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Question DetailsAsked on 3/1/2015

6.25 hp Briggs Engine. 12 yrs old. Starts fine, no smoke, runs 8 seconds and shuts down fast over & over.

New: carb, magnito, plug, filter, fuel. Can see good spark and fuel being sucked into intake. Can governor kick in and kill the engine?

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3 Answers

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1
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With you not saying what this is on - could be portable device or very small rider mower - check oill level - some fancier ones on air compressors and pressure washers and rider mowers have oil monitor that shuts it off if oil pressure does not build up.


Failed governor spring (rare - would be one of my last guesses) can do this, but when I had it happen the engine ran up to speed and died in only a second or two - not 8 seconds. Governor linkage usually an external metal rod with springs connecting to the carb - try watching it during the 8 seconds to see if it is yanking the throttle/choke setting back causing it to shut down. Also check to be sure not blocked by twig or leaves or such.


I would check the plug after it dies - if wet then you are pretty sure you are getting fuel, if dry then maybe it is only getting fuel from manual priming or automatic choke, but then the choke is cutting out and starving the engine for fuel. IF your model has accessible automatic choke linkage (or if choke plate is accessible if you remove air cleaner), try using a long tool (not fingers, in case of backfire through carb) to adjust choke angle to see if that will make it keep running - if you can see it after taking off air cleaner, it should be staying full on for probably 20-40 seconds before it opens up wide open if temperature contr4olled type, but the engine-speed type open up as soon as engine comes up to speed.


Another possibility - check air cleaner to be sure not plugged with dirt - wash (and with many models oil) per owners manual instructions.


Another common problem - starts on priming or choke, but when on regular fuel flow leans out and dies due to air leak in fuel line letting air in with fuel, or dirty screen in tank not letting enough fuel in to keep it running. Also, crudded up holes in the tank cap can cause vapor lock by not letting air in to tank to replace fuel consumed - remove and take out inner liner and check holes in liner and metal cap are open so you can see daylight through them.


Another possibility - look for debris, etc keeping throttle from opening up - could be idle screw jiggled loose and is set too low. Try screwing it in a half turn or so to see if keeps running - because might be starting on choke but then when choke starts opening up the idle setting is too low so throttle is not open enough to keep it running.


If consistently runs 8 seconds and dies, try turning the main needle valve 1/4 turn one way, then if tht does not work back the 1/4 turn plus 1/4 turn the other way (keep track of turns so you know where initial setting is) - if one or other makes better than adjust in that range for smooth running and no flooding or leaning out when you open throttle up.

Answered 2 years ago by LCD

0
Votes

Forgot to mention, it's on a Sears push mower. New air filter & oil. With the filter off & looking into the intake port, I watch the choke valve connected to the governor linkage as I have someone pull start it. It closes at first when pulling and opens as engine starts and revs up then 8 seconds later it starts to close and the engine dies. I've held it open with a screw driver but no difference. I've pull the fuel line off the new carburetor and let the fuel drain for about 30 seconds with the cap on tight. It flows really fast. When looking in the carb intake, you can really see fuel being sucked into the intake up from the bowl center jet right up until it dies. It even leaves a fuel fog from un-burnt fuel coming back away from the intake valve port after it dies. Thinking it's flooding out, I've tried blocking some of it that is coming up from the bowl in the center jet with a screw driver blade but still no difference. There's no smoke to show flooding out. Unfortunately there's no adjustment screws on the carb anywhere. The primer bulb is a strange one with a factory hole in the middle that forces air in when you pump with your finger over the hole. I tried taping it off to see it it'll help but still nothing. That stopped any fuel from spraying in the intake before starting and made it a little hard to start. I'll double check the plug if it's wet after dying. Thank you so much for your quick replay. Any ides or suggestions will be greatly appreciated.

Answered 2 years ago by RonParsley

1
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I am having real trouble with this one for two reasons - one I use B&S engines whenever possible becausee they just start and run for decades without problems, so most of my troubleshooting experience is with Hondas and Tecumsehs and other brands. Also, starting is usually the problem, so since yours is starting well and running for 8 seconds that really eliminates most of the normal possible causes.


