"The Lawn Mower Repair Man's Front Page"
I get a lot of questions on this so I thought, why not do a T.O.M. on it. So here it is. If I've sent you here rather then answer your question directly, my appologies for the impersonal approach, it's just much easier this way rather then repeat myself on a regular basis.
There are two primary cause of engine surge. The most common is a fuel system failure. The second is a throttle linkage problem. There is a rare third possibility which is a harmonic imballance of engine and governor spring.
A lean air fuel ratio due to a fuel system failure is the most common form of engine surging. The following are causes to look for when a lean run condition exists.
*Any kind of debris or fuel contamination. This can be dirt, grass, water or old fuel. All contamination must be cleaned or flushed from the fuel system, from the fuel tank to the carb. The carb may need anything from a simple draining to an overhaul.
*A lean run condition can be due to pin hole leaks in the fuel line, on engines using fuel pumps the pump may be bad. A fuel cap on the fuel tank may have it's breather clogged, preventing air to replace used up fuel. This creates a vacuum in the fuel tank and the stoppage of fuel flow.
*A lean run condition can be due to an intake manifold gasket failure or a poor sealing engine intake valve.
The most common causes of lean running is from old fuel causing a shellacing and narrowing of carb jets and plain old dirt obstructing a jet.
Worn linkage rods and joints between the governor arm and carb bellcrank will cause governor induced surge. When the linkage or linkage connect holes are worn and no longer tight, the governor senses that looseness as a demand to change rpm. Those un-needed demands to change sets off it's own pattern which feeds on itself and grows. This is rarely a governor problem itself so DON'T go to tweaking on the governor to try and correct. Governor adjustments are important to engine life and operator safety but the complexity of setting them goes beyond explanation without an ability to be there and show how it's done and without a tach that works on small engines. As a result, if you mess up the governor, you're on your own and headed for the repair shop.
This is rare but just thought I'd throw it in. The governor spring itself may have a harmonic resonance similar to the engine at one very specific rpm. This resonance or frequency, exactly the same idea as a tuning fork, can get the spring to vibrating wildly and setting off a surge. This will happen at one very specific rpm only such as on a generater. If you suspect this condition, first check all other conditions and then replace the spring with the same part number. Again don't go to tweaking the settings.
Diagnosing which is at fault;
Start and warm the engine up. Set the throttle speed to idle. Physically hold the throttle bellcrank against the idle speed screw. This eliminates all effects of the governor and throttle linkage. Observe how the engine runs. Next, continue holding the throttle against the idle screw but begin adjusting the idle speed screw to increase the speed. After each half turn of increase, observe how the engine runs. Continue increasing the idle speed screw while holding the throttle against it until you've reached about 3/4 the full throttle rpm. I've used the 3/4 figure because you probably don't have a tach so you'll be judging by ear and we don't want to over rev the engine.
What are you looking for? If at any of the speeds the engine was set at while holding the throttle steady, it wanted to die or falter, you have a carb/fuel system failure. This faltering is what is causing the governor to react and surge.
If the engine ran perfect at all points, the surge is being caused by worn, loose throttle linkage and linkage holes. Or in the rare condition, the harmonic imballance of the governor sping.
Armed with this info, you now know "what" needs to be repaired.
One final note. Todays emission engines for the most part, have no carb adjustments to compensate for worn engines. If your engine has high hours on it or has recieved poor maintenance, engine wear may be a major factor. Intake and exhaust valves not sealing well can cause for poor intake suction which will result in a lean run condition and set off an engine surge. In this way, non adjustable carbs prevents prolonging the life of a worn out engine which will pollute at a much higher level.
LMRM; Bob :<=