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bgsengineclinic
bgsengineclinic, Small Engine Technician
Category: Small Engine
Satisfied Customers: 220
Experience:  Over 20 years of experience in Small engine repair, Kohler Certified Expert Technician, Self-Employed in my own dealership.
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What causes my 17.5 Briggs & Stratton to backfire? Runs rough

Customer Question

What causes my 17.5 Briggs & Stratton to backfire? Runs rough & I changed spark plug,
engine oil, etc. Sprayed carburator cleaner in carb also. Is it a carb. adjustment?
Submitted: 5 years ago.
Category: Small Engine
Expert:  bgsengineclinic replied 5 years ago.

bgsengineclinic :

backfire out where? the carburetor or the exhaust?

bgsengineclinic :

runs rough as in black smoke from the exhaust, or speed changes up and down?

bgsengineclinic :

has it had any other service in the past, and by any chance was there a recent event like hitting a branch with the mower or anything?

bgsengineclinic :

Backfire can have a few causes. - the flywheel key for one can shear partially, causing ignition timing to be off.

bgsengineclinic :

depending on the nature of the backfire, bad gas and water in the gas can also cause this issue

bgsengineclinic :

if it hasnt run for some time , as in it was stored, the gas can get stale and then will not burn, so one thing you can do is drain the old gas and put in fresh 89 octane with a good fuel stabilizer.

bgsengineclinic :

may also experience backfire after the engine gets hot, if the cooling fins are blocked or plugged - mice love to build nests up in the engine cover on top of the engine, under the cover. overheating from this can cause backfire and burned valves, among other damage

JACUSTOMER-n6zqsb35- :

backfires carb I believe, don't notice black smoke, mower has

bgsengineclinic :

OK backfiring out the carburetor usually indicates bad fuel or ignition timing issue , I'd drain gas and refill with fresh 89 octane, check the cooling fins for debris , if that seems all OK then perhaps have to remove the flywheel nut and cup and inspect the key to see if keyway and crankshaft are lined up straight

bgsengineclinic :

other than that, it's pretty much ignition coil - you can unhook the wire from the coil when the blower housing is off, and run it briefly that way, you'll need to choke the engine to shut off , or use a screwdriver or insulated pliers to pull spark plug wire off spark plug to shut it off though

JACUSTOMER-n6zqsb35- :

mower was given to me recently by a neighbor, she no longer wanted it 6 yrs old, do not think it was maintained all that well I got it running but rough still after oil & spark plug change, runs smooth for a bit then starts to pop or backfire out of carb but no real black smoke

bgsengineclinic :

OK . do you know if it was in storage for a while then? if it has old gas in it - it does not burn well - may have to drain gas from carburetor by loosening bowl nut, drain gas from lines and tank, refill with fresh - it can make a difference.

bgsengineclinic :

also the air filter - check that as well. only other thing would likely be a mouse nest in the cooling fins- they will also chew the ignition and spark plug wires.

JACUSTOMER-n6zqsb35- :

Mower had no fuel in it when I got it the other day & woman said it had not been run in a few months so I put new fuel in it.

bgsengineclinic :

OK . only 2 things then - overheating from cooling problems, ignition coil or wiring, or valves need to be adjusted, or the valves may be sticking

bgsengineclinic :

if it was in our shop, probably would pull the engine cove r- blower housing and check for mouse nest, visually inspect coil, and remove flywheel nut and cup and check key to start off the diagnostics that can be done without special tools

bgsengineclinic :

if you had a leakdown tester, I'd say start off with that test :) but I assume you dont have one.

bgsengineclinic :

also, it is entirely possible for a brand new spark plug to be bad out of the box , but usually they wither work or they don't

bgsengineclinic :

-wither + either

JACUSTOMER-n6zqsb35- :

It starts backfiring when you start it up, also I took the air filter off & think the backfiring is from the carb but may be from the exhaust, I'm not 100% sure, it runs the same with the new or old spark plug

bgsengineclinic :

OK. runs rough as in like chugs? and no black smoke indicating rich mixture, and if it is backfiring right off the bat, it may also be exhaust , and it is getting too much fuel

bgsengineclinic :

which would also indicate a carburetor issue. basically backfire = ignition or fuel system - too much gas, bad gas, timing off, or a switch or device on the tractor causing intermittent short - doe sit backfore regularly or just every once in a while when it wants to?

JACUSTOMER-n6zqsb35- :

every once in a while when it wants

bgsengineclinic :

OK so it is not a consistent thing then. I'd be looking to isolate the issue from being in the tractor's wiring

bgsengineclinic :

easy way to do that would be to unhook the harness from the engine, if it will backfire at least once in the first minute of running consistently

bgsengineclinic :

it can be run unhooked for a minute or two but no more without causing problems with the charging system

bgsengineclinic :

although that wont do if it has an anti afterfire solenoid on the carburetor

bgsengineclinic :

could still have traces of water in float bowl, but unlikely

JACUSTOMER-n6zqsb35- :

I need to get back to you when I am at home, at work now & really have to go. Should I save & exit, do not know procedure?

bgsengineclinic :

Save and exit sure, I can convert to Q&A

JACUSTOMER-n6zqsb35- :

Thank you

bgsengineclinic :

I wont be around tomorrow, and partially tonight, but will work with you to get it going

Expert:  bgsengineclinic replied 5 years ago.
Just an update - if you want , you can download service manuals for most briggs engines at http://bgsengineclinic.com/downloads/ - free to download.