On reading your reply my first guess was going to be to suggest a cracked (so leaking air) fuel prime bulb - but you say it has a factory hole which you taped over. I have never seen a hole in a fuel prime bulb - instead of directly priming it must be pushing a piston or such so needs air hole to let the piston return to its initial position. I woulddouble-check the bulb to be sure it is not cracked and letting air in.


Because it starts and runs with choke, but not when holding choke open, I would guess a couple of possible causes:


1) main fuel jet clogged, so it starts on primed fuel but then does not pull fuel direct from tank throough main jet. I have never seen a gas engine without at least a main jet, if not both idle and main jets. Some B&S ones are tucked up under the air cleaner - did you look when air cleaner was off ?


2) air leak so it has proper fuel mixture for amount of air it is pulling when full of gas from priming, but then xxxx too much air and starves out after 8 seconds. You didnot say if you checked fuel cap - try with fuel cap loosened (not all the way off for safety) so air can get in around the threads just to rule that out.


3) it is possessed - so copy and paste the text from this question and answer and send to B&S to see if they will suggest something ?


What I would do at this point is try a light (maybe not full push) priming starting at maybe 6 seconds or the instant it starts shutting down to see if it will keep running - if so, that means it is starving for fuel or transitioning to running too lean - so not pulling fuel from tank or air leak into carb/intake. You could also manually fiddle with the choke at partial closure settings when it starts to open up, to see if you can keep it running in that case - if so, then maybe the choke mechanism needs adjustment. Some are thermostatic controlled (though not likely on that small an engine), some run off an air vane that is blown by the cooling fan so it basically chokes when RPM's are low, then the air from the fan opens it wide open at full speed. Could also be a sticky linkage or the linkage spring is too weak so choke plateis "bouncing" or not pulled on hard enough - I have seen the spring both at carb and at vane end. To see vane you have to remove the housing with the pull cord and recoil mechanism in it.


Checking spark plug after it dies - should be dry if starving, wet if flooding out. You could also look down in plug hole with light to see if cylinder/piston is wet and smell for excess gas smell in there.


You could also try starting without the air filter pad in the cleaner - if runs without that, then is not getting adequate airflow through filter pad. If that does not affect it, you could try partly blocking the air cleaner inlet holes by hand to increase the vacuum and make it run richer - if that keeps it running then is trying to run too lean so has a vacuum leak or not drawing enough gas through from the tank or partially blocked main jet.


Check your manual on how governor works - usually overrides throttle but I suppose there could be a design that cuts RPM's in some other way - like cutting out the magneto.


I presume you are starting at the right throttle setting - some makes/models start at idle, some at full throttle - check your manual, and make sure some kink or such is not causing throttle to be at wrong setting for starting, or making the actuall throttle setting different from what the throttle level says. I know my 6.5HP vertical-shaft Quantum B&S mower will not start at idle throttle - wants full throttle at startup as it transitions from primed to tank fuel. Mine also likes 3 pushes on the primer to start and keep running on first pull - one or two pushes it will start but then die after 3-6 seconds. However, my 4.5HP B&S Quantum pressure washer will only start on idle throttle setting. You might try over-priming to see if that keeps it running longer. If does and keeps going - great - just adjust your start procedure. If not - probably starving for fuel or has a vacuum leak.


Other than those, without hands-on I think I am tapped out. I really don't think it is an internal engine problem or blown head gasket, because those usually make for hard or impossible starting but might (if minor) then run OK once it is going - the opposite of your problem.


Google for your manual and for how-to tune up tips and where jets are with this search phrase, or similar phrasing if your is not a Quantum - briggs and stratton quantum

Answered 2 years ago by LCD

0
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Since you have a new carb and mag I would suspect an intake or exhaust valve sticking open just long enough to kill the engine,then closing making restart possible.It is rare but I have seen it happen.

Answered 1 year ago by classicobr




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