignition, which causes the engine to fire in bursts, and probably backfire when it restarts each time.

If the engine is really surging, not cutting out, and if it cold-starts fairly well but is reluctant to restart without choke when warm, all of your difficulties could be due to lean mixture. There are three likely causes of lean mixture with that engine:
1. Air leak between carburetor mounting flange and intake port.
2. Dirt in main jet.
3. Faulty fuel pump.

I suggest you look into the possibilities in the above order.

An air leak could be due either to loose mounting screws, or a damaged gasket between flange and port.

The main jet can be accessed by detaching the carburetor from the top of the fuel tank and looking at the underside of the carburetor:

Note that if your carburetor is Type A, it will have an idle mixture adjustment screw. This type has a vertical main jet. Types B and C have nonadjustable mixture, and horizontal main jets.

You have previous experience of overhauling the fuel pump, but we can go through that procedure if you wish.

Hi Ross, and a warm welcome back to OutdoorKing mate.

I do agree that this might be a vibration problem with the engine running roughly...but I still think it may be out of adjustment....the spring inside the clutch cone is quite strong and want to engage the clutch...it is the over-centering mechanism of the clutch handle that stops this from happening. That is where the adjustment needs to happen...a little trial and error.