Briggs Engine Repeatedly Dies, fouled plug

ThaiFarang

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I am in dire need of help to diagnose the engine problem of my MTD/B&S push mower. Any help will be much appreciated as here in Thailand knowledgeable repairers are hard to find.

The machine is a 3.5 hp, 20 inch, push mower with B&S engine.There are some 9 data items stamped on the mower, but as far as I can tell the B&S codes are : model# 98002; type # 1607B1; code # 00042855.

It has never been a smooth runner, but was ok until recently when a Thai "helper" ran it with no oil, resulting in a broken con-rod. This was repaired by local motorcycle mechanics. Then because it died completely another repairer serviced the carburetor and installed a new diaphragm. It appeared to run ok.

But at first use after the service, and ever since, it dies repeatedly (after running for about 15 minutes) and the spark plug is black. Cleaning the plug gives another 15 minutes of use, then it dies again. This happens repeatedly.

I am not a mechanic, and I am at a loss. I have a fear that the valve or ignition timing may be incorrect (after the con-rod repair), but really I have little clue as to what is wrong.

Please can someone offer knowledgeable advice?

ThaiFarang.
 

Grass ala Mowed

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Plug fouling to the point of quitting is usually due to running way too rich. Does the engine smoke when it is running? Black smoke is too much gas, blue smoke is burning oil. How did it run before the carb diaphram change? I am suspicious of the carb rebuild if it ran okay before then.
 

jmurray01

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Plug fouling to the point of quitting is usually due to running way too rich. Does the engine smoke when it is running? Black smoke is too much gas, blue smoke is burning oil. How did it run before the carb diaphram change? I am suspicious of the carb rebuild if it ran okay before then.
And not that it matters on this occasion, but grey smoke means it's running too lean :thumbsup:
 
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Joe79

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You should check your valves first. Easiest thing to check. Take of valve cover and check for any obvious problems. Are they way too loose... lift rod kicked off? Look online for the correct gapping of the valves, get a feeler gauge and adjust them. Check to see if the valve oil seals are gone or worn. This will let oil through and burn smoke.

If the valves are okay, then you should check your compression. Anything between 90 and 120 psi is good. If it's low then you probably need new rings. The rings could have went bad from the running with no oil, the con rod and it shakin all around in the chamber. This could cause smoke as well.
 

KennyV

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WELCOME to LMF...
I think your the first member from Thailand.

I'm with Grass ala Mowed...
Running to the point of failure from a fouled plug points to carb problems, way too rich...
also check if the air filter is clogged... or if the choke is less than full Open. :smile:KennyV
 

ThaiFarang

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Guys : thanks for your quick responses. Some further points re. the issues that you raised.....

1. There is not much smoke evident, not enough I think to suggest a problem.

2. After the con-rod repair the machine ran for a while and then developed problems as described, but subsequently died and would not start. Then a repairer replaced the carb. diaphragm. and it ran (at the shop). But at home, cutting grass, it again as before repeatedly dies with fouled plug, as described.

3. As far as I can tell, the air filter is ok. In any case I did try to start it (when it was dead, before the carb. rebuild) with the filter removed, but it would not fire at that time. I did clean the filter. Perhaps I should try running it again without the filter?

4. I am not sure if I have the competence, or the tools, to check the valves as suggested by Joe79. I will see if any easier ideas surface before contemplating this one!

Thanks again for your help so far. I am grateful.

ThaiFarang.
 

reynoldston

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gas or oil is fouling the plugs. Sure sound like a problem and no one to help you. I don't know what it like in Thailand but maybe the shop that did the repairs would be the place to help you. Is there any mower dealer with a service shop around you? Seeing you don't have tools or skills I would say try a new hotter range spark plug?
 

temarthers

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Run the engine without the fuel tanks cap on. If the engine runs normally and continuously, the vent hole on the gas cap is restricted or plugged.

Change the spark plug. Ensure it is the correctg plug and the Heat Range. Possible it is getting grounded out internally. A new plug may solve the problem.

I found this mentioned on a website, I know it is lengthy but good informaion:

One first things to check is whether plug correct type for engine. Then, check condition of electrodes. On new plug, bottom surface center electrode flat, surfaces lower electrode squared. Used plug in normal condition will look the same, but electrodes will be colored ashy grey or light tan from carbon deposits.

An oil fouled plug will have its end saturated with wet, sooty, black oil deposits. In four stroke engine, oil fouled plug may indicate rings aren稚 sealing cylinder properly. Or oil may be passing through valve stem area. Sometimes, clogged breather can cause oil fouled plug (remember that breather is vent in crankcase). Clogged breather will prevent crankcase from venting properly, and pressure will build up in crankcase. Pressure will cause oil to be pushed past piston rings, into combustion chamber where will foul plug.

On two stroke engines, oil fouling common. Is normal by product of engine operation when fuel and oil are burned together. Oil fouling in two stroke may also be caused by too much oil in fuel and oil mixture. In either two or four stroke engine, oil fouling may also be displayed as excessive smoke in exhaust.

Spark plug fouled by excessive fuel will have dry, black, fluffy deposits on plug electrodes. Deposits won稚 have caked or lumpy deposits of oil fouled plug. Fuel fouling most often caused by extended operation with air and fuel mixture that is too rich. Usually is carburetor problem; could also be blocked exhaust or faulty valve. If fuel fouling present, probably will be able to smell fuel on plug electrodes, especially if problem is severe.

Another possible cause fuel fouling is weak ignition. If high tension cable, points, condenser, electronic module, or coil faulty and spark weak, plug can be fuel fouled. Fuel fouling can also be caused by use of a too cold plug in engine.

Oil and fuel fouling can both cause condition known as bridged gap, where carbon or oil deposits build up in gap until gap completely blocked.

Deposits caused by fuel and oil fouling can usually be cleaned from plug and plug reinstalled in engine. However, since plugs inexpensive, should usually simply be replaced during tune up.

After many hours use, plug electrodes begin to erode. Eroded center electrode will appear rounded, while eroded side electrode will have curve on inside surface. Plugs with eroded electrodes should be replaced.

May find plug痴 electrode or insulator damaged. Electrodes may be heavily pitted, insulator broken or cracked. This usually caused by too hot a plug being used in engine. Physical impact can also damage plug. If piston or ring part breaks and hits plug, may find damaged or bent electrodes or cracked and broken insulators. If plug reach too long, piston head may strike electrodes. Most common cause damage is debris or foreign objects in cylinder. A bolt or washer may loosen and be sucked into cylinder. Will then strike plug electrodes when piston rises.

Plug heat ranges may be changed depending on condition of plug that痴 removed from cylinder head. Hotter plug generally installed if plug looks dirty. Cooler plug installed if plug displays heat damage such as cracking or chipping of insulator. Manufacturer痴 manual will provide recommendations about plug type. Should always follow these.

Never sand, sandblast, or file plug and then replace in engine. Sandpaper or file will leave tiny grooves on electrodes. Grooves will either burn off or collect deposits as engine operates. Sandblasting and filing will leave tiny particles sand or metal behind on electrodes; these will get into cylinder and cause damage.
 
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