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Hi all,

I've tried several things to resolve my problem. I'll list them first to try and get to the real source.

1) cleaned gas tank, found many bits of plastic and debris but did not solve the issue
2) cleaned and inspected carburetor down to the float and valve. Blew it out with air. Did not solve the problem
3) cleaned magneto of mouse nest. Improved start but intermittent sputtering continued.
4) added Marvel Mystery oil to gas and oil. Immediately the problem became worse. Thought the thinning of the oil was to blame, so tried drained it and refilled with fresh 15W-40 I had on hand. Continuous sputtering stopped immediately but returned intermittently with heavy oil burning and sputtering.

The motor starts and runs fine for a few minutes, then it begins to sputter and burn oil. Often it will subside and run smoothly again. It does not usually stall, but is difficult to start if I shut it down while it is sputtering. I believe oil is getting into the combustion chamber but not continuously. I cannot correlate this to any mowing conditions, slopes, heat or anything.

Please help with any ideas.

I need to add something. It is running smooth and perfect for about 5 mins. then starts the sputtering. It is becoming more consistent now that I have 15w-40 in it, but not as bad as with the marvel mystery oil additive. Possibly due to oil viscosity? Maybe I should try some 30W oil? Could it be coming through the valve seals? It seems to have good compression and starts on the first pull.

Two things come to mind that may help. First, Briggs engine should be getting 30W oil, not 10W-40. Second, have the carb ultrasonically cleaned and rebuilt with a new needle/seat kit.

Well, I changed over to 30W and the symptoms persist. The smoke is much worse when sputtering with the 30W. Now with three different loads of oil I am getting the same result but with different degrees of smoke.

I'd like to bring this up again as I may have put too much focus on oil burning. I don't know if the smoke is from oil in the chamber or just poor ignition (which is what I am now suspecting).

The engine runs perfectly for about 5 mins. Then it begins intermittent sputtering. It does not stall. Sometimes after stopping to empty the bag it will start perfectly, other times it will be a very hard start.

Could this be a failing magneto? Do they sometimes only fail at elevated temperature?

gccch said:

I'd like to bring this up again as I may have put too much focus on oil burning. I don't know if the smoke is from oil in the chamber or just poor ignition (which is what I am now suspecting).

The engine runs perfectly for about 5 mins. Then it begins intermittent sputtering. It does not stall. Sometimes after stopping to empty the bag it will start perfectly, other times it will be a very hard start.

Could this be a failing magneto? Do they sometimes only fail at elevated temperature?

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This sounds choke related to me .

I'm not sure how the choke works on this as it is spring loaded or automatic. But I don't see how it can be so predictable based on running perfectly for 5 mins. then sputter, then run perfect, then sputter, and seemingly random intervals.

What would I look for if this were the choke?

gccch said:

I'm not sure how the choke works on this as it is spring loaded or automatic. But I don't see how it can be so predictable based on running perfectly for 5 mins. then sputter, then run perfect, then sputter, and seemingly random intervals.

What would I look for if this were the choke?

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Take air filter off and when its sputtering look in the carb and see if the door is shut or open

It's possible you have two separate problems. 1) after 5 minutes of running the engine starts sputtering and stumbling. 2) smoke and oil consumption that varies according to oil viscosity. As for issue #1 it sounds like a coil that gets good electrical flow when it's cold, but as it heats up, resistance to the flow occurs and spark diminishes. It should get progressively worse as time goes on until it won't even fire up. As for #2 that could be anything from worn valve guides, too much blow-by, possible broken ring(s) or crankcase vent and tube stopped up.

Normally, black smoke is oil related, blue smoke is fuel related, and white smoke is water related. FWIW

Thanks. I think I will go ahead and replace the magneto. I have been suspecting this now as I struggle to find anything else.