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Curtis B.
Curtis B., Technician
Category: Small Engine
Satisfied Customers: 26016
Experience:  have worked on and around most engine models for 35 yrs./Polaris ATV expert
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OHV17 backfiring through. Have already checked for sticking

Customer Question

OHV17 backfiring through carburetor. Have already checked for sticking valve & burnt valve, carbon in head, flywheel key, set valve lash at .004, new carb, new ignition module.Carb is non adjustable.
Submitted: 3 months ago.
Category: Small Engine
Expert:  Briggs-Fan replied 3 months ago.

Hi my name is ***** ***** welcome to Just Answer. You have went over everything that would cause this normally. Did it continously backfire through the carb while it was running ?

Customer: replied 3 months ago.
no it will run for various shorts lengths of time ,backfire & quit.
Expert:  Briggs-Fan replied 3 months ago.

Ok can you check your oil , does it have a gas smell to it or is it thin ?

Customer: replied 3 months ago.
oil is o.k
Expert:  Briggs-Fan replied 3 months ago.

Has it been changed since the carburetorreplacement?

Customer: replied 3 months ago.
No
Expert:  Briggs-Fan replied 3 months ago.

Because you have covered alot of bases , the flywheel keyis good , valves are gapped the carburetor has been replaced. Besides a worn cam the other thought that comes to my mind is if the old cab was allowing gas to bypass the valves and into the crankcase and mix with the oil, that would cause backfiring , another is if you have a anti backfire solenoid and it's bad it will backfire when shutting it off

Customer: replied 3 months ago.
anti backfire solenoid was new with new carb
Expert:  Briggs-Fan replied 3 months ago.

Is this a briggs engine

Customer: replied 3 months ago.
No Tecumseh OHV17
Expert:  Briggs-Fan replied 3 months ago.

Ok , the coil was properly gapped right

Customer: replied 3 months ago.
coil gapped at .0125 as per repair manual
Expert:  Briggs-Fan replied 3 months ago.

Ok. This has the plastic type carb right ? , also how old is the gas in it and what grade of gas is it ?

Customer: replied 3 months ago.
plastic carb,fresh gas,regular unleaded
Expert:  Briggs-Fan replied 3 months ago.

Unless you got a faulty carburetor I'd start to suspect a cam issue or a valve sticking . Those plastic carburetors are known for not being great . You have done everything to a tee that would fiX a common issue . Did it just start backfiring out of the blue and does it stay running or backfire and die

Customer: replied 3 months ago.
The engine would not start when I got it. Rebuilt the old carburetor, put it back on and then that is when the backfiring
problem began. When it backfires, it dies. Sometimes when it backfires, I can choke it and it will keep running. Almost
acts like lean fuel mixture,but the carb is non-adjustable.
Expert:  Briggs-Fan replied 3 months ago.
But you replaced the carburetor with a new one right ? , brand new ?
Customer: replied 3 months ago.
carb was brand new, when it runs it runs perfect until it backfires
Expert:  Briggs-Fan replied 3 months ago.
Have you tried adding a water remover to the gas tank , like a cap full of heel water remover ?
Customer: replied 3 months ago.
no , using same gas in other lawn mowers no problem
Expert:  Briggs-Fan replied 3 months ago.
Water is heavier than gas so if any were in the tank etc it would sit , it just makes me wonder because choking it can keep it running
Customer: replied 3 months ago.
took gas tank off cleaned it out,new fuel line & filter
Expert:  Briggs-Fan replied 3 months ago.

Ok , im will opt out to see if any other techs have any other ideas. You really covered all bases so it has me stumped with being able to see it in person but if it was a cam issue it would do it constantly so I don't think it's that . I will opt out and sorry I couldn't help you resolve the issue

John

Expert:  Curtis B. replied 3 months ago.

How was the valves checked? Did you have the cylinder head off? Everything points to sticking valve.

Expert:  Curtis B. replied 3 months ago.

Backfire has to be spark occurring , when valve is open, and that is the only way that happens. Unless timing is changing, And with new coil, that is not moving.

Customer: replied 3 months ago.
Had cylinder head off,took valve out ,valve moves freely in guide,valve guide is not wore. I don't think this engine has had much use,no sign of wear. Valve seat & valve face in like new condition.
Expert:  Curtis B. replied 3 months ago.

But you didn't remove the valves from the head and clean the stems and the guides?

Customer: replied 3 months ago.
I found the problem, the new carb i put on is defective. engine will run if the choke is 1/2 to 3/4 closed when wide open engine backfires and quits.Need different carb.
Expert:  Curtis B. replied 3 months ago.

The needle valve passage is fully open, not plugged with debris from old fuel line?