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I have a b&s 9hp intek motor on a troy built 33" walk behind mower. The issue I have is that I am getting a backfire out of the carb and surging when the engine is run without choke. With the choke I can usually find the sweet spot by slowly applying choke until it stops backfiring. I have tried numerous things from people with suggestions to issues similar to mine. I have cleaned / replaced the carb with 0 improvement, check for vacuum leaks post carb by spraying choke cleaner around the seals with the engine running, check that the valves were opening and closing as well as adjust them, gapped the spark plug and the magneto. This is all with fresh gas. I thought the engine was running lean but when I spray a little bit of choke cleaner in the carb it chokes the engine out immediately so I think maybe its running too rich? I am kind of at a loss here and don't want to try just replacing multiple parts just in the hopes that I get lucky and find the issue. Any help would be a god send. Thanks!

make sure the exhaust is not obstructed with debris - mud dauber nests etc. Als omake sure that the fuel pump diaphragm is new and not old. You either are using the choke to pull gasoline to the carb, or the exhaust appears backed up. That is my 2 cents worth - others may feel different. While you are servicing it retorque the head bolts too.

If engine runs better with choke applied...then engine IS running lean, not rich.

Did you check your flywheel key while you were in there? Possible it’s out of timing a bit.

sgkent said:

make sure the exhaust is not obstructed with debris - mud dauber nests etc. Als omake sure that the fuel pump diaphragm is new and not old. You either are using the choke to pull gasoline to the carb, or the exhaust appears backed up. That is my 2 cents worth - others may feel different. While you are servicing it retorque the head bolts too.

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Thank you for your response. I removed the head and the head gasket looks to be perfectly intact and the valves appear to be sealing well ( poured water and nothing leaked past them) and moving easily. Push rods and valves are un-bent. I will try re-assembling and run the motor without the exhaust to see if that helps.

Russ2251 said:

If engine runs better with choke applied...then engine IS running lean, not rich.

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I would agree however when I spray carb cleaner in the carb it chokes the engine out.

I checked the flywheel key as well and it looked fine. There is a picture of it here as well as the cylinder head and piston.

https://photos.app.goo.gl/NfX9jtUKaosf5ZsM6

many of these engines also have a diaphragm that acts as a fuel pump. They go bad. Causes all sorts of problems like you describe. Your head gasket was leaking - see the darkened area. Maybe not bad enough to keep it from running but that causes issues too.

sgkent said:

many of these engines also have a diaphragm that acts as a fuel pump. They go bad. Causes all sorts of problems like you describe. Your head gasket was leaking - see the darkened area. Maybe not bad enough to keep it from running but that causes issues too.

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Where would this diaphragm be located? The path I see for the fuel goes straight from the tank to the carb. Also I don't see the area where the head gasket was leaking that you are talking about. The motor part number is 215802-0223-E1 FYI.

I spray carb cleaner in the carb it chokes the engine out.

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Of coarse!!! Because you OVER-richened the mix!!!

Russ2251 said:

Of coarse!!! Because you OVER-richened the mix!!!

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What else could be causing it to run lean though? Like I said I sprayed the gaskets post carb and there weren't any leaks there and I clean then replaced the carb and neither made a difference. Is there another way to test for lean conditions? Also I removed the fuel filter and fuel shutoff as well as the fuel tank cap to try to eliminate as many variables as possible. Also the backfiring occurs at all throttle positions.

scutzi128 said:

What else could be causing it to run lean though? Like I said I sprayed the gaskets post carb and there weren't any leaks there and I clean then replaced the carb and neither made a difference. Is there another way to test for lean conditions? Also I removed the fuel filter and fuel shutoff as well as the fuel tank cap to try to eliminate as many variables as possible.

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Might as well lap the valves while you got it down................. Yes I do see a leak also it's on the far right of the block..... That would be a low compression issue if it was leaking ...........

Let us know Mon Ami ~!~!

Boudreaux In Eunice La. said:

Might as well lap the valves while you got it down................. Yes I do see a leak also it's on the far right of the block..... That would be a low compression issue if it was leaking ...........

Let us know Mon Ami ~!~!

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Is this the area you are talking about?

https://photos.app.goo.gl/K2j7Yi8ci2Az3Cwj6

scutzi128 said:

Is this the area you are talking about?

https://photos.app.goo.gl/K2j7Yi8ci2Az3Cwj6

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No sir the area to the right of the pushrod chamber right below the middle.... I don't think its a shadow......

Boudreaux In Eunice La. said:

No sir the area to the right of the pushrod chamber right below the middle.... I don't think its a shadow......

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I just looked that area over really well and to me it looks like there was no burning and the gasket itself has not charing etc. I think it was just the picture.

It didn't look like bad burn and I see Briggs stepped up on making the areas bigger on the vangaurds the inteks have only a skinny area between the cylinder and pushrod chamber.... That one you got has a lot of meat on it for sure....

Lap the valves and if it still does it then check the travel of the rockers carefully.... A lobe on the cam might be worn down bad enough to do what your mower is doing.....

I'll post a link in a bit to show you what i'm talking about.........

Oops I forgot to come back and post the link.... Now I am not saying this is your issue but something to think about .....
Let us know Mon Ami ~!~!

https://youtu.be/fmbwImeiqaE

Also what carb do you have on your engine ??? I am guessing the Nikki Carb Depends on the age of the engine.... The code would tell me that. It's the last set of numbers, the first 2 numbers are the year.....

Those Nikki carbs are finicky......... I have done toooo many of them and the kits are pricey.....

Let us Know Mon Ami ~!~!