The carburetor is running too rich and fouling the spark plug. The idle on this 7hp engine also surges up and down, and doesn't seem to rev up to peak rpm. There is no idle mixture screw to adjust.

My first suspect is a faulty O2 sensor causing the computer to make the engine run rich (too much fuel). This would account for the fouled plugs and rough running. I'm assuming the exhaust smoke is also quite black. This condition will also, unfortunately, quickly ruin the catalytic convertors.

Define "kills spark plugs"? If they foul fast, their heat range maybe too cold not allowing them to burn off deposits. Be careful tampering with specified heat ranges as you can damage pistons. Are the new plugs the exact replacement part for that engine? I have switched brands of of spark plugs and have gotten "exact replacements" that were the wrong heat range. For me, I try to stick with the brand used at the factory where ever possible. This way, if it's wrong, it's THEIR fault. Colder range plugs will work fine but for much less time. Another thought. Does this have an automatic choke or a manual choke that you have to close and open? If it's manual, close it to start, open it part way to get going. Open it all the way in about a minute to run. If you run it with the choke shut too long, your fuel mixture will be way too rich once the engine warms up, in turn, fouling your spark plugs. If the choke is automatic, take the air cleaner off and look down the throat of the carburetor. When cold, the plate should be closed all the way. Start the engine and watch this plate. If it doesn't open within 1 or 2 minutes at the most, it's thermostatic actuator is either stuck, malfunctioning or not reacting to engine heat as it should. This will cause spark plug fouling in a hurry. A cold engine requires more fuel to run properly. Rather than try to make a carburetor that will deliver variable amounts of fuel according to ambient air temperature, It is much easier and cheaper to temporarily restrict the amount of available air with a choke plate to achieve this goal. If this choke stays closed for too long or if it is jammed shut, What is the right amount of fuel in a cold engine becomes way too much fuel once it warms up.This is the way that carburetors have worked for over 100 years. Good luck.

How do you know it is running rich? The computer controls fuel delivery. If it is running rich, one or more sensors may be giving the computer false readings resulting in a rich mixture. A leaking fuel pressure regulator internally will also cause a rich issue as well as a stuck open EGR valve. A scan test monitoring engine data can inform you of what may be the trouble

cold engine idle surge. (problem)
RPM range of surge not stated
300-700 or 800 to 1500? (range is critical to diagnosis)

bad fuel
running summer fuel in winter (running old fuel?)
needs a tune up bad, see the list in the operator guide.
idle speed controller,sticking or EGR valve same. (clean them)
location not stated, so cant guess about local air temps.?
no other symptoms, lack of engine power? etc?
was it ok last week, or better, when was the last time it was OK?
1week, 1year?
surge , causes, on a tuned engine might be:
1: too lean. so lean the ISC can not control idle. rpm low, stalling
2: too rich, flooding, stalling, spark plugs foul (look at them)
3: weak compression in 1 or more cylinders.
4: bad spark.
5: air leaks, in the plenum causing , ISC to lose control.
why high RPM?

especially?
well that means it fails hot too.
and there is a simple tests. for hot surge.
is engine fully hot? 180F and holds solid there?
say rpm is about 800
and you turn on head lights, heater fan and (not AC) and defrost.
the RPM must hold or the ISC is failing.
ISC -= idle speed controls. (all EFI has this function)

Hi Dale,

Several factors can contribute to spark plug fouling:

- The air/fuel ratio may be too rich as a result of incorrect carburetor adjustment or a poorly performing fuel injection system.
- Worn piston rings or valve seals may allow too much oil to leak into the combustion chamber, leading to oil fouling.
- The ignition system may not be performing properly.
- Prolonged idling or continuous low-speed driving may keep the spark plug from reaching its optimum operating temperature. Using too cold a spark plug can lead to the same problem.
- A dirty air cleaner can create a too-rich condition which can lead to fouling.

Have each of the above checked and corrected.

Fuel, oil and carbon fouling can all be the result of different causes but, once a spark plug is fouled, it will not provide adequate voltage to the firing tip and that cylinder will not fire properly.

In many cases, the spark plug cannot be cleaned sufficiently to restore normal operation. Hence, I always recommend that a plug be replace, once it becomes fouled.

Cheers,

Scott


"If this has helped you in any way, please be kind enough to rate this solution"

Hi Dale,

Several factors can contribute to spark plug fouling:

- The air/fuel ratio may be too rich as a result of incorrect carburetor adjustment or a poorly performing fuel injection system.
- Worn piston rings or valve seals may allow too much oil to leak into the combustion chamber, leading to oil fouling.
- The ignition system may not be performing properly.
- Prolonged idling or continuous low-speed driving may keep the spark plug from reaching its optimum operating temperature. Using too cold a spark plug can lead to the same problem.
- A dirty air cleaner can create a too-rich condition which can lead to fouling.

Have each of the above checked and corrected.

Fuel, oil and carbon fouling can all be the result of different causes but, once a spark plug is fouled, it will not provide adequate voltage to the firing tip and that cylinder will not fire properly.

In many cases, the spark plug cannot be cleaned sufficiently to restore normal operation. Hence, I always recommend that a plug be replace, once it becomes fouled.

Cheers,

Scott


"If this has helped you in any way, please be kind enough to rate this solution"

It sounds like the computer is operating the system in full rich mode. If all the plugs are fuel fouled then, it's something that effects all cylinders equally. If so, then check the ECT (engine coolant temperature) sensor. Make sure it is connected and in good condition. If the sensor lead is disconnected causing an open circuit, the computer will attempt to run the engine like it's at -274deg. kelvin. Also check the MAP sensor hose for cracks and damage. If you are checking the same plug each time then you may have a shorted injector or driver. Attach a noid light or a high impedance test light to the injector harness. If the light stays on with out blinking while the engine is running then the injector driver is shorted out and PCM replacement will be required. If that's OK then check the injector for a short using an ohm meter.

check spark plugs for fouling and injectors

HI, i recommend inspecting the following--

Inspect the hose at the PCV valve. this hose could be clogged or it may have collapsed.

Inspect the PCV valve as well. it may be plugged or damaged.

Inspect all fuel injectors and there operational value. this may be the issue. Your injection system may be faulty or the timing could be off. inspect thoroughly.

Inspect the plugs and wires. you may have fouled, damaged or, broken spark plugs. check each individual plug and wire for damage.

Inspect the Throttle position sensor. this sensor may be malfunctioning.

Inspect or replace the fuel pump. this is another known cause of surging and hesitation.

Please take the time to inspect all area thoroughly.

Try this first before having the engine rebuilt.

Go buy a bottle of Lucas engine treatment and add it to the oil, if in a couple hours of driving you do not gain more power, the engine is not running better Ect. then yes you need to have it rebuilt.

But try this first, this stuff im sending you to get is the leading engine treatment in the world.

I have actually cured engine knocks permantly with htis stuff.