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Need Help Eliminating Backfire and Rough Running Briggs & Stratton Engine


targa's Avatar
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07-23-14, 01:11 PM   #1  
Need Help Eliminating Backfire and Rough Running Briggs & Stratton Engine

I need some direction to solve a problem with my Briggs & Stratton 675 Series engine. If it will help here's the numbers from the tin shroud - 126T02-0290-B1 10032658.

First a little background.

- the motor is on a Swisher walk behind weed mower that's about 3 years old and has only been used 2-3 times.
- when last run, I drained all the gas and ran the engine dry (about 2 years ago) and then stored the machine in a dry shed until yesterday
- since I decided to sell the machine because we never use it and takes up storage space I used a relatively low pressure power washer to clean the grass and debris from under the machine along with some areas on the top surface but not intentionally directly into the motor

After putting fresh gas in the tank here's what the motor does

- it starts immediately but pops/backfires through the exhaust but mostly at idle speed
- the rpms rise when the throttle is moved to full open but the engine runs rough but doesn't pop/backfire


Here's what I've done so far

- removed and cleaned the air filter which was not dirty
- replaced the spark plug
- removed and disassembled the carb blowing out all passages and the jet areas with my air compressor
- put fresh gas in and restarted the engine which started immediately but actually seemed to pop/backfire through the exhaust at idle even more than before
- engine rpms still increase when throttle is moved toward wide open but still runs rough


Now I'm not sure where to look or what to try next.

I'm hoping my above description will allow someone with a lot more experience than me to give me some direction on how to pinpoint and solve my problem.

I would appreciate any suggestions or ideas.

Thank you

Dom

 
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07-23-14, 03:27 PM   #2  
Did you start it up this year . . . . before power washing it ?

 
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07-23-14, 03:48 PM   #3  
Hello Dom,


From your description, It sounds like while this machine was stored all this time carbon deposits on the head-piston and valves dried out. The first time this dried carbon got wet from fuel it popped loose and is now stuck under the exhaust valve seat, The exhaust valve not seating fully will cause backfiring and poor run.

To fix;

Remove the head and check for carbon buildup on the above mentioned components, You may be able to rotate the engine so each valve is fully open then clean the valve seat and face areas with a mini hand wire brush.

Once clean re-install the head with a new gasket and see if that solves the issue;






Good Luck

 
targa's Avatar
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07-23-14, 04:39 PM   #4  
Did you start it up this year . . . . before power washing it ?


Unfortunately, I did not. So, the problem may have already existed as 31YTech suggests.

Initially, I thought it might be an ignition problem but it starts perfectly every time and gains rpm's so I concluded it is probably something else and decided to write this thread.

Thank you both for your comments and suggestions. My next step will be to remove the head and check the valves and valve seats. Although there's probably less than 4 hours of run time on this engine, checking for carbon particles between the exhaust valve and seat makes sense.

I'll post my findings.

Thank you

 
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07-23-14, 05:36 PM   #5  
You could also try and blow a good liberal amount of seafoam through the carb and try to blow out any carbon obstruction on the valves. I would blast some deepcreep seafoam through the carb for a while then blow enough to stall out the motor. Remove the spark plug and spray a liberal amount in there as well. Let sit for 30 minutes, then start it up and blow some more through the carb.

You might get lucky and the seafoam may do the decarbon work for you.

 
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07-25-14, 12:59 PM   #6  
Update

I received OptsyEagle's suggestion to try
SeaFoam DeepCreep
just before I was about to remove the cylinder head and followed his recommended steps.

And it worked!

No more popping/backfire through the exhaust and it still starts immediately every time hot or cold. Also, most of the real roughness is gone but there is still
more than with most 4 cycle small engines I have. I did some research on the internet and engine roughness with the Briggs & Stratton 675 series is a fairly common complaint particularly when this engine is mounted on the Swisher trimmer I have. Anyone have any idea why?

Thanks for everyone's help.

Dom

 
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07-25-14, 01:07 PM   #7  
My guess the roughness is caused by the mower not having a blade, the blade works as a flywheel and the mass of it helps the smooth rotation of the engine. Have a good one. Geo

 
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07-25-14, 01:49 PM   #8  
My guess the roughness is caused by the mower not having a blade, the blade works as a flywheel and the mass of it helps the smooth rotation of the engine
Geo, I wondered about that as well. In fact, before I used the SeaFoam in the carb and cylinder, I removed the belt so the head wouldn't rotate while I was testing and running the engine. I thought the engine ran smoother without the belt and once I reinstalled it the roughness got worse so your theory makes sense. I even replaced one of the two strings in the cutter head because it was shorter than the other one and that seemed to help a bit.

Thank you for your comments.

 
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07-26-14, 06:20 AM   #9  
That seafoam is incredible stuff. This year I wanted to change the head gasket in my lawn tractor. This tractor also had a small leak in the carbs inlet needle that I bandaid fixed with a manual fuel shut off valve. I figured before I changed the head gasket I would put a high concentration of seafoam in the gas and run it for a while to see if it would dissolve or dislodge any obstruction in the inlet needle's seat. I used 15% seafoam to gas and ran about pint of that mixture through the motor, let it sit, ran it again, let it sit, ran it again and then added about a gallon of gas and ran it to dilute out the seafoam.

I have no idea whether it fixed my carburetor leak, since my manual fuel shut off valve has fixed that problem for me for now (very small leak that only caused a problem when stored for a week or so). What was amazing was when I popped the head to change the gasket. This picture is the look of the piston after 10 years of mowing about 1 hour per week for 5 months per year. Probably 200 hours of use. Look at how clean that piston is. If you have ever popped a cylinder head before you will have observed that they are usually cruded with carbon all over. This one you could almost eat off of. You can also see where my gasket had breached at around 2:30 o'clock. Small breach but it doesn't eat anymore oil with the new gasket. So all good.

Anyway, I have to assume the seafoam cleaned up that piston or me and hopefully blew out any carbon that might get in the piston rings. Maybe my carb is sealed now too, who knows. All I ask it that it cuts grass when I ask it to and at this time it does.

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