OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs and stratton 92908

hi guys

i picked up a scott bonnar 520 with a 92908 on it. it starts and runs fine but it does splutter sounds like its running fine then intermittently not getting fuel. adjusting the mixtures does make it run lean or rich but it still splutters.

any help would be appreciated.

regards Alex

I suggest you go through the standard procedure for finding an oddball fault, since the description could apply to quite a range of things.

First, I would test the spark. Because it starts easily, any ignition fault is going to be an odd one. Here is my process for finding those. Begin with a simple test. We each invent our own way of doing that, depending on how easy we want to make it and how much time we want to invest in making it easy. Here is my simple tester:


It consists of an old car spark plug (not an ex-2stroke spark plug) gapped to 0.060", screwed into a 14 mm nut welded to a 4-for-$2 Taiwanese clamp. You just clamp any convenient grounded metal part of the engine, and attach the spark plug lead to the old car spark plug. Set controls in the starting position, and pull the starter cord a couple of times, watching the gap in the spark plug. If you see an even series of blue sparks, the engine passes the simple ignition test. Unfortunately that will not find one particular fault: high speed ignition failure, usually caused by breakdown of the ignition coil secondary winding's insulation. So, if you have a problem of erratic running at speed despite passing the simple spark test, and you have already replaced the spark plug with a new one and that has had no effect, you may need an advanced spark test, using the second type of gadget.



This is a standard Briggs tester, with a 0.166" gap. You can attach it in series with the regular spark plug. That is, connect the red ground clip of the gadget to the tip of the spark plug, and attach the engine's plug lead to the protruding metal contact on the gadget. Start the engine, observing the huge 0.166" gap through the transparent panel. Rev the engine. When it sputters and breaks wind, see if the sparks seen through the window become erratic. If they continue smoothly when the engine sputters and stutters, the problem is not ignition-related.

Having either found that our fault is ignition or having proven that it is not, we can move on. If the ignition is misbehaving, we need to check for faults in the wiring: chafed insulation on the kill wire or breaker points wire, kill switch not opening properly, sick capacitor (condenser), points closed up or dirty, poor ground at the coil mounting, or incorrect clearance between coil yoke and flywheel. If it is ignition but isn't any of those, you should try a new capacitor. If that doesn't cure it, try a new coil, including a new spark plug lead.

If it isn't ignition, there are two remaining generic areas to be concerned about: air fuel mixture, and compression. I put them in that order because mixture is easier to check, not because it is more likely to be the cause of high-speed-only malfunction. Mixture will seldom cause an erratic fault, in which the carburetor can still be successfully tuned from rich to lean. However it can happen if, for example, we have a gravity-feed carburetor and the float mechanism is falling to pieces, or we have a Briggs and Stratton Pulsa-Jet carburetor and the fuel pump is failing. As it happens, the 92908 engine has a Pulsa-Jet.

To check the basic health of the Pulsa-Jet, you remove the entire carburetor and fuel tank as an assembly (which takes about 2 minutes if you take the time to do it carefully, much less if you don't) and unscrew the carburetor from the tank. Look at the underside of the carburetor. There are two fuel suction pipes with fine mesh screens. The longer one is prone to becoming clogged with crud from the bottom of the tank, especially grass clippings. If it is visually clear, put that possibility aside and look at the diaphragm, which is a gasket that also acts as fuel pump inlet and outlet valves. The flaps of the diaphragm should be rather flat and flexible. When you put it on the top of the tank, the flaps should lie flat, not be curled up. Usually, a defective diaphragm will cause no or only intermittent fuel flow out of the pump, whereas a clogged intake screen will cause heavy load fuel starvation, so the symptoms are different. However, make sure both screen and diaphragm are in good condition. If you haven't found a fault at this point, it is wise to clean out the body of the carburetor then reassemble it, and retest the engine. If it still has the fault, it is necessary to consider compression failure, which is a more difficult process.

Begin the final stage by doing the Briggs compression test: disconnect the spark plug making sure the lead can't flop back onto the plug, ensure the speed control is in the stop position, and turn the mower's blade plate backwards until you feel compression resisting its movement slightly, then flip the blade plate backwards to see if it will hit the compression and bounce back. If it will not bounce, you have so little compression that erratic running has to be expected. A Briggs engine will often start easily and run smoothly with so little compression that power output is woeful and high speed running unachievable.

If compression is OK, the final diagnostic step is to remove the air filter - not forgetting to put the retaining screw back into the carburetor - and run the engine through its speed range, observing for pulsing of gas outward from the air intake. This would indicate a sticking or leaking inlet valve. Then put the air filter back on and remove the muffler. Start the engine and run it through the range without the muffler, watching firstly whether it still misbehaves at speed, and secondly whether there are flames coming from the exhaust port. If the problem goes away when the muffler goes away, you have a blocked muffler. If flames come out the exhaust port, you have a sticking or leaking exhaust valve.

If you get to this point and have not found the problem, we will need a more detailed report to go further.

Hi grumpy

I managed to diagnose it last night. The diaphragm was faulty it's now running like a new one

Cheers

Thanks for reporting back, Alex. Problem solved, I'll close this thread.