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craftsman 46' with B&S 25 hp twin engine down

13 years ago

7 year old tractor. Suddenly starts backfiring and blowing blue smoke out of muffler. Changed fuel lines, filter, and spark plugs. Still have problem . . . will idle but when there is load . . . starts backfiring.

Maybe gasket? Anything else ? Will not cut grass. Help

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Comments (11)

walt2002
13 years ago
Could be, did you remove the valve covers and see if all was well there? Push rods in place?

Walt Conner
duster12
Original Author
13 years ago
walt connor
This is my first attempt at working on an engine. I saw a prior post of yours that pointed person listing to Briggs
twin cylinder shop manual via K&T and I downloaded that. It is a bit complicated to say the least. Section 5 is
on valve/cylinder properties . . . so I know where the covers are . . . . and once I take them off, what do I look for relative to push rods, etc.
From your post, I assume that my problem is most likely going to be in the valve/piston/gasket area?
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"I cannnot turn it over until it seem to want to cool down" TURN OVER means crank. Start means run- I'll assume you mean it won't RUN. There are 2 things that can cause this- Lack of spark to the spark plugs OR Lack of fuel. The fuel filter should be changed just for normal maintenance! IF it's too restricted (because that means it's working) there will be insufficient fuel flowing to the carburetor. The carb runs out of gas and the engine stalls. Wait for awhile, and the carb bowl SLOWLY fills back up and REPEAT. The other can be an ignition coil that's bad. Sometimes they get heat sensitive and quit working. Engine stalls, cools off, REPEAT. Check for spark IMMEDIATELY after the engine dies. You didn't say how long it runs before the problem occurs or how frequently it repeats??? Anyway, I'd change the fuel filter since it's OVERDUE!
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ab26, if your engine has a "fuel solenoid" in the carb bowl, you should shut your engine off at high RPM by turning off the IGNITION SWITCH. The solenoid has a plunger valve that shuts off the flow of gasoline to the MAIN JET. The solenoid is used to prevent the phenomenon known as "afterfire" (which is what you heard). It works like this: If you shut off you engine by moving the throttle to the idle/off position, you kill the spark ignition but gasoline continues to be fed to the intake of the engine (because the solenoid is still energized). This fuel air mix is now just passing through the cylinder and out the exhaust valve without ever combusting in the cylinder. The exhaust pipe and muffler are saturated with extreme heat from use. So long as the fuel/air mix is passing through the exhaust system at a pretty high velocity....no "afterfire, or ignition" can occur because the mix is whisked through without being able to heat to the point of spontaneous ignition. BUT, as soon as the crankshaft and pistons stop moving......so does the fuel air mix and at that point the blazing hot surfaces in the exhaust system instantly raise the temperature of the mix to the point of spontaneous combustion and BANG is the result. Now, when you shut down your solenoid equipped engine at high RPM by turning off the IGNITION SWITCH, this is what happens: Turn key off, fuel solenoid closes depriving the engine of its main jet fuel. Engine immediately burns the last puff of fuel mix in the normal fashion (because the spark ignition is still firing the plugs). As the last of the gas is burned, the engine RPM sudenly dives toward zero, all the while the engine is being purged and ventilated by the air that continues to flow (minus fuel of course) through the engine and out the exhaust. Because fuel is absent, when the engine finally stops at zero RPM under these conditions, there can be no "afterfire" bang.
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I'm not sure what the cross reference filter's part number was. It was just one that crossed and that was in stock. The outer diameter was 1/4" smaller than the B&S filter. Concerning the sequence of events - the clogged filter resulted in fouled plugs. I wasn't aware that the plugs were fouled but I did know that the filter was tremendously dirty so I changed it but didn't change the plugs until after I realized that the filter change had no effect. Once I noticed that the plugs were fouled I changed them and the mower seemed to be working properly but it then fouled the new plugs in after only running it for 1.5 hours. I went to the B&S website and submitted the information to them to see if they had any recommendations and their reply was to change the air filter. I wrote back and told them that I had already changed it but it didn't make a difference. The tech then wrote back and asked for the model number of my engine and when I provided that to him he responded by telling me that it was a documented problem with that engine and that there are several cross reference filters that will fit but the mower won't run well with. Twice he wrote to me and told me to change the filter to the B&S numbers and it would fix the problem. Obviously I had to change the plugs too since they were fouled and when I did that this weekend I could immediately tell that it was running much better. I used it for ~3.5 hours and it never missed a beat and I could tell too that it was no longer running rich. I'm sure it is fixed because even when it seemed to be running well when I had changed the plugs the first time it still ran rich. This time there were no signs of it running rich and it never once lacked power or backfired as it had before. The part number recommended by B&S is: Air filter 692519, pre filter 692520. BTW, the tech also stated that there are even hood stickers that call out the wrong air filter part number too. The printed material that I have for my engine even lists the wrong spark plug gap. I found that out when I had the tech verify all the numbers and specs for me before working on it this weekend. Lastly, the salesman at the parts store told me that they had experienced problems with that same engine when they had used other air filters. As he stated it, "that motor is very picky about the air filter that it uses". Strange but true! At least in this case. Thanks again. Scott
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duster12
Original Author
13 years ago
The new spark plugs that were installed got fouled very quickly. Would a blown cylinder gasket do this
Could this somehow route oil into the carb?
bill_kapaun
13 years ago
A blown head gasket could cause these type symptoms.

FIRST though, smell the oil for the presence of gasoline. IF the oil is extremely diluted by gasoline, it'll smoke profusely while causing damage. (It might really be a carburetor problem)
walt2002
13 years ago
" It is a bit complicated to say the least. Section 5 is
on valve/cylinder properties . . . so I know where the covers are . . . . and once I take them off, what do I look for relative to push rods, etc.
From your post, I assume that my problem is most likely going to be in the valve/piston/gasket area?"

Are you sure you want to tackle this yourself? It is very likely that a blown head gasket is the problem but you need to remove the valve cover to do that job anyway so there is a possibility that a push rod is bent, something out of place there.

To do this job right, you need to have or have access to a torque wrench, preferably an inch lbs torque wrench.

Walt Conner
duster12
Original Author
13 years ago
Walt . . . I have a torque wrench. Bought it way back when . . .I thought I would get into mechanical things a bit more. Once the valve covers come off, is the cylinder head easy to access. I am hoping that I don't have to dismantle much of the engine . . . Realize I will need new gaskets for both cylinders. I think the torque spec is in the manual I downloaded.
No one to help . . . so learning curve will be steep.
walt2002
13 years ago
"Once the valve covers come off, is the cylinder head easy to access. "

You should easily see the head bolts, both under and outside the valve cover. you will need to remove the Intake Manifold and Exhaust.

Walt Conner
duster12
Original Author
13 years ago
I will have to order new cylinder gaskets? Do I need to also order valve cover gaskets? Does the intake manifold also take a gasket? Is the exhaust gasket reusable? Should I also clean out the breather. Could that be causing this problem?
Thank you for your help
duster12
Original Author
13 years ago
I took off the valve cover on cylinder 2 and there are problems. The steel push rod was lying loose (separated) and the aluminum push rod is bent. what next?
tomplum
13 years ago
The most likely causes would be an issue with a rocker- possibly loose mount or tappet adjuster or the dreaded loose valve guide. If you look inside the valve springs, compare the inner guides to see if they project from the head the same amount or if one has come out further.
rustyj14
13 years ago
re: my letter to Felix:
What did i just say? Some of you took me to task about my advice to Felix!
Well, here we go again----

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