How JustAnswer Works:

  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.

Ask Hank F. Your Own Question

Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 13472
Experience:  Certified on Onan and Generac generators
19372534
Type Your Small Engine Question Here...
Hank F. is online now
A new question is answered every 9 seconds

what causes a small engine (b&s) to backfire when trying to

Customer Question

what causes a small engine (b&s) to backfire when trying to start it?
Submitted: 3 years ago.
Category: Small Engine
Expert:  Hank F. replied 3 years ago.

Hello!
Thank you for choosing Just Answer for the solution to your problem.
My name is Hank, and I am going to assist you with this.

 

The most common cause of this is a dirty carburetor.

 

As gas gets old, it turns to varnish and clogs up the passageways inside the carburetor, not allowing enough gas to get to the engine.

This condition is cumulative. Every time gas sits, the varnish builds up just a little more, like coats of paint, until eventually gas can not flow. It will not happen overnight, but the symptoms can show up over night.

The use of fuel additives, such as Sta-Bil or Sea Foam will not stop this process from happening. They will greatly slow it down, but the gas will still go bad.

 

When this happens, either the engine simply will not start, or it will not run without the choke on (this reduces the amount of air getting pulled into the engine, changing the fuel/air mixture), or it will run but surges.

 

Another issue that varnish in the carb can cause is that the varnish may not allow the float needle to seal properly against the seat, causing the flow of gas to not shut off when the bowl is full. The result will be gas overflowing the carb and running into the cylinder, and possibly out the air intake. If the gas gets into the cylinder, it will seep past the rings and down into the crankcase. This will be evidenced by your oil level being over-full and/or the oil smelling like gas.

 

The only 2 solutions are to either replace the carburetor or give it a good, thorough cleaning.

When removing the carb, make sure to take a good picture, or make a good drawing of where all springs and linkages are attached. This will make reassembly much easier.

Most people believe that cleaning a carb involves removing the bowl and wiping it out, then spraying some carb cleaner through it.

This is simply insufficient.

To properly clean the carb, you must remove it, disassemble it (making sure to remove all non-metal parts), and soak it in a commercial solvent for several hours. Soaking it overnight is even better.

Then clean all solvent off with a spray type carb cleaner, making sure to get lots of cleaner into every hole and passage there is. Pay special attention to the tiny holes in the bore of the carb, under the throttle plate for the carbs that have these holes. Use lots of cleaner. And make sure to wear safety goggles to avoid getting the over spray into your eyes. There will be over spray.

Dry the carb with low pressure compressed air.

When reassembling the carb, make sure to use a carb kit, when one is available for your carb.

 

Occasionally, even a good cleaning is not going to be sufficient, and you may end up having to replace the carb anyhow. Be prepared for this.

 

If for some odd reason this does not help, please let me know so I can assist you further.

 

JustAnswer in the News:

 
 
 
Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... Justanswer.com.
JustAnswer.com...has seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like justanswer.com/legal
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.
 
 
 

What Customers are Saying:

 
 
 
  • You did one super job of explaining to me everything there is to know about this fridge. I'm looking forward to asking you questions in the future. Jimmy Bagley, IA
< Last | Next >
  • You did one super job of explaining to me everything there is to know about this fridge. I'm looking forward to asking you questions in the future. Jimmy Bagley, IA
  • Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help. Mary C. Freshfield, Liverpool, UK
  • This expert is wonderful. They truly know what they are talking about, and they actually care about you. They really helped put my nerves at ease. Thank you so much!!!! Alex Los Angeles, CA
  • Thank you for all your help. It is nice to know that this service is here for people like myself, who need answers fast and are not sure who to consult. GP Hesperia, CA
  • I couldn't be more satisfied! This is the site I will always come to when I need a second opinion. Justin Kernersville, NC
  • Just let me say that this encounter has been entirely professional and most helpful. I liked that I could ask additional questions and get answered in a very short turn around. Esther Woodstock, NY
  • Thank you so much for taking your time and knowledge to support my concerns. Not only did you answer my questions, you even took it a step further with replying with more pertinent information I needed to know. Robin Elkton, Maryland
 
 
 

Meet The Experts:

 
 
 
  • Curtis B.

    Technician

    Satisfied Customers:

    6989
    have worked on and around most engine models for 35 yrs./Polaris ATV expert
< Last | Next >
  • http://ww2.justanswer.com/uploads/KO/koboma/2012-6-6_20917_DSCF1022.64x64.JPG Curtis B.'s Avatar

    Curtis B.

    Technician

    Satisfied Customers:

    6989
    have worked on and around most engine models for 35 yrs./Polaris ATV expert
  • http://ww2.justanswer.com/uploads/SM/SmEngPro/2012-8-5_3564_Rick1206290005Edit.fullres.logo.scrn.64x64.jpg Rick's Avatar

    Rick

    Factory Authorized Trainer

    Satisfied Customers:

    4126
    Outdoor Power Equipment technical trainer since 1990, covering eight states.
  • http://ww2.justanswer.com/uploads/GI/giftindex/2011-11-19_2715_JamesLWells500.64x64.jpg James W's Avatar

    James W

    Small Engine Troubleshooting Expert

    Satisfied Customers:

    2978
    7 years as Mechanic & Parts Manager for Brother who has Owned Lawn & Garden Repair shop for 35 years
  • http://ww2.justanswer.com/uploads/jonrk/2010-1-5_22551_jon.jpg Jon K.'s Avatar

    Jon K.

    Small Engine Diagnostic Expert

    Satisfied Customers:

    2950
    Gold certified Stihl tech, Service Manager for a rental/repair shop.Certified in most small engines
  • http://ww2.justanswer.com/uploads/MR/mr2cycle/2011-3-12_03458_sears.64x64.jpg Vince O.'s Avatar

    Vince O.

    Small Engine Technician

    Satisfied Customers:

    2374
    Over 30 years experience repairing lawn and garden equipment. Chainsaws, mowers,blowers and trimmers
  • http://ww2.justanswer.com/uploads/rheisnger/2010-12-31_010856_41385_100001225699183_7012_q.jpg RUSS's Avatar

    RUSS

    SERVICE TECHNICIAN

    Satisfied Customers:

    1723
    28 YEAR SERVICE TECHNICIAN
  • http://ww2.justanswer.com/uploads/BriggsEngines/2010-02-26_230200_n1834842973_4823.jpg CJBennett's Avatar

    CJBennett

    Small Engine Technician

    Satisfied Customers:

    623
    Briggs and Stratton Master Service Tech, Kohler, Tecumseh, Kawasaki certified. 18 years exp.
 
 
 
Chat Now With A Technician
Hank F.
Hank F.
Technician
13472 Satisfied Customers
Certified on Onan and Generac generators